Trade-offs between succulent and non-succulent epiphytes underlie variation in drought tolerance and avoidance.

Epiphyte communities comprise important components of many forest ecosystems in terms of biomass and diversity, but little is known regarding trade-offs that underlie diversity and structure in these communities or the impact that microclimate has on epiphyte trait allocation. We measured 22 functional traits in vascular epiphyte communities across six sites that span a microclimatic gradient in a tropical montane cloud forest region in Costa Rica. We quantified traits that relate to carbon and nitrogen allocation, gas exchange, water storage, and drought tolerance. Functional diversity was high in all but the lowest elevation site where drought likely limits the success of certain species with particular trait combinations. For most traits, variation was explained by relationships with other traits, rather than differences in microclimate across sites. Although there were significant differences in microclimate, epiphyte abundance, and diversity, we found substantial overlap in multivariate trait space across five of the sites. We found significant correlations between functional traits, many of which related to water storage (leaf water content, leaf thickness, hydrenchymal thickness), drought tolerance (turgor loss point), and carbon allocation (specific leaf area, leaf dry matter content). This suite of trait correlations suggests that the epiphyte community has evolved functional strategies along with a drought avoidance versus drought tolerance continuum where leaf succulence emerged as a pivotal overall trait.

Diverse functional responses to drought in a Mediterranean-type shrubland in South Africa.

• Mediterranean-type ecosystems contain 20% of all vascular plant diversity on Earth and have been identified as being particularly threatened by future increases in drought. Of particular concern is the Cape Floral Region of South Africa, a global biodiversity hotspot, yet there are limited experimental data to validate predicted impacts on the flora. In a field rainout experiment, we tested whether rooting depth and degree of isohydry or anisohydry could aid in the functional classification of drought responses across diverse growth forms. • We imposed a 6-month summer drought, for 2 yr, in a mountain fynbos shrubland. We monitored a suite of parameters, from physiological traits to morphological outcomes, in seven species comprising the three dominant growth forms (deep-rooted proteoid shrubs, shallow-rooted ericoid shrubs and graminoid restioids). • There was considerable variation in drought response both between and within the growth forms. The shallow-rooted, anisohydric ericoid shrubs all suffered considerable reductions in growth and flowering and increased mortality. By contrast, the shallow-rooted, isohydric restioids and deep-rooted, isohydric proteoid shrubs were largely unaffected by the drought. • Rooting depth and degree of iso/anisohydry allow a first-order functional classification of drought response pathways in this flora. Consideration of additional traits would further refine this approach.

Spectral analysis software improves confidence in plant and soil water stable isotope analyses performed by isotope ratio infrared spectroscopy (IRIS).

Previous studies have demonstrated the potential for large errors to occur when analyzing waters containing organic contaminants using isotope ratio infrared spectroscopy (IRIS). In an attempt to address this problem, IRIS manufacturers now provide post-processing spectral analysis software capable of identifying samples with the types of spectral interference that compromises their stable isotope analysis. Here we report two independent tests of this post-processing spectral analysis software on two IRIS systems, OA-ICOS (Los Gatos Research Inc.) and WS-CRDS (Picarro Inc.). Following a similar methodology to a previous study, we cryogenically extracted plant leaf water and soil water and measured the δ(2)H and δ(18)O values of identical samples by isotope ratio mass spectrometry (IRMS) and IRIS. As an additional test, we analyzed plant stem waters and tap waters by IRMS and IRIS in an independent laboratory. For all tests we assumed that the IRMS value represented the "true" value against which we could compare the stable isotope results from the IRIS methods. Samples showing significant deviations from the IRMS value (>2σ) were considered to be contaminated and representative of spectral interference in the IRIS measurement. Over the two studies, 83% of plant species were considered contaminated on OA-ICOS and 58% on WS-CRDS. Post-analysis, spectra were analyzed using the manufacturer's spectral analysis software, in order to see if the software correctly identified contaminated samples. In our tests the software performed well, identifying all the samples with major errors. However, some false negatives indicate that user evaluation and testing of the software are necessary. Repeat sampling of plants showed considerable variation in the discrepancies between IRIS and IRMS. As such, we recommend that spectral analysis of IRIS data must be incorporated into standard post-processing routines. Furthermore, we suggest that the results from spectral analysis be included when reporting stable isotope data from IRIS.

