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This paper provides a brief history of the development of poplar research in Canada within the broader North American context, as background to the present collection of papers on current Canadian poplar research. After the earliest times and European settlement, a few individual scientists played a pioneering role in early selection and breeding of poplars. The development of farm shelterbelts in the prairies over the last 100 years, including the widespread distribution of adapted poplars, has had a significant impact on the landscape. In the last 30 years, industrial strategies for the development and use of poplars have been the most important driver for poplar research. All of these components have in some way foreshadowed the present dramatic leading-edge research in poplar genomics. With the increasing diversity and sophistication of poplar research, particularly in recent years, a need is seen for identification of research priorities and coordination of research activities by disparate parties.

Phytoremediation (phyto) strategies employ trees, shrubs, and/or grasses for treating contaminated air, soil, or water. These strategies include buffers, vegetation filters, in situ phytoremediation plantings, and percolation controlling vegetative caps. The design parameter that separates phytoremediation from landscaping is purposefully placing and growing a root-zone reactor volume with predictable pollutant removal performance. This phyto reactor integrates with other engineered systems to cover landfills, treat petrochemical spills in soils, intercept a soluble subsurface plume, and capture non-point surface sediment entrained in urban or field runoff. There are many potential economic opportunities for biomass associated with phytoremediation, including bioenergy and traditional industrial products such as solid wood products and reconstituted products (i.e., paper, chip board, laminated beams, extruded trim). More intangibly, phyto creates environmental benefits such as soil erosion control, carbon sequestration, and wildlife habitat. Phyto also creates socio-economic benefits by diversify regional manufacturing into new products that employs local labor, thus building value-added industry. Alternative crops develop a greater diversity of products from the farmland, making the regional economy less exposed to global commodity crop price fluctuations. Thus, a strategic phyto treatment of non-point agricultural runoff would help diversify land use from annually tilled crops (corn, soybeans, wheat) into perennial, untilled tree crops. A landscape rebuilt using phyto would create diversity represented in business potential, healthier air and water, wildlife habitat, and aesthetics. Moreover, phyto provides local and current pollutant treatment. Such timely treatment of pollutants that would otherwise move to our downstream or downwind neighbors is key to the environmental justice concept. We present four case study summaries to illustrate installed commercial applications of phytoremediation.

Poplars and willows provide society with ecosystem services. These include direct economic benefits such as wood, fibre, fuelwood and other forest products. They also include environmental benefits improving the lives of people, such as through rehabilitation of degraded land, restoration of forest landscapes and mitigation of climate change. All of these services support rural livelihoods, enhance food security, alleviate poverty and contribute to sustainable land use and rural development, particularly in developing countries.

Worldwide, fuelwood demands, soil and groundwater contamination, and agriculture's impact on nature are growing concerns. Fast growing trees in short rotation woody crop (SRWC) systems may increasingly meet societal needs ranging from renewable energy to environmental mitigation and remediation. Phytoremediation, the use of plants for environmental cleanup, systems utilizing SRWCs have potential to remediate contaminated soil and groundwater. Non-hyperaccumulating, i.e., relatively low contaminant concentrating, species such as eucalypts (Eucalypts spp.), poplars (Populus spp.), and willows (Salix spp.) may phytoremediate while providing revenue from fuelwood and other timber products. Effective phytoremediation of contaminated sites by SRWCs depends on tree-contaminant interactions and on tree growth as influenced by silvicultural, genetic, and environmental factors. Locally adapted trees are essential for phytoremediation success. Among the different agroforestry practices, riparian buffers have the greatest opportunity for realizing the SRWC and phytoremediation potentials of fast growing trees. Agroforestry that combines SRWC and phytoremediation could be an emerging holistic approach for sustainable energy, agricultural development, and environmental mitigation globally.

We present the annual patterns of net ecosystem-atmosphere exchange (NEE) of CO2 and H2O observed from a 447m tall tower sited within a mixed forest in northern Wisconsin, USA. The methodology for determining NEE from eddy-covariance flux measurements at 30, 122 and 396m above the ground, and from CO2 mixing ratio measurements at 11, 30, 76, 122, 244 and 396m is described. The annual cycle of CO2 mixing ratio in the atmospheric boundary layer (ABL) is also discussed, and the influences of local NEE and large-scale advection are estimated. During 1997 gross ecosystem productivity (947 � 18g Cm � 2 yr � 1 ), approximately balanced total ecosystem respiration (963719g Cm � 2 yr � 1 ), and NEE of CO2 was close to zero (16719g Cm � 2 yr � 1 emitted into the atmosphere). The error bars represent the standard error of the cumulative daily NEE values. Systematic errors are also assessed. The identified systematic uncertainties in NEE of CO2 are less than 60g Cm � 2 yr � 1 . The seasonal pattern of NEE of CO2 was highly correlated with leaf-out and leaf-fall, and soil thaw and freeze, and was similar to purely deciduous forest sites. The mean daily NEE of CO2 during the growing season (June through August) was � 1.3g Cm � 2 day � 1 , smaller than has been reported for other deciduous forest sites. NEE of water vapor largely followed the seasonal pattern of NEE of CO2, with a lag in the spring when water vapor fluxes increased before CO2 uptake. In general, the Bowen ratios were high during the dormant seasons and low during the growing season. Evapotranspiration normalized by potential evapotranspiration showed the opposite pattern. The seasonal course of the CO2 mixing ratio in the ABL at the tower led the seasonal pattern of NEE of CO2 in time: in spring, CO2 mixing ratios began to decrease prior to the onset of daily net uptake of CO2 by the forest, and in fall mixing ratios began to increase before the forest became a net source for CO2 to the atmosphere. Transport as well as local NEE of CO2 are shown to be important components of the ABL CO2 budget at all times of the year.

Summary 1. The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34‐36 nL L − 1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3. After 3 years of exposure to 560 µ mol mol − 1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.

