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While tree rings have enabled interannual examination of the influence of climate on trees, this is not possible for most shrubs. Here, we leverage a multidecadal record of annual foliar carbon isotope ratio collections coupled with 39 y of survey data from two populations of the drought-deciduous desert shrub Encelia farinosa to provide insight into water-use dynamics and climate. This carbon isotope record provides a unique opportunity to examine the response of desert shrubs to increasing temperature and water stress in a region where climate is changing rapidly. Population mean carbon isotope ratios fluctuated predictably in response to interannual variations in temperature, vapor pressure deficit, and precipitation, and responses were similar among individuals. We leveraged the well-established relationships between leaf carbon isotope ratios and the ratio of intracellular to ambient CO2 concentrations to calculate intrinsic water-use efficiency (iWUE) of the plants and to quantify plant responses to long-term environmental change. The population mean iWUE value increased by 53 to 58% over the study period, much more than the 20 to 30% increase that has been measured in forests [J. Penuelas, J. G. Canadell, R. Ogaya, Glob. Ecol. Biogeogr. 20, 597–608 (2011)]. Changes were associated with both increased CO2 concentration and increased water stress. Individuals whose lifetimes spanned the entire study period exhibited increases in iWUE that were very similar to the population mean, suggesting that there was significant plasticity within individuals rather than selection at the population scale.

Significance As the proportion of land area covered by arid land vegetation continues to expand and water limitations for plants increase, understanding if and how desert shrubs are responding to environmental change has become more urgent. Among two populations of Mojave Desert shrubs, we found that intrinsic water-use efficiency has increased substantially over the last three decades in response to increasing aridity and CO2 concentration. While increases in intrinsic water-use efficiency have been widely assumed to mitigate negative effects of decreasing water availability, precise effects on plant productivity, reproduction, and survival remain unknown., While tree rings have enabled interannual examination of the influence of climate on trees, this is not possible for most shrubs. Here, we leverage a multidecadal record of annual foliar carbon isotope ratio collections coupled with 39 y of survey data from two populations of the drought-deciduous desert shrub Encelia farinosa to provide insight into water-use dynamics and climate. This carbon isotope record provides a unique opportunity to examine the response of desert shrubs to increasing temperature and water stress in a region where climate is changing rapidly. Population mean carbon isotope ratios fluctuated predictably in response to interannual variations in temperature, vapor pressure deficit, and precipitation, and responses were similar among individuals. We leveraged the well-established relationships between leaf carbon isotope ratios and the ratio of intracellular to ambient CO2 concentrations to calculate intrinsic water-use efficiency (iWUE) of the plants and to quantify plant responses to long-term environmental change. The population mean iWUE value increased by 53 to 58% over the study period, much more than the 20 to 30% increase that has been measured in forests [J. Peñuelas, J. G. Canadell, R. Ogaya, Glob. Ecol. Biogeogr. 20, 597–608 (2011)]. Changes were associated with both increased CO2 concentration and increased water stress. Individuals whose lifetimes spanned the entire study period exhibited increases in iWUE that were very similar to the population mean, suggesting that there was significant plasticity within individuals rather than selection at the population scale.

Germination, establishment, phenology, and death among three drought-deciduous shrubs were influenced by ENSO/PDO and precipitation, based on 37 years of annual surveys. Encelia farinosa forms near monospecific stands on slopes, whereas E. frutescens and Ambrosia salsola dominate wash habitats. All shrubs exhibited phenological coherence. While germination, establishment, and mortality patterns were similar among wash species, these dynamics contrasted with E. farinosa on slopes. Germination was associated with El Niño years. Slope plant establishment was dependent on precipitation in the subsequent year, but not evidently so in wash species. Major mortality events were episodic, with Encelia mortality just as likely to occur in years with below or above average precipitation. In both Encelia species, mortality was associated with transitions to a cold PDO phase. In E. frutescens this response was more rapid, whereas in E. farinosa mortality lagged 1 year, resulting in contrasting slope-wash mortality patterns. 50% of newly established shrubs died within 5, 5, and 18 years for E. frutescens, E. farinosa, and A. salsola, respectively. The 90% mortality ages were 26 years for E. frutescens, 24 years for E. farinosa, and 51 years for A. salsola. While maximum life expectancies are unknown, estimated maximum life expectancies were 56, 66, and 86 years for E. frutescens, E. farinosa, and A. salsola, respectively. Overall, as the climate has become more arid over the past four decades, the populations in both slope and wash habitats have exhibited similar responses: reduced shrub abundances and reduced total supportable leaf areas.

Our experimental design was formulated to determine whether or not bulb polarity (orientation) at the time of replanting of bulbs to salvage plants of Calochortus weedii A. W. Wood (Liliaceae) or Weed’s Mariposa Lily affected the success of the mitigation transplant effort. Polarity of bulbs at planting clearly did influence subsequent growth, most notably in the tip-down (D) treatment. Among these bulbs, 75% failed to emerge from dormancy and only four (20%) actually set mature fruit. This was in sharp contrast to the other three treatments where 100% of the bulbs successfully emerged in this season and between 80% (S) and 95% (UG and UN) set mature fruit. The data from this study do indicate that: 1) bulb planting orientation does influence survival and growth, and 2) proper bulb planting polarity (orientation) should be an important consideration in any transplantation of this or any sensitive bulb producing plant species for mitigation purposes.