First Molecular Evidence of Citrus psorosis virus and Citrus viroid III from Citrus spp. in New Zealand.

In December 2008, a collection of Citrus spp. in Kerikeri, New Zealand was surveyed for virus and viroid diseases. Symptoms characteristic of virus or viroid infection were not observed other than Citrus tristeza virus (CTV)-associated stem pitting when examined with the bark removed. Total RNA was extracted from bark samples of 273 trees using RLT buffer (Qiagen Inc., Chatsworth, CA) on a KingFisher mL workstation (Thermo Scientific, Waltham, MA) and tested by reverse transcription (RT)-PCR). Samples from three trees, two from sweet orange, Citrus × sinensis (L.) Osbeck (pro sp.) (maxima × reticulate) and one from tangerine, Citrus reticulata Blanco, tested positive for Citrus psorosis virus (CPsV), and two samples, one each from lemon, Citrus × limon (L.) Burm. F. (pro sp.) (medica × aurantifolia) and sweet orange, tested positive for Citrus viroid III (CVd-III) using previously published primers and PCR cycling conditions (2,4) in a one-step RT-PCR system. The 20-µl RT-PCR reaction was done with Verso Reddymix reagents (Thermo Scientific) containing 250 nM of specific primers and 300 µg/µl of bovine serum albumin (Sigma-Aldrich, St. Louis, MO). The CVd-III genome was completed using specific internal primers (forward: 5'-AACGCAGAGAGGGAAAGGGAA-3', reverse: 5'-TAGGGCTACTTCCCGTGGTC-3') with the following cycling conditions: 50°C for 15 min, 94°C for 2 min, then 40 cycles of 94°C for 10 s, 57°C for 30 s, and 68°C for 30 s. The three CPsV amplicons of 419 bp from the RNA-dependent RNA polymerase gene (GenBank Accession Nos. GQ388241 to GQ388243) had 96 to 100% nucleotide identity to each other. A 276-bp (nt position 48 to 323) fragment of the 419-bp sequence was used for comparison with sequences available on GenBank. The three 276-bp CPsV sequences had 89 to 97% nucleotide identity to other CPsV available in GenBank at the time of the analysis. The CVd-III genomes of 291 bp (GenBank Accession Nos. HQ219183 and JF521494) are identical and showed 94 to 99% nucleotide identity to other CVd-III available in GenBank. The presence of CPsV was confirmed in the three samples by a CPsV-specific double-antibody sandwich-ELISA kit (Agritest S.r.l., Valenzano, Italy), while the presence of CVd-III was confirmed only in the lemon sample by r-PAGE (3). The concentration of the viroid in the sweet orange sample may have been below the detection limit of the test. The incidence of the diseases is probably low since CPsV and CVd-III were detected in only a few trees which were planted between 1998 and 2002 at Kerikeri from budwoods of unknown sources imported between the 1970s and 1990s. New Zealand's growing conditions generally do not favor viroid replication in plants, whereas the temperatures may be suitable for CPsV disease. However, symptom characteristics to CPsV and CVd-III have never been observed on the infected trees. This is most likely because of the presence of CTV in the trees (data not shown). CPsV symptoms were thought to have been observed in the 1950s in New Zealand (1) but the causal agent had not been identified. To our knowledge, this is the first molecular and serological evidence of CPsV and the first report of the presence of CVd-III in New Zealand. References: (1) W. A. Fletcher. Orchard. N. Z. 30:33, 1957. (2) T. Ito et al. J. Virol. Methods 106:235, 2002. (3) C. Jeffries and C. James. OEPP/EPPO Bull. 35:125, 2005. (4) S. Martin et al. J. Gen. Virol. 87:3097, 2006.

Isolates of Citrus tristeza virus that overcome Poncirus trifoliata resistance comprise a novel strain.

