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

Lethal combination for seedlings: extreme heat drives mortality of drought-exposed high-elevation pine seedlings

BACKGROUND AND AIMS

Hotter drought- and biotically driven tree mortality are expected to increase with climate change in much of the western USA, and species persistence will depend upon ongoing establishment in novel conditions or migration to track ecological niche requirements. High-elevation tree species might be particularly vulnerable to increasing water stress as snowpack declines, increasing the potential for adult mortality and simultaneous regeneration failures. Seedling survival will be determined by ecophysiological limitations in response to changing water availability and temperature.

METHODS

We exposed seedlings from populations of Pinus longaeva, Pinus flexilis and Pinus albicaulis to severe drought and concurrent temperature stress in common gardens, testing the timing of drought onset under two different temperature regimes. We monitored seedling functional traits, physiological function and survival.

KEY RESULTS

The combined stressors of water limitation and extreme heat led to conservative water-use strategies and declines in physiological function, with these joint stressors ultimately exceeding species tolerances and leading to complete episodic mortality across all species. Growing conditions were the primary determinant of seedling trait expression, with seedlings exhibiting more drought-resistant traits, such as lower specific leaf area, in the hottest, driest treatment conditions. Water stress-induced stomatal closure was also widely apparent. In the presence of adequate soil moisture, seedlings endured prolonged exposure to high air and surface temperatures, suggesting broad margins for survival.

CONCLUSIONS

The critical interaction between soil moisture and temperature suggests that rising temperatures will exacerbate moisture stress during the growing season. Our results highlight the importance of local conditions over population- and species-level influences in shaping strategies for stress tolerance and resistance to desiccation at this early life stage. By quantifying some of the physiological consequences of drought and heat that lead to seedling mortality, we can gain a better understanding of the future effects of global change on the composition and distribution of high-elevation conifer forests.

Plant Species' Capacity for Range Shifts at the Habitat and Geographic Scales: A Trade-Off-Based Framework

Climate change is causing rapid shifts in the abiotic and biotic environmental conditions experienced by plant populations, but we lack generalizable frameworks for predicting the consequences for species. These changes may cause individuals to become poorly matched to their environments, potentially inducing shifts in the distributions of populations and altering species' habitat and geographic ranges. We present a trade-off-based framework for understanding and predicting whether plant species may undergo range shifts, based on ecological strategies defined by functional trait variation. We define a species' capacity for undergoing range shifts as the product of its colonization ability and the ability to express a phenotype well-suited to the environment across life stages (phenotype-environment matching), which are both strongly influenced by a species' ecological strategy and unavoidable trade-offs in function. While numerous strategies may be successful in an environment, severe phenotype-environment mismatches result in habitat filtering: propagules reach a site but cannot establish there. Operating within individuals and populations, these processes will affect species' habitat ranges at small scales, and aggregated across populations, will determine whether species track climatic changes and undergo geographic range shifts. This trade-off-based framework can provide a conceptual basis for species distribution models that are generalizable across plant species, aiding in the prediction of shifts in plant species' ranges in response to climate change.

Even modest climate change may lead to major transitions in boreal forests

The sensitivity of forests to near-term warming and associated precipitation shifts remains uncertain^1-9. Herein, using a 5-year open-air experiment in southern boreal forest, we show divergent responses to modest climate alteration among juveniles of nine co-occurring North American tree species. Warming alone (+1.6 °C or +3.1 °C above ambient temperature) or combined with reduced rainfall increased the juvenile mortality of all species, especially boreal conifers. Species differed in growth responses to warming, ranging from enhanced growth in Acer rubrum and Acer saccharum to severe growth reductions in Abies balsamea, Picea glauca and Pinus strobus. Moreover, treatment-induced changes in both photosynthesis and growth help explain treatment-driven changes in survival. Treatments in which species experienced conditions warmer or drier than at their range margins resulted in the most adverse impacts on growth and survival. Species abundant in southern boreal forests had the largest reductions in growth and survival due to climate manipulations. By contrast, temperate species that experienced little mortality and substantial growth enhancement in response to warming are rare throughout southern boreal forest and unlikely to rapidly expand their density and distribution. Therefore, projected climate change will probably cause regeneration failure of currently dominant southern boreal species and, coupled with their slow replacement by temperate species, lead to tree regeneration shortfalls with potential adverse impacts on the health, diversity and ecosystem services of regional forests.