Reflective leaf pubescence of the desert shrub Encelia farinosa (brittlebrush) reduces leaf temperature and plant water loss, and is considered adaptive in xeric environments. Yet, little is known about intraspecific variation in this trait. Among three populations in the northern range of E. farinosa, which span a very broad precipitation gradient, both leaf absorptance variation and differences in the timing of drought-induced leaf loss were broadly associated with climatic variability. Where mean annual rainfall was greatest, drought-induced leaf loss was earliest, but these plants also had higher population-level mean leaf absorptance values. Higher absorptance increases the relative dependence on latent heat transfer (transpirational cooling), but it also provides greater instantaneous carbon assimilation. Plants at the driest site reached lower leaf absorptance values and maintained leaves longer into the drought period. Lower leaf absorptance reduces water consumption, and extended leaf longevity may buffer against the unpredictability of growing conditions experienced in the driest site. These observations are consistent with a trade-off scenario in which plants from wetter regions might trade off water conservation for higher instantaneous carbon gain, whereas plants from drier regions reduce water consumption and extend leaf longevity to maintain photosynthetic activity in the face of unpredictable growing conditions.

Plant restoration activities can be positively or negatively affected by changes in the abiotic or biotic environment from that of the pre-disturbance condition, such as invasion by nonnative species and changes in aboveground microclimate, soil structure, or soil nutrients. A thorough understanding of the ecophysiological mechanisms of adaptation that describe the potential for a plant to persist in a habitat allows a more accurate assessment of the impact of an altered environment on future plant performance and restoration outcomes. This feature of plants is often referred to as tolerance. Plant species vary in their capacity to tolerate different biotic and abiotic stressors, and this tolerance can be the basis for why some species are capable of reestablishing themselves quickly in a restoration setting, whereas the reestablishment of other species proceeds more slowly, if at all. Individual plants also vary in the rate at which they take up carbon, nitrogen, and water, and these differences will influence rates of nutrient cycling and other ecosystem functions. Ecophysiological traits that influence ecosystem functioning are referred to as effect traits (Suding et al. 2008).

Summary •While studies of modern plants indicate negative responses to low [CO2] that occurred during the last glacial period, studies with glacial plant material that incorporate evolutionary responses are rare. In this study, physiological responses to changing [CO2] were compared between glacial (La Brea tar pits) and modern Juniperus trees from southern California. •Carbon isotopes were measured on annual rings of glacial and modern Juniperus. The intercellular : atmospheric [CO2] ratio (ci/ca) and intercellular [CO2] (ci) were then calculated on an annual basis and compared through geologic time. •Juniperus showed constant mean ci/ca between the last glacial period and modern times, spanning 50 000 yr. Interannual variation in physiology was greatly dampened during the last glacial period relative to the present, indicating constraints of low [CO2] that reduced responses to other climatic factors. Furthermore, glacial Juniperus exhibited low ci that rarely occurs in modern trees, further suggesting limiting [CO2] in glacial plants. •This study provides some of the first direct evidence that glacial plants remained near their lower carbon limit until the beginning of the glacial–interglacial transition. Our results also suggest that environmental factors that dominate carbon-uptake physiology vary across geologic time, resulting in major alterations in physiological response patterns through time.

Biological soil crusts (BSCs) are comprised of soil particles, bacteria, cyanobacteria, green algae, microfungi, lichens, and bryophytes and confer many ecosystem services in arid and semiarid ecosystems worldwide, including the highly threatened California sage scrub (CSS). These services, which include stabilizing the soil surface, can be adversely affected when BSCs are disturbed. Using field and greenhouse experiments, we tested the hypothesis that mechanical disturbance of BSC increases emergence of exotic vascular plants in a coastal CSS ecosystem. At Whiting Ranch Wilderness Park in southern California, 22 plots were established and emergence of exotic and native plants was compared between disturbed and undisturbed subplots containing BSC. In a separate germination study, seed fate in disturbed BSC cores was compared to seed fate in undisturbed BSC cores for three exotic and three native species. In the field, disturbed BSCs had significantly (>3×) greater exotic plant emergence than in undisturbed BSC, particularly for annual grasses. Native species, however, showed no difference in emergence between disturbed and undisturbed BSC. Within the disturbed treatment, emergence of native plants was significantly, and three times less than that of exotic plants. In the germination study, seed fates for all species were significantly different between disturbed and undisturbed BSC cores. Exotic species had greater emergence in disturbed BSC, whereas native plants showed either no response or a positive response. This study demonstrates another critical ecosystem service of BSCs—the inhibition of exotic plant species—and underscores the importance of BSC conservation in this biodiversity hotspot and possibly in other aridland ecosystems.

Invasive non-native species can create especially problematic restoration barriers in subtropical and tropical dry forests. Native dry forests in Hawaii presently cover less than 10% of their original area. Many sites that historically supported dry forest are now completely dominated by non-native species, particularly grasses. Within a grass-dominated site in leeward Hawaii, we explored the mechanisms by which non-native Pennisetum setaceum, African fountain grass, limits seedlings of native species. We planted 1,800 seedlings of five native trees, three native shrubs, and two native vines into a factorial field experiment to examine the effects of grass removal (bulldozed vs. clipped plus herbicide vs. control), shade (60% shade vs. full sun), and water (supplemental vs. ambient) on seedling survival, growth, and physiology. Both grass removal and shade independently increased survival and growth, as well as soil moisture. Seedling survival and relative growth rate were also significantly dependent on soil moisture. These results suggest that altering soil moisture may be one of the primary mechanisms by which grasses limit native seedlings. Grass removal increased foliar nitrogen content of seedlings, which resulted in an increase in leaf-level photosynthesis and intrinsic water use efficiency. Thus in the absence of grasses, native species showed increased productivity and resource acquisition. We conclude that the combination of grass removal and shading may be an effective approach to the restoration of degraded tropical dry forests in Hawaii and other ecologically similar ecosystems.