The economically important rootstock species Poncirus trifoliata is resistant to most isolates of Citrus tristeza virus (CTV), but not to members of the CTV resistance-breaking (RB) strain presently found in New Zealand. In this study, five known and suspected RB isolates were separated from field mixtures, and their genomes were sequenced in full. It was found that the RB isolates are members of a single phylogenetically distinct clade with an average of 90.3% genomic nucleotide sequence identity to the closest extant isolate, T36. These isolates also show evidence of multiple recombination events throughout their evolutionary history, with T36, T30 and VT-like isolates, and with each other. Finally, the genomic sequences of these isolates show that several genes contain unique polymorphisms that may or may not be involved in overcoming resistance. These data will aid in the understanding of host-virus interactions, and the mechanism of resistance in P. trifoliata.

Complete genome sequences of two distinct and diverse Citrus tristeza virus isolates from New Zealand.

Two Citrus tristeza virus (CTV) isolates from New Zealand that display distinct phenotypes were isolated, examined and sequenced in full. The first isolate, NZ-M16, is largely asymptomatic and non-transmissible by the aphid vector Toxoptera citricida, while the second, NZ-B18, is highly transmissible and induces very severe symptoms on C. sinensis and C. aurantii. Phylogenetic analysis of the genome sequences showed that both isolates were approximately 90-93% similar to the VT and T318 isolates but possessed only 89% identity to one another. Based on sequence identity, both isolates are VT subtypes, with NZ-M16 being T3-like, while NZ-B18 is a member of a novel subtype with B165 from India.

Intravesical treatments for painful bladder syndrome/ interstitial cystitis.

BACKGROUND: Painful Bladder Syndrome/Interstitial Cystitis (PBS/IC) occurs predominantly in women. It is a poorly-understood condition with symptoms of bladder pain, urinary frequency, urgency and nocturia. Treatments for PBS/IC include dietary/lifestyle interventions, oral medication, intravesical instillations and, in some cases, surgery. Success rates are generally modest and there is little consensus as to the best form of treatment for this condition. OBJECTIVES: To assess the effectiveness of intravesical treatment for PBS/IC. SEARCH STRATEGY: We searched the Cochrane Incontinence Group specialised trials register (30 May 2006) as well as reference lists of all selected trials. Recognised researchers in the field were contacted for any additional relevant material. SELECTION CRITERIA: Randomised or quasi-randomised controlled trials were included in the review if they had recruited participants with a clinical diagnosis of PBS/IC and if at least one arm of the trial was treatment with an intravesical preparation. Outcome measures were pre-determined, the primary ones being the effect on pain and bladder capacity. Others included symptomatic response to treatment, quality-of-life assessment, economic factors and adverse events. DATA COLLECTION AND ANALYSIS: Two reviewers independently assessed trial eligibility and quality, then extracted relevant data from the studies. MAIN RESULTS: Nine eligible trials were identified - six parallel group, one incomplete cross-over and two cross-over trials - with a total of 616 participants. Six trials compared an 'active' instillation with placebo instillation, two compared different types of instillation, and one was a comparison of an instillation plus bladder training versus bladder training alone. Altogether, the review included trials of six different types of intravesical instillation: Resiniferatoxin, Dimethyl sulfoxide, BCG, pentosanpolysulphate, oxybutin, and alkalinisation of urine pH. Confidence intervals were generally wide. Resiniferatoxin was not associated with sustained differences in the review outcomes reported but pain during instillation and withdrawal from treatment was significantly more common. The data available about Dimethyl sulfoxide (DMSO) were very limited but with no apparent differences from placebo. Groups treated with BCG tended to report less pain and fewer general symptoms. Although adverse events were commonly reported, these were no more common after BCG than after placebo instillation. The few data about Pentosanpolysulphate tended to favour the actively treated, but with wide confidence intervals; there was little information about adverse events. Oxybutinin instillation was associated with increased bladder capacity, reduced frequency, improved quality of life scores and fewer drop-outs. Alkalinisation of urine pH did not make any clear difference, but with potentially wide confidence intervals. AUTHORS' CONCLUSIONS: Overall, the evidence base for treating PBS/IC using intravesical preparations is limited and the potential for meta-analysis reduced by variation in the outcome measures used. The quality of trial reports was mixed and in some cases this precluded any meaningful data extraction. BCG and oxybutin are reasonably well-tolerated and evidence is most promising for these. Resiniferatoxin showed no evidence of effect for most outcomes and caused pain, which reduced treatment compliance. There is little evidence for the other treatments included in this review. Randomised controlled trials are still needed and study design should incorporate outcomes that are most relevant to these with PBS/IC and should be standardised.