ECOPHYS, an individual-based process model for poplar, requires a three-dimensional soil water redistribution model to simulate soil water dynamics, plant uptake, and root growth. SOILPSI is a potential-driven water redistribution model based on the RHIZOS rhizosphere simulator. It expands on RHIZOS by calculating water flux based on water potential, and has a macropore flow mode to allow rapid drainage of the soil. SOILPSI simulates water flux in three dimensions and accounts for slope. SOILPSI was evaluated by comparing model output to soil moisture data collected under bare soil conditions. AMMI analysis of a date × depth matrix of differences between simulated and observed soil moisture content showed that excluding the two shallowest soil layers resulted in a difference matrix that conformed to an additive model. The grand mean predicted values were within 2% of the observed values, and 50 of 56 predicted values were within 5% of the observed values. Better agreements between simulated and observed soil moisture content were observed deeper in the soil profile and later in the season. Agreement between SOILPSI and field conditions was consistently more accurate than RHIZOS. Improving simulation of evaporative flux at the soil surface would improve simulation accuracy in the upper horizons.  2002 Elsevier Science Ltd. All rights reserved.

1. Competition effects on growth of individual trees were examined for 4 years in aggrading, mixed-clone stands of trembling aspen (Populus tremuloides Michx.) at the Aspen-FACE free-air Co2 and O3 enrichment facility in northern Wisconsin, USA. During each growing season stands received one of four combinations of atmospheric [CO2] (ambient vs~56 Pa) and [O3] (ambient vs~1.5 x ambient). 2. Non-destructive measurements of annual tree growth were compared within and among clones and treatments in relation to an index of competitive status based on the difference between a tree's height and that of its four nearest neighbours. Competitive status strongly influenced tree growth, and the positive growth response to elevated [CO2] was greater for competitively advantaged individuals than for disadvantaged individuals of most clones. 3. The magnitude of O3 effects on growth depended on clone and competitive status: for some clones, negative O3 effects were stronger with competitive advantage while others showed stronger O3 effects with competitive disadvantage. The interactive effects of CO2 and O3 differed among clones, with negative effects of O3 amplified or ameliorated by elevated CO2, depending on clone and competitive status. 4. Treatments modified competitive interactions by affecting the magnitude of growth differences among clones. These modifications did not alter clone rankings of competitive performance, but when CO2 and 03 were both elevated, the differences in competitive performance among clones decreased.

Human activity causes increasing background concentrations of the greenhouse gases C02 and O3. Increased levels of C02 can be found in all terrestrial ecosystems. Damaging O3 concentrations currently occur over 29% of the world's temperate and subpolar forests but are predicted to affect fully 60% by 2100 (ref. 3). Although individual effects of C02 and O3, on vegetation have been widely investigated, very little is known about their interaction, and long-term studies on mature trees and higher trophic levels are extremely rare. Here we present evidence from the most widely distributed North American tree species, Populus tremuloides, showing that C02 and O3, singly and in combination, affected productivity, physical and chemical leaf defences and, because of changes in plant quality, insect and disease populations. Our data show that feedbacks to plant growth from changes induced by C02 and O3 in plant quality and pest performance are likely. Assessments of global change effects on forest ecosystems must therefore consider the interacting effects of C02 and O3 on plant performance, as well as the implications of increased pest activity.

To improve source–sink relationship based carbon-allocation models, the basic proportional model was extended to account for a well-known effect of individual source to sink distances: among different sinks of similar characteristics, the more distant from the source, the lower the allocation coefficient. This was achieved through multiplication of the sink strength value by a coefficient that is proportional to a decreasing, simple function of distance, f; the power form was chosen for both simplicity and theoretical reasons. The resulting model was parameterized and evaluated on the empirical allocation matrix of the ECOPHYS model, after grouping together several individual, small sinks of similar nature and close location to remove any phyllotaxy-related bias. Both goodness of fit and predictive value were significantly improved compared with the basic proportional model (f = constant). The f-extended model yielded even better results if segments of different nature or age on the source to sink pathway were assigned different weights in the expression of distance, whereas the default expression of f, with an exponent of –1 and no additive constant, was optimal with no further parameter required. Thus, only 7 parameters (3 for pathway segment weights and 4 for sink strength values) were sufficient to retrieve the original 68 independent experimental allocation coefficients with a reasonable degree of accuracy. Pathway segment weights likely reflect both intrinsic transport pathway properties and situation within the plant architecture; this is discussed in relation to the possibilities of generalization and practical use of the model.

We investigated the interaction of elevated CO2 and/or (Ozone) O3 on the occurrence and severity of aspen leaf rust (Malampsora medusae Thuem. f. sp. tremuloidae) on trembling aspen (Populus tremuloides MIchx.) Furthermore, we examined the role of changes in leaf surface properties induced by elevated CO2 and/or O3 in this host-pathogen interaction.

Leaf photosynthesis ( Ps ), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free-air CO 2 enrichment in Wisconsin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO 2 , respectively, at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloudfree days (PPFD day,sat ) around the lowermost branches was 16·8 ± ± ± 0·8 and 8·7 ± 0·2% of values at the top for the ambient and elevated CO 2 canopies, respectively. Elevated CO 2 significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFD day,sat throughout the canopies, and elevated CO 2 did not affect that relationship. Leaf Ps light-response curves responded differently to elevated CO 2 , depending upon canopy position. Elevated CO 2 increased Ps sat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO 2 . Consequently, estimated daily integrated Ps on cloudfree days ( Ps day,sat ) was stimulated by elevated CO 2 only in the upper canopy. Ps day,sat of the lowermost branches was actually lower with elevated CO 2 because of the darker light environment. The lack of CO 2 stimulation at the midand lower canopy was probably related to significant down-regulation of photosynthetic capacity; there was no down-regulation of Ps in the upper canopy. The relationship between Ps day,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole-canopy Ps or photosynthetic N use efficiency. Elevated CO 2 had no effect on the optimization of canopy N allocation.

