Showing papers on "Growing season published in 2013"

Energy exchange and water budget partitioning in a boreal minerogenic mire

Abstract: This study investigated patterns and controls of the seasonal and inter-annual variations in energy fluxes (i.e., sensible heat, H, and latent heat, lambda E) and partitioning of the water budget (i.e., precipitation, P; evapotranspiration, ET; discharge, Q; and soil water storage, Delta S) over five years (2001-2005) in a boreal oligotrophic fen in northern Sweden based on continuous eddy covariance, water table level (WTL), and weir measurements. For the growing season (May 1 to September 31), the 5 year averages (+/- standard deviation) of the midday (10:00 to 14:00 h) Bowen ratio (beta, i.e., H/lambda E) was 0.86 +/- 0.08. Seasonal and inter-annual variability of beta was mainly driven by lambda E which itself was strongly controlled by both weather (i.e., vapor pressure deficit, D, and net radiation, R-n) and physiological parameters (i.e., surface resistance). During the growing season, surface resistance largely exceeded aerodynamic resistance, which together with low mean values of the actual ET to potential ET ratio (0.55 +/- 0.05) and Priestley-Taylor alpha (0.89) suggests significant physiological constrains on ET in this well-watered fen. Among the water budget components, the inter-annual variability of ET was lower (199 to 298 mm) compared to Q (225 to 752 mm), with each accounting on average for 34 and 65% of the ecosystem water loss, respectively. The fraction of P expended into ET was negatively correlated to P and positively to R-n. Although a decrease in WTL caused a reduction of the surface conductance, the overall effect of WTL on ET was limited. Non-growing season (October 1 to April 30) fluxes of H, lambda E, and Q were significant representing on average -67%, 13%, and 61%, respectively, of their growing season sums (negative sign indicates opposite flux direction between the two seasons). Overall, our findings suggest that plant functional type composition, P and R-n dynamics (i.e., amount and timing) were the major controls on the partitioning of the mire energy and water budgets. This has important implications for the regional climate as well as for ecosystem development, nutrient, and carbon dynamics. Citation: Peichl, M., J. Sagerfors, A. Lindroth, I. Buffam, A. Grelle, L. Klemedtsson, H. Laudon, and M. B. Nilsson (2013), Energy exchange and water budget partitioning in a boreal minerogenic mire, J. Geophys. Res. Biogeosci., 118, 1-13, doi:10.1029/2012JG002073.

Large-scale variations in the vegetation growing season and annual cycle of atmospheric CO2 at high northern latitudes from 1950 to 2011.

Abstract: We combine satellite and ground observations during 1950-2011 to study the long-term links between multiple climate (air temperature and cryospheric dynamics) and vegetation (greenness and atmospheric CO(2) concentrations) indicators of the growing season of northern ecosystems (>45°N) and their connection with the carbon cycle. During the last three decades, the thermal potential growing season has lengthened by about 10.5 days (P 0.05). The photosynthetic growing season has closely tracked the pace of warming and extension of the potential growing season in spring, but not in autumn when factors such as light and moisture limitation may constrain photosynthesis. The autumnal extension of the photosynthetic growing season since 1982 appears to be about half that of the thermal potential growing season, yielding a smaller lengthening of the photosynthetic growing season (6.7 days at the circumpolar scale, P < 0.01). Nevertheless, when integrated over the growing season, photosynthetic activity has closely followed the interannual variations and warming trend in cumulative growing season temperatures. This lengthening and intensification of the photosynthetic growing season, manifested principally over Eurasia rather than North America, is associated with a long-term increase (22.2% since 1972, P < 0.01) in the amplitude of the CO(2) annual cycle at northern latitudes. The springtime extension of the photosynthetic and potential growing seasons has apparently stimulated earlier and stronger net CO(2) uptake by northern ecosystems, while the autumnal extension is associated with an earlier net release of CO(2) to the atmosphere. These contrasting responses may be critical in determining the impact of continued warming on northern terrestrial ecosystems and the carbon cycle.