Drainage channels are among the most conspicuous surficial features of deserts, but little quantitative analysis of their influence on plant distributions is available. We analysed the effects of desert stream channels (‘washes’) on Larrea tridentata and Ambrosia dumosa density and cover on an alluvial piedmont in the Mojave Desert, based on a spatial analysis of transect data encompassing a total length of 2775 m surveyed in 5 cm increments. Significant deviations from average transect properties were identified by bootstrapping. Predictably, shrub cover and density were much reduced inside washes, and elevated above average levels adjacent to washes. Average Larrea and Ambrosia cover and density peaked 1·2–1·6 m and 0·5–1·0 m from wash edges, respectively. We compared wash effects in runon-depleted (−R) sections, where washes had been cut off from runon and were presumably inactive, with those in runon-supplemented (+R) sections downslope from railroad culverts to help identify mechanisms responsible for the facilitative effect of washes on adjacent shrubs. Shrub cover and density near washes peaked in both + R and − R sections, suggesting that improved water infiltration and storage alone can cause a facilitative effect on adjacent shrubs. However, washes of < 2 m width in + R sections had larger than average effects on peak cover, suggesting that plants also benefit from occasional resource supplementation. The data suggest that channel networks significantly contribute to structuring plant communities in the Mojave Desert and their disruption has notable effects on geomorphic and ecological processes far beyond the original disturbance sites. Copyright © 2010 John Wiley & Sons, Ltd.

Hawaiian lowland dry forests have been reduced by >90% since first human contact. Restoration has focused on protection from fire and ungulates, and removal of invasive grasses as ways to stimulate native forest regeneration. Despite these efforts, natural regeneration of native plants has been infrequent. To assess effects of previous restoration treatments on natural regeneration, we monitored seed rain and dynamics of seedlings and juveniles for a period of 3 yr (2004–2007) within three restoration units within a remnant forest on the island of Hawai'i. All units had been protected from fire for many decades but differed in time since ungulate exclusion and grass removal. The units were as follows: (1) long-term restoration (fenced 1956, grass removal initiated 1995), (2) shortterm restoration (fenced and grass removal initiated 1997), and (3) unmanaged (fenced 1997, no grass removal). Overall juvenile plant abundance was highest in the short-term unit, but native abundance was highest in the...

Previous field studies of the reproductive biology of the San Fernando Valley spineflower, Chorizanthe parryi var. fernandina (S. Watson) Jeps. suggested that pollination by ants might be an important feature of this endangered polygonous taxon. This conclusion was based on observations that native ants were abundant floral visitors and constant to this species. We conducted the current study to explore more closely the possibility that native ants were facilitating pollination and resulting in viable seed set. Based on our data, ants can indeed be effective pollinators of spineflower. Fruit set was 57% higher in flowers exposed to ant visitation, compared to 27% in control flowers where ants were excluded. Further, a 25.7% germination rate was observed for achenes produced in the absence of ants, in contrast to a 61% rate in those produced in the presence of ants. We suggest that ant pollination may be more prevalent in drier climates, ant production of inhibitory substances may not be a severe ...

In response to conservation concerns, the reproductive biology of the San Fernando Valley Spineflower was investigated, focusing on pollination interactions and seed germination. Pollination by a variety of aerial visitors, as well as autogamy (a facultative selfer, showing about 25% selfing), appear to contribute significantly to fruit/seed set. There was a significant correlation between the numerous different floral visitors (many went uncollected) and the invertebrate fauna in the immediately surrounding coastal sage scrub community indicating that this taxon is visited by a substantial variety of potential pollinators and is probably not pollinator-limited. Although there were many potential pollinators, only six species, including three species of ants, made up the vast majority of visits to the flowers at the two study sites. Many of the invertebrate visitors to the flowers of the San Fernando Valley Spineflower exhibited a high rate of constancy. An overall generalist strategy is suggeste...

Summary 1. Tropical dry forests are among the Earth’s most threatened ecosystems. On the Island of Hawaii the African bunchgrass Pennisetum setaceum (fountain grass) dominates the understorey of the few remaining fragments of native dry forests and is contributing to the degradation of this once diverse ecosystem. In this study, we examined the impacts of Pennisetum on water use and productivity of the dominant native canopy tree, Diospyros sandwicensis . 2. Over a 3-year period, measurements were made on tree growth rates, and physiological and morphological responses of the most common dry forest native tree, D. sandwicensis , growing with an understorey dominated by Pennisetum , and on trees growing in plots maintained free of grasses. 3. Analysis of stable oxygen isotope ratios indicated that trees growing in the absence of Pennisetum used a higher proportion of water from shallow soil sources. They also sustained higher mid-day water potentials, especially during drier periods. At the leaf level, no significant differences were found in gas exchange measurements between Diospyros trees growing with or without Pennisetum . However, trees growing without Pennisetum had 30% lower leaf mass per unit area and 40% higher diameter growth than trees growing with Pennisetum . 4. These results demonstrate that invasion by Pennisetum has a pronounced negative impact on resource acquisition and use by the dominant native tree of this dry forest ecosystem. Although tree death due to these impacts would be much slower than through more immediate processes such as fire and grazing, our findings suggest that long-term conservation of ecosystems threatened by invasions, such those in Hawaiian dry forests, will ultimately require active management of the invading species.

Non-native plants are invading terrestrial ecosystems across the globe, yet little is known about how invasions impact carbon (C) cycling or how these impacts will be influenced by climate change. We quantified the effect of a non-native C4 grass invasion on soil C pools and fluxes in a Hawaiian tropical dry forest over 2 years in which annual precipitation was average (Year 1) and � 60% higher than average (Year 2). Work was conducted in a series of forested plots where the grass understory was completely removed (removal plots) or left intact (grass plots) for 3 years before experiment initiation. We hypothesized that grass invasion would: (i) not change total soil C pools, (ii) increase the flux of C into and out of soils, and (iii) increase the sensitivity of soil C flux to variability in precipitation. In grass plots, grasses accounted for 25‐34% of litter layer C and � 70% of fine root C. However, no differences were observed between treatments in the size of any soil C pools. Moreover, grass-derived C constituted a negligible fraction of the large mineral soil C pool (o3%) despite being present in the system for � 50 years. Tree litterfall was � 45% lower in grass plots, but grass-derived litterfall more than compensated for this reduction in both years. Annual cumulative soil-surface CO2 efflux (Rsoil) was � 40% higher in grass plots in both years, and increased in both treatments by � 36% in the wetter Year 2. Despite minimal grassderived mineral soil C, 475% of Rsoil in grass plots was of C4 (i.e. grass) origin. These results demonstrate that grass invasion in forest ecosystems can increase the flux of C into and out of soils without changing total C pools, at least over the short term and as long as the native tree canopy remains intact, and that invasion-mediated changes in belowground C cycling are sensitive to precipitation.