Warming impacts on photosynthetic processes in dominant plant species in a subtropical forest

Climate warming could shift some subtropical regions to a tropical climate in the next 30 years. Yet, climate warming impacts on subtropical species and ecosystems remain unclear. We conducted a passive warming experiment in a subtropical forest in Florida, USA, to determine warming impacts on four species differing in their climatic distribution, growth form, and functional type: Serenoa repens (palm), Andropogon glomeratus (C₄ grass), Pinus palustris (needled evergreen tree), and Quercus laevis (broadleaved deciduous tree). We hypothesized that warming would have neutral-positive effects on photosynthetic processes in monocot species with warmer climatic distributions or adaptations to warmer temperatures, but negative effects on photosynthesis in tree species. We also hypothesized that periods of low soil moisture would alter photosynthetic responses to warming. In both monocot species, warming had no significant effect on net photosynthesis (A) or stomatal conductance (g_s ) measured at prevailing temperatures, or photosynthetic capacity measured at a common temperature. In P. palustris, warming reduced A (-15%) and g_s (-28%), and caused small reductions in Rubisco carboxylation and RuBP regeneration. Warming had little effect on photosynthetic processes in Q. laevis. Interestingly, A. glomeratus showed little sensitivity to reduced soil moisture, and all C₃ species reduced A and g_s as soil moisture declined and did so consistently across temperature treatments. In subtropical forests of the southeastern US, we conclude that climate warming may have neutral or slightly positive effects on the performance of grasses and broadleaved species but negative effects on P. palustris seedlings, foreshadowing possible changes in community and ecosystem properties.

New insight into the divergent responses of plants to warming in the context of root endophytic bacterial and fungal communities

Plant adaptation under climate changes is critical to the maintenance of terrestrial ecosystem structure and function. Studying the response of the endophytic community to climate warming is a novel way to reveal the mechanism of host environmental adaptability because of the prominent role endophytes play in host nutrient acquisition and stress tolerance. However, host performance was generally neglected in previous relevant research, which limits our understanding of the relationships between the endophytic community and host responses to climate warming. The present study selected two plants with different responses to climate warming. Elymus nutans is more suitable for growing in warm environments at low altitude compared to Kobresia pygmaea. K. pygmaea and E. nutans were sampled along an altitude gradient in the natural grassland of Qinghai-Tibet Plateau, China. Root endophytic bacterial and fungal communities were analyzed using high throughput sequencing. The results revealed that hosts growing in more suitable habitats held higher endophytic fungal diversity. Elevation and host identity significantly affected the composition of the root endophytic bacterial and fungal community. 16S rRNA functional prediction demonstrated that hosts that adapted to lower temperatures recruited endophytic communities with higher abundance of genes related to cold resistance. Hosts that were more suitable for warmer and drier environments recruited endophytes with higher abundance of genes associated with nutrient absorption and oxidation resistance. We associated changes in the endophytic community with hosts adaptability to climate warming and suggested a synchronism of endophytic communities and hosts in environmental adaptation.

Seeds and Seedlings in a Changing World: A Systematic Review and Meta-Analysis from High Altitude and High Latitude Ecosystems

The early life-history stages of plants, such as germination and seedling establishment, depend on favorable environmental conditions. Changes in the environment at high altitude and high latitude regions, as a consequence of climate change, will significantly affect these life stages and may have profound effects on species recruitment and survival. Here, we synthesize the current knowledge of climate change effects on treeline, tundra, and alpine plants’ early life-history stages. We systematically searched the available literature on this subject up until February 2020 and recovered 835 potential articles that matched our search terms. From these, we found 39 studies that matched our selection criteria. We characterized the studies within our review and performed a qualitative and quantitative analysis of the extracted meta-data regarding the climatic effects likely to change in these regions, including projected warming, early snowmelt, changes in precipitation, nutrient availability and their effects on seed maturation, seed dormancy, germination, seedling emergence and seedling establishment. Although the studies showed high variability in their methods and studied species, the qualitative and quantitative analysis of the extracted data allowed us to detect existing patterns and knowledge gaps. For example, warming temperatures seemed to favor all studied life stages except seedling establishment, a decrease in precipitation had a strong negative effect on seed stages and, surprisingly, early snowmelt had a neutral effect on seed dormancy and germination but a positive effect on seedling establishment. For some of the studied life stages, data within the literature were too limited to identify a precise effect. There is still a need for investigations that increase our understanding of the climate change impacts on high altitude and high latitude plants’ reproductive processes, as this is crucial for plant conservation and evidence-based management of these environments. Finally, we make recommendations for further research based on the identified knowledge gaps.

Effects of postfire climate and seed availability on postfire conifer regeneration

Large, severe fires are becoming more frequent in many forest types across the western United States and have resulted in tree mortality across tens of thousands of hectares. Conifer regeneration in these areas is limited because seeds must travel long distances to reach the interior of large burned patches and establishment is jeopardized by increasingly hot and dry conditions. To better inform postfire management in low elevation forests of California, USA, we collected 5-yr postfire recovery data from 1,234 study plots in 19 wildfires that burned from 2004-2012 and 18 yrs of seed production data from 216 seed fall traps (1999-2017). We used these data in conjunction with spatially extensive climate, topography, forest composition, and burn severity surfaces to construct taxon-specific, spatially explicit models of conifer regeneration that incorporate climate conditions and seed availability during postfire recovery windows. We found that after accounting for other predictors both postfire and historical precipitation were strong predictors of regeneration, suggesting that both direct effects of postfire moisture conditions and biological inertia from historical climate may play a role in regeneration. Alternatively, postfire regeneration may simply be driven by postfire climate and apparent relationships with historical climate could be spurious. The estimated sensitivity of regeneration to postfire seed availability was strongest in firs and all conifers combined and weaker in pines. Seed production exhibited high temporal variability with seed production varying by over two orders of magnitude among years. Our models indicate that during droughts postfire conifer regeneration declines most substantially in low-to-moderate elevation forests. These findings enhance our mechanistic understanding of forecasted and historically documented shifts in the distribution of trees.

