Intraspecific variation in the water relations of Salix arctica, an arctic-alpine dwarf willow

Climatic variation allows montane willows to escape an adaptive trade-off

Adaptive responses to climate change, based on heritable variation in stress tolerance, may be important for plant population persistence. It is unclear which populations will mount the strongest future adaptive responses. It may be fruitful to identify populations that have escaped trade-offs among performance traits, which can hinder adaptation. Barring strong genetic constraints, the extent of trade-offs may depend on spatial relationships among climate variables shaping different traits. Here, we test for climate-driven ecotypic variation and trade-offs among drought and freezing sensitivity, and growth, for Lemmon's willow (Salix lemmonii) in a common garden study of 90 genotypes from 38 sites in the Sierra Nevada, USA. Salix lemmonii exhibits ecotypic variation in leaf turgor loss point, a measure of drought sensitivity, from -0.95 to -0.74 MPa along a gradient of spring snowpack. We also find variation in spring freezing sensitivity with minimum May temperature. However, we find no trade-off, as the climatic gradients shaping these traits are spatially uncorrelated in our study region, despite being negatively correlated across the Sierra Nevada. Species may escape adaptive trade-offs in geographic regions where climate variables are spatially decoupled. These regions may represent valuable reservoirs of heritable adaptive phenotypic variation.

Differential ecophysiological response of deciduous shrubs and a graminoid to long-term experimental snow reductions and additions in moist acidic tundra, Northern Alaska

Changes in winter precipitation that include both decreases and increases in winter snow are underway across the Arctic. In this study, we used a 14-year experiment that has increased and decreased winter snow in the moist acidic tussock tundra of northern Alaska to understand impacts of variation in winter snow depth on summer leaf-level ecophysiology of two deciduous shrubs and a graminoid species, including: instantaneous rates of leaf gas exchange, and δ(13)C, δ(15)N, and nitrogen (N) concentrations of Betula nana, Salix pulchra, and Eriophorum vaginatum. Leaf-level measurements were complemented by measurements of canopy leaf area index (LAI) and depth of thaw. Reductions in snow lowered summer leaf photosynthesis, conductance, and transpiration rates by up to 40% compared to ambient and deep snow conditions for Eriophorum vaginatum, and reduced Salix pulchra conductance and transpiration by up to 49%. In contrast, Betula nana exhibited no changes in leaf gas exchange in response to lower or deeper snow. Canopy LAI increased with added snow, while reduced winter snow resulted in lower growing season soil temperatures and reduced thaw depths. Our findings indicate that the spatial and temporal variability of future snow depth will have individualistic consequences for leaf-level C fixation and water flux by tundra species, and that these responses will be manifested over the longer term by changes in canopy traits, depth of thaw, soil C and N processes, and trace gas (CO2 and H2O) exchanges between the tundra and the atmosphere.

Gender differences in Salix myrsinifolia at the pre-reproductive stage are little affected by simulated climatic change

Females of dioecious species are known often to prioritize defense, while males grow faster. As climatic change is known to influence both growth and defense in plants, it would be important to know whether it affects the sexes of dioecious species differently. This could have impacts on future sex ratios in nature. We grew four clones of each sex of Salix myrsinifolia in greenhouse chambers under ambient conditions, enhanced temperature, enhanced CO2 or enhanced temperature  +  enhanced CO2 . The females had the greatest growth and also the highest levels of phenolic compounds in twigs, while in leaves some compounds were higher in males, some in females. Enhanced CO2 increased growth equally in both sexes, while growth was not affected by elevated temperature. Phenolic compounds in twigs were, however, lowered under elevated temperature. The gender differences were not strongly affected by the simulated climatic changes, but the effects seen on some highly concentrated compounds may be important. We interpret the intensive growth at pre-reproductive phase as a strategy in females to get an initial advantage before later periods with fewer resources available for growth.