Cabin (2007) asks whether formal science is an effective framework and methodology for designing and implementing ecological restoration programs. He argues that beyond certain ancillary benefits, restoration science has little of practical value to offer the practice of restoration. He goes on to suggest that restoration science most often represents an impediment to restoration practice because an ‘‘ivory tower’’ mentality limits the utility of experiments and diverts research dollars away from answering practical questions. His conclusion is that a nonscientific gardening approach may be more effective at restoring degraded ecosystems. We disagree with this perspective because: (1) restoration science has moved beyond exclusively using ‘‘square grids’’ placed on small patches of land to examine treatment effects on species representation; (2) Cabin’s critique greatly undervalues the contribution of science to restoration practice even where the input of restoration scientists is not directly evident; and (3) the practice of restoration is unlikely to advance beyond small-scale and truly haphazard successes without well-designed studies that can provide peer-reviewed and widely accessible published information on the mechanisms underlying both successes and failures. We conclude that through integration with other disciplines, restoration science increasingly will provide novel approaches and tools needed to restore ecosystem composition, structure, and function at stand to landscape scales. As with the broader role of science in the human enterprise (Sagan 1996), the contribution of restoration science to restoration practice can only grow as the discipline matures.

The N cycle of arid ecosystems is influenced by low soil organic matter, high soil pH, and extremes in water potential and temperature that lead to open canopies and development of biological soil crusts (biocrusts). We investigated the effects of N amendment on soil microbial dynamics in a Larrea tridentata-Ambrosia dumosa shrubland site in southern Nevada USA. Sites were fertilized with a NO3-NH4 mix at 0, 7, and 15 kg N ha(-1) y(-1) from March 2012 to March 2013. In March 2013, biocrust (0-0.5 cm) and bulk soils (0-10 cm) were collected beneath Ambrosia canopies and in the interspaces between plants. Biomass responses were assessed as bacterial and fungal SSU rRNA gene copy number and chlorophyll a concentration. Metabolic responses were measured by five ecoenzyme activities and rates of N transformation. By most measures, nutrient availability, microbial biomass, and process rates were greater in soils beneath the shrub canopy compared to the interspace between plants, and greater in the surface biocrust horizon compared to the deeper 10 cm soil profile. Most measures responded positively to experimental N addition. Effect sizes were generally greater for bulk soil than biocrust. Results were incorporated into a meta-analysis of arid ecosystem responses to N amendment that included data from 14 other studies. Effect sizes were calculated for biomass and metabolic responses. Regressions of effect sizes, calculated for biomass, and metabolic responses, showed similar trends in relation to N application rate and N load (rate × duration). The critical points separating positive from negative treatment effects were 88 kg ha(-1) y(-1) and 159 kg ha(-1), respectively, for biomass, and 70 kg ha(-1) y(-1) and 114 kg ha(-1), respectively, for metabolism. These critical values are comparable to those for microbial biomass, decomposition rates and respiration reported in broader meta-analyses of N amendment effects in mesic ecosystems. However, large effect sizes at low N addition rates indicate that arid ecosystems are sensitive to modest increments in anthropogenic N deposition.

Hawaiian tropical dry forests are a unique and highly endangered ecosystem. Remaining fragments are heavily impacted by invasive plant species, particularly the perennial bunchgrass Pennisetum setaceum (Forssk.) Chiov. (fountain grass). Little is known about the impact of invasive species on carbon cycling in terrestrial ecosystems. Biomass estimates are a critical first step in understanding the effects of invasive species on carbon dynamics. Biomass data can be used to quantify carbon pools and fluxes, as well as the impacts of land cover change on carbon sequestration. The objectives of our work were to compare: (1) population structure of the dominant native tree species and (2) carbon pools in aboveground live biomass among three land cover types: native—native dominated, largely intact dry forest; invaded—intact overstory, but understoryheavilyinvaded byP.setaceum;andconverted—a formerlyforestedsite thathasbeenconvertedtograssland dominatedbyP.setaceum. Invasion of Hawaiian tropical dry forest by P. setaceum leads to an unsustainable population of native trees characterized by a conspicuous absence of saplings and smaller diameter individuals. Aboveground tree biomass did not differ between native (108.1 Mg ha � 1 ) and invaded (107.0 Mg ha � 1 ) forests due to the preponderance of wood biomass in large Diospyros sandwicensis trees at both sites. Grass invaded forest had � 7� more understory biomass than the native forest, but no differences were observed in total aboveground live biomass (tree + understory) between native (108.9 Mg ha � 1 ) and invaded forests (112.1 Mg ha � 1 ). However, total aboveground live biomass was � 93% lower at the converted site (7.8 Mg ha � 1 ), which is the eventual fate of invaded forests with no natural regeneration of native canopy species. Native forests contained significantly more individuals and leaf biomass for the mid-canopy tree Psydrax odorata, which increased overall stand leaf area index. This structural difference appears to have prevented P. setaceum from invading the native site by reducing understory light levels. These results indicate that large changes in the sequestration of carbon in aboveground biomass have occurred across the landscape following widespread grass invasion and conversion of Hawaiian dry forests to grasslands. These large losses of carbon have important implications for quantifying the effects of invasive species and land cover change on ecosystem carbon storage at landscape and regional scales. Crown Copyright # 2006 Published by Elsevier B.V. All rights reserved.