The temperature optima for tree seedling photosynthesis and growth depend on water inputs

Understanding how tree growth is affected by rising temperature is a key to predicting the fate of forests in future warmer climates. Increasing temperature has direct effects on plant physiology, but there are also indirect effects of increased water limitation because evaporative demand increases with temperature in many systems. In this study, we experimentally resolved the direct and indirect effects of temperature on the response of growth and photosynthesis of the widely distributed species Eucalyptus tereticornis. We grew E. tereticornis in an array of six growth temperatures from 18 to 35.5°C, spanning the climatic distribution of the species, with two watering treatments: (a) water inputs increasing with temperature to match plant demand at all temperatures (W_incr ), isolating the direct effect of temperature; and (b) water inputs constant for all temperatures, matching demand for coolest grown plants (W_const ), such that water limitation increased with growth temperature. We found that constant water inputs resulted in a reduction of temperature optima for both photosynthesis and growth by ~3°C compared to increasing water inputs. Water limitation particularly reduced the total amount of leaf area displayed at T_opt and intermediate growth temperatures. The reduction in photosynthesis could be attributed to lower leaf water potential and consequent stomatal closure. The reduction in growth was a result of decreased photosynthesis, reduced total leaf area display and a reduction in specific leaf area. Water availability had no effect on the response of stem and root respiration to warming, but we observed lower leaf respiration rates under constant water inputs compared to increasing water inputs at higher growth temperatures. Overall, this study demonstrates that the indirect effect of increasing water limitation strongly modifies the potential response of tree growth to rising global temperatures.

Large variation in branch and branch-tip hydraulic functional traits in Douglas-fir (Pseudotsuga menziesii) approaching lower treeline

Few studies have quantified intraspecific variation of hydraulic functional traits in conifers across elevation gradients that include range boundaries. In the Intermountain West, USA, the lower elevational limit of forests (lower treeline) is generally assumed to be caused by water limitations to growth and water relations, yet few studies directly show this. To test this assumption, we measured changes in a suite of traits that characterize drought tolerance such as drought-induced hydraulic vulnerability, hydraulic transport capacity and morphological traits in branch tips and branches of Douglas-fir (Pseudotsuga menziesii var. glauca (Mirb.) Franco) along a 400-m elevation gradient in southeastern Idaho that included lower treeline. As elevation decreased, vulnerability to hydraulic dysfunction and maximum conductivity both decreased in branches; some hydraulic safety-efficiency trade-offs were evident. In branch tips, the water potential at the turgor loss point decreased, while maximum conductance increased with decreasing elevation, highlighting that branch-tip-level responses to less moisture availability accompanied by warmer temperatures might not be coordinated with branch responses. As the range boundary was approached, we did not observe non-linear changes in parameters among sites or increased variance within sites, which current ecological hypotheses on range limits suggest. Our results indicate that there is substantial plasticity in hydraulic functional traits in branch tips and branches of Douglas-fir, although the direction of the trends along the elevation gradient sometimes differed between organs. Such plasticity may mitigate the negative impacts of future drought on Douglas-fir productivity, slowing shifts in its range that are expected to occur with climate change.

Wildfires and climate change push low-elevation forests across a critical climate threshold for tree regeneration

Climate change is increasing fire activity in the western United States, which has the potential to accelerate climate-induced shifts in vegetation communities. Wildfire can catalyze vegetation change by killing adult trees that could otherwise persist in climate conditions no longer suitable for seedling establishment and survival. Recently documented declines in postfire conifer recruitment in the western United States may be an example of this phenomenon. However, the role of annual climate variation and its interaction with long-term climate trends in driving these changes is poorly resolved. Here we examine the relationship between annual climate and postfire tree regeneration of two dominant, low-elevation conifers (ponderosa pine and Douglas-fir) using annually resolved establishment dates from 2,935 destructively sampled trees from 33 wildfires across four regions in the western United States. We show that regeneration had a nonlinear response to annual climate conditions, with distinct thresholds for recruitment based on vapor pressure deficit, soil moisture, and maximum surface temperature. At dry sites across our study region, seasonal to annual climate conditions over the past 20 years have crossed these thresholds, such that conditions have become increasingly unsuitable for regeneration. High fire severity and low seed availability further reduced the probability of postfire regeneration. Together, our results demonstrate that climate change combined with high severity fire is leading to increasingly fewer opportunities for seedlings to establish after wildfires and may lead to ecosystem transitions in low-elevation ponderosa pine and Douglas-fir forests across the western United States.