Drought response of five conifer species under contrasting water availability suggests high vulnerability of Norway spruce and European larch

Abstract: The ability of tree species to cope with anticipated decrease in water availability is still poorly understood. We evaluated the potential of Norway spruce, Scots pine, European larch, black pine, and Douglas-fir to withstand drought in a drier future climate by analyzing their past growth and physiological responses at a xeric and a mesic site in Central Europe using dendroecological methods. Earlywood, latewood, and total ring width, as well as the d13C and d18O in early- and latewood were measured and statistically related to a multiscalar soil water deficit index from 1961 to 2009. At the xeric site, d13C values of all species were strongly linked to water deficits that lasted longer than 11 months, indicating a long-term cumulative effect on the carbon pool. Trees at the xeric site were particularly sensitive to soil water recharge in the preceding autumn and early spring. The native species European larch and Norway spruce, growing close to their dry distribution limit at the xeric site, were found to be the most vulnerable species to soil water deficits. At the mesic site, summer water availability was critical for all species, whereas water availability prior to the growing season was less important. Trees at the mesic were more vulnerable to water deficits of shorter duration than the xeric site. We conclude that if summers become drier, trees growing on mesic sites will undergo significant growth reductions, whereas at their dry distribution limit in the Alps, tree growth of the highly sensitive spruce and larch may collapse, likely inducing dieback and compromising the provision of ecosystem services. However, the magnitude of these changes will be mediated strongly by soil water recharge in winter and thus water availability at the beginning of the growing season.

Earlier springs decrease peak summer productivity in North American boreal forests

Abstract: In the northern high latitudes, alternative hypotheses with regards to how warming-related shifts in seasonality influence ecosystem productivity exist. Increased plant growth associated with a longer growing season may enhance ecosystem productivity, but shifts to earlier springs may also negatively influence soil moisture status and productivity during the peak of the growing season. Here, we analyzed nearly three decades (1982?2008) of observational records and derived products, including satellite microwave and optical imagery as well as upscaled ecosystem flux observations, to better understand how shifts in seasonality impact hydrology and productivity in the North American boreal forests. We identified a dominant adverse influence of earlier springs on peak summer forest greenness, actual evapotranspiration and productivity at interannual time scales across the drier western and central sections of the North American boreal forests. In the vast regions where this spring onset mechanism operates, ecosystem productivity gains from earlier springs during the early portion of the growing season are effectively cancelled through corresponding losses in the later portion. Our results also indicate that recent decadal shifts towards earlier springs and associated drying in the midst of the growing season over western North American boreal forests may have contributed to the reported declines in summer productivity and increases in tree mortality and fire activity. With projections of accelerated northern high-latitude warming and associated shifts to earlier springs, persistent soil moisture deficits in peak summer may be an effective mechanism for regional-scale boreal forest dieback through their strong influence on productivity, tree mortality and disturbance dynamics.

Leaf-wax n-alkanes record the plant–water environment at leaf flush

Abstract: Leaf-wax n-alkanes 2H/1H ratios are widely used as a proxy in climate reconstruction. Although the broad nature of the relationship between n-alkanes δ2H values and climate is appreciated, the quantitative details of the proxy remain elusive. To examine these details under natural environmental conditions, we studied a riparian broadleaf angiosperm species, Populus angustifolia, growing on water with a constant δ2H value and monitored the δ2H values of leaf-wax n-alkanes and of stem, leaf, stream, and atmospheric waters throughout the entire growing season. Here we found the δ2H values of leaf-wax n-alkanes recorded only a 2-wk period during leaf flush and did not vary for the 19 weeks thereafter when leaves remained active. We found δ2H values of leaf-wax n-alkanes of P. angustifolia record conditions earlier in the season rather than fully integrating the entire growing season. Using these data, we modeled precipitation δ2H values during the time of wax synthesis. We observed that the isotope ratios of this precipitation generally were 2H-enriched compared with mean annual precipitation. This model provides a mechanistic basis of the often-observed 2H-enrichment from the expected fractionation values in studies of broadleaf angiosperm leaf-wax δ2H. In addition, these findings may have implications for the spatial and temporal uses of n-alkane δ2H values in paleoapplications; when both plant community and growth form are known, this study allows the isolation of the precipitation dynamics of individual periods of the growing season.

