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Interactions between tropical cyclones and wildfires occur widely and can tip closed forests into open-canopy structures that initiate a 'grass–fire' cycle. We examined cyclone–fire interactions in a monodominant tropical montane pine forest in the Dominican Republic using remotely-sensed imagery to quantify damage from fires between 1986 and 2004, a category 1 cyclone in 1998, and an extensive wildfire in 2005. We also measured forest structure and composition 14.7 years after the 2005 fire. The area inside the 2005 burn scars (fire perimeters) totaled 25,206 ha, of which 81% burned and 14% was cyclone damaged. Cyclone damage made the fire markedly more extensive and severe—high-severity fires were > 3 times more frequent with high-severity cyclone damage than no cyclone damage—but these markedly synergistic effects were restricted to areas that had not burned for at least 19 years before the 2005 fire. Though earlier fires from 1986 to 2004 were small and low-severity, they were sufficient, when present, to prevent high-severity fire in 2005 irrespective of cyclone severity. In areas with strong cyclone–fire interactions, there was a complete loss of pine canopies, yet these stands had abundant pine canopy recruitment by 2019 and showed no evidence of compositional shifts toward open-canopy structures with pyrogenic herbaceous understories, illustrating the resilience of this ecosystem to a range of cyclone–fire synergies. However, the future resilience of tropical montane pine forests to cyclone–fire synergies is uncertain as climate change increases the intensity of cyclones and frequency of drought-triggered fires in these ecosystems. [ABSTRACT FROM AUTHOR]

Recent observational studies report weak or flat temperature − growth relationships for many tree species in temperate forests. In contrast, distribution limits of trees are strongly shaped by temperature, and studies show marked short‐term temperature effects on leaf‐level ecophysiology. To better determine the effects of warming on trees, we planted one‐year‐old seedlings of one lower montane (ponderosa pine), two upper montane (quaking aspen and lodgepole pine), and one subalpine tree species (subalpine fir) in in situ experimental gardens on an elevation gradient in the Rocky Mountains (USA) which span a 6°C range in temperature but have approximately uniform precipitation. Seedlings were lightly watered the first three growing seasons to facilitate establishment, and growth and survivorship were followed for four years. We expected a trade‐off between growth and survivorship, as seedlings in high temperatures grow faster (e.g., with a longer growing season), but have higher mortality from heat stress. Compared to the coldest site, aspen (+256% wider, +337% taller), ponderosa pine (+234% wider, 270% taller), and lodgepole pine (+235% wider, 283% taller) all had strikingly higher cumulative diameter and height growth in the warmest site by the end of the study. Linear models of cumulative and annual growth in the montane species showed strong, positive relationships with growing‐season temperature, but no significant relationships with growing‐season precipitation. In contrast, growth of subalpine fir did not vary significantly with temperature, but increased slightly with higher growing‐season precipitation. Accelerated growth did not come at the expense of survivorship in the montane species: cumulative four‐year survivorship of the montane species remained robust (71.4–94.4%) in high temperatures, but caused complete mortality of subalpine fir. As long as precipitation remains adequate, these results indicate that warming is likely to strongly increase growth in seedlings of montane species with only modest decreases in survivorship despite higher evapotranspiration, especially in cooler and wetter portions of their current distributions where hydric stress is low. In contrast, warming may negatively affect seedling growth and survival in hotter and drier areas of the Rockies, and warming of +3–6°C may endanger the persistence of subalpine fir over much of its current distribution. [ABSTRACT FROM AUTHOR]

Mountains have a long history in the study of diversity. Like macroscopic taxa, soil microbes are hypothesized to be strongly structured by montane gradients, and recently there has been important progress in understanding how microbes are shaped by these conditions. Here, we summarize this literature and synthesize patterns of microbial diversity on mountains. Unlike flora and fauna that often display a mid-elevation peak in diversity, we found a decline (34% of the time) or no trend (33%) in total microbial diversity with increasing elevation. Diversity of functional groups also varied with elevation (e.g. saprotrophic fungi declined 83% of the time). Most studies (82%) found that climate and soils (especially pH) were the primary mechanisms driving shifts in composition, and drivers differed across taxa—fungi were mostly determined by climate, while bacteria (48%) and archaea (71%) were structured primarily by soils. We hypothesize that the central role of soils—which can vary independently of other abiotic and geographic gradients—in structuring microbial communities weakens diversity patterns expected on montane gradients. Moving forward, we need improved cross-study comparability of microbial diversity indices (i.e. standardizing sequencing) and more geographic replication using experiments to broaden our knowledge of microbial biogeography on global gradients. [ABSTRACT FROM AUTHOR]