In the arid and semiarid regions of North America, discrete precipitation pulses are important triggers for biological activity. The timing and magnitude of these pulses may differentially affect the activity of plants and microbes, combining to influence the C balance of desert ecosystems. Here, we evaluate how a "pulse" of water influences physiological activity in plants, soils and ecosystems, and how characteristics, such as precipitation pulse size and frequency are important controllers of biological and physical processes in arid land ecosystems. We show that pulse size regulates C balance by determining the temporal duration of activity for different components of the biota. Microbial respiration responds to very small events, but the relationship between pulse size and duration of activity likely saturates at moderate event sizes. Photosynthetic activity of vascular plants generally increases following relatively larger pulses or a series of small pulses. In this case, the duration of physiological activity is an increasing function of pulse size up to events that are infrequent in these hydroclimatological regions. This differential responsiveness of photosynthesis and respiration results in arid ecosystems acting as immediate C sources to the atmosphere following rainfall, with subsequent periods of C accumulation should pulse size be sufficient to initiate vascular plant activity. Using the average pulse size distributions in the North American deserts, a simple modeling exercise shows that net ecosystem exchange of CO2 is sensitive to changes in the event size distribution representative of wet and dry years. An important regulator of the pulse response is initial soil and canopy conditions and the physical structuring of bare soil and beneath canopy patches on the landscape. Initial condition influences responses to pulses of varying magnitude, while bare soil/beneath canopy patches interact to introduce nonlinearity in the relationship between pulse size and soil water response. Building on this conceptual framework and developing a greater understanding of the complexities of these eco-hydrologic systems may enhance our ability to describe the ecology of desert ecosystems and their sensitivity to global change.

Because leaf pubescence of the desert shrub Encelia farinosa increases in response to drought and influences photosynthesis and transpiration, we hypothesized that differences in water availability across the range of this species may result in genetic differentiation for pubescence and associated productivity traits. We examined maternal family variation of pubescence-moderated light absorption (absorptance) in three populations of E. farinosa. Absorptance was always greatest for plants from the high-rainfall environment and lowest for those from the driest site, but the rate of absorptance change in response to drought was similar among all populations. Similar patterns were found when we compared families within populations-all genotypes had similar initial leaf absorptances, differentiated very early in seasonal growth, then had concordant changes in absorptance thereafter. However, family-level variance was greatest for plants from the driest site, a region with highly heterogeneous precipitation patterns, whereas low variance was found for plants from the wettest, least heterogeneous site. The concordance of leaf absorptance changes, within and among populations, may be due to integration with other drought-related traits; however, the differences in absorptance values within and among populations suggest that variation of leaf pubescence results from selection associated with geographical and local patterns of water availability.

Previous studies of the desert shrub Encelia farinosa have shown variation of morpholological and physiological integration that appears to match environmental differences among populations. Such findings led us to ask if there is a genetic basis for such differentiation that may be related to physiological control of intercellular CO(2) concentrations as indicated by carbon isotope discrimination (Delta) values, and if genetic variance for Delta is detectable within populations. Under common environment conditions, Delta values were compared between two populations of E. farinosa from desert regions with contrasting rainfall patterns: Superior, Ariz., a region with high annual rainfall and droughts of short duration, and Oatman, Ariz. a region with lower annual rainfall and longer drought periods. Superior plants had consistently greater mean Delta values than Oatman plants across a broad range of soil water potentials, indicating that there is a genetic basis for Delta variation between these populations. At the intrapopulation level only Oatman plants showed detectable genetic variance of Delta based on: (1) consistent individual-rank values for Delta among soil-drought stages, and (2) evidence of heritable genetic variance for Delta during one drought stage. No genetic variance in Delta was evident for the Superior population. It is hypothesized that the high spatio-temporal heterogeneity of water availability at Oatman may facilitate the maintenance of genetic variance for carbon isotope discrimination within this population. Both the inter- and intra-population level findings suggest that selection associated with rainfall and drought has resulted in genetic divergence of the physiological factors involved in Delta determination for these populations. There appears to be strong differences of water-use and carbon-gain strategies among populations, and broader functional breadth among plants in the habitat of greatest environmental heterogeneity.

Although the destruction of tropical rain forests receives much attention, tropical dry forests are in general far more threatened and endangered. Eliminating grazing ungulates is often considered a key first step toward protecting these ecosystems, but few studies have investigated the long-term effects of this tech- nique. We examined the effects of ungulate exclusion from a 2.3-ha native dry-forest preserve on the island of Hawaii by comparing its present flora to the flora of an adjacent area subjected to continuous grazing since the preserve was fenced over 40 years ago. Relative to this adjacent area, the fenced preserve contained a more diverse flora with substantially greater coverage of native overstory and understory species. Until re- cently, however, regeneration of native canopy trees within the preserve appears to have been thwarted by a dominant herbaceous cover of alien fountain grass ( Pennisetum setaceum ) and predation by alien rodent species. Our results indicate that although ungulate exclusion may be a necessary and critical first step, it is not sufficient to adequately preserve and maintain Hawaii's remaining tropical dry forest remnants. Our re- cent efforts to control the dominant alien species within the fenced preserve suggest that this practice may fa- cilitate both the regeneration of native species and the colonization and potential invasion of new alien plants. Comparisons of seedlings of the dominant native canopy tree Diospyros sandwicensis growing in sites both dominated by and free of fountain grass suggested that fountain grass inhibits Diospyros seedling growth and photosynthesis but may increase survival if seedlings are protected from ungulates.