Root niche partitioning among grasses, saplings, and trees measured using a tracer technique.

Abstract: Niche partitioning of resources by plants is believed to be a fundamental aspect of plant coexistence and biogeochemical cycles; however, measurements of the timing and location of resource use are often lacking because of the difficulties of belowground research. To measure niche partitioning of soil water by grasses, planted saplings, and trees in a mesic savanna (Kruger National Park, South Africa), we injected deuterium oxide into 102,000 points in 15, 154-m2 plots randomly assigned to one of five depths (0–120 cm) and one of three time periods during the 2008/2009 growing season. Grasses, saplings and trees all demonstrated an exponential decline in water uptake early in the season when resources were abundant. Later in the season, when resources were scarce, grasses continued to extract the most water from the shallowest soil depths (5 cm), but saplings and trees shifted water uptake to deeper depths (30–60 cm). Saplings, in particular, rapidly established roots to at least 1 m and used these deep roots to a greater extent than grasses or trees. Helping to resolve contradictory observations of the relative importance of deep and shallow roots, our results showed that grasses, saplings and trees all extract the most water from shallow soils when it is available but that woody plants can rapidly shift water uptake to deeper soils when resources are scarce. Results highlight the importance of temporal changes in water uptake and the problems with inferring spatial and temporal partitioning of soil water uptake from root biomass measurements alone.

Comparison Between the Water and Salt Stress Effects on Plant Growth and Development

Abstract: Abiotic stress limits crop productivity [1], and plays a major role in determining the distri‐ bution of plant species across different types of environments. Abiotic stress and its effects on plants in both natural and agricultural settings is a topic that is receiving increasing at‐ tention because of the potential impacts of climate change on rainfall patterns and tempera‐ ture extremes, salinization of agricultural lands by irrigation, and the overall need to maintain or increase agricultural productivity on marginal lands. In the field, a plant may experience several distinct abiotic stresses either concurrently or at different times through the growing season [2].

Causes for the unimodal pattern of biomass and productivity in alpine grasslands along a large altitudinal gradient in semi-arid regions

Abstract: Questions: How can we understand the limitations to plant growth at high altitudes? Our aimwas to test the hypotheses that for alpine grasslands along a large altitudinal gradient in semi-arid regions, plant growth is mainly limited by drought at low altitudes but by low temperature at high altitudes, resulting in a unimodal pattern of biomass and productivity associated with an optimal combination of temperature and precipitation. Such knowledge is important to understanding the response of alpine ecosystems to climate change. Location: We conducted a 5-yr livestock exclosure experiment along the southfacing slope of the Nyaiqentanglha Mountains, central Tibetan Plateau. Methods: We measured above- and below-ground biomass, species richness, leaf d13C and water potential, and related climate and soil variables across 42 fenced and unfenced quadrats near seven HOBO weather stations along the slope. The vegetation changed from alpine steppe-meadow at 4390–4500 m to alpinemeadow at 4600–5210 m. Results: Total above- and below-ground biomass across fenced and unfenced quadrats increased with increasing altitude up to 4950–5100 m, and then decreased above 5100 m. Altitudinal trends in leaf d13C and water potential of dominant species also showed a unimodal pattern corresponding to that of vegetation biomass. Total above- and below-ground biomass as well as sedge above-ground biomass all showed a quadratic relationship with mean temperatures and the ratio of growing season precipitation (GSP) to
5 °C accumulated temperature (AccT; R2 = 0.830.88, P < 0.001). In general, above- and below-ground biomass increased with increasing water availability when the GSP/AccT ratio was lower than the threshold level of 0.80–0.84, but decreased when the GSP/AccT ratio was higher than this threshold level. No significant relationship was found between residuals of above-ground biomass and species richness after removing the effects of climate factors on both stand variables. Conclusions: The results support our hypotheses, further suggesting a threshold ofwater limitation that is consistent with themodel prediction over the Tibetan Plateau. Species richness per se appears to weakly affect community-level productivity. The response of alpine grasslands to climate warming may vary with altitude because of altitudinal shifts in factors limiting plant growth. Keywords: Alpine grassland; Biomass; Functional groups; Leaf water potential; Net primary productivity; Plant–climate interactions; Species richness