Coupled ecohydrology and plant hydraulics modeling predicts ponderosa pine seedling mortality and lower treeline in the US Northern Rocky Mountains

We modeled hydraulic stress in ponderosa pine seedlings at multiple scales to examine its influence on mortality and forest extent at the lower treeline in the northern Rockies. We combined a mechanistic ecohydrologic model with a vegetation dynamic stress index incorporating intensity, duration and frequency of hydraulic stress events, to examine mortality from loss of hydraulic conductivity. We calibrated our model using a glasshouse dry-down experiment and tested it using in situ monitoring data on seedling mortality from reforestation efforts. We then simulated hydraulic stress and mortality in seedlings within the Bitterroot River watershed of Montana. We show that cumulative hydraulic stress, its legacy and its consequences for mortality are predictable and can be modeled at local to landscape scales. We demonstrate that topographic controls on the distribution and availability of water and energy drive spatial patterns of hydraulic stress. Low-elevation, south-facing, nonconvergent locations with limited upslope water subsidies experienced the highest rates of modeled mortality. Simulated mortality in seedlings from 2001 to 2015 correlated with the current distribution of forest cover near the lower treeline, suggesting that hydraulic stress limits recruitment and ultimately constrains the low-elevation extent of conifer forests within the region.

Effects of climate warming on photosynthesis in boreal tree species depend on soil moisture

Climate warming will influence photosynthesis via thermal effects and by altering soil moisture^1-11. Both effects may be important for the vast areas of global forests that fluctuate between periods when cool temperatures limit photosynthesis and periods when soil moisture may be limiting to carbon gain^4-6,9-11. Here we show that the effects of climate warming flip from positive to negative as southern boreal forests transition from rainy to modestly dry periods during the growing season. In a three-year open-air warming experiment with juveniles of 11 temperate and boreal tree species, an increase of 3.4 °C in temperature increased light-saturated net photosynthesis and leaf diffusive conductance on average on the one-third of days with the wettest soils. In all 11 species, leaf diffusive conductance and, as a result, light-saturated net photosynthesis decreased during dry spells, and did so more sharply in warmed plants than in plants at ambient temperatures. Consequently, across the 11 species, warming reduced light-saturated net photosynthesis on the two-thirds of days with driest soils. Thus, low soil moisture may reduce, or even reverse, the potential benefits of climate warming on photosynthesis in mesic, seasonally cold environments, both during drought and in regularly occurring, modestly dry periods during the growing season.

Snowmelt Timing Regulates Community Composition, Phenology, and Physiological Performance of Alpine Plants

The spatial patterning of alpine plant communities is strongly influenced by the variation in physical factors such as temperature and moisture, which are strongly affected by snow depth and snowmelt patterns. Earlier snowmelt timing and greater soil-moisture limitations may favor wide-ranging species adapted to a broader set of ecohydrological conditions than alpine-restricted species. We asked how plant community composition, phenology, plant water relations, and photosynthetic gas exchange of alpine-restricted and wide-ranging species differ in their responses to a ca. 40-day snowmelt gradient in the Colorado Rocky Mountains (Lewisia pygmaea, Sibbaldia procumbens, and Hymenoxys grandiflora were alpine-restricted and Artemisia scopulorum, Carex rupestris, and Geum rossii were wide-ranging species). As hypothesized, species richness and foliar cover increased with earlier snowmelt, due to a greater abundance of wide-ranging species present in earlier melting plots. Flowering initiation occurred earlier with earlier snowmelt for 12 out of 19 species analyzed, while flowering duration was shortened with later snowmelt for six species (all but one were wide-ranging species). We observed >50% declines in net photosynthesis from July to September as soil moisture and plant water potentials declined. Early-season stomatal conductance was higher in wide-ranging species, indicating a more competitive strategy for water acquisition when soil moisture is high. Even so, there were no associated differences in photosynthesis or transpiration, suggesting no strong differences between these groups in physiology. Our findings reveal that plant species with different ranges (alpine-restricted vs. wide-ranging) could have differential phenological and physiological responses to snowmelt timing and associated soil moisture dry-down, and that alpine-restricted species' performance is more sensitive to snowmelt. As a result, alpine-restricted species may serve as better indicator species than their wide-ranging heterospecifics. Overall, alpine community composition and peak % cover are strongly structured by spatio-temporal patterns in snowmelt timing. Thus, near-term, community-wide changes (or variation) in phenology and physiology in response to shifts in snowmelt timing or rates of soil dry down are likely to be contingent on the legacy of past climate on community structure.

