Organismal responses to habitat change: herbivore performance, climate and leaf traits in regenerating tropical dry forests.

The ecological effects of large-scale climate change have received much attention, but the effects of the more acute form of climate change that results from local habitat alteration have been less explored. When forest is fragmented, cut, thinned, cleared or otherwise altered in structure, local climates and microclimates change. Such changes can affect herbivores both directly (e.g. through changes in body temperature) and indirectly (e.g. through changes in host plant traits). We advance an eco-physiological framework to understand the effects of changing forests on herbivorous insects. We hypothesize that if tropical forest caterpillars are climate and resource specialists, then they should have reduced performance outside of mature forest conditions. We tested this hypothesis with a field experiment contrasting the performance of Rothschildia lebeau (Saturniidae) caterpillars feeding on the host plant Casearia nitida (Salicaceae) in two different aged and structured tropical dry forests in Area de Conservación Guanacaste, Costa Rica. Compared to more mature closed-canopy forest, in younger secondary forest we found that: (1) ambient conditions were hotter, drier and more variable; (2) caterpillar growth and development were reduced; and (3) leaves were tougher, thicker and drier. Furthermore, caterpillar growth and survival were negatively correlated with these leaf traits, suggesting indirect host-mediated effects of climate on herbivores. Based on the available evidence, and relative to mature forest, we conclude that reduced herbivore performance in young secondary forest could have been driven by changes in climate, leaf traits (which were likely climate induced) or both. However, additional studies will be needed to provide more direct evidence of cause-and-effect and to disentangle the relative influence of these factors on herbivore performance in this system.

Contrasting responses in the growth and energy utilization properties of sympatric Populus and Salix to different altitudes: implications for sexual dimorphism in Salicaceae.

An interesting ecological and evolutionary puzzle arises from the observations of male-biased sex ratios in genus Populus, whereas in the taxonomically related Salix, females are generally more dominant. In the present study, we combined results from a field investigation into the sex ratios of the Salicaceous species along an altitudinal gradient on Gongga Mountain, and a pot experiment by monitoring growth and energy utilization properties to elucidate the mechanisms governing sexual dimorphism. At middle altitudes 2000 and 2300 m, the sex ratios were consistent with a 1:1 equilibrium in sympatric Populus purdomii and Salix magnifica. However, at the lower and higher ends of the altitudinal gradient, skewed sex ratios were observed. For example, the male:female ratios were 1.33 and 2.36 in P. purdomii at 1700 and 2600 m respectively; for S. magnifica the ratio was 0.62 at 2600 m. At 2300 m, the pot-grown seedlings of both species exhibited the highest biomass accumulation and total leaf area, simultaneously with the balanced sex ratios in the field. At 3300 m, the specific leaf area in male P. purdomii was 23.9% higher than that of females, which may be the morphological cause for the observed 19.3% higher nitrogen allocation to Rubisco, and 20.6% lower allocation to cell walls. As such, male P. purdomii showed a 32.9% higher foliar photosynthetic capacity, concomitant with a 12.0% lower construction cost. These properties resulted in higher photosynthetic nitrogen- and energy-use efficiencies, and shorter payback time (24.4 vs 40.1 days), the time span that a leaf must photosynthesize to amortize the carbon investment. Our results thus suggested that male P. purdomii evolved a quicker energy-return strategy. Consequently, these superior energy gain-cost related traits and the higher total leaf area contributed to the higher growth rate and tolerance in stress-prone environments, which might, in part, shed new light on the male-biased sex ratios in Populus. However, no significant sexual difference was observed in S. magnifica for all the above parameters, thereby implying that the female-biased sex ratios in Salix cannot be explained in terms of the energy-use properties studied here.

Contrasting effects of plant species traits and moisture on the decomposition of multiple litter fractions.