(Z)-7-tricosene and monounsaturated ketones as sex pheromone components of the Australian guava moth Coscinoptycha improbana: identification, field trapping, and phenology.

Pheromone gland extracts of the Australian guava moth Coscinoptycha improbana (Lepidoptera: Carposinidae), contained four compounds that elicited responses from male moth antennae in gas chromatography-electroantennogram detection (GC-EAD) analyses. These were identified by GC-mass spectrometry as (Z)-7-tricosene (Z7-23Hy), (Z)-7-octadecen-11-one (Z7-11-one-18Hy), (Z)-7-nonadecen-11-one (Z7-11-one-19Hy), and (Z)-7-tricosen-11-one (Z7-11-one-23Hy) at a ratio of 65:23.5:1.5:10, respectively. Z7-23Hy, Z7-11-one-18Hy, and Z7-11-one-23Hy have not previously been reported as lepidopteran sex pheromone components. Z7-11-one-18Hy was active as a single component, and was synergized by Z7-11-one-23Hy but not Z7-11-one-19Hy, although the latter compound was weakly attractive as a single component. Addition of Z7-23Hy further increased attraction. The amount of the major pheromone component, Z7-11-one-18Hy in female pheromone gland extracts was estimated to be 16.4 ng/female (N = 8). Phenological data gathered over a 12-mo period in 2002 and 2003 using the binary blend indicated that moths are active throughout the year. The pheromone has already been employed to monitor the spread of C. improbana in New Zealand and detect its presence in Queensland, Australia.

The importance of seed reserves for seedling performance: an integrated approach using morphological, physiological, and stable isotope techniques.

To investigate how seed reserves affect early seedling performance, we conducted a factorial greenhouse experiment using Lithocarpus densiflora (Tanoak). Seedlings were grown from large (5.8+/-0.7 g) and small (3.2+/-0.4 g) seeds and, following shoot emergence, seeds were either removed or left attached. Seedlings were harvested for quantification of biomass and delta13C at seven time periods following seed removal (2, 4, 8, 16, 32, 64, 128 days) and seedling photosynthesis was measured three separate time periods (2-4, 49-82, 95-128 days after seed removal). Biomass increased for all seedlings, but the increase was significantly larger for seedlings with attached seeds than with removed seeds. Seed removal just after shoot emergence significantly decreased seedling biomass, but seed removal 64 days after shoot emergence had no effect on seedling biomass. Seedling photosynthesis per unit leaf area varied by time and seed presence, but not by seed size. At the first period, seedlings with attached seeds had significantly higher photosynthetic rates than seedlings with removed seeds, at the second period there was no effect of seed removal, and at the third time period seedlings with attached seeds had significantly lower photosynthetic rates than seedlings with removed seeds. Despite temporal variation in photosynthesis per unit leaf area, seedlings with attached seeds always had significantly greater leaf area than seedlings with removed seeds, resulting in significantly higher total plant photosynthesis at all three time periods. The delta13C values of both the leaves and roots were more similar to that of the seed for seedlings with attached seeds than for seedlings with removed seeds, however, seed removal and seed size strongly affected root delta13C. This study demonstrates that seed reserves have important effects on the early growth, physiology, and delta13C of L. densiflora seedlings.

Hydraulic lift in Acacia tortilis trees on an East African savanna.