The Intergovernmental Panel of Climate Change (IPCC) has concluded that the greenhouse gases carbon dioxide (CO2) and tropospheric ozone (O3) are increasing concomitantly globally. Little is known about the effect of these interacting gases on growth, survival, and productivity of forest ecosystems. In this study we assess the effects of three successive years of exposure to combinations of elevated CO2 and O3 on growth responses in a five trembling aspen (Populus tremuloides) clonal mixture in a regenerating stand. The experiment is located in Rhinelander, Wisconsin, USA (45 N8 9W) and employs free air carbon dioxide and ozone enrichment (FACE) technology. The aspen stand was exposed to a factorial combination of four treatments consisting of elevated CO2 (560 ppm), elevated O3 (episodic exposure-90 m 11 1 hour 1 ), a combination of elevated CO2 and O3, and ambient control in 30 m treatment rings with three replications. Our overall results showed that our three growth parameters including height, diameter and volume were increased by elevated CO2, decreased by elevated O3, and were not significantly different from the ambient control under elevated CO2+O3. However, there were significant clonal differences in the responses; all five clones exhibited increased growth with elevated CO2, one clone showed an increase with elevated O3, and two clones showed an increase over the control with elevated CO2+O3, two clones showed a decrease, and one was not significantly different from the control. Notably, there was a significant increase in current terminal shoot dieback with elevated CO2 during the 1999–2000 dormant season. Dieback was especially prominent in two of the five clones, and was attributed to those clones growing longer into the autumnal season where they were subject to frost. Our results show that elevated O3 negates expected positive growth effects of elevated CO2 in Populus tremuloides in the field, and suggest that future climate model predictions should take into account the offsetting effects of elevated O3 on CO2 enrichment when estimating future growth of trembling aspen stands. # 2001 Published by Elsevier Science Ltd. All rights reserved.

To determine whether elevated CO 2 reduces or exacerbates the detrimental effects of O 3 on aspen ( Populus tremuloides Michx.), aspen clones 216 and 271 (O 3 tolerant), and 259 (O 3 sensitive) were exposed to ambient levels of CO 2 and O 3 or elevated levels of CO 2 , O 3 , or CO 2 +O 3 in the FACTS II (Aspen FACE) experiment, and physiological and molecular responses were measured and compared. Clone 259, the most O 3 -sensitive clone, showed the greatest amount of visible foliar symptoms as well as significant decreases in chlorophyll, carotenoid, starch, and ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) concentrations and transcription levels for the Rubisco small subunit. Generally, the constitutive (basic) transcript levels for phenylalanine ammonia-lyase ( PAL ) and chalcone synthase ( CHS ) and the average antioxidant activities were lower for the ozone sensitive clone 259 as compared to the more tolerant 216 and 271 clones. A significant decrease in chlorophyll a , b and total ( a + b ) concentrations in CO 2, O 3 , and CO 2+ O 3 plants was observed for all clones. Carotenoid concentrations were also significantly lower in all clones; however, CHS transcript levels were not significantly affected, suggesting a possible degradation of carotenoid pigments in O 3 -stressed plants. Antioxidant activities and PAL and 1-aminocyclopropane-1-carboxylic acid (ACC)-oxidase transcript levels showed a general increase in all O 3 treated clones, while remaining low in CO 2 and CO 2+ O 3 plants (although not all differences were significant). Our results suggest that the ascorbate-glutathione and phenylpropanoid pathways were activated under ozone stress and suppressed during exposure to elevated CO 2 . Although CO 2+ O 3 treatment resulted in a slight reduction of O 3 -induced leaf injury, it did not appear to ameliorate all of the harmful affects of O 3 and, in fact, may have contributed to an increase in chloroplast damage in all three aspen clones.

Rising atmospheric CO 2 may stimulate future forest productivity, possibly increasing carbon storage in terrestrial ecosystems, but how tropospheric ozone will modify this response is unknown. Because of the importance of fine roots to the belowground C cycle, we monitored fine-root biomass and associated C fluxes in regenerating stands of trembling aspen, and mixed stands of trembling aspen and paper birch at FACTS-II, the Aspen FACE project in Rhinelander, Wisconsin. Free-air CO 2 enrichment (FACE) was used to elevate concentrations of CO2 (average enrichment concentra- tion 535 µl l -1 ) and O 3 (53 nl l -1 ) in developing forest stands in 1998 and 1999. Soil respiration, soil pCO 2 , and dissolved organic carbon in soil solution (DOC) were monitored biweekly. Soil respiration was measured with a portable infrared gas analyzer. Soil pCO 2 and DOC samples were collected from soil gas wells and tension lysimeters, respectively, at depths of 15, 30, and 125 cm. Fine-root biomass averaged 263 g m-2 in con- trol plots and increased 96% under elevated CO 2. The increased root biomass was accompanied by a 39% in- crease in soil respiration and a 27% increase in soil pCO 2 . Both soil respiration and pCO 2 exhibited a strong seasonal signal, which was positively correlated with soil temperature. DOC concentrations in soil solution averaged ~12 mg l -1 in surface horizons, declined with depth, and were little affected by the treatments. A sim- plified belowground C budget for the site indicated that native soil organic matter still dominated the system, and that soil respiration was by far the largest flux. Ozone decreased the above responses to elevated CO2, but effects were rarely statistically significant. We con- clude that regenerating stands of northern hardwoods have the potential for substantially greater C input to soil due to greater fine-root production under elevated CO 2 . Greater fine-root biomass will be accompanied by greater soil C efflux as soil respiration, but leaching losses of C will probably be unaffected.