Environmental variation in moisture directly influences plant litter decomposition through effects on microbial activity, and indirectly via plant species traits. Whether the effects of moisture and plant species traits are mutually reinforcing or counteracting during decomposition are unknown. To disentangle the effects of moisture from the effects of species traits that vary with moisture, we decomposed leaf litter from 12 plant species in the willow family (Salicaceae) with different native habitat moisture preferences in paired mesic and wetland plots. We fit litter mass loss data to an exponential decomposition model and estimated the decay rate of the rapidly cycling litter fraction and size of the remaining fraction that decays at a rate approaching zero. Litter traits that covaried with moisture in the species' native habitat significantly influenced the decomposition rate of the rapidly cycling litter fraction, but moisture in the decomposition environment did not. In contrast, for the slowly cycling litter fraction, litter traits that did not covary with moisture in the species' native habitat and moisture in the decomposition environment were significant. Overall, the effects of moisture and plant species traits on litter decomposition were somewhat reinforcing along a hydrologic gradient that spanned mesic upland to wetland (but not permanently surface-saturated) plots. In this system, plant trait and moisture effects may lead to greater in situ decomposition rates of wetland species compared to upland species; however, plant traits that do not covary with moisture will also influence decomposition of the slowest cycling litter fraction.

Identification of an SCAR marker related to female phenotype in Idesia polycarpa Maxim.

Idesia polycarpa Maxim. is a dioecious species. Because of the lack of morphological and cytological methods available for identifying its sex during the long juvenile stage, the application of molecular markers in sex identification may facilitate sex determination in the seedling stage. The objective of this study was to use sequence-related amplified polymorphism to identify sex-linked markers in I. polycarpa and convert these markers into sequence-characterized amplified region markers, which are much easier to identify. A total of 342 primer combinations were screened and 2770 bands were examined. Only me14/em8 could amplify a specific fragment (210 base pairs) in all female but none in male plants. We analyzed this fragment using GenBank and found that the sequence similarity was 80% to the Populus trichocarpa clone POP006-H09 (sequence ID: gb|AC212923.1|) and that of the deduced amino acid sequence was 73% to the integrase of Mendicago truncatula (sequence ID: gb|ABD28291.1|) and 71% to the predicted retrotransposon integrase-like protein 1-like in Cicer arietinum (sequence ID: ref|XP 004515460.1|) (NCBI database through December 17, 2013). This fragment was converted into a stable and simple sequence-characterized amplified region marker approximately 200 base pairs in length. This marker can be utilized for early sexual identification in I. polycarpa, which will facilitate future breeding programs.

High diversity and low specificity of chaetothyrialean fungi in carton galleries in a neotropical ant-plant association.

New associations have recently been discovered between arboreal ants that live on myrmecophytic plants, and different groups of fungi. Most of the - usually undescribed - fungi cultured by the ants belong to the order Chaetothyriales (Ascomycetes). Chaetothyriales occur in the nesting spaces provided by the host plant, and form a major part of the cardboard-like material produced by the ants for constructing nests and runway galleries. Until now, the fungi have been considered specific to each ant species. We focus on the three-way association between the plant Tetrathylacium macrophyllum (Salicaceae), the ant Azteca brevis (Formicidae: Dolichoderinae) and various chaetothyrialean fungi. Azteca brevis builds extensive runway galleries along branches of T. macrophyllum. The carton of the gallery walls consists of masticated plant material densely pervaded by chaetothyrialean hyphae. In order to characterise the specificity of the ant-fungus association, fungi from the runway galleries of 19 ant colonies were grown as pure cultures and analyzed using partial SSU, complete ITS, 5.8S and partial LSU rDNA sequences. This gave 128 different fungal genotypes, 78% of which were clustered into three monophyletic groups. The most common fungus (either genotype or approximate species-level OTU) was found in the runway galleries of 63% of the investigated ant colonies. This indicates that there can be a dominant fungus but, in general, a wider guild of chaetothyrialean fungi share the same ant mutualist in Azteca brevis.

Phenological cues drive an apparent trade-off between freezing tolerance and growth in the family Salicaceae.