Combined enhancements of temperature and UVB influence growth and phenolics in clones of the sexually dimorphic Salix myrsinifolia

Although several climatic factors are expected to change simultaneously in the future, the effect of such combined changes on plants have seldom been tested under field conditions. We report on a field experiment with dark-leaved willow, Salix myrsinifolia, subjected to enhancements in ultraviolet-A (UVA), UVB radiation and temperature, setup in Joensuu, Eastern Finland. S. myrsinifolia is a dioecious species, known as an important food plant for many herbivores. Cuttings of eight clones, four of each sex, of dark-leaved willow were planted in the field in spring 2009. In both 2009 and 2010, the total biomass increased significantly with temperature, and in 2010 there was an additive effect of UVB radiation. Both height and diameter increased with temperature in 2009, while the effect on height growth ceased in 2010. Males had greater diameter growth than females in 2010. Most phenolic compounds in the leaves decreased under enhanced temperature in both growing seasons. In 2010, four of six salicylates increased in response to enhanced temperature. Some quercetin derivatives increased under enhanced UVB radiation. Females had higher concentrations of chlorogenic acids than males, and while enhanced temperature reduced chlorogenic acid in females only, luteolins were reduced only in males. In summary, the combined enhancements gave no effects in addition to those that appeared under the single-factor treatments, except for the additive effect of UVB on temperature-increased biomass. The few gender-related differences found in response to climate change do not allow any marked expectations of future climate-induced changes in sex ratios.

Variability of stomatal conductance in a small and isolated population of silver fir (Abies alba Mill.)

We analyzed the response to drought of 420 individuals from eight half-sib families from a small and isolated population of silver fir (Abies alba Mill.): 105 of them were kept in well-watered conditions as control while the remaining 315 were exposed to drought for 27 days. A model describing stomatal behavior derived from Monteith and developed in beech by Leonardi et al. was fitted to experimental transpiration data obtained simply from the difference between two daily pot weighings. The estimated parameters were maximum stomatal conductance, maximum transpiration in well-watered conditions and sensitivity to soil water deficit. The model worked well: convergence for all but four individuals and concordance between experimental and fitted data were good (R(2)=0.86). Inter-individual variability for all three estimated parameters was high and two of them (maximum stomatal conductance and sensitivity to soil water deficit) were significantly different among families, suggesting genetic control. Our results validate the simplified method used to evaluate individual stomatal parameters. We also show that in the small and isolated population of our study substantial adaptive variability remains, a crucial prerequisite to endure environmental conditions determined by climatic change foreseen for the next decades.

Streamside trees: responses of male, female and hybrid cottonwoods to flooding

Cottonwoods, riparian poplars, are dioecious and prior studies have indicated that female poplars and willows can be more abundant than males in low-elevation zones, which are occasionally flooded. We investigated the response to flooding of clonal saplings of 12 male and 9 female narrowleaf cottonwoods (Populus angustifolia) grown for 15 weeks in a greenhouse, along with three females of a co-occurring native hybrid (Populus × jackii = Populus deltoides × Populus balsamifera). Three water-level treatments were provided, with substrate inundation as the flood treatment. In the non-flooded condition, the hybrids produced about four-fold more dry weight (DW) than the narrowleaf cottonwoods (P < 0.01). In both cottonwood taxa, flooding reduced stem height and DW, root and leaf area and weight, leaf chlorophyll and stomatal conductance (all P < 0.01). Inundation increased the foliar carbon-to-nitrogen ratio (+11%; P < 0.05) but did not significantly alter leaf water potential (mean -1.5 MPa), or foliar δ(13)C, which was lower in P. angustifolia (-32.8‰) than P. × jackii (-31.5‰; P < 0.05). Water level influenced the root distribution as roots were sparse in the saturated substrate and abundant in the capillary fringe above. The male and female P. angustifolia genotypes grew similarly with the favorable water levels, but the males tended to be more inhibited by flooding. Sapling DW of males was reduced by 56% compared with a 44% reduction for females (P = 0.1), and there were similar lower reductions for leaf, stem and root DW in females. These results demonstrate the inundation response of floodplain trees and suggest relative flood tolerance as: P. angustifolia female > P. angustifolia male > P. × jackii female. This indicates that narrowleaf cottonwoods are relatively flood tolerant and suggests that females are more flood tolerant than males. We propose the concept of 'strategic positioning', whereby the seed-producing females could be better adapted to naturally flooded, low-elevation streamside zones where seedling recruitment generally occurs.