Recent studies suggest that savanna trees in semi-arid areas can increase understorey plant production. We hypothesized that one of the mechanisms that explains the facilitation between trees and grasses in East African savannas is hydraulic lift (HL). HL in large Acacia tortilis trees was studied during the first 3 months of the dry season during a relatively wet year (1998) and a very dry year (2000). In 1998, we found distinct diel fluctuation in soil water potential (psi(s)), with increasing values during the night and decreasing again the following day. These fluctuations in psi(s )are consistent with other observations of HL and in A. tortilis were found up to 10 m from the tree. In 2000, during a severe drought, psi(s) measurements indicated that HL was largely absent. The finding that HL occurred in wetter years and not in drier years was supported by data obtained on the delta(18)O values in soil, rain and groundwater. The delta(18)O of water extracted from the xylem water of grasses indicated that when they grew near trees they had values similar to those of groundwater. This could be because they either (1) use water from deeper soil layers or (2) use hydraulically lifted water provided by the tree; this was not seen in the same grass species growing outside tree canopies. While our data indicate that HL indeed occurs under Acacia trees, it is also true that psi(s) was consistently lower under trees when compared to outside tree canopies. We believe that this is because tree-grass mixtures take up more water from the upper soil layers than is exuded by the tree each night. This limits the beneficial effect of HL for understorey grasses and suggests that in savannas both facilitation via HL and competition are active processes. The importance of each process may depend upon how wet or dry that particular site or year is.

Responses of Acer negundo genders to interannual differences in water availability determined from carbon isotope ratios of tree ring cellulose.

Understanding the responses of riparian trees to water availability is critical for predicting the effects of changes in precipitation on riparian ecosystems. Dioecious Acer negundo L. (box elder) is a common riparian tree that is highly sensitive to water stress. Earlier studies indicated that the genders of A. negundo respond differently to water availability, with males being more conservative in their water use than females. To assess the potential effects of changes in precipitation on the sex ratio of riparian trees, we extended earlier studies of A. negundo by analyzing responses of male and female genotypes to interannual differences in water availability in a common garden. We measured growth of tree rings and used stable carbon isotope analysis of tree ring alpha-cellulose to integrate physiological responses to annual water treatments. During dry years, male and female trees exhibited similar growth and physiological responses. However, during wet years, females exhibited higher growth rates and lower carbon isotope ratios (indicating less conservative water use) than did males. Furthermore, we found that male trees exhibited similar stomatal behavior (inferred from integrated carbon isotope ratios) whether years were wet or dry, whereas females did not exhibit a consistent response to changes in water availability. We predict that with increasing precipitation and soil water availability, the representation of females will be favored because of shifts in the competitive interactions of the genders. Such changes may affect the reproductive output of these riparian trees and may influence overall water flux from riparian ecosystems. In addition, this study demonstrates the utility of carbon isotope analysis for assessing long-term responses of tree populations to shifts in water availability.

Root water uptake and transport: using physiological processes in global predictions.

Plant water loss, regulated by stomata and driven by atmospheric demand, cannot exceed the maximum steady-state supply through roots. Just as an electric circuit breaks when carrying excess current, the soil-plant continuum breaks if forced to transport water beyond its capacity. Exciting new molecular, biophysical and ecological research suggests that roots are the weakest link along this hydraulic flow path. We attempt here to predict rooting depth and water uptake using the hydraulic properties of plants and the soil, and also to suggest how new physiological tools might contribute to larger-scale studies of hydraulic lift, the water balance and biosphere-atmosphere interactions.

The perception and recognition of natural object shape from deforming and static shadows.

In this study of the informativeness of shadows for the perception of object shape, observers viewed shadows cast by a set of natural solid objects and were required to discriminate between them. In some conditions the objects underwent rotation in depth while in other conditions they remained stationary, thus producing both deforming and static shadows. The orientation of the light source casting the shadows was also varied, leading to further alterations in the shape of the shadows. When deformations in the shadow boundary were present, the observers were able to reliably recognize and discriminate between the objects, invariant over the shadow distortions produced by movements of the light source. The recognition performance for the static shadows depended critically upon the content of the specific views that were shown. These results support the idea that there are invariant features of shadow boundaries that permit the recognition of shape (cf Koenderink, 1984 Perception 13 321-330).

The effects of age upon the perception of depth and 3-D shape from differential motion and binocular disparity.

The ability of younger and older adults to perceive the 3-D shape, depth, and curvature of smooth surfaces defined by differential motion and binocular disparity was evaluated in six experiments. The number of points defining the surfaces and their spatial and temporal correspondences were manipulated. For stereoscopic sinusoidal surfaces, the spatial frequency of the corrugations was also varied. For surfaces defined by motion, the lifetimes of the individual points in the patterns were varied, and comparisons were made between the perception of surfaces defined by points and that of more ecologically valid textured surfaces. In all experiments, the older observers were less sensitive to the depths and curvatures of the surfaces, although the deficits were much larger for motion-defined surfaces. The results demonstrate that older adults can extract depth and shape from optical patterns containing only differential motion or binocular disparity, but these abilities are often manifested at reduced levels of performance.