With increasing concern about the ecological consequences of global climate change, there has been renewed interest in understanding the processes that determine species range limits. We tested a long-hypothesized trade-off between freezing tolerance and growth rate that is often used to explain species range limits. We grew 24 willow and poplar species (family Salicaceae) collected from across North America in a greenhouse common garden under two climate treatments. Maximum entropy models were used to describe species distributions and to estimate species-specific climate parameters. A range of traits related to freezing tolerance, including senescence, budburst, and susceptibility to different temperature minima during and after acclimation were measured. As predicted, species from colder climates exhibited higher freezing tolerance and slower growth rates than species from warmer climates under certain environmental conditions. However, the average relative growth rate (millimeters per meter per day) of northern species markedly increased when a subset of species was grown under a long summer day length (20.5 h), indicating that genetically based day-length cues are required for growth regulation in these species. We conclude that the observed relationship between freezing tolerance and growth rate is not driven by differences in species' intrinsic growth capacity but by differences in the environmental cues that trigger growth. We propose that the coordinated evolution of freezing tolerance and growth phenology could be important in circumscribing willow and poplar range limits and may have important implications for species' current and future distributions.

Are vegetative reproduction capacities the cause of widespread invasion of Eurasian Salicaceae in Patagonian river landscapes?

In recent decades, invasive willows and poplars (Salicaceae) have built dense floodplain forests along most of the rivers in Patagonia, Argentina. These invasion processes may affect Salix humboldtiana as the only native floodplain tree species in this region. It is assumed, that the property to reproduce vegetatively can play an important role in the establishment of invasive species in their new range. Thus, in order to contribute to a better understanding of willow and poplar invasions in riparian systems and to assess the potential impacts on S. humboldtiana the vegetative reproduction capacities of native and invasive Salicaceae were analysed. In a greenhouse experiment, we studied cutting survival and growth performance of the three most dominant invasive Salicaceae of the Patagonian Río Negro region (two Salix hybrids and Populus spec.), as well as S. humboldtiana, taking into account three different moisture and two different soil conditions. In a subsequent experiment, the shoot and root biomass of cuttings from the former experiment were removed and the bare cuttings were replanted to test their ability to re-sprout. The two invasive willow hybrids performed much better than S. humboldtiana and Populus spec. under all treatment combinations and tended to re-sprout more successfully after repeated biomass loss. Taking into account the ecology of vegetative and generative recruits of floodplain willows, the results indicate that the more vigorous vegetative reproduction capacity can be a crucial property for the success of invasive willow hybrids in Patagonia being a potential threat for S. humboldtiana.

Phenolic glycosides of the Salicaceae and their role as anti-herbivore defenses.

Since the 19th century the phytochemistry of the Salicaceae has been systematically investigated, initially for pharmaceutical and later for ecological reasons. The result of these efforts is a rich knowledge about the phenolic components, especially a series of glycosylated and esterified derivatives of salicyl alcohol known as "phenolic glycosides". These substances have received extensive attention with regard to their part in plant-herbivore interactions. The negative impact of phenolic glycosides on the performance of many generalist herbivores has been reported in numerous studies. Other more specialized feeders are less susceptible and have even been reported to sequester phenolic glycosides for their own defense. In this review, we attempt to summarize our current knowledge about the role of phenolic glycosides in mediating plant-herbivore interactions. As background, we first review what is known about their basic chemistry and occurrence in plants.

Modeling the effect of photosynthetic vegetation properties on the NDVI--LAI relationship.