SUMMARY Traits associated with leaf energy balance are especially important for desert plants because decoupling leaf and ambient temperatures can be critical for maximizing productivity and survival in these hot, dry regions. Deserts are also one of the most climatically unpredictable and heterogeneous biomes; thus, variation of energy balance characters due either to plasticity or genetic differentiation might also be crucial to the success of desert plant species. In a common garden environment we examined variation in the traits associated with energy balance and productivity in Encelia farinosa A. Gray (brittlebush) plants. Comparing two populations from contrasting rainfall and drought climates, we found that plants from the wetter site (Superior, Arizona) always maintained higher leaf absorptances than plants from the more xeric region (Oatman, Arizona) when at similar water potentials. Superior plants also increased stomatal conductance and photosynthesis in response to mid-spring rainfall, whereas Oatman plants did not. Oatman plants, however, tended to have greater leaf areas and continuous spring growth, which made them larger than Superior plants, yet both populations produced the same number of flower heads. The differences for these traits, and the associations among them, agree with predictions based on the contrasting drought and rainfall environments of these two populations. Our results suggest that the differences might represent alternative suites of characters of selective importance for maximizing carbon gain over the climatically variable range of this widespread species. Furthermore, these differences, detected in common environment conditions, might have a genetic basis, which might contribute to a greater potential for local adaptation in this species.

Question: Is there evidence of a trade-off between drought and shade tolerance in coastal desert shrubs? Location: Arid scrubland, Atacama desert, north-central Chile. Methods: One-week-old seedlings of six coastal desert shrub species (Centaurea chilensis, Encelia canescens, Flourensia thurifera, Pleocarphus revolutus, Senna cumingii and Haplopappus parvifolius) were planted in the field under six treatment combinations: three light environments (low, intermediate and high radiation) and two water levels (natural rainfall (55.4 mm) and natural rainfall plus 100 mm irrigation). We quantified seedling survival for each species during 22 weeks, estimated relative growth rate, and examined how light and water affect wholeplant responses, particularly specific leaf area, leaf mass ratio and root–shoot ratio. Results: We found species-specific differences in the temporal pattern of mortality. Water and/or light levels affected seedling survival of all species, excluding C. chilensis. Relative growth rate (RGR) increased in low-light conditions in C. chilensis and P. revolutus, but otherwise did not vary in response to differences in either light or water, independently or to their interaction. Across species, the effect of water on specific leaf area (SLA) was inconsistent, increasing both in drought conditions (C. chilensis) and in treatments with supplemental water (S. cumingii). Additionally, SLA tended to increase with decreasing light levels for most species (F. thurifera, H. parvifolius, C. chilensis). In our study, only F. thrurifera and C. chilensis showed changes in leaf mass ratio (LMR) and only with respect to light levels; specifically, LMR tended to increase with decreasing light level. Biomass allocation was independent of light and water for all species except F. thurifera, which showed an increase in root biomass in drought conditions. Conclusions: Overall, we did not find evidence to support a trade-off between drought and shade tolerance during early plant establishment. Our results suggest that water is the most important resource that limits recruitment in the coastal desert. Therefore, increased aridity in this system is likely to severely hinder seedling recruitment of the native coastal desert shrubs.

Intrapopulation genetic variation in four dominant, perennial plant species from the deserts of south-western North America was assessed and compared using isozyme techniques. In each of two localities, a warm desert and a cold desert environment, one population of the most common long-lived perennial was compared with a nearby population of a dominant, comparatively short-lived perennial. The warm desert species were Encelia farinosa and Larrea tridentata while Gutierrezia microcephala and Coleogyne ramosissima were examined at the cold desert site. All samples were electrophoresed and stained for 18 enzyme systems. Mean values for these four species were 0·173 for gene diversity, 66·9% for polymorphism, and 2·02 for number of alleles per locus. These values are significantly greater than published means from a recent survey of the plant isozyme literature, and are comparable to or higher than levels of genetic variation in other widespread plant species. Differences in level of variation among the species were not significant, thus failing to indicate any relationship between life expectancy and genetic variation. These results demonstrate that aridland plant populations can harbor high levels of genetic variation, and suggest that environmental heterogeneity may be important in the development and maintenance of this diversity.

• While studies of modern plants indicate negative responses to low [CO₂] that occurred during the last glacial period, studies with glacial plant material that incorporate evolutionary responses are rare. In this study, physiological responses to changing [CO₂] were compared between glacial (La Brea tar pits) and modern Juniperus trees from southern California. • Carbon isotopes were measured on annual rings of glacial and modern Juniperus. The intercellular:atmospheric [CO₂] ratio (c(i) /c(a) ) and intercellular [CO₂] (c(i) ) were then calculated on an annual basis and compared through geologic time. • Juniperus showed constant mean c(i) /c(a) between the last glacial period and modern times, spanning 50,000 yr. Interannual variation in physiology was greatly dampened during the last glacial period relative to the present, indicating constraints of low [CO₂] that reduced responses to other climatic factors. Furthermore, glacial Juniperus exhibited low c(i) that rarely occurs in modern trees, further suggesting limiting [CO₂] in glacial plants. • This study provides some of the first direct evidence that glacial plants remained near their lower carbon limit until the beginning of the glacial-interglacial transition. Our results also suggest that environmental factors that dominate carbon-uptake physiology vary across geologic time, resulting in major alterations in physiological response patterns through time.

We examined carbon-gain, water-use, and leaf-allocation traits for six tree species of a Hawaiian dry forest to better understand the functional diversity within this threatened ecosystem. Tropical dry forests are among the most endangered ecosystems on Earth, and in Hawaii, as elsewhere, declining biodiversity threatens ecosystem processes that may depend on forest functional diversity. We found broad variation among species including a two-fold difference for mean photosynthetic rate, a greater than three-fold difference for predawn water potential, and a nearly three-fold difference for leaf life span. Principal component analysis showed a clear separation of species based on carbon-gain vs. water-use related axes, and d 13 C analysis revealed differing limitations (supply vs. demand) on carbon assimilation. The broad functional variation not only spanned traditional classifications (avoiders vs. tolerators), but also included unusual strategies (e.g., fast growth with drought tolerance). Correlations among traits, including leaf life span, leaf mass per area, and %N, followed typical global patterns, but some exceptions appeared as a result of unique life-history characteristics, such as latex-rich sap and root parasitism. Elucidating functional variation provides important information that can be used to link plant biodiversity with ecosystem processes and also facilitate the management and preservation of tropical dry forests and other threatened communities.