In southern Wisconsin, U.S.A., tree growth and associated canopy traits were compared among five native and hybrid genotypes of poplar (Populus spp.) in replicated, monoclonal stands planted at a 1 × 1 m spacing. The overall objective of this study was to assess clonal suitability to cultural conditions entailing high levels of intracanopy competition (such as high-density plantations or long rotations) and to identify selection criteria suitable to such conditions. Two of the clones were Populus deltoides Bartr., two were P. deltoides × Populus nigra L. (DN) crosses, and the fifth was a P. nigra × Populus maximowiczii A. Henry (NM) cross. In the third year after establishment, variation in aboveground biomass gain (ANBG) was analyzed in relation to canopy light interception (IPAR) and canopy light-use efficiency (LUE) during a 31-day period when growing conditions were most favorable (late June through late July). ANBG in this interval varied by twofold among genotypes (2.76–5.78 Mg·ha–1), and it was highest in the two P. deltoides clones, followed by the NM and DN hybrids, respectively. Across genotypes, ANBG was unrelated to IPAR, which varied by only 5%. Instead, it was strongly and positively related (r2 = 0.99) to the twofold variation in LUE (1.06–2.22 g·MJ–1). Among measured canopy traits, the best predictor of LUE (r2 = 0.88) was an additive combination of factors associated to the optimization of canopy photosynthesis: LUE was negatively related to both the canopy light-extinction coefficient and compensation irradiance at the canopy base. We infer from these findings that poplar genotypes can vary considerably in LUE and, correspondingly, in the extent to which photosynthesis is optimized in dense canopies. Furthermore, the low LUE among hybrid genotypes at this level of intracanopy competition may reflect a bias in "tree improvement" efforts towards maximizing biomass production under conditions of relatively low competition.

Two aspen (Populus tremuloides Michx.) clones, differing in O3 tolerance, were grown in a free-air CO2 enrichment (FACE) facility near Rhinelander, Wisconsin, and exposed to ambient air, elevated CO2, elevated O3 and elevated CO2+O3. Leaf instantaneous light-saturated photosynthesis (PS) and leaf areas (A) were measured for all leaves of the current terminal, upper (current year) and the current-year increment of lower (1-year-old) lateral branches. An average, representative branch was chosen from each branch class. In addition, the average photosynthetic rate was estimated for the short-shoot leaves. A summing approach was used to estimate potential whole-plant C gain. The results of this method indicated that treatment differences were more pronounced at the plant- than at the leaf- or branch-level, because minor effects within modules accrued in scaling to plant level. The whole-plant response in C gain was determined by the counteracting changes in PS and A. For example, in the O3-sensitive clone (259), inhibition of PS in elevated O3 (at both ambient and elevated CO2) was partially ameliorated by an increase in total A. For the O3-tolerant clone (216), on the other hand, stimulation of photosynthetic rates in elevated CO2 was nullified by decreased total A.

We present results from a Populus Regional Testing Program that has been conducted in Minnesota, Iowa, Wisconsin, and Michigan over the past six years. Our objectives have been to: 1) identify highly productive, disease resistant intra- and inter-specific clonal selections and 2) understand patterns of genotype × environment interactions within the region that would, logically, govern commercial deployment of new clones. Clones were developed by breeding and selection programs at the University of Illinois, Iowa State University, University of Minnesota, and the USDA Forest Service for experiments established in 1995. We report results of analyses of variance and principal component analyses of tree diameters and estimated above-ground biomass that demonstrate significant genotype main effects and significant genotype × environment interactions. Maximum mean annual above-ground biomass increments have surpassed 16 Mg ha−1 y−1, exceeding previously reported yields of poplars grown under similar conditions in the north-central U.S. We also discuss the breeding and selection of poplars in general with specific attention to regional research needs. Key words: Populus, biomass, multi-trait selection, genotype, genotype × environment interaction

Leaf gas exchange parameters and the content of ribulose1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2-year-old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO2 and/or O3. The plants were exposed either to ambient air (control), elevated CO2 (560 p.p.m.) elevated O3 (55 p.p.b.) or a mixture of elevated CO2 and O3 in a free air CO2 enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light-saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age-related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO2 and decreased by O3 at both control and elevated CO2. The relative stomatal limitation to photosynthesis (ls) was in both clones about 10% under control and elevated O3. Exposure to elevated CO2 + O3 in both clones and to elevated CO2 in clone 259, decreased ls even further ‐ to about 5%. The corresponding changes in Rubisco content and the stability of Ci/Ca ratio suggest that the changes in photosynthesis in response to elevated CO2 and O3 were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O3 tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable Ci under the given treatment, that indicates close coupling between stomatal and mesophyll processes.

The episodic or flushing growth habit of northern red oak (Quercus rubra L.) has a significant influence on carbon fixation, carbon transport from source leaves, and carbon allocation within the plant; however, the impact of episodic growth on carbon partitioning among chemical fractions is unknown. Median-flush leaves of the first and second flush were photosynthetically labeled with 14CO2, and partitioning of 14C into lipids and pigments, sugars, amino acids, organic acids, protein, starch, and structural carbohydrates of source leaves, stem, and roots was determined. In addition, four chemical fractions (sugars, starch, amino acids, and total structural carbohydrates) were quantitatively analyzed in leaves, stems, and roots. Chemical changes in source leaves reflected leaf maturation, changing sink demand during a growth cycle, and leaf senescence. Starch and sugar storage in leaves, stems, and roots during lag and bud growth stages indicate a feedback response of these tissues to decreasing sink strength and temporary storage of both starch and sugar in these plant tissues. Northern red oak, with episodic shoot growth patterns, provides an experimental system in which large changes in sink strength occur naturally and require no plant manipulation. Metabolic changes in leaf, stem, and root tissue of red oak have broad application for other oak species and for both temperate and tropical tree species with cyclic growth habits.