Developing a relationship between the normalized difference vegetation index (NDVI) and the leaf area index (LAI) is essential to describe the pattern of spatial or temporal variation in LAI that controls carbon, water, and energy exchange in many ecosystem process models. Photosynthetic vegetation (PV) properties can affect the estimation of LAI, but no models integrate the effects of multiple species. We developed four alternative NDVI-LAI models, three of which integrate PV effects: no PV effects, leaf-level effects, canopy-level effects, and effects at both levels. The models were fit to data across the natural range of variation in NDVI for a widespread High Arctic ecosystem. The weight of evidence supported the canopy-level model (Akaike weight, wr = 0.98), which includes species-specific canopy coefficients that primarily scale fractional PV cover to LAI by accounting for the area of unexposed PV. Modeling the canopy-level effects improved prediction of LAI (R2 = 0.82) over the model with no PV effect (R2 = 0.71) across the natural range of variation in NDVI but did not affect the site-level estimate of LAI. Satellite-based methods to estimate species composition, a variable in the model, will need to be developed. We expect that including the effects of PV properties in NDVI-LAI models will improve prediction of LAI where species composition varies across space or changes over time.

[Microclimatic effect and soil moisture change of poplar-wheat intercropping systems in Huaibei Plain].

In this paper, the microclimatic effect and the soil moisture of poplar-wheat intercropping systems were studied. The results indicated that the strip intercropping, comparing with the contrast, could increase the relative humidity by 2-8%, decrease the surface temperature by 1-7 degrees C, and reduce wind speed. The illumination hours were decreased with the time and the distance between the rows, and the varying range was within 4.1-15.3%. In shelterbelt network, the relative humidity was increased by 6.4-11.6%, and the illumination hours were decreased by 8.5-11.7%. In strip intercropping, the intercropping systems could improve soil moisture, which was varied with the intensity of tree, and generally, the increase of soil moisture was about in a range of 0.67-3.87%. In shelterbelt network, the soil moisture was related to the orientation and the distance from the shelterbelt, and there existed negative correlation between the soil moisture and the distance in all orientations.

Understanding the role of the cytoskeleton in wood formation in angiosperm trees: hybrid aspen (Populus tremula x P. tremuloides) as the model species.

The involvement of microfilaments and microtubules in the development of the radial and axial components of secondary xylem (wood) in hybrid aspen (Populus tremula L. x P. tremuloides Michx.) was studied by indirect immunofluorescent localization techniques. In addition to cambial cells, the differentiated cell types considered were early- and late-wood vessel elements, axial parenchyma, normal-wood fibers and gelatinous fibers, and contact and isolation ray cells. Microfilaments were rare in ray cambial cells, but were abundant and axially arranged in their derivatives once cell elongation had begun, and persisted in that orientation in mature ray cells. Microfilaments were axially arranged in fusiform cambial cells and persisted in that orientation in all xylem derivatives of those cells. Microtubules were randomly oriented in ray and fusiform cells of the cambial zone. Dense arrays of parallel-aligned microtubules were oriented near axially in the developing gelatinous fibers, but at a wide range of angles in normal-wood fibers. Ellipses of microfilaments were associated with pit development in fiber cells and isolation ray cells. Rings of co-localized microtubules and microfilaments were associated with developing inter-vessel bordered pits and vessel-contact ray cell contact pits, and, in the case of bordered pits, these rings decreased in diameter as the over-arching pit border increased in size. Although only microtubules were seen at the periphery of the perforation plate of vessel elements, a prominent meshwork of microfilaments overlaid the perforation plate itself. A consensus view of the roles of the cytoskeleton during wood formation in angiosperm trees is presented.

Rate of stomatal opening, shoot hydraulic conductance and photosynthetic characteristics in relation to leaf abscisic acid concentration in six temperate deciduous trees.

