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Arbuscular mycorrhizal fungi (AMF) are mutualistic fungi that play important roles in plant nutrition and soil ecosystem functions. While AMF are known to benefit diverse host plants under a range of conditions, little is known about their presence in commercial almond orchards and how frequently used management practices regulate AMF root colonisation. A large-scale survey of almond orchards in the Central Valley of California was conducted to determine the extent of mycorrhizal associations with roots and the impact of orchard management practices and soil properties on AMF root colonisation rates. The roots in all orchards were colonised, with an overall average rate of 64.4%. Organically managed orchards had higher AMF root colonisation rates (73.2%) as compared with conventionally managed orchards (62.1%), primarily due to the presence of soil vegetative cover rather than organic matter inputs. Choice of rootstock and fumigation had only marginal effects while inoculation at planting increased AMF root colonisation of young trees by 27% compared to non-inoculated control. These results highlighted the ubiquitous presence of AMF in commercial almond orchards and significant interacting influences of common management practices on AMF root colonisation under field conditions. Further research into the functional implications of mycorrhizal associations in these orchards will help guide the development of management practices that increase AMF abundance and root colonisation to improve the sustainability of this rapidly expanding industry.

Information on root growth patterns is crucial to understand cultivar adaptations to deficit irrigation, particularly for subsurface drip-irrigated crops, where root systems are more confined than furrow or sprinkler systems. A 2-year field experiment evaluated the effect of deficit irrigation (50% vs. 100% crop evapotranspiration, ETc) on root growth of melon (Cucumis melo L.) cultivars (Mission, Da Vinci, and Super Nectar) on a mollisol soil in southwestern Texas, USA. Root length intensity (La; mm cm−2), and root length density (RLD; cm cm−3) were measured at different growth stages using the minirhizotron and soil core methods, respectively. A higher La was recorded in 2012 than in 2011 (3.71 mm cm−2 vs. 0.50 mm cm−2), due to differences in temporal distribution of rainfall between years. Overall, deficit irrigation increased La as compared to 100% ETc, but significant effects occurred at depth (> 40 cm below the soil surface and > 25 cm below the subsurface drip tape). In 2012, moderate water deficit promoted deep root growth at the fruit setting stage but not in 2011, while root growth responses to cumulative deficit varied with cultivar. In 2012, deficit irrigation enhanced root growth in cv. Mission (cantaloupe; reticulatus) throughout the season, it was maintained in cv. Da Vinci (Tuscan; reticulatus), while it decreased in cv. Super Nectar (honeydew; inodorus) at the final harvest stage. Thus, the reticulatus melon cvs. Mission and Da Vinci can better adjust to moderate water-deficit conditions as compared to the inodorus melon cv. Super Nectar. Minirhizotron data provided useful information on root growth dynamics in deeper soil layers in a non-destructive way, which makes it a valuable tool in screening cultivars for water deficit adaptation.

The inclusion of a smother crop used as a cash crop in an intercropping system may be an effective cultural control strategy for the management of weeds in organic production systems. In addition, a multilayered canopy created when intercropping species with different growth forms may limit germination cues for weed seeds and can allow for a more efficient utilization of resources that reduce competition to target crops from weeds. Watermelon (Citrullus lanatus) was evaluated for its ability to reduce weeds in a low-input organic system in Texas when planted alone or in various intercropping combinations that also included peanut (Arachis hypogaea), okra (Abelmoschus esculentus), cowpea (Vigna unguiculata), and hot pepper (Capsicum annuum). Watermelon significantly reduced total weed biomass when planted in monoculture and in all intercropping combinations compared with peanut, okra, cowpea, and pepper monocultures in year 1 of the 2-year study. Total weed biomass was reduced by 81%, 83%, 88%, and 92% in treatments containing watermelon on average as compared with pepper, peanut, okra, and cowpea grown in monoculture, respectively. Less effective weed suppression was obtained with watermelon in year 2. Pepper grown in monoculture had significantly higher weed biomass than all other treatments in year 2. Broadleaf weeds were effectively suppressed across all intercropping treatments in year 1, but nutsedges (Cyperus sp.) were consistently reduced both years, particularly when compared with monocrops with small leaf area such as pepper. The three and four species intercropping combinations consistently had high leaf area index (LAI) values, whereas pepper monoculture had significantly lower LAI values than all other treatments except for cowpea monoculture. There was a significant negative relationship between LAI and total weed biomass 33 d after last planting (r = −0.51, P < 0.01). There was a significant negative relationship between total weed biomass and total fruit yield in year 1 (r = −0.64, P < 0.01) but no significant relationship in year 2. Although findings were inconsistent in year 2 because of changes in precipitation amounts and in relative planting dates, these findings suggest that incorporating a multifunctional intercropping system that includes a low-growing vining crop such as watermelon or at least an architecturally complex mixture can optimize canopy density to reduce weed pressure from resilient perennial weeds such as nutsedge. This may offer organic producers another management tool for the control of perennial weeds.

Interspecies specific interactions are generally regarded as drivers of plant productivity in multispecies agroecosystems. Complementary use of resource in diverse communities can enhance community productivity through optimal use of plant-available resources and positive interactions such as facilitation can ameliorate high abiotic stress conditions. We studied the effects on physiological response, leaf traits and water use efficiency of a multifunctional species intercropping system consisting of peanut (Arachis hypogaea L.), watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai], okra [Abelmoschus esculentus (L.) Moench], cowpea [Vigna unguiculata (L.) Walp.], and pepper (Capsicum annuum L.) planted alone or in various intercropping combinations in a low fertilizer input system in the peak of summer heat in Texas. Differences in gas exchange measurements were detected only in watermelon in year 2 of the study when okra was the dominant crop. This same year watermelon specific leaf area (SLA) was significantly higher when okra was present in a treatment and particularly in the three and four species combinations, Wpwo and Wpwoc, 27.5 and 31.0 m2 kg−1, respectively, as compared to watermelon grown in monoculture, strip intercropped with peanut (Spw) and within row intercropped with peanut (Wpw), 20.4, 20.1, and 19.8 m2 kg−1, respectively. This corresponds with an increase watermelon leaf N concentration and a decrease in leaf C:N ratio in Wpwo and Wpwoc treatments. No differences in d13C composition, a measure of water use efficiency over the leaf lifespan, were detected across cropping system for each species. Water use efficiency based on per plant production (WUEyield) indicated an increase in water use efficiency in dominant crops such as watermelon in 2011 and okra in 2012, but a reduction in WUEyield subordinate crops such as cowpea and pepper both years of the study. Peanut grown in monoculture and strip intercropped with watermelon had significantly lower leaf water potential values in 2012, −2.2 and −2.1 MPa, respectively, as compared to intercropping systems increasing in level of integration (Wpw = − 1.1, Wpwo = − 0.6, Wpwoc = − 1.3, Wall = −1.1 MPa), indicating peanut benefited from alterations to microclimate and facilitative interactions with companion crops in some intercropping systems through a reduction in plant water stress. The results from this study suggest there may be a benefit to a multifunctional intercropping system in the form of increased food production per unit of water input in dominant crops and reduced water stress for some component species. This is important to producers; showing a method to increase overall crop production without increasing water inputs.