Relatively little is known about the changing carbon allocation patterns in species with episodic growth cycles such as northern red oak (Quercus rubra L.). To examine such changing allocation and growth patterns, northern red oak plants were grown from seed in controlled environment chambers through four cycles of growth. 14CO2 was supplied to leaves of the first, second, or third flushes at different Quercus morphological index growth stages within each flush, and the distribution of 14C within the plant was analyzed. Carbon allocation from source leaves of the first and second flush was primarily upward during the subsequent cycle of shoot growth and downward during lag and bud growth stages. All leaves within a flush did not respond the same. Upper leaves allocated most 14C-photosynthate upward during leaf and shoot growth while lower leaves supplied more 14C to lower stem and roots. During the third and fourth flushes, differential allocation from leaves within a flush resulted in essentially equal upward and downward carbon allocation. Growth and allometric relationships reflected these changes in carbon allocation.

Seasonal patterns of photosynthates export were followed on 1- and 2-year-old trees of four Populus clones, belonging to the species Populus trichocarpa Torr. & Gray, Populus deltoides Bartr., and their interspecific hybrids grown in the field under short rotation. Different types of branches (including sylleptic and proleptic) and cohorts of leaves on the main stem were exposed to 14CO2 at monthly and bimonthly intervals in the first and second growing season. Patterns of photosynthates export were influenced by phenology and differed markedly among clones. Differences of translocation patterns were also found among various portions of the crown. Sylleptic branches exported carbon mainly to the lower stem and the roots, whereas main stem leaves, before bud set, contributed much to the height growth of the tree. Sylleptic branches also had greater translocation efficiency than proleptic, contributing more, on a per unit mass basis, to the growth of the tree. Within branches of the same order and among branches of different order, export of assimilates followed patterns similar to those found in the main stem. There was little export of assimilates between adjacent branches of the same order or between branches and main stem leaves. Clones 11-11 and 1-12, which had the highest number of sylleptic branches, were also the most productive clones, respectively, for the hybrids and the parental species.

With the increased use of variables such as canopy cover, photosynthetically active radiation (PAR) and overstory leaf area index (LAI) in forestry research, relationships between these variables and traditional forestry variables must be defined before recommended levels of these research variables can be achieved by forestry practitioners on the ground. We measured basal area, canopy cover, Ozalid percent full light, PAR, and overstory LAI in thinned and unthinned plots within oak and pine stands with the objectives of: (1) determining the relationships between these variables in two common forest types, (2) investigating the feasibility of using basal area to estimate and achieve recommended levels of canopy cover, PAR, and LAI in the field, and (3) examining the possibility of using direct canopy cover and Ozalid light measurements for estimating PAR and LAI. Very strong relationships (r² > 0.90 and P < 0.0001) were indicated between basal area and canopy cover, PAR, and LAI. Direct canopy cover and Ozalid light measurements were also strongly related to PAR and LAI. It is likely that the even-aged structure of the stands studied contributed to these results. The strength of the relationships between the measures examined suggest that practical variables such as basal area could potentially be used by forestry practitioners to estimate and achieve recommended levels of canopy cover, PAR, and LAI in similar oak and pine stands. The possibility also exists for strong relationships between these variables in other stand types that resemble those studied in terms of overstory structure. North. J. Appl. For. 16(1):25-32.

Over the years, a series of trembling aspen (Populus tremuloides Michx.) clones differing in O3 sensitivity have been identified from OTC studies. Three clones (216 and 271[(O3 tolerant] and 259 [O3 sensitive]) have been characterized for O3 sensitivity by growth and biomass responses, foliar symptoms, gas exchange, chlorophyll content, epicuticular wax characteristics, and antioxidant production. In this study we compared the responses of these same clones exposed to O3 under field conditions along a natural O3 gradient and in a Free-Air CO2 and O3 Enrichment (FACE) facility. In addition, we examined how elevated CO2 affected O3 symptom development. Visible O3 symptoms were consistently seen (5 out of 6 years) at two of the three sites along the O3 gradient and where daily one-hour maximum concentrations were in the range of 96 to 125 ppb. Clonal differences in O3 sensitivity were consistent with our OTC rankings Elevated CO2 (200 ppm over ambient and applied during daylight hours during the growing season) reduced visible foliar symptoms for all three clones from 31 to 96% as determined by symptom development in elevated O3 versus elevated O3 + CO2 treatments. Degradation of the epicuticular wax surface of all three clones was found at the two elevated O3 gradient sites. This degradation was quantified by a coefficient of occlusion which was a measure of stomatal occlusion by epicuticular waxes. Statistically significant increases in stomatal occlusion compared to controls were found for all three clones and for all treatments including elevated CO2, elevated O3, and elevated CO2 + O3. Our results provide additional evidence that current ambient O3 levels in the Great Lakes region are causing adverse effects on trembling aspen. Whether or not elevated CO2 in the future will alleviate some of these adverse effects, as occurred with visible symptoms but not with epicuticular wax degradation, is unknown.

A wide variety of hybrid poplar clones are being introduced for intensive culture biomass production, but the potential clonal or genotypic response to increasing tropospheric carbon dioxide (CO2), ozone (O3), and their interactions are unknown. To study these effects, we exposed five different hybrid Populus clones to increased concentrations of CO2, O3, and CO2 + O3 in open-top chambers for one growing season and determined growth responses. Exposure to elevated CO2 increased height growth, dry mass, and basal area; exposure to O3 decreased all three of these growth responses. Exposure impact differed among the different plant parts (leaf, stem, and roots) and among the clones. These differences were associated with different growth strategies or carbon allocation patterns inherent in the different clones. The fastest growing clones had the greatest response to O3 treatment. The addition of CO2 to the O3 exposure counteracted the negative impact of O3 in all plant components except leaf mass (e.g., CO2 + O3 plant mass equaled control plant mass) in all of the clones. But correspondingly, added O3 negated increased growth from CO2. Genetic variation in response to atmospheric pollutants must be considered even in closely related genotypes found in Populus culture.