Correlations between leaf abscisic acid concentration ([ABA]), stomatal conductance (gs), rate of stomatal opening in response to an increase in leaf water potential (si), shoot hydraulic conductance (L) and photosynthetic characteristics were examined in saplings of six temperate deciduous tree species: Acer platanoides L., Padus avium Mill., Populus tremula L., Quercus robur L., Salix caprea L. and Tilia cordata Mill. Species-specific values of foliar [ABA] were negatively related to the mean values of gs, si, L and light- and CO2- saturated net photosynthesis (P(max)), thus providing strong correlative evidence of a scaling of foliar gas exchange and hydraulic characteristics with leaf endogenous [ABA]. In addition, we suggest that mean gs, si, L and Pmax for mature leaves may partly be determined by the species-specific [ABA] during leaf growth. The most drought-intolerant species had the lowest [ABA] and the highest gs, suggesting that interspecific differences in [ABA] may be linked to differences in species-specific water-use efficiency. Application of high concentrations of exogenous ABA led to large decreases in gs, si and P(max), further underscoring the direct role of ABA in regulating stomatal opening and photosynthetic rate. Exogenous ABA also decreased L, but the decreases were considerably smaller than the decreases in gs, si and Pmax. Thus, exogenous ABA predominantly affected the stomata directly, but modification of L by ABA may also be an important mechanism of ABA action. We conclude that interspecific variability in endogenous [ABA] during foliage growth and in mature leaves provides an important factor explaining observed differences in L, gs, si and Pmax among temperate deciduous tree species.

Ethylene enhances water transport in hypoxic aspen.

Water transport was examined in solution culture grown seedlings of aspen (Populus tremuloides) after short-term exposures of roots to exogenous ethylene. Ethylene significantly increased stomatal conductance, root hydraulic conductivity (L(p)), and root oxygen uptake in hypoxic seedlings. Aerated roots that were exposed to ethylene also showed enhanced L(p). An ethylene action inhibitor, silver thiosulphate, significantly reversed the enhancement of L(p) by ethylene. A short-term exposure of excised roots to ethylene significantly enhanced the root water flow (Q(v)), measured by pressurizing the roots at 0.3 MPa. The Q(v) values in ethylene-treated roots declined significantly when 50 microM HgCl(2) was added to the root medium and this decline was reversed by the addition of 20 mM 2-mercaptoethanol. The results suggest that the response of Q(v) to ethylene involves mercury-sensitive water channels and that root-absorbed ethylene enhanced water permeation through roots, resulting in an increase in root water transport and stomatal opening in hypoxic seedlings.

Measurement carbon dioxide concentration does not affect root respiration of nine tree species in the field.

Inhibition of respiration has been reported as a short-term response of tree roots to elevated measurement CO2 concentration ([CO2]), calling into question the validity of root respiration rates determined at CO2 concentrations that differ from the soil [CO2] in the rooting zone. Our objectives were to validate previous observations of a direct CO2 effect on root respiration in sugar maple (Acer saccharum Marsh.) and to determine if high [CO2] also inhibited root respiration in other tree species. Root respiration rates for nine common North American tree species were measured in the field at ambient soil temperature at both 350 and 1000 microl CO2 l-1. No evidence of direct inhibition of root respiration by elevated measurement [CO2] was found for any of the species tested. The ratio of respiration rates at 1000 and 350 microl CO2 l-1 ranged from 0.97 to 1.07, and the 95% confidence intervals for this ratio included unity for all species tested. Tests of a respiration cuvette used in earlier experiments suggested that gas leakage from the cuvette/IRGA system created an apparent direct CO2 effect on respiration of sugar maple roots when none actually existed. Small sample masses used in those experiments exacerbated the error. Careful attention to the possibility of gas leaks and the avoidance of small sample masses should produce data that will allow researchers to accurately assess whether direct effects of measurement [CO2] exist. Our findings of no direct CO2 effect on respiration of roots of a wide variety of species suggest that such effects may be less common than previously thought for tree roots.

Leaf area dynamics in a closed poplar plantation under free-air carbon dioxide enrichment.