Above- and belowground controls on water use by trees of different wood types in an eastern US deciduous forest.

Abstract: Stomata control tree transpiration by sensing and integrating environmental signals originating in the atmosphere and soil, and co-occurring species may differ in inherent stomatal sensitivity to these above- and belowground signals and in the types of signals to which they respond. Stomatal responsiveness to environmental signals is likely to differ across species having different types of wood (e.g., ring-porous, diffuse-porous and coniferous) because each wood type differs in the structure, size and spatial distribution of its xylem conduits as well as in the scaling of hydraulic properties with stem diameter. The objective of this study was to evaluate the impact of variation in soil water availability and atmospheric evaporative demand on stomatal regulation of transpiration in seven co-occurring temperate deciduous forest species representing three wood types. We measured whole-tree sap flux and soil and atmospheric variables in a mixed deciduous forest in central Pennsylvania over the course of a growing season characterized by severe drought and large fluctuations in atmospheric vapor pressure deficit ( D). The relative sensitivity of sap flux to soil drying was ~2.2–2.3 times greater in the diffuse-porous and coniferous species than in the ring-porous species. Stomata of the ring-porous oaks were only about half as responsive to increased D as those of trees of the other two wood types. These differences in responsiveness to changes in the belowand aboveground environment implied that regulation of leaf water potential in the ring-porous oaks was less stringent than that in the diffuse-porous angiosperms or the conifers. The results suggest that increases in the frequency or intensity of summer droughts in the study region could have multiple consequences for forest function, including altered successional time courses or climax species composition and cumulative effects on whole-tree architecture, resulting in a structural and physiological legacy that would restrict the ability of trees to respond rapidly to more favorable growth conditions.

Differential ecophysiological response of a major Mediterranean pine species across a climatic gradient.

Abstract: The rate of migration and in situ genetic variation in forest trees may not be sufficient to compete with the current rapid rate of climate change. Ecophysiological adjustments of key traits, however, could complement these processes and allow sustained survival and growth across a wide range of climatic conditions. This was tested in Pinus halepensis Miller by examining seven physiological and phenological parameters in five provenances growing in three common garden plots along a climatic transect from meso-Mediterranean (MM) to thermo-Mediterranean (TM) and semi-arid (SA) climates. Differential responses to variations in ambient climatic conditions were observed in three key traits: (i) growing season length decreased with drying in all provenances examined (from 165 under TM climate to 100 days under SA climate, on average); (ii) water use efficiency (WUE) increased with drying, but to a different extent in different provenances, and on average from 80, to 95, to 110 µmol CO(2) mol(-1) H(2)O under MM, TM and SA climates, respectively; (iii) xylem native embolism was stable across climates, but varied markedly among different provenances (percent loss of conductivity, was below 5% in two provenances and above 35% in others). The results indicated that changes in growing season length and WUE were important contributors to tree growth across climates, whereas xylem native embolism negatively correlated with tree survival. The results indicated that irrespective of slow processes (e.g., migration, genetic adaptation), the capacity for ecophysiological adjustments combined with existing variations among provenances could help sustain P. halepensis, a major Mediterranean tree species, under relatively extreme warming and drying climatic trends.