In a two year minirhizotron study (2011 and 2012), the root growth, yield and fruit quality responses of six melon genotypes to two locations with different soil types were investigated. The field experiments were conducted in south Texas at two locations viz. Uvalde with clayey (hyperthermic aridic calciustolls) and Weslaco with sandy loam (hyperthermic typic calciustolls) type soils, with subsurface drip irrigated using black plastic mulch. Root length intensity ( L a , cm cm −2 ), marketable (MFY), total fruit yield (TFY; t ha −1 ), fruit firmness (N) and soluble solids content (SSC; °Bx) of four TAMU inbreds and two commercial hybrids, Mission and Journey were measured. Root length intensity was lower in 2011 than in 2012, possibly due to the variations in climatic, and soil conditions, and minirhizotron tube artifacts. Sandy loam soil produced 77% higher L a (1.6 vs. 0.9 cm cm −2 ) as compared to clay soil. Under sandy loam soils, root growth distribution was deeper (40–70 cm) while it was confined at the depth of drip tape ( L a to most popular commercial cultivar Mission and higher L a than Journey. These results reveal that under high input intensive production systems large root systems may not be required for high yielding potential, however under sandy loam soils cultivars vary in their root distribution patterns across soil depth. Further, the great rooting ability of TAMU breeding lines under different soil types and equivalent yield potential to commercial hybrids confirms their suitability as a potential parent for developing genetically improved and stable cultivars for a wide range of environments.

A greenhouse experiment was conducted to evaluate the effects of water stress on leaf water potential, plant growth, and photosynthesis in purple passionflower (Passiflora incarnata). Twenty 4-L pots with two plants in each pot were arranged in a completely randomized design. Ten pots received a daily irrigation dose of 100% evapotranspiration (ET) throughout the 43-day experiment (control). The other 10 pots were subjected to a reduced irrigation (RI) treatment, which was implemented stepwise to achieve a gradual increase in stress, by irrigating them with 50% ET first, then with 25% ET and, finally, with 10% ET. The last stress phase was followed by a recovery phase in which all treatments received the same amount of water (100% ET). A lower water potential was obtained at 10% ET compared with control plants (−2.51 and −0.98 MPa, respectively). Plants in both 25% and 10% ET irrigation treatments had reduced net CO2 assimilation rates (4.25 and 3.50 μmol·m−2·s−1, respectively) than plants watered with 100% ET (8.53 and 6.77 μmol·m−2·s−1, respectively). Values of maximum carboxylation rate allowed by rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), calculated 31 days after treatment (DAT) application (when RI plants were irrigated with 10% ET) decreased by ≈60%, whereas rate of photosynthetic electron transport and triose phosphate use (TPU) were reduced by ≈30% and ≈45% in the stress treatment compared with the control during the 10% ET irrigation period, respectively. Values of water potential and net CO2 assimilation rates in previously stressed plants were not different from the control treatment in the recovery phase, suggesting that P. incarnata plants could adapt well to landscaping situations where periods of extreme drought can be expected.

Premise While root-order approaches to fine-root classification have shown wide utility among wild plants, they have seen limited use for perennial crop plants. Moreover, inadequate characterization of fine roots across species of domesticated perennial crops has led to a knowledge gap in the understanding of evolutionary and functional patterns associated with different fine-root orders. Methods We examined fine-root traits of common horticultural fruit and nut crops: Malus ×domestica, Prunus persica, Vitus vinifera, Prunus dulcis, and Citrus ×clementina. Additional roots were sampled from 33 common perennial horticultural crops, native to tropical, subtropical, and temperate regions, to examine variation in 1st- and 2nd-order absorptive roots. Results First-order roots of grape and 1st- and 2nd-order roots of apple and peach were consistently thin, nonwoody, mycorrhizal, and had high N:C ratios. In contrast, 4th- and 5th-order roots of grape and 5th-order roots of apple and peach were woody, nonmycorrhizal, had low N:C ratios, and were thicker than lower-order roots. Among the 33 horticultural species, diameter of 1st- and 2nd-order roots varied about 15-fold, ranging from 0.04 to 0.60 mm and 0.05 to 0.89 mm respectively. This variation generally was phylogenetically conserved across plant lineages. Conclusions Collectively, our research shows that root-order characterization has considerably more utility than an arbitrary diameter cutoff for identifying roots of different functions in perennial horticultural crops. In addition, much of the variation in root diameter among species can be predicted by evolutionary relationships.

Subtropical and tropical climates are becoming more vulnerable to drought and salinity problems. Of the crops produced in these climates, citrus is especially sensitive to increasing salinity. In times of water scarcity and periodic drought alternative water sources that may be lower in quality may be required for irrigation, leading to reduced yield unless a mechanism of increasing salinity tolerance is found. The salinity tolerance of citrus rootstocks differs depending on species and cultivar in their ability to exclude or sequester toxic ions. Identifying rootstocks that are more salt tolerant is important to citrus production in susceptible areas. ‘Olinda’ Valencia trees budded to three rootstocks (sour orange [SO], C22, and C146) and the ungrafted rootstocks were subjected to 6 months of saline irrigation. Simulated brackish water at 1, 3, 5, and 10 dS m −1 along with a 0 dS m −1 control were used to irrigate trees over the course of the study. Throughout this time period chlorophyll fluorescence, stomatal conductance, electrolyte leakage, and SPAD were measured to identify how trees responded to different levels of salinity stress. Chlorophyll fluorescence, stomatal control, and SPAD decreased with increased salinity, while electrolyte leakage increased. These responses were more pronounced in grafted trees than ungrafted trees. C22 and C146 rootstocks showed more tolerance to increases in salinity than the SO rootstocks. The ‘Olinda’ Valencia scion reduces rootstock tolerance to salinity, however, trees grafted to the C22 and C146 rootstocks showed more salinity tolerance than those grafted to SO rootstocks. This study emphasizes that both rootstock and scion should be considered when selecting trees for saline conditions.