Biotic and abiotic drivers of tree seedling recruitment across an alpine treeline ecotone

Treeline responses to climate change ultimately depend on successful seedling recruitment, which requires dispersal of viable seeds and establishment of individual propagules in novel environments. In this study, we evaluated the effects of several abiotic and biotic drivers of early tree seedling recruitment across an alpine treeline ecotone. In two consecutive years, we sowed seeds of low- and high-elevation provenances of Larix decidua (European larch) and Picea abies (Norway spruce) below, at, and above the current treeline into intact vegetation and into open microsites with artificially removed surface vegetation, as well as into plots protected from seed predators and herbivores. Seedling emergence and early establishment in treatment and in control plots were monitored over two years. Tree seedling emergence occurred at and several hundred metres above the current treeline when viable seeds and suitable microsites for germination were available. However, dense vegetation cover at lower elevations and winter mortality at higher elevations particularly limited early recruitment. Post-dispersal predation, species, and provenance also affected emergence and early establishment. This study demonstrates the importance of understanding multiple abiotic and biotic drivers of early seedling recruitment that should be incorporated into predictions of treeline dynamics under climate change.

Leap frog in slow motion: Divergent responses of tree species and life stages to climatic warming in Great Basin subalpine forests

In response to climate warming, subalpine treelines are expected to move up in elevation since treelines are generally controlled by growing season temperature. Where treeline is advancing, dispersal differences and early life stage environmental tolerances are likely to affect how species expand their ranges. Species with an establishment advantage will colonize newly available habitat first, potentially excluding species that have slower establishment rates. Using a network of plots across five mountain ranges, we described patterns of upslope elevational range shift for the two dominant Great Basin subalpine species, limber pine and Great Basin bristlecone pine. We found that the Great Basin treeline for these species is expanding upslope with a mean vertical elevation shift of 19.1 m since 1950, which is lower than what we might expect based on temperature increases alone. The largest advances were on limber pine-dominated granitic soils, on west aspects, and at lower latitudes. Bristlecone pine juveniles establishing above treeline share some environmental associations with bristlecone adults. Limber pine above-treeline juveniles, in contrast, are prevalent across environmental conditions and share few environmental associations with limber pine adults. Strikingly, limber pine is establishing above treeline throughout the region without regard to site characteristic such as soil type, slope, aspect, or soil texture. Although limber pine is often rare at treeline where it coexists with bristlecone pine, limber pine juveniles dominate above treeline even on calcareous soils that are core bristlecone pine habitat. Limber pine is successfully "leap-frogging" over bristlecone pine, probably because of its strong dispersal advantage and broader tolerances for establishment. This early-stage dominance indicates the potential for the species composition of treeline to change in response to climate change. More broadly, it shows how species differences in dispersal and establishment may result in future communities with very different specific composition.

Climate and landscape drive the pace and pattern of conifer encroachment into subalpine meadows

Mountain meadows have high biodiversity and help regulate stream water release following the snowmelt pulse. However, many meadows are experiencing woody plant encroachment, threatening these ecosystem services. While there have been field surveys of individual meadows and remote sensing-based landscape-scale studies of encroachment, what is missing is a broad-scale, ground-based study to understand common regional drivers, especially at high elevations, where land management has often played a less direct role. With this study, we ask: What are the climate and landscape conditions conducive to woody plant encroachment at the landscape scale, and how has historical climate variation affected tree recruitment in subalpine meadows over time? We measured density of encroaching trees across 340 subalpine meadows in the central Sierra Nevada, California, USA, and used generalized additive models (GAMs) to determine the relationship between landscape-scale patterns of encroachment and meadow environmental properties. We determined ages of trees in 30 survey meadows, used observed climate and GAMs to model the relationship between timing of recruitment and climate since the early 1900s, and extrapolated recruitment patterns into the future using downscaled climate scenarios. Encroachment was high among meadows with lodgepole pine (Pinus contorta Douglas ex Loudon var. murrayana (Balf.) Engelm.) in the immediate vicinity, at lower elevations, with physical conditions favoring strong soil drying, and with maximum temperatures above or below average. Climatic conditions during the year of germination were unimportant, with tree recruitment instead depending on a 3-yr seed production period prior to germination and a 6-yr seedling establishment period following germination. Recruitment was high when the seed production period had high snowpack, and when the seedling establishment period had warm summer maximum temperatures, high summer precipitation, and high snowpack. Applying our temporal model to downscaled output from four global climate models indicated that the average meadow will shift to forest by the end of the 21st century. Sierra Nevada meadow encroachment by conifers is ubiquitous and associated with climate conditions increasingly favorable for tree recruitment, which will lead to substantial changes in subalpine meadows and the ecosystem services they provide.