Climate change is causing significant shifts in tree species distributions to higher elevations and latitudes. Seed germination and seedling establishment are particularly important steps in tree range expansion under warmer conditions, yet seedling establishment is influenced by a range of factors beyond temperature, including herbivory, microenvironment, and the timing and amount of precipitation. We conducted an experiment to assess how augmented precipitation regimes, wildlife herbivory, and microclimate influence germination and first-season survival of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) near the upper elevational limit of its range in the southern Rocky Mountains. Germination was strongly influenced by moisture, with over three times higher germination in watered treatments. Seedling survival was similar across watered treatments but was negatively associated with microenvironments with higher maximum temperatures. These results indicate that soil moisture effects on germination and the negative impact of hot growing-season temperatures on seedling survival limit initial seedling establishment in Douglas-fir, even at the cooler and wetter end of its range, suggesting that the planting of this species will be most successful in cooler and wetter microsites. Taken together, this study suggests that continued warming and projected increases in droughts may strongly limit Douglas-fir regeneration and thus its ability to shift upwards with climate change. [ABSTRACT FROM AUTHOR]

Aim: The rate and magnitude of climate‐induced tree range shifts may be influenced by range‐wide variation in recruitment, which acts as a bottleneck in tree range dynamics. Here, we compare range predictions made using standard species distribution models (SDMs) and an integrated metamodelling approach that assimilates data on adult occurrence, seedling recruitment dynamics, and seedling survival under both current and future climate, and evaluate the degree to which information provided by seedling data can improve predictions of range dynamics. Location: The interior west region of the United States. Time period: 1990–2015. Major taxa studied: Five widespread conifer tree species. Methods: We used a previously published metamodelling framework to combine information from SDMs of adult tree occurrence and sub‐models describing seedling recruitment dynamics and seedling survival into a single set of predictions for the probability of occurrence for each species. The integrated framework links sub‐models to a SDM to generate cohesive predictions that consider information and uncertainty contained in all datasets. We then compared predictions from the integrated model to SDM predictions. Results: Integration of seedling information served primarily to improve characterization of model uncertainty, particularly in regions where recruitment may be limited by temperatures that exceed seedling tolerance. Integration constrained response curves very slightly across most climate gradients, particularly across temperature gradients. These differences were primarily attributable to the isolated effects of temperature on seedling survival and not to recruitment dynamics. Main conclusions: Our results indicate that range‐wide variation in recruitment both now and in the future is most uncertain along the edges of occupied regions, which increases uncertainty in projections of future species occurrence along range margins. Overall, the broad‐scale climatic dependence of the regeneration niche appears weaker than that of the adult climatic niche, and this enhances uncertainty in predicting range‐wide responses of these species to climate change. [ABSTRACT FROM AUTHOR]

Highlights • Osmotic potential was highly influenced by available soil moisture and seasonality. • Timing of osmotic adjustment varied by site, with the coldest site shifting last. • Electrolyte leakage from freezing peaked at highest growth efficiency site. • Growth efficiency declined under warming or cooling from native conditions. Abstract Higher temperatures and evaporative demand forecasted for Colorado forests by the end of the century suggest that soil water limitation increasingly will negatively impact whole plant performance. At the same time, upslope or poleward migration of plant ranges in response to warming may result in species experiencing cooler overnight temperatures, particularly if extremes in climate increase. In 2014 we established three experimental gardens along a temperature/elevation gradient to test the implications of shifting temperatures on tree function. Quaking aspen (Populus tremuloides) seedlings from a mid-elevation population were established in all three gardens and leaves were sampled monthly during the 2017 growing season. From these, we quantified the magnitude and timing of osmotic regulation, the relationships between leaf osmotic potential (Ψ osm), midday leaf water potential (Ψ mid), and soil moisture conditions under different temperature regimes, and growth efficiency (change in basal area/growing day). We observed a strong relationship between Ψ osm and soil moisture, and a strong seasonal decline in Ψ osm at the warmest and intermediate sites while the coldest site experienced a later increase in osmolytes associated with the highest degree of freeze tolerance. Growth efficiency was highest at the intermediate-temperature site – closest in elevation to the seed source location – but declined asymmetrically with warming or cooling. The novel abiotic conditions at both non-local sites resulted in declines in growth efficiency, suggesting that aspen will experience suboptimal conditions whether it stays in areas experiencing warming temperatures or if it migrates upslope to areas with colder overnight temperatures. [ABSTRACT FROM AUTHOR]