Leaf temperature (TL) is tightly coupled with the rate of transpirational water loss from the leaf. The temperatures of wet and dry reference leaf surfaces (Twet and Tdry, respectively) are commonly used to normalize temperature measurements for current environmental conditions and then calculate a crop water stress index (CWSI). Since it is often impractical to directly measure Tdry and Twet, the goals of this work were to: i) determine a suitable artificial reference surface that makes application of the CWSI faster and easier in the field, ii) develop a model for Tdry and Twet based on the reference surface temperature that allows for calculation of standard CWSIs, iii) test the technique for a range of weather conditions and tree species, and iv) analyze the sensitivity of these two models to Tdry and Twet, and their impact on the estimation of four different CWSIs. Our results showed that both Tdry and Twet are linearly related to the thermal temperature of green paper across a wide range of environmental conditions. Although there was a significant effect of the light conditions on Tdry and Twet, the same models could be used in both the sun and shade to relate Tdry and Twet to Tref. Moreover, results indicated that a new CWSI dependent only on TL and Twet was least sensitive to errors in Twet, but most sensitive to TL.

Saline irrigation water can lead to salt buildup and reduced crop yields. Halophytic plants are known to accumulate excess salts in tissues, removing them from the immediate environment. This two-phase experiment explored the feasibility of intercropping watermelon (Citrullus lanatus (Thunb.) Matsum. and Nakai var. lanatus) with halophytic species to mitigate the negative effects of saline irrigation water while providing a value-added crop. In the first experiment, six greenhouse-grown species were irrigated with water that was either deionized (0 dS m−1) or contained 3 or 6 dS m−1 of salts for 41 days and screened for growth and salt removal. Two halophytes were selected to be additively intercropped with watermelon under field conditions and irrigated with the same saline irrigation levels as the first experiment. Results indicated that garden orache (Atriplex hortensis L.) exhibited the highest growth rates and purslane (Portulaca oleracea L.) accumulated high amounts of sodium in plant tissues under saline irrigation. The field experiment showed that watermelon yields, stem water potential, and fruit quality were not affected by saline irrigation; however, the watermelon/orache intercropping treatment had significantly higher yields. These results suggest intercropping with halophytes has the potential to contribute a value-added crop without reducing watermelon yields.

A greenhouse study was conducted to evaluate the growth and development of poinsettia ‘Prestige Red’ (Euphorbia pulcherrima) grown in a commercial peat-based potting mix (Sunshine Mix #1) amended with biochar at 0%, 20%, 40%, 60%, 80%, or 100% (by volume) at four different fertigation regimes: F1: 100 to 200 mg·L−1 nitrogen (N), F2: 200 to 300 mg·L−1 N (control), F3: 300 to 400 mg·L−1 N, or F4: 400 to 500 mg·L−1 N. The experiment was a two-factor factorial design with 10 replications for each combination of biochar by fertigation. As the percentage of biochar increased, root substrate pore space and bulk density increased, while container capacity decreased. Root rot and red bract necrosis only occurred in F4 combined with 100% biochar. Plants grown in 40% biochar had a similar growth and development to those in 0% biochar. Up to 80% biochar, plants exhibited no significant change, except in terms of dry weight, which decreased at higher biochar percentages (60% and 80%). In summary, at a fertigation rate of 100 mg·L−1 N to 400 mg·L−1 N, up to 80% biochar could be used as an amendment to peat-based root substrate with acceptable growth reduction and no changes in quality.

In natural ecosystems, plant communities composed of functionally diverse species produce more biomass overall than less diverse communities. This increased biomass production is thought to occur due to complementary use of resources such as nutrients and water, and facilitation during sub-optimal environmental conditions. Using the same concept in a crop setting may lead to increased yield (overyielding) in diverse cropping systems when compared to monocultures. Different combinations of peanut, watermelon, okra, cowpea, and pepper planted alone or in various intercropping combinations were investigated over two growing seasons in a low-input system in the peak of summer heat in Texas. Each species was selected to perform a specific function within the system. Results from land equivalent ratio (LER) indicate that the within-row combination with peanut, watermelon and okra (Wpwo) and peanut, watermelon, okra and cowpea (Wpwoc) consistently overyielded in 2011 and 2012. LER values were 1.17 each for Wpwo and Wpwoc in 2011 and 1.17 and 1.20 in 2012, respectively. In 2011, watermelon was the dominant crop and was up-regulated in all intercropping combinations while all other component crops were down-regulated. Watermelon per plant production was significantly higher in the combination containing all species (Wall) when compared to its monoculture, 5.50 and 2.09 kgfruit plant−1, respectively. In 2012, okra was the dominant crop and was up-regulated in all intercropping combinations while watermelon, cowpea, and pepper were down-regulated. Okra per plant production was significantly higher in Wpwoc and Wall than in monoculture, 2.28, 2.46, and 1.13 kgfruit plant−1, respectively. These findings suggest that three and four species intercropping combinations, whereby each crop is selected to perform a specific function within the system, may provide small-scale sustainably-minded producers a model system that can be utilized in suboptimal conditions and allow them to reduce inputs while increasing overall yields.

As population and agricultural pressure on high quality fresh water resources increases, it becomes more important to identify rootstocks and production techniques that allow the usage of lower quality water in irrigated agriculture. Water of 0, 1, 3, 5 and 10 dS m −1 salinity was applied to grafted (with Olinda Valencia ( Citrus sinensis var. Olinda) scion) and ungrafted sour orange ( Citrus aurantium L.), C22, and C146 ( Citrus sunki Hort. Ex Tan. × Poncirus trifoliata L. Raf. ‘Swingle’) trees to evaluate the effect of grafting and rootstock variety on salinity tolerance. Grafted rootstocks had a lower survival rate at higher salinity concentration than ungrafted trees and effects were dependent upon tree variety. Height growth rate and standing leaf area of grafted citrus trees declined with increased salinity concentrations while the response of ungrafted and grafted citrus trees was variety dependent. Mortality increased and standing leaf area decreased with increasing tissue Na + and Cl − concentrations. Mortality in response to salinity was greater in grafted rootstocks than mortality of ungrafted trees. Overall, C22 and C146 rootstocks and ungrafted trees were found to be more tolerant of saline conditions than grafted SO rootstock or ungrafted SO trees.