Drought sensitivity of Norway spruce is higher than that of silver fir along an altitudinal gradient in southwestern Germany

Abstract: Context For Central Europe, climate projections foresee an increase in temperature combined with decreasing summer precipitation, resulting in drier conditions during the growing season. This might negatively affect forest growth, especially at sites that are already water-limited, i.e., at low elevation. At higher altitudes trees might profit from increasing temperatures.

Seasonal Dynamics of Aboveground and Belowground Biomass and Nutrient Accumulation and Remobilization in Giant Reed ( Arundo donax L.): A Three-Year Study on Marginal Land

Abstract: Giant reed (Arundo donax L.) is a perennial rhizomatous grass that shows promise as a bioenergy crop in the Mediterranean environment. The species has spread throughout the world, catalyzed by human activity, though also as a result of its intrinsic robustness, adaptability, and versatility. Giant reed is able to thrive across a wide range of soil types and is tolerant to drought, salinity, and flooding. This tolerance to environmental stresses is significant and could mean that growing energy crops on marginal land is one possible strategy for reducing competition for land with food production and for improving soil quality. We devised an experiment in which we cultivated giant reed in a sandy loam soil with low nutrient availability. Our goal was to evaluate the dynamics of aboveground and belowground biomass and assess the nutrient dynamics of this grass species, focusing particularly on nutrient accumulation and remobilization. The species demonstrated good productivity potential: In the third year, aboveground dry biomass yield reached around 20 t ha−1, with a corresponding rhizome dry biomass yield of 16 t ha−1. Results for this species were characterized by low nutrient contents in the aboveground biomass at the end of the growing season, and its rhizome proved able to support growth over the spring period and to store nutrients in the autumn. Nevertheless, the adaptability of giant reed to marginal land and the role of its belowground biomass should be investigated over the long-term, and any further research should focus on its potential to reduce greenhouse gas emissions and maintain soil fertility.

Experimental warming studies on tree species and forest ecosystems: a literature review

Abstract: Temperature affects a cascade of ecological processes and functions of forests. With future higher global temperatures being inevitable it is critical to understand and predict how forest ecosystems and tree species will respond. This paper reviews experimental warming studies in boreal and temperate forests or tree species beyond the direct effects of higher temperature on plant ecophysiology by scaling up to forest level responses and considering the indirect effects of higher temperature. In direct response to higher temperature (1) leaves emerged earlier and senesced later, resulting in a longer growing season (2) the abundance of herbivorous insects increased and their performance was enhanced and (3) soil nitrogen mineralization and leaf litter decomposition were accelerated. Besides these generalizations across species, plant ecophysiological traits were highly species-specific. Moreover, we showed that the effect of temperature on photosynthesis is strongly dependent on the position of the leaf or plant within the forest (canopy or understory) and the time of the year. Indirect effects of higher temperature included among others higher carbon storage in trees due to increased soil nitrogen availability and changes in insect performance due to alterations in plant ecophysiological traits. Unfortunately only a few studies extrapolated results to forest ecosystem level and considered the indirect effects of higher temperature. Thus more intensive, long-term studies are needed to further confirm the emerging trends shown in this review. Experimental warming studies provide us with a useful tool to examine the cascade of ecological processes in forest ecosystems that will change with future higher temperature.