• Subalpine forest tree seedlings were experimentally droughted in situ. • Gas exchange and survival were monitored across canopy gap and shaded microsites. • Shading partly ameliorated drought-induced mortality under moderate drought. • Shading exacerbated loss of carbon gain and mortality under intense drought. • Drought alters historical species-specific microsite suitability for regeneration. Rates of tree mortality continue to rise with climate change, particularly in high elevation subalpine forests. In these systems, which depend on a narrow range of microsite conditions for successful seedling establishment, growing drought frequency and intensity may impose regeneration bottlenecks that further exacerbate population declines. However, it is unclear how regeneration processes tied to species-specific stress tolerances and microsite preferences are affected by varying magnitudes and durations of drought. To simulate moderate (50%) and severe (100% precipitation exclusion) growing-season drought, we installed precipitation exclusion shelters in shade and canopy gap light microenvironments over naturally-regenerating seedlings across a range of size-delimited establishment cohorts spanning first year emergents to large and well-established seedlings of two dominant subalpine forest species – Engelmann spruce (Picea engelmannii Parry ex Engelmann) and subalpine fir (Abies lasiocarpa (Hook) Nutt.). Seedling survival and gas exchange physiology was assessed to test the interacting effects of drought and light microenvironment on seedling performance, with contrasts to each species' historically preferred microsite under ambient precipitation conditions. Shade partially ameliorated drought effects when precipitation reductions were moderate as illustrated by rates of seedling mortality that were statistically indistinguishable from reference rates for both species, whereas moderate drought in canopy gaps significantly or nearly-significantly increased seedling mortality in several cohorts of both spruce and fir (1.9–2.9 and 2.3–7.5 times greater mortality risk in spruce and fir, respectively). When drought was severe, however, shade induced prolonged net-negative photosynthetic carbon assimilation as low as a −1.5 μmol CO 2 ·m−2·s−1 (growing-season average), which occurred alongside significantly increased mortality in spruce (3.2–9.4 times greater mortality risk). In contrast, mortality was significantly higher in canopy gaps under severe drought for fir (5.5–23.1 times greater mortality risk). Larger seedling size was strongly associated with lower mortality risk and more conservative stomatal behavior (e.g., predominantly <0.1 mol·H 2 O·m−2·s−1 growing-season averages), illustrating the sensitivity of new seedlings to microsite environments. However, persistent declines in photosynthetic carbon uptake observed across all seedling sizes in shaded microsites suggests the potential for lagged mortality and greater susceptibility to recurring drought even for larger and seemingly well-established seedlings. These results demonstrate how acute, intense droughts may alter historical patterns of spruce-fir seedling regeneration and highlight the importance of maintaining suitable microsites when implementing adaptive management strategies to avoid regeneration bottlenecks in forests that are already vulnerable to accelerating drought-induced mortality. [ABSTRACT FROM AUTHOR]