Variable internal plant nutrient content may confound plant response to environmental stress. Plant nutrient content may be controlled with relative addition rate techniques in solution culture. However, because raising large numbers of plants in flowing solution culture is difficult, we investigated the feasibility of raising plants in soil mix using relative fertilizer additions. Aspen (Populus tremuloides Michx.) clones (216, 259 and 271) planted in pots containing a peat, sand and vermiculite (2:1:1, v/v/v) soil mix were grown with exponentially increasing fertilizer concentrations and harvested periodically to assess growth. Addition rate treatments ranged from 0.01 to 0.05 day -1 . The lag phase of growth, in which plants adjusted to the fertilizer regime, lasted 40 days after which plants entered the experimental period characterized by constant relative growth rates equivalent to applied fertilizer addition rates. Total plant nutrient concentration was (1) unique for each addition rate, (2) linearly related to addition rate and growth rate, and (3) tended to increase at the highest, and decrease at the lowest addition rates. Regardless, the plants appeared to have attained steady-state conditions. Allocation of carbon to roots increased with lower addition rate treatments and was not dependent upon ontogeny. There were no treatment differences in growth response among aspen clones. Yet there were treatment differences in leaf chlorophyll and photosynthesis within the clones. For the 0.05 day -1 addition rate treatment, chlorophyll, leaf N concentration and photosynthetic rate were strongly correlated with one another, were at a maximum in recently mature leaves, and rapidly declined with leaf age. The rate of decline in these leaf characteristics was slowest in clone 271, consistent with the leaf longevity stress response reported elsewhere. Plant responses from these relative fertilizer addition trials in soil mix agree closely with those run in hydroponics, indicating that steady-state nutrition can be achieved with a technically simple experimental assemblage.

We exposed trembling aspen (Populus tremuK&s Michx.) clones differing in tropospheric ozone (O$ tolerance in various opentop chamber studies for rhree growing seasons, and examined the effects of 03, COz, and O3 + CO2 on growth and physiological processes. Ozone in the range of 80 ppm hr (Sum 00) per growing season decreased height, diameter, and stem and leaf biomass slightly in a tolerant clone but severely in a sensitive clone. Elevated CO2 (150 ppm over ambient) did not compensate for the O3 effects. Antioxidant enzyme analysis showed elevated SOD levels in the tolerant clone but not in the sensitive clone following O3 exposure. Northern blot analysis indicated that the chloroplastic and cytosolic CwZn SOD’s were significantly increased in response to O3 in the tolerant but not the sensitive clone. Currently, we are conducting molecular analysis to determine the functional significance of SOD’s in regulaling O3 tolerance in aspen. 0 1997 Elsevier

The decreased ability of northern red oak (Quercus rubra) to regenerate throughout its range in the eastern United States has important ecological and economic implications. We studied regeneration of northern red oak in oak and pine stands on moderately productive sites in northern Lower Michigan. Our objectives were (1) to investigate the hypothesis that regeneration of northern red oak is more successful in pine than in oak stands and (2) to test whether removal of potential overstory and understory competitors increases regeneration success on moderately productive sites. Northern red oak acorns and 2-yr-old nursery seedlings were planted in spring 1991 in three natural oak stands and three red pine (Pinus resinosa) plantations on comparable, moderately productive sites. Each stand contained four canopy cover treatments: clearcut, 25% cover (50% the first year), 75% cover, and uncut. Each canopy cover treatment contained four understory treatments: herb-layer removal, shrub-layer removal, litter remov...

La croissance et les composantes de la biomasse de quatre clones de peuplier, incluant Populus deltoides Bartr., Populus trichocarpa Torr. & A. Gray et leurs hybrides interspecifiques, ont ete etudiees pendant 4 annees consecutives d'une revolution de courte duree dans l'ouest de l'Etat de Washington, aux Etats-Unis. Les resultats confirment les observations precedentes mettant en evidence le potentiel de productivite elevee des clones hybrides de P. trichocarpa x P. deltoides; le taux maximum de production de biomasse totale a atteint 21 Mg.ha -1 .an -1 a la fin de la deuxieme annee et la production maximum de biomasse epigee a atteint 35 Mg.ha -1 .an -1 a la fin de la quatrieme annee. Chez deux des clones d'hybrides testes, 11-11 et 44-136, des differences importantes dans la distribution de la biomasse entre les differentes composantes des arbres ainsi que dans le patron de croissance etaient evidentes telles qu'illustrees par l'indice de recolte et les ratios racines/tige. Un examen des coefficients de distribution de la biomasse a revele que les differences clonales dans l'activite des zones d'accumulation de biomasse dans un arbre variaient dans le temps. Dans ce cas egalement les clones d'hybrides avaient des comportements differents durant la premiere annee; le clone 11-11 accumulait plus de biomasse dans les feuilles et les branches tandis que le clone 44-136 accumulait plus de biomasse dans les racines, ce qui contribuait a une croissance rapide des feuilles et de la tige tot dans la deuxieme saison de croissance. Les resultats suggerent que les differences clonales, dans la biomasse totale, dans l'allocation vers differentes parties des arbres et dans l'indice de recolte, ont des consequences importantes pour les programmes futurs d'hybridation du peuplier bases sur des idees recues.