Glucose metabolism and milk yield of cows infused abomasally or ruminally with starch.

The effect of ruminal or abomasal starch infusion on milk yield and glucose metabolism of early lactation cows was measured. Four cows were continuously infused in the rumen or abomasum with partially hydrolyzed starch (1500 g/d) or were not infused (control) for three 14-d periods during wk 4 to 12 postpartum. Milk yield averaged over 40 kg/d throughout the experiment. Milk and milk lactose yields tended to increase when starch was infused and DMI was decreased, regardless of the site of infusion. Starch infusion increased mean insulin concentration and tended to decrease the concentration of serum nonesterified fatty acids. Ruminal starch infusion did not affect glucose irreversible loss rate but tended to increase glucagon concentration and decrease glucose oxidation. The increased milk yield that occurred when starch was infused ruminally relative to the milk yield of control cows could be a result of increased microbial protein supply or increased energy availability. Compared with ruminal starch infusion, abomasal starch infusion tended to increase the irreversible loss rate of glucose and to increase glucose oxidation. Abomasal infusion tended to increase plasma insulin concentration and to decrease the nonesterified fatty acid concentration relative to ruminal infusion. Infusion of starch abomasally resulted in increases of most uses of glucose, including milk lactose production, glucose oxidation, and the possible storage of glucose as body fat, which indicates that the early lactation dairy cow has a greater capacity for glucose metabolism than is provided by voluntary feed intake of average diets, but that not all available glucose is partitioned to the mammary gland. These data should be useful in testing current concepts and equations in nutritional and metabolic models of dairy cattle.

Fog in the California redwood forest: ecosystem inputs and use by plants.

Fog has been viewed as an important source of moisture in many coastal ecosystems, yet its importance for the plants which inhabit these ecosystems is virtually unknown. Here, I report the results of a 3-year investigation of fog inputs and the use of fog water by plants inhabiting the heavily fog inundated coastal redwood (Sequoia sempervirens) forests of northern California. During the study period, 34%, on average, of the annual hydrologic input was from fog drip off the redwood trees themselves (interception input). When trees were absent, the average annual input from fog was only 17%, demonstrating that the trees significantly influence the magnitude of fog water input to the ecosystem. Stable hydrogen and oxygen isotope analyses of water from fog, rain, soil water, and xylem water extracted from the dominant plant species were used to characterize the water sources used by the plants. An isotopic mixing model was employed to then quantify how much fog water each plant used each month during the 3-year study. In summer, when fog was most frequent, ∼19% of the water within S.sempervirens, and ∼66% of the water within the understory plants came from fog after it had dripped from tree foliage into the soil; for S.sempervirens, this fog water input comprised 13-45% of its annual transpiration. For all plants, there was a significant reliance on fog as a water source, especially in summer when rainfall was absent. Dependence on fog as a moisture source was highest in the year when rainfall was lowest but fog inputs normal. Interestingly, during the mild El Niño year of 1993, when the ratio of rainfall to fog water input was significantly higher and fog inputs were lower, both the proportion and coefficient of variation in how much fog water was used by plants increased. An explanation for this is that while fog inputs were lower than normal in this El Niño year, they came at a time when plant demand for water was highest (summer). Therefore, proportional use of fog water by plants increased. The results presented suggest that fog, as a meteorological factor, plays an important role in the water relations of the plants and in the hydrology of the forest. These results demonstrate the importance of understanding the impacts of climatic factors and their oscillations on the biota. The results have important implications for ecologists, hydrologists, and forest managers interested in fog-inundated ecosystems and the plants which inhabit them.

Seasonal air and soil temperature effects on photosynthesis in red spruce (Picea rubens) saplings.