Climate is widely assumed to influence physiological and demographic processes in trees, and hence forest composition, biomass and range limits. Growth in trees is an important barometer of climate change impacts on forests as growth is highly correlated with other demographic processes including tree mortality and fecundity., We investigated the main drivers of diameter growth for five common tree species occurring in the Rocky Mountains of the western United States using nonlinear regression methods. We quantified growth at the individual tree level from tree core samples collected across broad environmental gradients. We estimated the effects of both climate variation and biotic interactions on growth processes and tested for evidence that disjunct populations of a species respond differentially to climate., Relationships between tree growth and climate varied by species and location. Growth in all species responded positively to increases in annual moisture up to a threshold level. Modest linear responses to temperature, both positive and negative, were observed at many sites. However, model results also revealed evidence for differentiated responses to local site conditions in all species. In severe environments in particular, growth responses varied nonlinearly with temperature. For example, in northerly cold locations pronounced positive growth responses to increasing temperatures were observed. In warmer southerly climates, growth responses were unimodal, declining markedly above a threshold temperature level., Net effects from biotic interactions on diameter growth were negative for all study species. Evidence for facilitative effects was not detected. For some species, competitive effects more strongly influenced growth performance than climate. Competitive interactions also modified growth responses to climate to some degree., Synthesis. These analyses suggest that climate change will have complex, species-specific effects on tree growth in the Rocky Mountains due to nonlinear responses to climate, differentiated growth processes that vary by location and complex species interactions that impact growth and potentially modify responses to climate. Thus, robust model simulations of future growth responses to climate trends may need to integrate realistic scenarios of neighbourhood effects as well as variability in tree performance attributed to differentiated populations. [ABSTRACT FROM AUTHOR]

Increasing temperatures worldwide, as a primary manifestation of climate change, may cause substantial alterations in forests, presenting a major challenge to predicting responses in forest composition and function. Yet, recent empirical research on climate and forests has found patterns at odds with theoretical and modeling expectations. Indeed, the need for an improved mechanistic understanding of climate’s effects on forests is clear but controlled field studies that address this issue are lacking. Montane and subalpine systems may be particularly sensitive to changes in temperature yet quantifying temperature effects in real-world conditions can be challenging. We used three common gardens arrayed over a 1200 m elevation and 6 °C mean annual temperature gradient in the Front Range of Colorado to evaluate the temperature responses of seedlings of three widespread and dominant tree species in Colorado: lodgepole pine ( Pinus contorta var. latifolia ), ponderosa pine ( Pinus ponderosa ) and aspen ( Populus tremuloides ). Seedlings for the gardens were sourced from populations naturally occurring at the intermediate elevation on this gradient and were planted into identical soils in all three gardens. We focused on in situ photosynthetic performance of seedlings, plasticity in spring phenology, and adjustments in leaf morphology. We found no evidence for clear temperature sensitivities in photosynthetic rates of the two coniferous species, neither across a 6 °C range in growing season temperatures between sites nor across manipulated leaf temperatures of 15–30 °C within sites. Likewise, lodgepole pine exhibited uniform leaf size across the temperature gradient; however, ponderosa pine leaf size did increase significantly at the warmest site. In contrast, aspen displayed pronounced temperature sensitivity in photosynthesis and leaf morphology, with maximum observed values at intermediate temperatures which both declined at the colder or warmer temperatures. Relative to the conifers, aspen also showed reduced phenological responses to warming with ∼12% fewer growing days at the intermediate site, and 6% at the warmest site. These divergent responses suggest that warming temperatures can alter seedling success in a number of different ways, and taken together, are likely to alter forest composition of Colorado in favor of greater dominance by montane conifer species. [ABSTRACT FROM AUTHOR]

Seed production by Picea engelmannii was monitored at 13 sites distributed across a ˜670 m elevation gradient for 40 years. Time series of annual seed output was investigated for evidence of masting behaviour and trends in seed abundance over time., We used regression models in a likelihood framework to examine climate effects on seed production for critical periods in the species' reproductive cycle., We rigorously evaluated the performance of two gridded climate data sets, PRISM and TopoWx, before using associated variables as predictors in the seed models., Seed production at these sites does not strictly conform to the classic masting concept. Seed abundance was highly variable over time and strongly synchronized among sites, but mast years could not be objectively identified due to intermediate levels of seed output., Model results indicate that climate conditions across multiple years cumulatively determine reproductive output. High seed rain is associated with elevated summer temperatures in the year that seeds are dispersed, low spring snowfall in the year preceding seed dispersal when buds are initiated, and reduced spring snowfall in a so-called priming year two years prior to seed dispersal. Low spring precipitation putatively increases growing season length and resource accumulation in seed trees., Linear models identified significant positive trends in seed output over time. Anomalous aridity and summer warmth in the latter half of the study period were highly favourable for seed production and were associated with increases in seed abundance., Synthesis. The increases in seed output observed in this study may promote population fitness of P. engelmannii in the face of changing climate regimes and increasing frequencies of fire- and insect-related tree mortality in the Rocky Mountains. Since this species lacks a persistent seed bank, re-colonization of disturbed areas or dispersal to shifting habitats depends on adequate production of seed by surviving trees, which according to these analyses may be moderately enhanced by current climate trends. However, some evidence also indicates that increases in seed output will ultimately be constrained by threshold high temperatures in the seed maturation year. [ABSTRACT FROM AUTHOR]