Water scarcity associated with intense and frequent droughts has increased the need for the implementation of drought adaptation strategies that can save water and sustain crop productivity in water limited environments. A two season (2011 and 2012) study evaluated root growth, yield and fruit quality responses of cvs. Mission (muskmelon; reticulatus ), Da Vinci (tuscan; reticulatus ) and Super Nectar (honeydew; inodorus ) of melon ( Cucumis melo L.) to two irrigation rates (100% and 50% crop evapotranspiration (ETc)) on a silty clay soil under the semi-arid conditions of Texas. Deficit irrigation (50% ETc) increased root length density (RLD) in Mission, decreased in Da Vinci and did not affect in Super Nectar. Marketable fruit yield at 100% ETc irrigation was 77.1 t ha −1 in 2011 and 78.7 t ha −1 in 2012, but deficit irrigation caused a 30% decrease in marketable yield in both seasons, mainly due to a reduction in fruit size. Yield responses to deficit irrigation also varied with cultivar. A significant yield reduction of 43% in 2011 and 33% in 2012 was measured in Super Nectar, while for cvs. Mission and Da Vinci the reduction in yield was 24% and 30%, respectively in 2012. Deficit irrigation had no adverse impact on melon fruit quality; rather it increased total soluble solids content (23%) in Mission and β-carotene content (25%) in Da Vinci in 2011. At 50% ETc, agronomic water use efficiency (WUE; kg ha −1 mm −1 ) was improved in Mission (13%) in 2012 while it decreased in Super Nectar (21%) in 2011. These results showed that deficit irrigation can save 37–45% of irrigation water in Mission and Da Vinci cultivars ( reticulatus ) with a moderate reduction in economic yield. This practice may not be applicable for cv. Super Nectar ( inodorus ), as it reduced yield without improving water use efficiency.

As green roofs continue to grow in popularity more research will be needed in new ecoregions to support development of policy, regulation, and incentives. Extensive green roofs represent the majority of new construction each year and understanding their performance expectations is critical for regulation. In our experiment we tested the ability of three monocultures of low stature CAM plants (Sedum kamtschaticum, Delosperma cooperi and Talinum calycinum syn. Phemeranthus calycinus) to improve runoff reduction efficiency of un-irrigated modular extensive green roofs exposed to a range of precipitation event sizes in a warm and dry southern US climate. Stormwater runoff data were collected for 15 rainfall events that ranged from a minimum of 4.1 mm to a maximum of 102.9 mm. An average precipitation event retention efficiency of 78% was recorded. On average, the presence of T. calycinum enhanced retention efficiency by an additional 7.5% compared to unvegetated modules. Substrate volumetric water content affected retention capacity of unvegetated modules only.

Root zones of mature Sweetgum paved with either pervious or impervious concrete after 15–18 years of tree growth exhibited much reduced standing fine root length compared to unpaved plots 3 years after pavement installation. The objective of this paper was to determine whether these observed reductions in standing root length were due to reduced root production rates or due to enhanced root mortality rates in response to the presence of either pavement. We measured both fine root production and root death over a 15-month period using biweekly nondestructive observations of the root zones at four depths. In addition, we used proportional hazards analysis to determine how the presence of pavement affected chances of root mortality. We found that new root production was more negatively affected by the presence of either pavement than root mortality while chances of root mortality were increased by the presence of pavement, leading to much reduced standing net root length in paved plots at any given time. Surprisingly, even though root production and mortality were strongly altered by pavement, there was no significant effect on tree diameter growth. These results suggest that while Sweetgum root dynamics are negatively affected by pavement, there is enough plasticity within this species to adapt to altered root zone dynamics without affecting aboveground growth.

After an outbreak of blotch leafminer (Cameraria caryaefoliella) on field-grown pecan (Carya illinoinensis) trees in 2010, an experiment was conducted to evaluate the consequences of the injury on carbon assimilation and photosynthetic efficiency, and, in particular, to assess if low-to-moderate injury induces a compensatory increase in photosynthesis. Gas exchange and light-adapted fluorescence were measured on non-injured portions of the leaflet lamina adjacent to the injured area as well as on portions of leaflets that included leafminer injury. Results indicate that damage of the photosynthetic apparatus did not extend beyond the injured areas by leafminers. Furthermore, although a strong relationship between the proportion of leafminer injury and area-based net CO2 assimilation rate of injured leaflet tissue was found, there was no evidence that pecan leaves were able to compensate for leafminer injury by upregulating CO2 assimilation in leaflet tissue that was unaffected.

We conducted a study to determine if inoculation with arbuscular mycorrhizal fungi (AMF) would enhance the tolerance of vinca [Catharanthus roseus (L.) G. Don] plants to sodium chloride (NaCl)-induced salinity in irrigation water. Vinca tolerated salinity levels up to 40 mM. Chlorophyll concentration, proline synthesis, and total antioxidant activity were increased with saline irrigation, while leaf potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), and boron (B) content decreased, suggesting a detrimental salinity effect. Despite the limited effect of increasing salinity on N content, NaCl-stressed vinca plants showed a marked decrease in nitrate reductase activity, which was associated with decreased leaf K and Fe total content. Mycorrhizal inoculation resulted in reduced growth when compared to non-AMF plants, regardless of salinity level. The K/sodium (Na) ratio decreased dramatically with increasing salinity regardless of AMF treatment. Suggesting, that the AMF isolate was not able to selectivel...

Modular green roofs were investigated to better understand surface and membrane level temperature expectations of unirrigated green roofs during hot summer conditions in south-central Texas. We used three succulent monocultures, Sedum kamtschaticum, Delosperma cooperi, Talinum calycinum syn. Phemeranthus calycinus and one unplanted control module, each replicated 3 times. Media surface and below media temperatures were monitored, as well as soil water content and general weather conditions (RH, air temperature). Temperatures at the surface and below the media surface were compared with temperatures of a standard roof surface. We found that diurnal surface temperature reductions were very stable throughout the summer. Much larger temperature reductions were achieved below the modules than at the soil surface. Temperature reductions at the soil surface were predominantly driven by soil volumetric water content (VWC) and, to a lesser degree, air temperature while species and percent cover had small modifying effects through interactions with VWC and air temperature. Temperature reductions below the modules were driven by surface soil temperature, while increasing VWC led to a small decrease in temperature reductions at the membrane level. Mean daily temperature reductions achieved were 18.0 °C at the soil surface and 27.5 °C below the module, thus demonstrating that unirrigated, succulent-based green roofs can provide significant rooftop temperature reductions during hot, dry summer conditions.