Warming and provenance limit tree recruitment across and beyond the elevation range of subalpine forest

Climate niche models project that subalpine forest ranges will extend upslope with climate warming. These projections assume that the climate suitable for adult trees will be adequate for forest regeneration, ignoring climate requirements for seedling recruitment, a potential demographic bottleneck. Moreover, local genetic adaptation is expected to facilitate range expansion, with tree populations at the upper forest edge providing the seed best adapted to the alpine. Here, we test these expectations using a novel combination of common gardens, seeded with two widely distributed subalpine conifers, and climate manipulations replicated at three elevations. Infrared heaters raised temperatures in heated plots, but raised temperatures more in the forest than at or above treeline because strong winds at high elevation reduced heating efficiency. Watering increased season-average soil moisture similarly across sites. Contrary to expectations, warming reduced Engelmann spruce recruitment at and above treeline, as well as in the forest. Warming reduced limber pine first-year recruitment in the forest, but had no net effect on fourth-year recruitment at any site. Watering during the snow-free season alleviated some negative effects of warming, indicating that warming exacerbated water limitations. Contrary to expectations of local adaptation, low-elevation seeds of both species initially recruited more strongly than high-elevation seeds across the elevation gradient, although the low-provenance advantage diminished by the fourth year for Engelmann spruce, likely due to small sample sizes. High- and low-elevation provenances responded similarly to warming across sites for Engelmann spruce, but differently for limber pine. In the context of increasing tree mortality, lower recruitment at all elevations with warming, combined with lower quality, high-provenance seed being most available for colonizing the alpine, portends range contraction for Engelmann spruce. The lower sensitivity of limber pine to warming indicates a potential for this species to become more important in subalpine forest communities in the coming centuries.

Carbon Costs of Constitutive and Expressed Resistance to a Non-Native Pathogen in Limber Pine

Increasing the frequency of resistance to the non-native fungus Cronartium ribicola (causative agent of white pine blister rust, WPBR) in limber pine populations is a primary management objective to sustain high-elevation forest communities. However, it is not known to what extent genetic disease resistance is costly to plant growth or carbon economy. In this study, we measured growth and leaf-level physiology in (1) seedling families from seed trees that have previously been inferred to carry or not carry Cr4, the dominant R gene allele conferring complete, gene-for-gene resistance to WPBR in limber pine, and (2) populations that were and were not infected with C. ribicola. We found that, in the absence of C. ribicola exposure, there was no significant difference in carbon relations between families born from seed trees that harbor the resistance allele compared to those that lack it, either to plant growth and phenology or leaf-level photosynthetic traits. However, post-infection with C. ribicola, growth was significantly reduced in inoculation survivors expressing complete resistance compared to uninoculated seedlings. Furthermore, inoculation survivors exhibited significant increases in a suite of traits including photosynthetic rate, respiration rate, leaf N, and stomatal conductance and a decrease in photosynthetic water-use efficiency. The lack of constitutive carbon costs associated with Cr4 resistance in non-stressed limber pine is consistent with a previous report that the R gene allele is not under selection in the absence of C. ribicola and suggests that host resistance may not bear a constitutive cost in pathosystems that have not coevolved. However, under challenge by C. ribicola, complete resistance to WPBR in limber pine has a significant cost to plant growth, though enhanced carbon acquisition post-infection may offset this somewhat. These costs and effects on performance further complicate predictions of this species’ response in warmer future climates in the presence of WPBR.

Limber Pine (Pinus flexilis James), a Flexible Generalist of Forest Communities in the Intermountain West

As forest communities continue to experience interactions between climate change and shifting disturbance regimes, there is an increased need to link ecological understanding to applied management. Limber pine (Pinus flexilis James.), an understudied species of western North America, has been documented to dominate harsh environments and thought to be competitively excluded from mesic environments. An observational study was conducted using the Forest Inventory and Analysis Database (FIAD) to test the competitive exclusion hypothesis across a broad elevational and geographic area within the Intermountain West, USA. We anticipated that competitive exclusion would result in limber pine's absence from mid-elevation forest communities, creating a bi-modal distribution. Using the FIAD database, limber pine was observed to occur with 22 different overstory species, which represents a surprising number of the woody, overstory species commonly observed in the Intermountain West. There were no biologically significant relationships between measures of annual precipitation, annual temperature, or climatic indices (i.e. Ombrothermic Index) and limber pine dominance. Limber pine was observed to be a consistent component of forest communities across elevation classes. Of the plots that contained limber pine regeneration, nearly half did not have a live or dead limber pine in the overstory. However, limber pine regeneration was greater in plots with higher limber pine basal area and higher average annual precipitation. Our results suggest limber pine is an important habitat generalist, playing more than one functional role in forest communities. Generalists, like limber pine, may be increasingly important, as managers are challenged to build resistance and resilience to future conditions in western forests. Additional research is needed to understand how different silvicultural systems can be used to maintain multi-species forest communities.