A relationship between forest vegetation patterns and climate has been proposed for Caribbean mountains, but mesoscale temperature, precipitation (PPT), humidity, and cloud formation patterns are poorly documented. Half-hourly temperature and humidity observations were obtained from 2001 to 2011 from a network of 10 data-logging instruments ranging in elevation from 1500 to 2800 m on the windward slopes of the Cordillera Central, Dominican Republic. We report diurnal, seasonal, and annual patterns in temperature, PPT, humidity, and the trade wind inversion (TWI) along the elevation gradient. The elevational gradient in mean air temperature was non-linear during the dry season, with lapse rates decreasing to -0.5 °C km-1 between 1500 and 1900 m and -0.8 °C km-1 between 2100 and 2400 m. Relative humidity reached a maximum at 2100 m (mean of 91%), but remains above 85% over the entire gradient until 2600 m, above which it drops steeply. Relative humidity also showed marked seasonality but only at the highest elevations, dropping markedly above 2400 m and especially above 2600 m in the dry season, while remaining high at lower elevations throughout the year. PPT declined only slightly with elevation on windward slopes, but was markedly lower in leeward areas. Dry season PPT was lower on windward and leeward slopes at all elevations, except at ∼2400 m on windward slopes where it remained nearly as high as the rest of the year. Sub-zero temperatures occurred at elevations ≥2325 m and increased markedly in frequency ≥2600 m. These observations support the hypothesis that the discrete vegetation ecotone between the cloud forest and subalpine pine forest at ∼2200 m on windward slopes results from climatic discontinuities, especially during the dry season. In particular, the TWI effect on mesoclimatic patterns (especially moisture) regulates the elevational maximum of cloud forest flora and likely will represent a strong barrier to the future migration of cloud forest flora to higher elevations in response to warmer temperatures. Together with increased moisture stress due to higher temperatures, climate change in the high elevations of tropical mountains is therefore likely to disrupt the dynamics and distributions of tropical montane forests. [ABSTRACT FROM AUTHOR]

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The boundary between the humid eastern and the arid western regions of the Great Plains of North America is of great economic interest and historic intrigue, yet its location is controversial. Areas to the east of this boundary have historically enjoyed the benefits of fertile soil coupled with more favorable rainfall and reliable surface water, permitting conventional agriculture to flourish over a remarkably large percentage of the eastern Great Plains. The expansion of population and agriculture during the nineteenth century across the western Great Plains tested the extent that non-irrigated, row crop agriculture could be successful in areas where year-to-year rainfall was unreliable. In this paper, we quantify the historic annual variability of soil moisture and hydrologic conditioning in the Great Plains resulting from climatic variability, show the regions that historically demonstrate unreliable precipitation, and identify the extent of arid regions of the central United States based on modeled annual soil moisture variability. We asked how arid climates have influenced soil formation patterns at small cartographic scales, and how soil properties buffer or enhance soil moisture regimes (at the udic–ustic boundary) to climate variability at larger cartographic scales. At small cartographic scales, a climate-only model worked nearly as well as a climate-and-soils model in mapping the region's soil moisture boundary; however, a climate-only model missed important local soil influences. Finally, we demonstrate that long-term climate and climate variability are reflected in the depth and concentration of the calcic soil properties. From a practical standpoint, our work highlights that soils with higher water holding capacity dampen periodic short-term rainfall deficits, while soils with lower water holding capacity can exhibit edaphic drought during otherwise normal climate years. [ABSTRACT FROM AUTHOR]