[1] Isotopic analyses of cactus spines grown serially from the apex of long-lived columnar cactuses may be useful for climatological and ecological studies if time series can be reliably determined from spines. To characterize the timescales over which spines may record this information, we measured spine growth in saguaro cactus over days, months, and years with time-lapse photography, periodic marking, and postbomb radiocarbon dating and then analyzed isotopic variability over these same timescales and compared these measurements to local climate. We used daily increments of growth, visible as transverse bands of light and dark tissue in spines, as chronometers to develop diurnally resolved δ13C and δ18O records from three spines grown in series over a 70 day period. We also constructed a 22 year record of δ13C variations from spine tips arranged in chronological sequence along the side of a 4 m tall, single-stemmed saguaro. We evaluated two mechanisms potentially responsible for daily, weekly, and annual variability in δ13C values of spines; both related to vapor pressure deficit (VPD). Our data suggest that stomatal conductance is unlikely to be the determinant of δ13C variation in spines. We suggest that either VPD-induced changes in the balance of nighttime- and daytime-assimilated CO2 or mesophyll-limited diffusion of CO2 at night are the most likely determinant of δ13C variation in spines. Intra-annual and interannual variability of δ18O in spine tissue appears to be controlled by the mass balance of 18O-depleted water taken up after rain events and evaporative enrichment of 18O in tissue water between rains. We were able to estimate the annual growth and areole generation rate of a saguaro cactus from its 22 yearlong isotopic record because VPD, rainfall, and evaporation exhibit strong annual cycles in the Sonoran Desert and these variations are recorded in the oxygen and carbon isotope ratios of spines.

This is the peer reviewed version of the following article: Nippert, Jesse B.; Hooten, Mevin B.; Sandquist, Darren R.; Ward, Joy K. (2010). "A Bayesian model for predicting local El Nino events using tree ring widths and cellulose δ18O." Journal of Geophysical Research: Biogeosciences, 115(G1):G01011. http://www.dx.doi.org/10.1029/2009JG001101., which has been published in final form at http://doi.org/10.1029/2009JG001101. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

Carbon isotope discrimination (Δ) was compared between populations of dominant perennial plant species, differing in life expectancy, in two deserts with contrasting vegetation types. In both deserts, plants of the shorter-lived species showed significantly higher Δ and greater intrapopulation variance in this character compared to the long-lived species. These results indicate underlying differences in gas-exchange physiology, and suggest a positive correlation between water-use efficiency and lifespan in desert plants. Differences in variance for this character may reflect greater microenvironmental variation experienced by shorter-lived plants and/or different forms of selection acting on water-use traits. Spatial distributions were significantly clustered for the shorter-lived species and significantly uniform for the long-lived species, indicating that competition has been important in the development of the long-lived populations. The long-lived Larrea tridentata showed a significant, negative correlation between Δ and Thiessen polygon area, suggesting a positive relationship between water-use efficiency and longevity within this species. This relationship was weakly supported in the other warm desert species, Encelia farinosa, but was not observed within populations of the cold desert species, Gutierrezia microcephala and Coleogyne ramosissima. These results suggest that Δ reflects key aspects of plant metabolism related to lifespan; these differences may ultimately influence interactions among desert plants and the structure of desert plant communities.

The carbon isotope composition (13C/'2C) of C3 plant tissues provides a long-term, integrated measure of photosynthetic metabolism. Quantitative genetic methods were used to study the inheritance of carbon isotope composition and several morphological characters in Gutierrezia microcephala, a short-lived desert perennial. Open-pollinated seed was collected from a population located near Lee's Ferry, Arizona, in an area that was disturbed approximately 20 years ago. Seeds were germinated and seedlings grown in a common greenhouse environment. Carbon isotope discrimination (A) and all morphological characters varied significantly among maternal families. Heritability of A was estimated as 81% in the greenhouse and as 92% using the regression of family mean A on parent A from field samples. Using both field and greenhouse data, we estimated a lower bound for heritability in nature of 54%. Offspring size and biomass both showed significant, positive correlations with A. The corresponding genetic correlations were similar in magnitude and direction, but did not differ significantly from zero. These results imply the existence of heritable differences in physiology associated with carbon assimilation and water loss within populations, and the potential for microevolutionary change through natural selection.

The primary purpose was to determine the diurnal course of carbon uptake by I. bolanderi which inhabits high elevation oligotrophic lakes. Specifically we address the role of CAM and its contribution to carbon uptake by the shoots of I. bolanderi. For comparative purposes, one sample of Fontinalis antipyretica, a non-CAM aquatic species found within the same macrophyte community as I. bolanderi, was also tested for carbon assimilation characteristics