Heterotrophic respiration does not acclimate to continuous warming in a subtropical forest

As heterotrophic respiration (R_H) has great potential to increase atmospheric CO₂ concentrations, it is important to understand warming effects on R_H for a better prediction of carbon–climate feedbacks. However, it remains unclear how R_H responds to warming in subtropical forests. Here, we carried out trenching alone and trenching with warming treatments to test the climate warming effect on R_H in a subtropical forest in southwestern China. During the measurement period, warming increased annual soil temperature by 2.1 °C, and increased annual mean R_H by 22.9%. Warming effect on soil temperature (WE_T) showed very similar pattern with warming effect on R_H (WE_RH), decreasing yearly. Regression analyses suggest that WE_RH was controlled by WE_T and also regulated by the soil water content. These results showed that the decrease of WE_RH was not caused by acclimation to the warmer temperature, but was instead due to decrease of WE_T. We therefore suggest that global warming will accelerate soil carbon efflux to the atmosphere, regulated by the change in soil water content in subtropical forests.

Early establishment of trees at the alpine treeline: idiosyncratic species responses to temperature-moisture interactions

On a global scale, temperature is the main determinant of arctic and alpine treeline position. However on a local scale, treeline form and position vary considerably due to other climatic factors, tree species ecology and life-stage-dependent responses. For treelines to advance poleward or uphill, the first steps are germination and seedling establishment. These earliest life stages may be major bottlenecks for treeline tree populations and will depend differently on climatic conditions than adult trees. We investigated the effect of soil temperature and moisture on germination and early seedling survival in a field experiment in the French Alps near the local treeline (2100 m a.s.l.) using passive temperature manipulations and two watering regimes. Five European treeline tree species were studied: Larix decidua, Picea abies, Pinus cembra, Pinus uncinata and Sorbus aucuparia In addition, we monitored the germination response of three of these species to low temperatures under controlled conditions in growth chambers. The early establishment of these trees at the alpine treeline was limited either by temperature or by moisture, the sensitivity to one factor often depending on the intensity of the other. The results showed that the relative importance of the two factors and the direction of the effects are highly species-specific, while both factors tend to have consistent effects on both germination and early seedling survival within each species. We show that temperature and water availability are both important contributors to establishment patterns of treeline trees and hence to species-specific forms and positions of alpine treelines. The observed idiosyncratic species responses highlight the need for studies including several species and life-stages to create predictive power concerning future treeline dynamics.

Local-scale topoclimate effects on treeline elevations: a country-wide investigation of New Zealand's southern beech treelines

Although treeline elevations are limited globally by growing season temperature, at regional scales treelines frequently deviate below their climatic limit. The cause of these deviations relate to a host of climatic, disturbance, and geomorphic factors that operate at multiple scales. The ability to disentangle the relative effects of these factors is currently hampered by the lack of reliable topoclimatic data, which describe how regional climatic characteristics are modified by topographic effects in mountain areas. In this study we present an analysis of the combined effects of local- and regional-scale factors on southern beech treeline elevation variability at 28 study areas across New Zealand. We apply a mesoscale atmospheric model to generate local-scale (200 m) meteorological data at these treelines and, from these data, we derive a set of topoclimatic indices that reflect possible detrimental and ameliorative influences on tree physiological functioning. Principal components analysis of meteorological data revealed geographic structure in how study areas were situated in multivariate space along gradients of topoclimate. Random forest and conditional inference tree modelling enabled us to tease apart the relative effects of 17 explanatory factors on local-scale treeline elevation variability. Overall, modelling explained about 50% of the variation in treeline elevation variability across the 28 study areas, with local landform and topoclimatic effects generally outweighing those from regional-scale factors across the 28 study areas. Further, the nature of the relationships between treeline elevation variability and the explanatory variables were complex, frequently non-linear, and consistent with the treeline literature. To our knowledge, this is the first study where model-generated meteorological data, and derived topoclimatic indices, have been developed and applied to explain treeline variation. Our results demonstrate the potential of such an approach for ecological research in mountainous environments.

Moisture rivals temperature in limiting photosynthesis by trees establishing beyond their cold-edge range limit under ambient and warmed conditions

Climate change is altering plant species distributions globally, and warming is expected to promote uphill shifts in mountain trees. However, at many cold-edge range limits, such as alpine treelines in the western United States, tree establishment may be colimited by low temperature and low moisture, making recruitment patterns with warming difficult to predict. We measured response functions linking carbon (C) assimilation and temperature- and moisture-related microclimatic factors for limber pine (Pinus flexilis) seedlings growing in a heating × watering experiment within and above the alpine treeline. We then extrapolated these response functions using observed microclimate conditions to estimate the net effects of warming and associated soil drying on C assimilation across an entire growing season. Moisture and temperature limitations were each estimated to reduce potential growing season C gain from a theoretical upper limit by 15-30% (c. 50% combined). Warming above current treeline conditions provided relatively little benefit to modeled net assimilation, whereas assimilation was sensitive to either wetter or drier conditions. Summer precipitation may be at least as important as temperature in constraining C gain by establishing subalpine trees at and above current alpine treelines as seasonally dry subalpine and alpine ecosystems continue to warm.