Planting depth (location of root collar in relation to substrate surface grade) varies in the container production industry. Optimum planting depth varies for different species. Most container-produced plants are grown in a soilless media to allow better aeration and drainage conditions, so the effects of planting depth observed in landscape establishment may be different from those seen in container-production. This study tested if inoculation with arbuscular mycorrhizal fungi (AMF) would enhance the tolerance of container-grown Ulmus parvifolia Jacq. (lacebark elm) to above and/or below grade planting depths. The AMF plants and non-AMF plants were planted into 2.1 L black plastic containers with their root collars at substrate surface (grade), 3 cm above grade, 1 cm above grade, 1 cm below grade, or 3 cm below grade under greenhouse conditions. Growth (stem diameter, total plant dry mass) and physiological responses (leaf nutrient concentration, chlorophyll concentration, total antioxidant and nitrate reductase activities, and total phenolic content) were measured. In general, AMF plants had greater growth when planted above or below grade when compared to non-AMF plants at the same planting depth, but this was not correlated with enhanced nutrient tissue levels. Non-AMF plants planted at grade had greater growth than AMF plants at grade. Planting at grade resulted in optimum growth for U. parvifolia. Poor transplanting techniques during container production (above or below grade) were partially alleviated by inoculating plants with AMF.

Mature trees with large canopies help reduce problems of excessive heat and surface runoff in urban areas through shading, transpirational cooling, and interception and absorption of precipitation. Trees in paved impervious areas often suffer from a poor root zone environment. The objective of this experiment was to test if use of pervious concrete could be more beneficial for root growth and consequently a better alternative towards preserving existing mature trees during urban development. We measured root activity through CO 2 flux, and also measured root production of American sweetgum under impervious concrete, pervious concrete and no pavement conditions (control). Soil under impervious concrete had lower oxygen concentrations than soil under pervious concrete and control treatments, particularly under wet conditions. Soil under pervious concrete had a 0.020 m 3 m −3 (2% volumetric water content (VWC)) greater volumetric water content than soils paved with impervious concrete. Soil CO 2 efflux rates and soil CO 2 concentrations were extremely high under both concrete treatments. Standing live root length at the end of the experiment was greater under the control treatment than under both concrete treatments and these changes in root production were likely due to high soil CO 2 concentrations under both pervious and impervious concrete. The soil type at our site, which is a Ships clay with a very low permeability rate, may have limited CO 2 exchange through the pervious pavement. On this heavy clay soil, pervious concrete did not convey any measurable root growth benefits over impervious concrete.

The effect of planting depth, defined as the location of the root collar relative to soil surface, is of particular concern for tree growth, development, and performance in the landscape and seems to be dependent on soil conditions. The objective of this study was to determine the effect of three planting depths and four soil amendments on live oak (Quercus virginiana Mill.) growth and visual quality. Trees were planted with root collars at one of three planting depths (grade, 7.6 cm above grade, or 7.6 cm below grade). Soil amendments were: incorporated sand (30% by volume), incorporated composted peat (30% by volume), sandy topsoil in raised (20 cm) bed, and native sandy loam soil (control). Planting at grade or below grade resulted in 0% mortality, while planting live oak trees above grade resulted in 12.5% mortality. The container produced trees were top-heavy (high shoot:root ratio) and were thus susceptible to wind damage when planted above grade. Trunk diameter growth and relative growth rate were smaller when trees were planted with root collars below grade compared to those planted above grade or at grade. Visual quality of live oak roots was improved when trees were planted in raised beds with sandy topsoil compared to the control soil. Shoot visual quality was improved when trees were planted in the incorporated sand and raised beds with sandy topsoil compared to the control and incorporated peat sections. Thus, it is recommended to plant live oaks with the root collar at grade and in sandy top soils in raised beds to improve tree quality after establishment.

Installation of intensive and extensive green roofs is becoming popular for reducing runoff from impervious surfaces in many cities around the world. Most studies on runoff quality from green roofs have been conducted in cooler northern climates. We examined the losses and gains of nutrients, cations and selected anions in planted and unplanted growth medium and compared these to initial growth medium (IGM) typically used for green roof modules in south-central Texas. Water extracts of growth medium and leachate from three replicates of unplanted growth medium and three planted species (Sedum kamtschaticum, Delosperma cooperi and Talinum calycinum) were examined. During the first 6 months after establishment we observed high losses of nitrate (25 to 44 mg kg−1), dissolved organic carbon (DOC: 155 to 190 mg kg−1) and nitrogen (DON: 9.0 to 11.2 mg kg−1) and orthophosphate-P (1 to 2 mg kg−1). Average leachate concentrations based on four rain events 6 months after establishment ranged from 0.3 to 6.6 mg L−1 in planted modules and 6.3 mg L−1 in unplanted modules for nitrate-N, 38 to 42 mg L−1 in planted modules and 32 mg L−1 in unplanted modules for DOC, 2.1 to 3.1 mg L−1 in planted modules and 2.1 mg L−1 in unplanted modules for DON and 0.27 to 0.37 mg L−1 in planted modules and 0.40 mg L−1 in unplanted modules for orthophosphate-P. We suggest that after the establishment of green roofs, leachate losses may contribute some runoff concentrations of nitrogen, carbon and phosphorous in urban areas.

A green roof is a vegetated roof or deck designed to provide urban greening for buildings, people, or the environment. Made popular across Europe over the past few decades, green roofs are now becoming more familiar to North Americans as some cities have built green roof pilot projects and adopted incentives for using green roofs or even require their use. Green roof standards and guidelines are also emerging to be used for governance and project specification. Although much is known about the application of green roofs across Europe, much less is known about their application across North America's diverse ecological regions. When considering the many decisions required in applying green roof technology to a specific place, there are few choices more critical to their success than the selection of appropriate vegetation. We conducted a review of green roof research to investigate what is known about the application of plants on green roofs across North America and their ecological implications. Results indicate that investigation sites across ecoregions begin to reveal differences in plant survival. Although ecological investigations are limited, their results show improved plant performance and ecological services with diverse green roofs. We conclude that as green roofs continue to become regulated and adopted in policy, further development of standards and guidelines is needed. To date, there is no common ground for reporting of green roof research, and we make recommendations for facilitating such efforts for improved research, policy development and their management across North America's diverse ecological regions.