Revisiting factors controlling methane emissions from high-Arctic tundra

Abstract: The northern latitudes are experiencing disproportionate warming relative to the mid-latitudes, and there is growing concern about feedbacks between this warming and methane production and release from high-latitude soils. Studies of methane emissions carried out in the Arctic, particularly those with measurements made outside the growing season, are underrepresented in the literature. Here we present results of 5 yr (2006–2010) of automatic chamber measurements at a high-Arctic location in Zackenberg, NE Greenland, covering both the growing seasons and two months of the following freeze-in periods. The measurements show clear seasonal dynamics in methane emission. The start of the growing season and the increase in CH 4 fluxes were strongly related to the date of snowmelt. Within each particular growing season, CH 4 fluxes were highly correlated with the soil temperature ( R 2 > 0.75), which is probably explained by high seasonality of both variables, and weakly correlated with the water table. The greatest variability in fluxes between the study years was observed during the first part of the growing season. Somewhat surprisingly, this variability could not be explained by commonly known factors controlling methane emission, i.e. temperature and water table position. Late in the growing season CH 4 emissions were found to be very similar between the study years (except the extremely dry 2010) despite large differences in climatic factors (temperature and water table). Late-season bursts of CH 4 coinciding with soil freezing in the autumn were observed during at least three years. The cumulative emission during the freeze-in CH 4 bursts was comparable in size with the growing season emission for the year 2007, and about one third of the growing season emissions for the years 2009 and 2010. In all three cases the CH 4 burst was accompanied by a corresponding episodic increase in CO 2 emission, which can compose a significant contribution to the annual CO 2 flux budget. The most probable mechanism of the late-season CH 4 and CO 2 bursts is physical release of gases accumulated in the soil during the growing season. In this study we discuss possible links between growing season and autumn fluxes. Multiannual dynamics of the subsurface CH 4 storage pool are hypothesized to be such a link and an important driver of intearannual variations in the fluxes, capable of overruling the conventionally known short-term control factors (temperature and water table). Our findings suggest the importance of multiyear studies with a continued focus on shoulder seasons in Arctic ecosystems.

Revisiting factors controlling methane emissions from high-arctic tundra

Abstract: The northern latitudes are experiencing disproportionate warming relative to the mid-latitudes, and there is growing concern about feedbacks between this warming and methane production and release from high-latitude soils. Studies of methane emissions carried out in the Arctic, particularly those with measurements made outside the growing season, are underrepresented in the literature. Here we present results of 5 yr (2006–2010) of automatic chamber measurements at a high-Arctic location in Zackenberg, NE Greenland, covering both the growing seasons and two months of the following freeze-in periods. The measurements show clear seasonal dynamics in methane emission. The start of the growing season and the increase in CH 4 fluxes were strongly related to the date of snowmelt. Within each particular growing season, CH 4 fluxes were highly correlated with the soil temperature ( R 2 > 0.75), which is probably explained by high seasonality of both variables, and weakly correlated with the water table. The greatest variability in fluxes between the study years was observed during the first part of the growing season. Somewhat surprisingly, this variability could not be explained by commonly known factors controlling methane emission, i.e. temperature and water table position. Late in the growing season CH 4 emissions were found to be very similar between the study years (except the extremely dry 2010) despite large differences in climatic factors (temperature and water table). Late-season bursts of CH 4 coinciding with soil freezing in the autumn were observed during at least three years. The cumulative emission during the freeze-in CH 4 bursts was comparable in size with the growing season emission for the year 2007, and about one third of the growing season emissions for the years 2009 and 2010. In all three cases the CH 4 burst was accompanied by a corresponding episodic increase in CO 2 emission, which can compose a significant contribution to the annual CO 2 flux budget. The most probable mechanism of the late-season CH 4 and CO 2 bursts is physical release of gases accumulated in the soil during the growing season. In this study we discuss possible links between growing season and autumn fluxes. Multiannual dynamics of the subsurface CH 4 storage pool are hypothesized to be such a link and an important driver of intearannual variations in the fluxes, capable of overruling the conventionally known short-term control factors (temperature and water table). Our findings suggest the importance of multiyear studies with a continued focus on shoulder seasons in Arctic ecosystems.

