Mean Annual Temperature Gradient Research Articles

Contrasting foliar nitrogen nutrition of coexisting temperate and boreal trees across a modest temperature cline

The temperate-boreal forest ecotone represents a transition zone from temperate to boreal forest where nitrogen (N) is frequently limiting tree growth, which is highly sensitive to climate change. However, the spatial patterns and potential drivers of plant N nutrition and soil N availability remain poorly understood. To address this, we conducted a field investigation along a temperate-boreal forest ecotone in northeastern China, characterized by a modest mean annual temperature gradient (∼1°C) within the range of current climate warming. Our goal was to evaluate the spatial variation in foliar N nutrition and soil N availability, and the potential driving factors for Mongolian oak (Quercus mongolica) and Dahurian larch (Larix gmelinii), the dominant trees of the local temperate and adjacent boreal forests, respectively. Our results revealed no significant spatial trend in topsoil N availability across the sampling transect. Overall, foliar N concentration was significantly higher, but foliar δ15N was lower, for Mongolian oak than Dahurian larch. More important, foliar N concentration for Mongolian oak increased significantly toward the boreal forest, driven by lower mean annual temperatures and mean annual precipitation, with no significant trend observed for Dahurian larch. Additionally, foliar Δδ15N (foliar δ15N−soil δ15N) decreased significantly for Mongolian oak as it approached the boreal forest, while it increased significantly for Dahurian larch toward the temperate forest. Furthermore, the foliar N concentration, δ15N, and Δδ15N for Dahurian larch were all higher with an increasing basal area proportion of Mongolian oak. Our findings reveal contrasting patterns of foliar N nutrition between co-occurring temperate and boreal trees across a temperate-boreal forest ecotone which experiences a modest climatic gradient. These results underscore the importance of incorporating interspecific interactions to enhance our understanding of future N cycling in southern boreal forests in the context of climate warming.

Evidence that spatial scale and environment factors explain grassland community assembly in woodland-grassland ecotones.

How communities of living organisms assemble has long been a central question in ecology. The impact of habitat filtering and limiting similarity on plant community structures is well known, as both processes are influenced by individual responses to environmental fluctuations. Yet, the precise identifications and quantifications of the potential abiotic and biotic factors that shape community structures at a fine scale remains a challenge. Here, we applied null model approaches to assess the importance of habitat filtering and limiting similarity at two spatial scales. We used 63 natural vegetation plots, each measuring 5 × 5 m, with three nested subplots measuring 1 × 1 m, from the 2021 field survey, to examine the alpha diversity as well as beta diversity of plots and subplots. Linear mixed-effects models were employed to determine the impact of environmental variables on assembly rules. Our results demonstrate that habitat filtering is the dominant assembly rules at both the plot and subplot levels, although limiting similarity assumes stronger at the subplot level. Plot-level limiting similarity exhibited a positive association with fine-scale partitioning, suggesting that trait divergence originated from a combination of limiting similarity and spatial partitioning. Our findings also reveal that the community assembly varies more strongly with the mean annual temperature gradient than the mean annual precipitation. This investigation provides a pertinent illustration of non-random assembly rules from spatial scale and environmental factors in plant communities in the loess hilly region. It underscores the critical influence of spatial and environmental constraints in understanding the assembly of plant communities.

Warming Reduces Priming Effect of Soil Organic Carbon Decomposition Along a Subtropical Elevation Gradient

AbstractThe priming effects (PEs) of soil organic carbon (SOC) decomposition is a crucial process affecting the C balance of terrestrial ecosystems. However, there is uncertainty about how PEs will respond to climate warming. In this study, we sampled soils along a subtropical elevation gradient in China and conducted a 126‐day lab‐incubation experiment with and without the addition of 13C‐labeled high‐bioavailability glucose or low‐bioavailability lignin. Based on the mean annual temperature (MAT) of each elevation (9.3–16.4°C), a temperature increase of 4°C was used to explore how PEs mediate the decomposition of SOC in response to warming. Our results showed that the magnitude of glucose‐induced PEs (PEglucose) was higher than lignin‐induced PEs (PElignin), with both PEs linearly increasing with MAT. Across the MAT (i.e., elevation) gradient, short‐term warming had a constant magnitude of negative effects on PEglucose, whereas rising MAT exacerbated the negative effects of short‐term warming on PElignin. Moreover, the temperature sensitivity of SOC decomposition decreased after adding glucose and lignin across the MAT gradient, suggesting that fresh C inputs may prime the microbial breakdown of labile SOC under warming. Taken together, warming alleviated SOC loss due to PEs through varying mechanisms depending on substrate bioavailability. Warming mediated the PEglucose by increasing available nitrogen and weakening microbial nitrogen‐mining but inhibited the PElignin by shifting from microbial nitrogen‐mining to microbial co‐metabolization. Our findings highlight the role of warming in regulating the PEs and suggest that incorporating the suppression effect of warming on PEs can contribute to the accurate prediction of soil C dynamics in a warming world.

Limits of thermal and hydrological tolerance in a foundation tree species (Populus fremontii) in the desert southwestern United States.