Tree transplanting practices influence plant survival, establishment, and subsequent landscape value. The inability to adequately quantify effects of transplanting practices threatens long-term sustainability of landscape trees. Planting depth [i.e., location of the root collar relative to soil grade (soil surface)], is of particular concern for tree growth, development, and landscape performance. The authors of this study investigated the effects of planting depth and transplant season on landscape establishment of baldcypress [Taxodium distichum (L.) Rich.] and effects of planting depth and irrigation practices on landscape establishment of sycamore (Platanus occidentalis L.). Baldcypress planted above grade had reduced relative growth rate in height and diameter compared to those planted at or below grade during the first growing season, regardless of transplant season. Sycamore trees planted below grade had increased mortality and decreased growth compared to trees planted at grade or above grade, regardless of irrigation treatment. Even though trees of both species were grown under similar conditions, baldcypress was much more tolerant to belowgrade planting than sycamore. We suggest that this is related to the native habitat of both species, where baldcypress is frequently exposed to hypoxic conditions while sycamore is more prevalent on well-drained soils. Thus, it may be important to consider the native habitat of a species when evaluating the effect of planting depth.

Planting depth during container production may influence plant growth, establishment, and subsequent landscape value. A lack of knowledge about the effects of common transplanting practices may lead to suboptimal performance of planted landscape trees. Planting depth, i.e., location of the root collar relative to soil grade, is of particular concern for posttransplant tree growth both when transplanted to larger containers during production and after transplanting into the landscape. It is unknown whether negative effects of poor planting practices are compounded during the production phases and affect subsequent landscape establishment. This study investigated effects of planting depth during two successive phases of container production (10.8 L and 36.6 L) and eventual landscape establishment using lacebark elm (Ulmus parvifolia Jacq.). Tree growth was greater when planted at grade during the initial container (10.8 L) production phase and was reduced when planted 5 cm below grade. In the second container production phase (36.6 L), trees planted above grade had reduced growth compared with trees planted at grade or below grade. For landscape establishment, transplanting at grade to slightly below or above grade produced trees with greater height on average when compared with planting below grade or substantially above grade, whereas there was no effect on trunk diameter. Correlations between initial growth and final growth in the field suggested that substantial deviations of the original root to shoot transition from at-grade planting was more of a factor in initial establishment of lacebark elm than the up-canning practices associated with planting depth during container production.

Climate warming and drought may alter tree establishment in savannas through differential responses of tree seedlings and grass to intermittent rainfall events. We investigated leaf gas exchange responses of dominant post oak savanna tree (Quercus stellata and Juniperus virginiana) and grass (Schizachyrium scoparium ,C 4 grass) species to summer rainfall events under an ambient and intensified summer drought scenario in factorial combination with warming (ambient, 11.5 1C) in both monoculture and tree-grass mixtures. The three species differed in drought resistance and response of leaf gas exchange to rainfall events throughout the summer. S. scoparium experienced the greatest decrease in Aarea (� 56% and � 66% under normal and intensified drought, respectively) over the summer, followed by Q. stellata (� 44%, � 64%), while J. virginiana showed increased Aarea under normal drought (113%) and a small decrease in Aarea when exposed to intensified summer drought (� 10%). Following individual rainfall events, mean increases in Aarea were 90% for S. scoparium, 26% for J. virginiana and 22% for Q. stellata. The responsiveness of Aarea of S. scoparium to rainfall events initially increased with the onset of drought, but decreased dramatically as summer drought progressed. For Q. stellata, Aarea recovery decreased as drought progressed and with warming. In contrast, J. virginiana showed minimal fluctuations in Aarea following rainfall events, in spite of declining water potential, and warming enhanced recovery. J. virginiana will likely gain an advantage over Q. stellata during establishment under future climatic scenarios. Additionally, the competitive advantage of C4 grasses may be reduced relative to trees, because grasses will likely exist below a critical water stress threshold more often in a warmer, drier climate. Recognition of unique species responses to critical global change drivers in the presence of competition will improve predictions of grass–tree interactions and tree establishment in savannas in response to climate change.

During the development of parking lots and pedestrian areas, mature trees are often removed from the site and replaced with saplings after development is completed. This research aims to study the possibility of using pervious porous concrete as an alternative pavement that may help to keep existing mature trees, and all the benefits they can provide, alive after site development. Three different pavement treatments: (1) no pavement; (2) impervious standard concrete; and, (3) pervious porous concrete were compared over 2 years to test their ability to modify growth and leaf gas exchange of existing 15–18-year-old American sweetgum ( Liquidamber styraciflua ) trees. Soil water and temperature dynamics of the pervious plots were closely correlated with those in the control plots. Plots treated with pervious concrete had greater soil water content than plots with standard concrete in the deeper soil layers in some seasons, but not during the summer. Tree diameter (dbh) relative growth rates were similar across the three pavement types. There was no effect of the treatments on leaf water potential or leaf gas exchange. The data presented in this paper suggest that pervious porous concrete does not provide additional benefits in ensuring growth and survival of pre-existing mature trees in new urban developments.

We investigated seasonal root production and root turnover of fertilized and wellwatered monocultures of Phalaris for 2 years using minirhizotrons installed in six newly designed temperature gradient tunnels, combined with sequential soil coring. Elevated atmospheric CO2 treatments were combined with two cutting frequencies and three warming scenarios: no warming, 13.0/13.0 and 12.2/14.01C (day/night) atmospheric warming. The elevated CO2 treatment increased both new and net root length production primarily when combined with atmospheric warming, where the constant warming treatment had a greater positive effect than the increased night-time warming treatment. Responses to elevated CO2 were greater when the swards were cut more frequently and responsiveness varied with season. For Phalaris swards, 17% of total net primary productivity went belowground. On account of root turnover, only one-third of the new roots produced in the year following establishment could be expected, on average, to be recovered from soil cores. The interaction between the effects of CO2 and warming, combined with the differential effects of the two warming treatments, has important implications for modelling belowground responses to projected climate change.