Linking carbon and water relations to drought-induced mortality in Pinus flexilis seedlings

Survival of tree seedlings at high elevations has been shown to be limited by thermal constraints on carbon balance, but it is unknown if carbon relations also limit seedling survival at lower elevations, where water relations may be more important. We measured and modeled carbon fluxes and water relations in first-year Pinus flexilis seedlings in garden plots just beyond the warm edge of their natural range, and compared these with dry-mass gain and survival across two summers. We hypothesized that mortality in these seedlings would be associated with declines in water relations, more so than with carbon-balance limitations. Rather than gradual declines in survivorship across growing seasons, we observed sharp, large-scale mortality episodes that occurred once volumetric soil-moisture content dropped below 10%. By this point, seedling water potentials had decreased below -5 MPa, seedling hydraulic conductivity had decreased by 90% and seedling hydraulic resistance had increased by >900%. Additionally, non-structural carbohydrates accumulated in aboveground tissues at the end of both summers, suggesting impairments in phloem-transport from needles to roots. This resulted in low carbohydrate concentrations in roots, which likely impaired root growth and water uptake at the time of critically low soil moisture. While photosynthesis and respiration on a leaf area basis remained high until critical hydraulic thresholds were exceeded, modeled seedling gross primary productivity declined steadily throughout the summers. At the time of mortality, modeled productivity was insufficient to support seedling biomass-gain rates, metabolism and secondary costs. Thus the large-scale mortality events that we observed near the end of each summer were most directly linked with acute, episodic declines in plant hydraulic function that were linked with important changes in whole-seedling carbon relations.

Expression of functional traits during seedling establishment in two populations of Pinus ponderosa from contrasting climates

First-year tree seedlings represent a particularly vulnerable life stage and successful seedling establishment is crucial for forest regeneration. We investigated the extent to which Pinus ponderosa P. & C. Lawson populations from different climate zones exhibit differential expression of functional traits that may facilitate their establishment. Seeds from two populations from sites with contrasting precipitation and temperature regimes east (PIPO dry) and west (PIPO mesic) of the Oregon Cascade mountains were sown in a common garden experiment and grown under two water availability treatments (control and drought). Aboveground biomass accumulation, vegetative phenology, xylem anatomy, plant hydraulic architecture, foliar stable carbon isotope ratios (δ(13)C), gas exchange and leaf water relations characteristics were measured. No treatment or population-related differences in leaf water potential were detected. At the end of the first growing season, aboveground biomass was 74 and 44% greater in PIPO mesic in the control and drought treatments, respectively. By early October, 73% of PIPO dry seedlings had formed dormant buds compared with only 15% of PIPO mesic seedlings. Stem theoretical specific conductivity, calculated from tracheid dimensions and packing density, declined from June through September and was nearly twice as high in PIPO mesic during most of the growing season, consistent with measured values of specific conductivity. Intrinsic water-use efficiency based on δ(13)C values was higher in PIPO dry seedlings for both treatments across all sampling dates. There was a negative relationship between values of δ(13)C and leaf-specific hydraulic conductivity across populations and treatments, consistent with greater stomatal constraints on gas exchange with declining seedling hydraulic capacity. Integrated growing season assimilation and stomatal conductance estimated from foliar δ(13)C values and photosynthetic CO2-response curves were 6 and 28% lower, respectively, in PIPO dry seedlings. Leaf water potential at the turgor loss point was 0.33 MPa more negative in PIPO dry, independent of treatment. Overall, PIPO dry seedlings exhibited more conservative behavior, suggesting reduced growth is traded off for increased resistance to drought and extreme temperatures.

Forecasting distributional responses of limber pine to climate change at management-relevant scales in Rocky Mountain National Park

Resource managers at parks and other protected areas are increasingly expected to factor climate change explicitly into their decision making frameworks. However, most protected areas are small relative to the geographic ranges of species being managed, so forecasts need to consider local adaptation and community dynamics that are correlated with climate and affect distributions inside protected area boundaries. Additionally, niche theory suggests that species' physiological capacities to respond to climate change may be underestimated when forecasts fail to consider the full breadth of climates occupied by the species rangewide. Here, using correlative species distribution models that contrast estimates of climatic sensitivity inferred from the two spatial extents, we quantify the response of limber pine (Pinus flexilis) to climate change in Rocky Mountain National Park (Colorado, USA). Models are trained locally within the park where limber pine is the community dominant tree species, a distinct structural-compositional vegetation class of interest to managers, and also rangewide, as suggested by niche theory. Model forecasts through 2100 under two representative concentration pathways (RCP 4.5 and 8.5 W/m(2)) show that the distribution of limber pine in the park is expected to move upslope in elevation, but changes in total and core patch area remain highly uncertain. Most of this uncertainty is biological, as magnitudes of projected change are considerably more variable between the two spatial extents used in model training than they are between RCPs, and novel future climates only affect local model predictions associated with RCP 8.5 after 2091. Combined, these results illustrate the importance of accounting for unknowns in species' climatic sensitivities when forecasting distributional scenarios that are used to inform management decisions. We discuss how our results for limber pine may be interpreted in the context of climate change vulnerability and used to help guide adaptive management.