Three young northern temperate forest communities in the north-central United States were exposed to factorial combinations of elevated carbon dioxide (CO2 ) and tropospheric ozone (O3 ) for 11 years. Here, we report results from an extensive sampling of plant biomass and soil conducted at the conclusion of the experiment that enabled us to estimate ecosystem carbon (C) content and cumulative net primary productivity (NPP). Elevated CO2 enhanced ecosystem C content by 11%, whereas elevated O3 decreased ecosystem C content by 9%. There was little variation in treatment effects on C content across communities and no meaningful interactions between CO2 and O3 . Treatment effects on ecosystem C content resulted primarily from changes in the near-surface mineral soil and tree C, particularly differences in woody tissues. Excluding the mineral soil, cumulative NPP was a strong predictor of ecosystem C content (r(2) = 0.96). Elevated CO2 enhanced cumulative NPP by 39%, a consequence of a 28% increase in canopy nitrogen (N) content (g N m(-2) ) and a 28% increase in N productivity (NPP/canopy N). In contrast, elevated O3 lowered NPP by 10% because of a 21% decrease in canopy N, but did not impact N productivity. Consequently, as the marginal impact of canopy N on NPP (∆NPP/∆N) decreased through time with further canopy development, the O3 effect on NPP dissipated. Within the mineral soil, there was less C in the top 0.1 m of soil under elevated O3 and less soil C from 0.1 to 0.2 m in depth under elevated CO2 . Overall, these results suggest that elevated CO2 may create a sustained increase in NPP, whereas the long-term effect of elevated O3 on NPP will be smaller than expected. However, changes in soil C are not well-understood and limit our ability to predict changes in ecosystem C content.

Current projections indicate steady increases in both trophospheric ozone and carbon dioxide well into the next century with concurrent increases in plant stress. Because information about effects of these interacting stresses on forest trees is limited, we have conducted ozone and carbon dioxide experiments using ozone-tolerant and ozone-sensitive trembling aspen (Populustremuloides Michx.) clones (clones 216 and 259, respectively). Aspen plants were grown either in pots (square-wave study) or in the ground (episodic study) in open-top chambers. Plants in the square-wave study were exposed for a single growing season to charcoal-filtered air (CF) or to CF plus elevated carbon dioxide (CO2), ozone (O3), or O3 plus CO2 (O3 + CO2). Plants in the episodic study were exposed for three growing seasons to CF, twice simulated ambient (2x) O3 (2x O3), or 2x O3 plus CO, (2x O3 + CO2). Photosynthetic measurements were made either in the open-top chambers at treatment CO2 concentrations or in controlled-environment cuvettes with various CO2 concentrations, producing assimilation versus intercellular CO2 concentration (A/Ci) curves. Ozone decreased photosynthetic rate and stomatal conductance and accelerated leaf senescence. Elevated CO2 increased photosynthetic rate and decreased stomatal conductance when measured at treatment CO2 concentrations, and exacerbated the negative effect of O3 on photosynthesis. For example, for clone 259, photosynthesis decreased 9% for the O3 treatment compared with the CF treatment, but decreased 24% for the O3 + CO2 treatment compared with the CF treatment. Similar decreases for clone 216 of 2% and 6% for O3 and O3 + CO2, respectively, were not significant. A/Ci curves showed that O3 decreased carboxylation efficiency and maximum photosynthetic rate and that photosynthetic inhibition in response to O3 was greater with elevated CO2. The simultaneous declines in all factors of photosynthetic gas exchange measurements suggest that the equilibrium between stomatal conductance, carboxylation, and light harvesting systems was not disrupted by O3 and O3× CO2 interactions. Carbon dioxide did not ameliorate the detrimental effects of O3 on the leaf photosynthetic apparatus. In fact, the O3-tolerant clone appeared more sensitive to O3 with elevated CO2.

The effects of single-season tropospheric ozone (O3) exposures on growth, leaf abscission, and biomass of trembling aspen (Populustremuloides Michx.) rooted cuttings and seedlings were studied. Plants were grown in the Upper Peninsula of Michigan in open-top chambers with O3 exposures that ranged from 7 to 92 ppm-h. Depending on the genotype, total seasonal O3 exposure in the range of 50–92 ppm-h had negative impacts on stem, retained leaf, and root biomass accumulation and on diameter growth. Leaf abscission generally increased with increasing O3 exposure and was the principal cause of the decrease in leaf biomass of the O3-treated plants. Considerable genetic variation in O3 responses occurred, as shown by differences in sensitivities among clones and among seedlings. However, the responses to O3 of rooted cuttings and seedlings were similar when seedling means were compared with clonal means for leaf abscission, diameter growth, retained leaf biomass, and root biomass. Comparison of a single square-wave treatment (52 ppm-h) with 70 and 92 ppm-h episodic exposures suggested that the plant response to the square-wave exposure was similar to the response to the highest episodic exposure even though the 92 ppm-h episodic exposure was almost twice the square-wave exposure. Our results are consistent with previous studies that show that P. tremuloides is highly responsive to O3 exposure and this response has a strong genetic component.

In order to effectively deploy short-rotation woody crop plantations for energy and fiber production at regional scales, biologically sound-field-level models of plant growth are required. Individual tree growth process models have proven effective at predicting growth based on environmental driving variables, but in the past these models have been too complex to use at the plantation scale. In this paper we describe an object-oriented strategy for scaling ECOPHYS, an individual tree growth process model for hybrid poplar, to a plantation. Included in this strategy are methods of scaling from an individual shoot to a tree, from individual trees to a patch, and from patches to a plantation. In describing interactions among trees, both above- and below-ground processes are considered. Finally, we describe methods for integrating the plantation level ECOPHYS with EPIC, a field scale model of soil productivity and erosion used in regional assessment.