Three Populus genotypes (P. alba L. (Clone 2AS-11), P. nigra L. (Clone Jean Pourtet) and P. x euramericana (Clone I-214)) growing in a managed, high-density forest plantation were exposed to free-air CO(2) enrichment (FACE) at CO(2) concentrations expected to occur in the future (550 ppm). Leaf area index (optical LAI), measured with a fish-eye-type plant canopy analyzer, was not significantly affected by FACE after canopy closure in the second growing season. However, when stands of similar size were compared, optical LAI and number of main stem leaves were reduced by FACE. Allometric relationships, which were established to scale-up leaf area to the stand level (allometric LAI), did not differ between the FACE and control plots. Allometric LAI increased in response to FACE, as a result of increased tree dimensions and increased individual leaf size. We postulate that, although FACE increased allometric LAI, FACE had no effect on optical LAI after canopy closure, because FACE caused increased shading and competition resulting in enhanced leaf fall or leaf turnover. Specific leaf area (SLA) was unaffected by FACE. Allometric relationships and relative responses to atmospheric CO(2) enrichment were genotype-dependent.

Ecotypic and genetic variation in poplar bark storage protein gene expression and accumulation.

Bark storage proteins (BSP) store nitrogen (N) translocated from senescing leaves in autumn, and supply reduced N for spring growth. Expression of bsp and BSP accumulation are associated with short day photoperiod. To determine if photoperiod-associated bsp expression varies among poplars native to different latitudes, Populus deltoides Bartr. clones originating from six latitudes were grown under natural conditions at a common location. Relative amounts of BSP mRNA in these clones were measured at 2-week intervals from August 7 to October 16. The date of maximum BSP mRNA accumulation was correlated with latitude of origin, and maximum accumulation of BSP mRNA occurred earlier in clones native to northern latitudes than in clones native to southern latitudes. This pattern of variation is consistent with photoperiodic responses of plants native to temperate climates. Genotypic variations in BSP accumulation, bark protein concentration and bark N concentration were compared among clones of six hybrid poplar (Populus trichocarpa Torr. and Gray x P. deltoides) full-sib families (three F(2) families, two F(1) families and one BC(1) family) after 6 weeks in a short day photoperiod and at midwinter. Significant differences in BSP accumulation occurred among clones within four of the six full-sib families after 6 weeks in a short day photoperiod and also at midwinter for outdoor-grown plants. Bark protein and bark N concentrations also varied significantly among clones within certain families. In general, the greatest variation was found in F(2) and BC(1) families. Within several families, relative BSP amounts were positively correlated with bark protein concentration and total bark N concentration. These results indicate a role of photoperiod in regulating bsp expression and demonstrate a genetic component underlying seasonal BSP accumulation. The results could have significance in selecting for clones with improved N storage capacity and N-use efficiency.

Volatile compounds from Salix spp. varieties differing in susceptibility to three willow beetle species.

The volatile compounds emitted by leaves of 10 willow varieties that differ in their susceptibility to damage by blue (Phratora vulgatissima), brassy (P. vitellinae), and brown (Galerucella lineola) willow beetles were examined both before and after mechanical damage and correlated with feeding preferences of these beetles determined under laboratory conditions. Three compounds were identified from intact undamaged leaves of six willow varieties, namely cis-3-hexenyl acetate, cis-3-hexenol, and benzaldehyde. After mechanical damage, the yield and number of volatile compounds increased for all varieties. There were significant differences among willow varieties for both the concentration of cis-3-hexenyl acetate and the relative proportion of this compound to cis-3-hexenol (green leaf volatile ratio). The 10 varieties collectively showed a significant negative correlation between the relative resistance of each variety to blue and brown willow beetles and the yield of cis-3-hexenyl-acetate from damaged plants. The green leaf volatile ratio of damaged plants was also negatively correlated with the relative resistance of willow variety to these two beetle species.

Genotypic variation in growth and physiological responses of Finnish hybrid aspen (Populus tremuloides x P. tremula) to elevated tropospheric ozone concentration.