Net photosynthesis and stomatal conductance were measured in ten red spruce (Picea rubens Sarg.) saplings, growing near Ithaca, New York, throughout the early spring and late-fall growing periods. Gas exchange and daily minimum and maximum soil and air temperatures were also measured. Linear regression analysis showed that rates of net photosynthesis were positively correlated with both minimum daily soil and air temperatures but that minimum soil temperature was a better predictor of net photosynthesis. Moreover, net photosynthesis was more sensitive to changes in soil temperature than to changes in air temperature, and photosynthesis was approximately twice as sensitive to temperature changes during the fall than during the spring.

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

The seasonal and diurnal water relations were investigated within arctic and alpine populations of the dwarf willow Salix arctica. Marked differences that were habitat dependent (e.g. xeric vs. mesic) occured both within and between the populations. The environmental variables that most affected plant water balance and the bulk tissue water relations were soil water potential (Ψsoil) and the leafto-air water vapor pressure gradient (Δw), however, low soil temperature (<4.0° C) also had a marked effect in the wet to mesic habitats. The effects of declining Ψsoil and increased ‡w were most pronounced in the plants growing in xiric habitats in both populations. Stomatal response to increased ‡w was two-fold greater in alpine versus arctic plants and is hypothesized to have arisen in response to more frequent exposure to the higher evaporative conditions of alpine existance. Seasonal fluctuations in the osmotic potential closely followed changes in Ψsoil, suggesting that these were active rather that passive changes. Additionally, plants from xeric habiats had a lower bulk tissue elastic modulus (more elastic tissues) in both arctic and alpine populations. The osmotic and elastic properties enhanced turgor maintenance over a broad range of leaf water potentials and during periods when ‡w was high. Turgor maintenance also correlated to continued transpiration despite fluctuations in soil and atmospherically induced water deficits. Arctic habitats have a shorter growing season, lower soil temperatures due to the presence of permafrost, but higher soil water potentials and lower leaf-to-air vapor pressure gradients than alpine habitats. The observed variation in patterns of stomatal conductance and in values of tissue water relations characteristics between arctic and alpine populations of S. arctica is hypothesized to have arisen in response to these different environmental regimes which represent different selective regimes that occur along the arctic-alpine environmental continuum inhabited by this wide ranging species.

Patterns of water use and the tissue water relations in the dioecious shrub, Salix arctica: the physiological basis for habitat partitioning between the sexes.

Within the high arctic of Canada, Salix arctica, a dioecious, dwarf willow exhibits significant spatial segregation of the sexes. The overall sex ratio is female-biased and female plants are especially common in wet, higher nutrient, but lower soil temperature habitats. In contrast, male plants predominate in more xeric and lower nutrient habitats with higher soil temperatures that can be drought prone. Associated with the sex-specific habitat differences were differences in the seasonal and diurnal patterns of water use as measured by stomatal conductance to water vapor and the bulk tissue water relations of each gender. Within the wet habitats, female plants maintained higher rates of stomatal conductance (g) than males when soil and root temperatures were low (<4° C). In contrast, within the xeric habitats, male plants maintained higher g and had lower leaf water potentials Ψleaf at low soil water potentials and a high leaf-to-air vapor pressure gradient (Δw) when compared to females. Female plants had more positive carbon isotope ratios than males indicating a lower internal leaf carbon dioxide concentration and possibly higher water use efficiency relative to males. Tissue osmotic and elastic properties also differed between the sexes. Male plants demonstrated lower tissue osmotic potentials near full tissue hydration and at the turgor loss point and a lower bulk tissue elastic modulus (higher tissue elasticity) than female plants. Males also demonstrated a greater ability to osmotically adjust on a diurnal basis than females. These properties allowed male plants to maintain higher tissue turgor pressures at lower tissue water contents and Ψsoil over the course of the day. The sex-specific distributional and ecophysiological characteristics were also correlated with greater total plant growth and higher fecundity of females in wet habitats, and males in xeric habitats respectively. The intersexual differences in physiology persisted in all habitats. These results and those obtained from growth chamber studies suggest that sex-specific differences have an underlying genetic basis. From these data we conjecture that selection maintaining the intersexual differences may be related to different costs associated with reproduction that can be most easily met through physiological specialization and spatial segregation of the sexes among habitats of differing conditions.