Clones of aspen (Populus tremuloides Michx.) were identified that differ in biomass production in response to O(3) exposure. (14)Carbon tracer studies were used to determine if the differences in biomass response were linked to shifts in carbon allocation and carbon partitioning patterns. Rooted cuttings from three aspen Clones (216, O(3) tolerant; 271, intermediate; and 259, O(3) sensitive) were exposed to either charcoal-filtered air (CF) or an episodic, two-times-ambient O(3) profile (2x) in open-top chambers. Either recently mature or mature leaves were exposed to a 30-min (14)C pulse and returned to the treatment chambers for a 48-h chase period before harvest. Allocation of (14)C to different plant parts, partitioning of (14)C into various chemical fractions, and the concentration of various chemical fractions in plant tissue were determined. The percent of (14)C retained in recently mature source leaves was not affected by O(3) treatment, but that retained in mature source leaves was greater in O(3)-treated plants than in CF-treated plants. Carbon allocation from source leaves was affected by leaf position, season, clone and O(3) exposure. Recently mature source leaves of CF-treated plants translocated about equal percentages of (14)C acropetally to growing shoots and basipetally to stem and roots early in the season. When shoot growth ceased (August 16), most (14)C from all source leaves was translocated basipetally to stem and roots. At no time did mature source leaves allocate more than 6% of (14)C translocated within the plant to the shoot above. Ozone effects were most apparent late in the season. Ozone decreased the percent (14)C translocated from mature source leaves to roots and increased the percent (14)C translocated to the lower stem. In contrast, allocation from recently mature leaves to roots increased. Partitioning of (14)C among chemical fractions was affected by O(3) more in source leaves than in sink tissue. In source leaves, more (14)C was incorporated into the sugar, organic acid and lipids + pigments fractions, and less (14)C was incorporated into starch and protein fractions in O(3)-treated plants than in CF-treated plants. In addition, there were O(3) treatment interactions between leaf position and clones for (14)C incorporation into different chemical fractions. When photosynthetic data were used to convert percent (14)C transported to the total amount of carbon transported on a mass basis, it was found that carbon transport was controlled more by photosynthesis in the source leaves than proportional changes in allocation to the sinks. Ozone decreased the total amount of carbon translocated to all sink tissue in the O(3)-sensitive Clone 259 because of decreases in photosynthesis in both recently mature and mature source leaves. In contrast, O(3) had no effect on carbon transport from recently mature leaves to lower shoots of either Clone 216 or 271, had no significant effect on transport to roots of Clone 216, and increased transport to roots of Clone 271. The O(3)-induced increase in transport to roots of Clone 271 was the result of a compensatory increase in upper leaf photosynthesis and a relatively greater shift in the percent of carbon allocated to roots. In contrast to those of Clone 271, recently mature leaves of Clone 216 maintained similar photosynthetic rates and allocation patterns in both the CF and O(3) treatments. We conclude that Clone 271 was more tolerant to O(3) exposure than Clone 216 or 259. Tolerance to chronic O(3) exposure was directly related to maintenance of high photosynthetic rates in recently mature leaves and retention of lower leaves.

We evaluated several sampling schemes for estimating instantaneous whole-tree photosynthesis of 1-year-old Populus clones. Growth of two clones was simulated under varying weather conditions and leaf orientation scenarios providing photosynthetic data on a leaf-by-leaf basis throughout the growing season. Simple random sampling, stratified random sampling and a series of physiologically based sampling schemes were evaluated using either whole-leaf photosynthesis or photosynthetic rate (i.e., photosynthesis per unit leaf area) as the sampling attribute. Ratio and regression estimators with leaf area as an auxiliary variable were also studied. On the basis of their bias and accuracy in estimating instantaneous whole-tree photosynthesis (mg CO(2)), the physiologically based sampling schemes were superior for all combinations of clone type, weather condition and leaf orientation. Aspects of extending the sampling process to estimate daily and seasonal photosynthesis are also elaborated.

Nous avons explore la possibilite d'utiliser une estimation de type Monte-Carlo comme alternative aux techniques traditionnelles d'inventaires par echantillonnage pour l'estimation de la photosynthese journaliere et saisonniere de peupliers clonaux entiers âges de 1 an. Nous avons applique plusieurs techniques dites de reduction de variance au probleme de l'estimation, et les avons comparees a l'aide de la racine des carres moyens de l'erreur. Nous avons aussi considere le gain de precision par rapport a l'estimateur par expansion au premier terme (l'estimation Monte Carlo sous sa forme la plus simple). La variance a ete reduite en ajustant a la courbe de photosynthese une fonction connue et simple a integrer. Les estimateurs sont non biaises, quel que soit l'ajustement de la fonction. Nous demontrons comment ces methodes de reduction de variance peuvent etre utilisees pour ameliorer la precision de l'estimation de la photosynthese journaliere et saisonniere de l'arbre entier. Ces methodes sont des alternatives utiles aux methodes actuelles d'echantillonnage, methodes qui ont un potentiel eleve de biais et d'erreur

Height and diameter growth, stem volume production, leaf phenology and leaf number, and number of branches of Populustrichocarpa Torr. & Gray, Populusdeltoides Bartr., and their F1 hybrids (P. trichocarpa × P. deltoides) were studied for 4 years in a research plantation in western Washington, United States. Twelve clones (three of each species and six of the hybrids) grew under a short-rotation silviculture regime in monoclonal plots at spacings of 1 × 1 m (10 000 stems/ha). Clones represented a north-south gradient within the geographic distribution of both the two North American poplar species and the parentage of the hybrid material. The results support earlier work by contributing additional evidence for the superiority of the hybrids. However, the relative hybrid superiority in these monoclonal plots was less pronounced than that found earlier in field trials with single-tree plots because of heightened intraclonal competition. After 4 years, mean estimated stem volume of the hybrids was 1.5 times that of P. trichocarpa and 2.3 times that of P. deltoides. Total tree height of the hybrids was 1.1 times that off. trichocarpa and 1.3 times that off. deltoides. Clonal variation was the dominant theme in height and diameter growth, stem volume productivity, time of bud break and bud set, tree mortality, and number of branches. Populustrichocarpa had the highest number of sylleptic branches, P. deltoides had the lowest, and hybrids were intermediate. Significant clone by replicate interactions were observed in height, diameter, and volume growth. Phenological traits, such as the dates of bud break and bud set, and the length of growing period only partly explained the observed differences in growth between the P. trichocarpa × P. deltoides hybrids and the parental species.