The stable isotope composition of spines produced serially from the apex of columnar cacti has the potential to be used as a record of changes in climate and physiology. To investigate this potential, we measured the delta(18)O, delta(13)C and F(14)C values of spines from a long-lived columnar cactus, saguaro (Carnegiea gigantea). To determine plant age, we collected spines at 11 different heights along one rib from the stem apex (3.77 m height) to the base of a naturally occurring saguaro. Fractions of modern carbon (F(14)C) ranged from 0.9679 to 1.5537, which is consistent with ages between 1950 and 2004. We observed a very strong positive correlation (r = 0.997) between the F(14)C age of spines and the age of spines determined from direct and repeated height measurements taken on this individual over the past 37 years. A series of 96 spines collected from this individual had delta(18)O values ranging from 38 per thousand to 50 per thousand [Vienna standard mean ocean water (VSMOW)] and delta(13)C values from -11.5 per thousand to -8.5 per thousand [Vienna Peedee belemnite (VPDB)]. The delta(18)O and delta(13)C values of spines were positively correlated (r = 0.45, P0.0001) and showed near-annual oscillations over the approximately 15-year record. This pattern suggests that seasonal periods of reduced evaporative demand or greater precipitation input may correspond to increased daytime CO(2) uptake. The lowest delta(18)O and delta(13)C values of spines observed occurred during the 1983 and 1993 El Niño years, suggesting that the stable isotope composition recorded in spine tissue may serve as a proxy for these climate events. We compared empirical models and data from potted experimental cacti to validate these observations and test our hypotheses. The isotopic records presented here are the first ever reported from a chronosequence of cactus spines and demonstrate that tissues of columnar cacti, and potentially other long-lived succulents, may contain a record of past physiological and climatic variation.

A number of operationally defined methods exist for pretreating plant tissues in order to measure C, N, and O isotopes. Because these isotope measurements are used to infer information about environmental conditions that existed at the time of tissue growth, it is important that these pretreatments remove compounds that may have exchanged isotopes or have been synthesized after the original formation of these tissues. In stable isotope studies, many pretreatment methods focus on isolating "cellulose" from the bulk tissue sample because cellulose does not exchange C and O isotopes after original synthesis. We investigated the efficacy of three commonly applied pretreatment methods, the Brendel method and two variants of the Brendel method, the Jayme-Wise method and successive acid/base/acid washes, for use on three tissue types (wood, leaves, roots). We then compared the effect of each method on C and O isotope composition (13C,14C,18O), C and N content, and chemical composition of the residue produced (using13C nuclear magnetic resonance (NMR)). Our results raised concerns over use of the Brendel method as published, as it both added C and N to the sample and left a residue that contains remnant lipids and waxes. Furthermore, this method resulted in18O values that are enriched relative to the other methods. Modifying the Brendel method by adding a NaOH step (wash) solved many of these problems. We also found that processed residues vary by tissue type. For wood and root tissues, the13C NMR spectra and the18O and13C data showed only small differences between residues for the Jayme-Wise and modified Brendel methods. However, for leaf tissue,13C NMR data showed that Jayme-Wise pretreatments produced residues that are more chemically similar to cellulose than the other methods. The acid/base/acid washing method generated13C NMR spectra with incomplete removal of lignin for all tissues tested and both isotopic, and13C NMR results confirmed that this method should not be used if purified cellulose is desired. © 2005 American Chemical Society.

The carbon isotope ratios (delta(13)C) of 160 beers from around the world ranged from -27.3 to -14.9 per thousand, primarily due to variation in the percentage of C(3) or C(4) plant carbon in the final product. Thirty-one percent of beers had a carbon signature of C(3) plants (barley, rice, etc.), whereas the remaining 69% contained some C(3)-C(4) mixture (mean of mixtures, 39 +/- 11% C(4) carbon). Use of C(4) carbon (corn, cane sugar, etc.) was not confined to beers from any particular region (Pacific Rim, Mexico, Brazil, Europe, Canada, and the United States). However, the delta(13)C of European beers indicated mostly C(3) plant carbon. In contrast, U.S. and Canadian beers contained either only C(3) or C(3)-C(4) mixtures; Brazilian, Mexican, and Pacific Rim beers were mostly C(3)-C(4) mixtures. Among different lagers, U.S.-style lagers generally contained more C(4) carbon than did imported pilsners. Among different ales, those brewed by large high-production breweries contained significant proportions of C(4) carbon, while C(4) carbon was not detected in microbrewery or home-brew ales. Furthermore, inexpensive beers generally contained more C(4) carbon than expensive beers.

Seasonal changes in the hydrogen isotope ratios of xylem waters were measured to determine water sources used for growth in desert plants of southern Utah. While all species used winter-spring recharge precipitation for spring growth, utilization of summer rains was life-form dependent. Annuals and succulent perennials exhibited a complete dependence on summer precipitation. Herbaceous and woody perennial species simultaneously utilized both summer precipitation and remaining winter-spring precipitation, with herbaceous species much more reliant on the summer precipitation component. Several of the woody perennials exhibited no response to summer precipitation. Currently, precipitation in southern Utah is evenly partitioned between winter and summer time periods; however, global circulation models predict that summer precipitation will increase in response to anticipated climate change. Our data indicate that components within the community will differentially responde to the change in precipitation patterns. These results are discussed in relation to impact on competition and possible changes in community structure.

functional diversity, which underlies variation in growth rates, productivity, population and community dynamics, and ecosystem function. The broad congruence of these variations with climatic and environmental conditions on local, regional, and global scales has fostered the concept that plant ecophysiological characteristics are well adapted to their local circumstances. For example, the repeated occurrence of plants with CAM (Crassulacean Acid Metabolism) photosynthesis and succulent leaves or stems in severely water-limited environments, and the independent evolution of these traits in numerous plant lineages, provides compelling evidence of the physiological evolution of these water-conserving traits under the influence of natural selection (Ehleringer and Monson 1993). Similarly, studies of the evolution of heavy metal tolerance confirm that natural selection may cause rapid ecophysiological evolution in just a few generations, leading to local adaptation in populations just a few meters apart (Antonovics et al. 1971). Many ecophysiological traits—considered here as all aspects of resource uptake and utilization, including biochemistry, metabolism, gas exchange, leaf structure and function, nutrient and biomass allocation, canopy structure, and growth—are likely to influence fitness and undergo adaptive evolution. Traits affecting the assimilation and use of resources such as carbon, water, and nutrients directly influence plant growth. Patterns of resource allocation to growth, reproduction, defense, and stress tolerance are also likely to be under strong selection. Phenotypic plasticity, the expression of different phenotypes by