Stage-dependent patterns of drought tolerance and gas exchange vary between sexes in the alpine willow, Salix glauca

Females and males of sexually dimorphic species have distinct resource demands due to differential allocation to reproduction. Sexual allocation theory predicts that functional traits will diverge between sexes to support these demands. However, such dimorphism may be masked by the impact of current reproduction on source-sink interactions between vegetative and reproductive organs. We ask whether natural selection has led to genetic dimorphism in homologous physiological traits between sexes of the dioecious willow shrub, Salix glauca. In a common garden experiment we compared physiological responses to drought stress by male and female ramets in the absence of confounding demands from reproductive structures. Ramets experienced similar pre-dawn leaf water status (Psi(l)) as parental genets in flower within the natural population, indicating that experimental dry-down mirrored environmental conditions in nature. Male and female ramets achieved similar instantaneous water use efficiency, based on the ratio of carbon gain to water loss, under wet and dry conditions. However, female ramets experienced greater water stress (i.e., more negative Psi(l)) than males under dry conditions. Lower Psi(l) for female ramets may partly reflect the maintenance of conductance under drought; males, in contrast, maintain Psi(l) under drought by reducing conductance. Differences between sexes in terms of conductance and leaf water status of the vegetative ramets were absent in a concomitant comparison of parental flowering plants. Our results show (1) genetic divergence in physiology between sexes of S. glauca occurs in the absence of gender-specific reproductive sinks, (2) males are the more physiologically plastic sex with respect to water use, and (3) paradoxically, divergence in water relations between sexes is not detectable at sexual maturity under natural conditions.

Ecological correlates of secondary sexual dimorphism in Salix glauca (Salicaceae)

The ecological causation hypothesis for secondary sexual dimorphism was tested in Salix glauca, a dioecious willow shrub. Plants growing in a Colorado Rocky Mountain (USA) krummholz mosaic of mesic and xeric patches were monitored for four consecutive years. Ecological causation is predicated on unique resource demands associated with sexual function. In S. glauca, seeds have twofold higher N and P concentrations compared to pollen. P, but not N, allocation costs differed between sexes at plant and flower scales. Ecological causation also predicts spatial segregation of sexes along underlying habitat gradients. In five populations of S. glauca, sexes displayed significant spatial segregation. The theory also predicts that sexes differ in performance across gradients of environmental stress or resource availability. Consistent with this hypothesis, females had lower drought tolerance than males under years of extreme aridity. Furthermore, over 10 years, annual shoot growth for females was greatest in mesic habitat patches, while males grew at a consistent rate regardless of habitat aridity. Because current shoot growth is correlated with future catkin production, habitat specialization likely provides a fitness payoff in females. Overall, this long-term study provides some of the strongest evidence to date for ecological causation of secondary sexual dimorphism in plants.

Variation in leaf physiology of Salix arctica within and across ecosystems in the High Arctic: test of a dual isotope (Δ13C and Δ18O) conceptual model

Leaf carbon isotope discrimination (Delta(13)C) varies with the balance between net photosynthesis (A) and stomatal conductance (g ( s )). Inferences that can be made with Delta(13)C are limited, as changes could reflect variation in A and/or g ( s ). Investigators have suggested that leaf delta(18)O enrichment above source water (Delta(18)O) may enable differentiation between sources of variation in Delta(13)C, as leaf Delta(18)O varies with transpiration rate (E), which is closely correlated with g ( s ) when leaves experience similar leaf to air vapor pressure differences. We examined leaf gas exchange of Salix arctica at eight sites with similar air temperatures and relative humidities but divergent soil temperatures and soil water contents near Pituffik, Greenland (76 degrees N, 38 degrees W). We found negative correlations at the site level between g ( s ) and Delta(18)O in bulk leaf tissue (r (2) = 0.62, slope = -17.9 per thousand/mol H(2)O m(-2) s(-1), P = 0.02) and leaf alpha-cellulose (r (2) = 0.83, slope = -11.5 per thousand mol H(2)O m(-2) s(-1), P < 0.01), consistent with the notion that leaf water enrichment declines with increasing E. We also found negative correlations at the site-level between intrinsic water-use efficiency (iWUE) and Delta(13)C in bulk leaf tissue (r (2) = 0.65, slope = -0.08 per thousand/mumol CO(2) /mol H(2)O, P = 0.02) and leaf alpha-cellulose (r (2) = 0.50, slope = -0.05 per thousand/[mumol CO(2) /mol H(2)O], P = 0.05). When increasing Delta(13)C was driven by increasing g ( s ) alone, we found negative slopes between Delta(13)C and Delta(18)O for bulk leaf tissue (-0.664) and leaf alpha-cellulose (-1.135). When both g ( s ) and A (max) increased, we found steeper negative slopes between Delta(13)C and Delta(18)O for bulk leaf tissue (-2.307) and leaf alpha-cellulose (-1.296). Our results suggest that the dual isotope approach is capable of revealing the qualitative contributions of g ( s ) and A (max) to Delta(13)C at the site level. In our study, bulk leaf tissue was a better medium than leaf alpha-cellulose for application of the dual isotope approach.