Transpiration Response of Maize Hybrids to Atmospheric Vapour Pressure Deficit

Abstract: Maize (Zea mays L.) yield is often restricted by low soil water availability, particularly late in the growing season. To increase yields, genetic options for more effective use of available soil water are being explored. One option is to select genotypes that have restricted transpiration rate under high vapour pressure deficit (VPD) conditions so that soil water is conserved for use later in the growing season. While genetic variation for this trait has been identified within several crop species, such variation has never been explored in maize. The objective of this study was to examine transpiration rate of 35 single-cross hybrids to determine whether hybrids can be identified that express limited transpiration under high VPD. Two sets of experiments were undertaken in which plants were exposed to a range of VPD in chambers. A two-phase transpiration response was observed in 11 hybrids in which there was a threshold VPD above which transpiration rate was restricted. The VPD threshold varied from 1.7 to 2.5 kPa among these hybrids. Eight hybrids were included in both sets of experiments, and the same results were obtained in both experiments, indicating that expression of the trait was consistent.

Growing season and spatial variations of carbon fluxes of Arctic and boreal ecosystems in Alaska (USA).

Abstract: To better understand the spatial and temporal dynamics of CO2 exchange between Arctic ecosystems and the atmosphere, we synthesized CO2 flux data, measured in eight Arctic tundra and five boreal ecosystems across Alaska (USA) and identified growing season and spatial variations of the fluxes and environmental controlling factors. For the period examined, all of the boreal and seven of the eight Arctic tundra ecosystems acted as CO2 sinks during the growing season. Seasonal patterns of the CO2 fluxes were mostly determined by air temperature, except ecosystem respiration (RE) of tundra. For the tundra ecosystems, the spatial variation of gross primary productivity (GPP) and net CO2 sink strength were explained by growing season length, whereas RE increased with growing degree days. For boreal ecosystems, the spatial variation of net CO2 sink strength was mostly determined by recovery of GPP from fire disturbance. Satellite-derived leaf area index (LAI) was a better index to explain the spatial variations of GPP and NEE of the ecosystems in Alaska than were the normalized difference vegetation index (NDVI) and enhanced vegetation index (EVI). Multiple regression models using growing degree days, growing season length, and satellite-derived LAI explained much of the spatial variation in GPP and net CO2 exchange among the tundra and boreal ecosystems. The high sensitivity of the sink strength to growing season length indicated that the tundra ecosystem could increase CO2 sink strength under expected future warming, whereas ecosystem compositions associated with fire disturbance could play a major role in carbon release from boreal ecosystems.

Warming and the dependence of limber pine (Pinus flexilis) establishment on summer soil moisture within and above its current elevation range.

Abstract: Continued changes in climate are projected to alter the geographic distributions of plant species, in part by affecting where individuals can establish from seed We tested the hypothesis that warming promotes uphill redistribution of subalpine tree populations by reducing cold limitation at high elevation and enhancing drought stress at low elevation We seeded limber pine (Pinus flexilis) into plots with combinations of infrared heating and water addition treatments, at sites positioned in lower subalpine forest, the treeline ecotone, and alpine tundra In 2010, first-year seedlings were assessed for physiological performance and survival over the snow-free growing season Seedlings emerged in midsummer, about 5–8 weeks after snowmelt Low temperature was not observed to limit seedling photosynthesis or respiration between emergence and October, and thus experimental warming did not appear to reduce cold limitation at high elevation Instead, gas exchange and water potential from all sites indicated a prevailing effect of summer moisture stress on photosynthesis and carbon balance Infrared heaters raised soil growing degree days (base 5 °C, p < 0001) and August–September mean soil temperature (p < 0001) Despite marked differences in vegetation cover and meteorological conditions across sites, volumetric soil moisture content (θ) at 5–10 cm below 016 and 008 m3 m−3 consistently corresponded with moderate and severe indications of drought stress in midday stem water potential, stomatal conductance, photosynthesis, and respiration Seedling survival was greater in watered plots than in heated plots (p = 001), and negatively related to soil growing degree days and duration of exposure to θ < 008 m3 m−3 in a stepwise linear regression model (p < 00001) We concluded that seasonal moisture stress and high soil surface temperature imposed a strong limitation to limber pine seedling establishment across a broad elevation gradient, including at treeline, and that these limitations are likely to be enhanced by further climate warming