This research shows that elevated CO2, climate warming, and management impact shoot and root nitrogen concentrations in different ways in managed pastures. Management (clipping frequency) had the strongest impact on aboveground tissue N concentrations, while the impact of the climate change drivers on shoot N concentration was interactive and varied seasonally. Green leaf N concentrations were decreased by elevated CO2 and increased by more frequent clipping. Both warming treatments increased leaf N concentrations under ambient CO2 concentrations, but did not significantly alter leaf N concentrations under elevated CO2 concentrations. Fine root N concentrations were mostly unaffected by the treatments, although elevated CO2 decreased root N concentration in deeper soil layers. The interactive nature of the climate change drivers through time, as well as the fact that root N concentration response to the treatments was entirely different from aboveground responses, highlights the complexity in predicting plant N nutrition responses to projected climate change., Forecasting the effects of climate change on nitrogen (N) cycling in pastures requires an understanding of changes in tissue N. We examined the effects of elevated atmospheric CO2 concentration, atmospheric warming and simulated grazing (clipping frequency) on aboveground and belowground tissue N concentrations and C : N ratios of a C3 pasture grass. Phalaris aquatica L. cv. ‘Holdfast’ was grown in the field in six transparent temperature gradient tunnels (18 × 1.5 × 1.5 m each), three at ambient atmospheric CO2 and three at 759 p.p.m. CO2. Within each tunnel, there were three air temperature treatments: ambient control, +2.2/+4.0 °C above ambient day/night warming and +3.0 °C continuous warming. A frequent and an infrequent clipping treatment were applied to each warming × CO2 combination. Green leaf N concentrations were decreased by elevated CO2 and increased by more frequent clipping. Both warming treatments increased leaf N concentrations under ambient CO2 concentrations, but did not significantly alter leaf N concentrations under elevated CO2 concentrations. Nitrogen resorption from leaves was decreased under elevated CO2 conditions as well as by more frequent clipping. Fine root N concentrations decreased strongly with increasing soil depth and were further decreased at the 10–60 cm soil depths by elevated CO2 concentrations. The interaction between the CO2 and warming treatments showed that leaf N concentration was affected in a non-additive manner. Changes in leaf C : N ratios were driven by changes in N concentration. Overall, the effects of CO2, warming and clipping treatments on aboveground tissue N concentrations were much greater than on belowground tissue.

Summary • This study examined the effects of warming, elevated atmospheric CO 2 and cutting regimen on the growth of Phalaris aquatica cv. Holdfast swards. • Six temperature gradient tunnels (TGT) were used to manipulate both air temperature and atmospheric CO 2 concentrations (ambient and 750 ppm). Within each tunnel, there were three temperature treatments: no warming, constant warming of +3.0 ° C and a daytime warming of 2.2 ° C combined with a night-time warming of 4.0 ° C and two defoliation frequencies. •A veraged across 20 months of growth, there was a positive effect of elevated atmospheric CO 2 (+11%), no effect of either warming treatment and a negative effect of frequent cutting ( − 19%) on total above ground biomass production. The responses to all treatments, however, were strongly seasonal. • Positive responses to CO 2 were statistically significant only in the spring, when plant growth was strongest. No evidence was found that high night-time warming had different effects on plant growth and plant responses to elevated atmospheric CO 2 , than constant warming.

Polar-desert plants experience low average air temperatures during their short growing season (4–8 °C mean July temperature). In addition, low availability of inorganic nitrogen in the soil may also limit plant growth. Our goals were to elucidate which N sources can be acquired by polar-desert plants, and how growth and N-uptake are affected by low growth temperatures. We compared rates of N-uptake and increases in mass and leaf area of two polar-desert species (Cerastium alpinum L. and Saxifraga caespitosa L.) over a period of 3 weeks when grown at two temperatures (6 °C vs. 15 °C) and supplied with either glycine, NH4 + or NO3 −. At 15 °C, plants at least doubled their leaf area, whereas there was no change in leaf area at 6 °C. Measured mean N-uptake rates varied between 0.5 nmol g−1 root DM s−1 on glycine at 15 °C and 7.5 nmol g−1 root DM s−1 on NH4 + at 15 °C. Uptake rates based upon increases in mass and tissue N concentrations showed that plants had a lower N-uptake rate at 6 °C, regardless of N source or species. We conclude that these polar-desert plants can use all three N sources to increase their leaf area and support flowering when grown at 15 °C. Based upon short-term (8 h) uptake experiments, we also conclude that the short-term capacity to take up inorganic or organic N is not reduced by low temperature (6 °C). However, net N-uptake integrated over a three-week period is severely reduced at 6 °C.

ently maintain or very quickly recover P and water uptake capacity. This behaviour is consistent with an The effects of prolonged exposure to dry surface soil overall rooting strategy where essentially no surface on the capacity of roots to take up water and phos- roots are shed following prolonged exposure to dry phorus were examined in mycorrhizal sour orange soil. (Citrus aurantium L.) seedlings grown in pots with upper and lower portions separated hydraulically. In Key words: Drought, split-root, 32P, citrus, uptake kinetics, the first experiment, upper portions of the pots were phosphorus uptake, water uptake, sour orange, Citrus either irrigated every 2‐3 d, droughted for 14 d, aurantium. droughted for 43 d, or droughted for 42 d followed by 8 d re-irrigation. Lower portions of the pots were irrig- Introduction ated and fertilized every 2‐3 d. Phosphorus uptake capacity was estimated in excised roots using 32 Pi n The soil surface layers, with their accumulation of roots, aerated 50, 750, and 1500 mM P solutions. Exposure organic matter and nutrients, commonly undergo large to dry soil had no appreciable effect on P uptake fluctuations in temperature, water content, and nutrient capacity. In the second experiment, the ability of intact availability. These fluctuations promote pulses of nutrient roots to acquire water and P in the 8 d following release by increasing microbial biomass turnover (Reid, rewatering after roots were exposed to localized 1974; Stark, 1994; Cui and Caldwell, 1997). Moisture drought for 14 and 43 d was examined. Roots were and temperature fluctuations may cause opportunities for observed non-destructively using small transparent nutrient uptake by roots in the litter, organic and surface tubes (2 cm diameter) and a rigid borescope. Soil water mineral layers to be brief and unpredictable (Grime, depletion was monitored using time-domain reflectro- 1994). There are at least two basic strategies for exploiting metry. Phosphorus (32P) was added at various depths resources in soil surface layers that periodically dry and in the soil in the upper compartment and uptake was which may develop high temperatures. One strategy is to assessed by non-destructively counting beta particle grow rapidly relatively thin inexpensive roots that have emissions from leaves using a scintillation probe. high absorptive capacity when the soil is wet, and then Similar to the first experiment, localized drought had rapidly shed these roots when the soil becomes dry or no effect on P uptake and soil water depletion in citrus otherwise unfavourable. This kind of root strategy is roots compared to continuously irrigated plants. Water commonly found in hot desert climates with desert succuand P uptake in the first few days apparently occurred lents (Huang and Nobel, 1992) and in certain desert from existing roots because of delayed production of half-shrubs like Cryptantha (personal observation). In new roots in the droughted treatment. Thus, citrus addition, many annual crops seem to exhibit this strategy ( Eissenstat and Yanai, 1997). roots exposed to extended periods of dry soil appar