Ecophysiological analysis of two arctic sedges under reduced root temperatures

Shoot physiological activity in arctic vascular plants may be controlled by low soil temperatures. While leaves may be exposed to moderate temperatures during the growing season, root temperatures often remain near freezing. In this study, two tundra sedges, Eriophorum vaginatum and Carex bigellowii, were subjected to reduced soil temperatures, and photosynthetic parameters (light saturated photosynthesis A(max), variable to maximal fluorescence and F(v)/F(m) stomatal conductance) and abscisic acid concentrations were determined. Stomatal conductance and A(max) for both E. vaginatum and C. bigellowii strongly decreased with declining soil temperatures. Decreasing soil temperature, however, impacted F(v)/F(m) to a much lesser degree. Root and leaf ABA concentrations increased with decreasing root temperature. These observations support the contention that soil temperature is a significant photosynthetic driving factor in arctic sedges exposed to variable root and shoot temperatures. Because these two species comprise approximately 30% of the vascular ground cover of wet tussock tundra, the soil temperature responses of these sedges potentially scale up to significant effects on ecosystem carbon exchange.

Leaf photosynthetic characteristics and net primary production of the polar willow (Salix polaris) in a high arctic polar semi-desert, Ny-Ålesund, Svalbard

Photosynthetic characteristics and their leaf-age dependence were examined to estimate ecophysiological effects on net primary production (NPP) of a polar willow (Salix polaris), a dominant dwarf shrub species in a polar semi-desert area of Ny-Ålesund, Svalbard. Leaves of S. polaris emerged just after snowmelt in early July in 2000; flowers were initiated within 1 week, and fruits in late July. Light-saturated rate of photosynthesis and stomatal conductance to water vapor increased rapidly to their maximum values within 1 week after leaf emergence and then decreased gradually. Depending on the leaf age, photosynthetic rates saturated at photosynthetically active photon flux density (PPFD) of 200–400 µmol·m–2·s–1, which is the light level usually available in the natural habitat. Optimum leaf temperature of photosynthesis ranged from 10 to 18°C, while air temperature in the habitat ranged from 8 to 20°C. These light and temperature responses of photosynthesis of S. polaris would be suitable for efficient carbon gain in the natural habitat characterized by highly variable light and temperature conditions. Using the photosynthetic and respiratory characteristics, biomass distribution, and meteorological data, NPP of S. polaris in the current year was estimated to be 26.1 g C·m–2. A model simulation of rising temperature conditions predicted a reduction of NPP because of a large increase in respiration. It was suggested that temperature condition and leaf phenological aspects strongly influence the carbon fixation by plants in the high arctic area studied.Key words: arctic semi-desert, climate change, net ecosystem production (NEP), net primary production (NPP), Salix polaris, photosynthesis.

Water availability alters the relative performance of Salix sericea,Sralix eriocephala, and their F1 hybrids

Seedlings of Salix eriocephala Michx., Salix sericea Marsh., and their F1 hybrids were tested for their growth under contrasting water conditions. One-year old seedlings were grown in pots and subjected to one of two experiments, a scheduled-watering experiment (watered every 1, 4, 8, or 12 days) or a dry-down experiment (no watering after an initial period without water limitation). In the scheduled-watering experiment, short-term growth of hybrids was suppressed relative to the parents. However, subsequent watering allowed hybrids to recover. By the end of the 48-day experiments, hybrids exhibited heterosis across all treatments. In the dry-down experiment, hybrids again were highly susceptible to the onset of drought conditions. From these results, we hypothesize that temporal and spatial variability in water availability could determine the survivorship of hybrids and the frequency of introgression; high water availability may enhance the relative performance of hybrids, while low water availability may limit hybrids.Key words: Salicaceae, willows, hybridization, water relations, performance, heterosis.