Saplings of six Finnish hybrid aspen (Populus tremuloides Michx. x P. tremula L.) clones were exposed to 0, 50, 100 and 150 ppb ozone (O3) for 32 days in a chamber experiment to determine differences in O3 sensitivity among genotypes. Based on the chamber experiment, three clones with intermediate sensitivity to O3 were selected for a free-air O3 enrichment experiment in which plants were exposed for 2 months to either ambient air (control) or air containing 1.3 x the ambient O3 concentration. We measured stem height and radial growth, number of leaves, dry mass and relative growth rate of leaves, stem and roots, visible leaf injuries, net photosynthesis and stomatal conductance of the clones. There was high clonal variation in susceptibility to O3 in the chamber experiment, indicated by foliar injuries and differential reductions in growth and net photosynthesis. In the free-air O3 enrichment experiment, ozone caused a shift in resource allocation toward stem height growth, thereby altering the shoot to root balance. In both experiments, low O3 concentrations tended to stimulate growth of most clones, whereas 100 and 150 ppb O3 in the chamber experiment impaired growth of most clones. However, growth of the most O3-tolerant clone was not significantly affected by any O3 treatment.

Evidence for increased sensitivity to nutrient and water stress in a fast-growing hybrid willow compared with a natural willow clone.

The hypothesis that fast-growing breeds of willow (Salix spp.) are more sensitive to nutrient and water stress and less efficient in nutrient- and water-use than slower-growing natural willow clones was tested. Cuttings of a natural clone of S. viminalis L. collected in Sweden (L78183) and a hybrid clone of S. schwerinii E. Wolf. x S. viminalis L. ("Tora") were grown outdoors in pots under various experimental conditions in a full-factorial design. The experimental conditions included three fertilization, two irrigation and two temperature regimes. Classical growth analysis techniques, based on an initial and a final harvest, were used as a screening method, together with calculation of intrinsic water-use efficiency (foliar carbon isotope ratio; delta13C). In addition, nitrogen-use efficiency was calculated as the product of nitrogen productivity and mean residence time of nitrogen on an annual basis. There were significant differences in plant structural parameters (leaf area ratio, specific leaf area) and water-use efficiency between the clones. Furthermore, several clone x treatment interaction effects on various growth parameters indicated that the clones adapted to specific environments in different ways. "Tora" plants produced up to 25% more shoot biomass than plants of the natural clone in response to high rates of fertilization and irrigation, whereas clone ranking was reversed in most other treatments. The results support the hypothesis that fast-growing hybrids are more sensitive to nutrient and water stress than slower-growing natural clones. The hypothesis that natural clones have higher resource-use efficiency than fast-growing hybrids was supported with respect to water, but not nitrogen.

Physiological variation among Populus fremontii populations: short- and long-term relationships between delta13C and water availability.

Different populations of widely distributed species can experience dramatically different climatic conditions that may influence physiological activity, specifically carbon assimilation and water use. Populus fremontii Wats. (Fremont cottonwood) populations are found near rivers of varying size along a precipitation gradient from New Mexico to northern California. Climatic differences among populations may lead to physiological differences because P. fremontii is sensitive to water availability. To assess physiological variation among populations, we collected foliage and wood samples from 13 populations that experience different precipitation and stream flow regimes and analyzed the samples for carbon isotope composition (delta13C). Wood delta13C served as a lifetime-averaged indicator of water-use efficiency (WUE), whereas foliage delta13C provided as an estimate of WUE during the growing season of collection. We found approximately 3.4 per thousand variation in delta13C among populations for both foliage (-31.1 to -27.9 per thousand) and wood (-28.3 to -24.7 per thousand). Wood delta13C was, on average, 2.8 per thousand more enriched than foliage. Some of the variation in wood delta13C can be explained by variation in elevation of the study sites. We constructed total precipitation and mean stream flow variables based on the length of the growing season at each study site and analyzed for a relationship between delta13C, precipitation and stream flow. A significant relationship between foliage delta13C and precipitation was found, but water availability did not explain a significant fraction of the variation in wood delta13C. The data suggest that water availability can account for some of the delta13C variation among populations but, given the large residual variances, other factors are important.