Aquatic seedlings of Ranunculus peltatus Schrank were subjected to three levels of droughl (dry, saturated and flooded) at two different phenological stages, vegetative stage and at onset of flowering. Changes in growth form, timing of flowering, number of flowers and fiuits and the number of seeds per fruit were monitored to test the effect of drought on reproduction. Plants that were transferred to a terrestrial, but water-saturated, environment at a vegetative stage rapidly changed from an aquatic, spreading, elongated growth form to a prostrate growth form and produced only slightly fewer seeds per pot (86) than the flooded control plants (93). Total seed production was decreased by 1000 seeds per pot in both saturated treatments compared to the control treatment. There was no significant difference in total seed production between plants encountering drought at a vegetative or a flowering stage (2471 vs. 2348 seeds per pot). Plants subjected to air drying of the soil (no water added) at both phenological stages changed growth form, but flower, fruit and seed production was negligible. The fact that total seed production was still relatively high under water-saturated conditions at both phenological stages may explains partially why this species is successful in temporary marshes where drought often occurs during its lifecycle. ~_3 1997 Elsevier Science B.V.

Savanna tree–grass interactions may be particularly sensitive to climate change. Establishment of two tree canopy dominants, post oak (Quercus stellata) and eastern redcedar (Juniperus virginiana), grown with the dominant C4 perennial grass (Schizachyrium scoparium) in southern oak savanna of the United States were evaluated under four climatic scenarios for 6 years. Tree–grass interactions were examined with and without warming (+1.5 °C) in combination with a long-term mean rainfall treatment and a modified rainfall regime that redistributed 40% of summer rainfall to spring and fall, intensifying summer drought. The aim was to determine: (1) the relative growth response of these species, (2) potential shifts in the balance of tree–grass interactions, and (3) the trajectory of juniper encroachment into savannas, under these anticipated climatic conditions. Precipitation redistribution reduced relative growth rate (RGR) of trees grown with grass. Warming increased growth of J. virginiana and strongly reduced Q. stellata survival. Tiller numbers of S. scoparium plants were unaffected by warming, but the number of reproductive tillers was increasingly suppressed by intensified drought each year. Growth rates of J. virginiana and Q. stellata were suppressed by grass presence early, but in subsequent years were higher when grown with grass. Quercus stellata had overall reduced RGR, but enhanced survival when grown with grass, while survival of J. virginiana remained near 100% in all treatments. Once trees surpassed a threshold height of 1.1 m, both tiller number and survival of S. scoparium plants were drastically reduced by the presence of J. virginiana, but not Q. stellata. Juniperus virginiana was the only savanna dominant in which neither survival nor final aboveground mass were adversely affected by the climate scenario of warming and intensified summer drought. These responses indicate that climate warming and altered precipitation patterns will further accelerate juniper encroachment and woody thickening in a warm-temperate oak savanna.

In recent years, increased awareness of the potential interactions between rising atmospheric CO2 concentrations ([ CO2 ]) and temperature has illustrated the importance of multifactorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and [ CO2 ] manipulation, and compares it with those obtained in single factor [ CO2 ] and temperature manipulation experiments. Across all combined elevated [ CO2 ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [ CO2 ]-only treatment than to those in the warming-only treatment. In contrast to warming-only experiments, both the combined and the [ CO2 ]-only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [ CO2 ]-only treatment, possibly due to the warming-induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [ CO2 ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [ CO2 ] and warming, i.e. the response to the combined treatment was usually less-than-additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long-term) multifactor manipulation experiments. Because single factor CO2 responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.

We only have a limited understanding of the nutrient uptake physiology of individual roots as they age. Despite this shortcoming, the importance of nutrient uptake processes to our understanding of plant nutrition and nutrient cycling cannot be underestimated. In this study, we used a

Coupled surface–atmosphere models are being used with increased frequency to make predictions of tropospheric chemistry on a ‘future’ earth characterized by a warmer climate and elevated atmospheric CO 2 concentration. One of the key inputs to these models is the emission of isoprene from forest ecosystems. Most models in current use rely on a scheme by which global change is coupled to changes in terrestrial net primary productivity (NPP) which, in turn, is coupled to changes in the magnitude of isoprene emissions. In this study, we conducted measurements of isoprene emissions at three prominent global change experiments in the United States. Our results showed that growth in an atmosphere of elevated CO 2 inhibited the emission of isoprene at levels that completely compensate for possible increases in emission due to increases in aboveground NPP. Exposure to a prolonged drought caused leaves to increase their isoprene emissions despite reductions in photosynthesis, and presumably NPP. Thus, the current generation of models intended to predict the response of isoprene emission to future global change probably contain large errors. A framework is offered as a foundation for constructing new isoprene emission models based on the responses of leaf biochemistry to future climate change and elevated atmospheric CO 2 concentrations.

Changes in function as an individual root ages has important implications for understanding resource acquisition, competitive ability and optimal lifespan. Both nitrate uptake and respiration rates of differently aged fine roots of grape (Vitis rupestris x V. riparia cv. 3309 C) were measured. The resulting data were then used to simulate nitrate uptake efficiency and nutrient depletion as a function of root age. Both nitrate uptake and root respiration declined remarkably quickly with increasing root age. The decline in both N uptake and root respiration corresponded with a strong decline in root N concentration, suggesting translocation of nitrogen out of the roots. For simulations where no nutrient depletion occurs at the root surface, daily uptake efficiency was maximal at root birth and lifetime nitrate uptake efficiency slowly increased as the roots aged. Simulations of growth of roots into unoccupied soil using a solute transport model indicated the advantage of high uptake capacity in new roots under competitive conditions where nitrate availability is very transitory.