Populus fremontii is among the most dominant, and ecologically important riparian tree species in the western United States and can thrive in hyper-arid riparian corridors. Yet, P. fremontii forests have rapidly declined over the last decade, particularly in places where temperatures sometimes exceed 50°C. We evaluated high temperature tolerance of leaf metabolism, leaf thermoregulation, and leaf hydraulic function in eight P. fremontii populations spanning a 5.3°C mean annual temperature gradient in a well-watered common garden, and at source locations throughout the lower Colorado River Basin. Two major results emerged. First, despite having an exceptionally high Tcrit (the temperature at which Photosystem II is disrupted) relative to other tree taxa, recent heat waves exceeded Tcrit , requiring evaporative leaf cooling to maintain leaf-to-air thermal safety margins. Second, in midsummer, genotypes from the warmest locations maintained lower midday leaf temperatures, a higher midday stomatal conductance, and maintained turgor pressure at lower water potentials than genotypes from more temperate locations. Taken together, results suggest that under well-watered conditions, P. fremontii can regulate leaf temperature below Tcrit along the warm edge of its distribution. Nevertheless, reduced Colorado River flows threaten to lower water tables below levels needed for evaporative cooling during episodic heat waves.

A critical thermal transition driving spring phenology of Northern Hemisphere conifers.

Despite growing interest in predicting plant phenological shifts, advanced spring phenology by global climate change remains debated. Evidence documenting either small or large advancement of spring phenology to rising temperature over the spatio-temporal scales implies a potential existence of a thermal threshold in the responses of forests to global warming. We collected a unique data set of xylem cell-wall-thickening onset dates in 20 coniferous species covering a broad mean annual temperature (MAT) gradient (-3.05 to 22.9°C) across the Northern Hemisphere (latitudes 23°-66° N). Along the MAT gradient, we identified a threshold temperature (using segmented regression) of 4.9 ± 1.1°C, above which the response of xylem phenology to rising temperatures significantly decline. This threshold separates the Northern Hemisphere conifers into cold and warm thermal niches, with MAT and spring forcing being the primary drivers for the onset dates (estimated by linear and Bayesian mixed-effect models), respectively. The identified thermal threshold should be integrated into the Earth-System-Models for a better understanding of spring phenology in response to global warming and an improved prediction of global climate-carbon feedbacks.

Habitat-specific responses of soil organic matter decomposition to Spartina alterniflora invasion along China's coast.

Plant invasions cause a fundamental change in soil organic matter (SOM) turnover. Disentangling the biogeographic patterns and key drivers of SOM decomposition and its temperature sensitivity (Q10 ) under plant invasion is a prerequisite for making projections of global carbon feedback. We collected soil samples along China's coast across saltmarshes to mangrove ecosystems invaded by the smooth cordgrass (Spartina alterniflora Loisel.). Microcosm experiments were carried out to determine the patterns of SOM decomposition and its thermal response. Soil microbial biomass and communities were also characterized accordingly. SOM decomposition constant dramatically decreased along the mean annual temperature gradient, whereas the cordgrass invasion retarded this change (significantly reduced slope, p < 0.05). The response of Q10 to invasion and the soil microbial quotient peaked at midlatitude saltmarshes, which can be explained by microbial metabolism strategies. Climatic variables showed strong negative controls on the Q10 , whereas dissolved carbon fraction exerted a positive influence on its spatial variance. Higher microbial diversity appeared to weaken the temperature-related response of SOM decomposition, with apparent benefits for carbon sequestration. Inconsistent responses to invasion were exhibited among habitat types, with SOM accumulation in saltmarshes but carbon loss in mangroves, which were explained, at least in part, by the SOM decomposition patterns under invasion. This study elucidates the geographic pattern of SOM decomposition and its temperature sensitivity in coastal ecosystems and underlines the importance of interactions between climate, soil, and microbiota for stabilizing SOM under plant invasion.

Agricultural intensification with seasonal fallow land promotes high bee diversity in Afrotropical drylands

Abstract The exponential increase in the human population in tandem with increased food demand has caused agriculture to be the global‐dominant form of land use. Afrotropical drylands are currently facing the loss of natural savannah habitats and agricultural intensification with largely unknown consequences for bees. Here we investigate the effects of agricultural intensification on bee assemblages in the Afrotropical drylands of northern Tanzania. We disentangled the direct effects of agricultural intensification and temperature on bee richness from indirect effects mediated by changes in floral resources. We collected data from 24 study sites representing three levels of management intensity (natural savannah, moderate intensive and highly intensive agriculture) spanning an extensive gradient of mean annual temperature (MAT) in northern Tanzania. We used ordinary linear models and path analysis to test the effects of agricultural intensity and MAT on bee species richness, bee species composition and body‐size variation of bee communities. We found that bee species richness increased with agricultural intensity and with increasing temperature. The effects of agricultural intensity and temperature on bee species richness were mediated by the positive effects of agriculture and temperature on the richness of floral resources used by bees. During the off‐growing season, agricultural land was characterized by an extensive period of fallow land holding a very high density of flowering plants with unique bee species composition. The increase in bee diversity in agricultural habitats paralleled an increasing variation of bee body sizes with agricultural intensification that, however, diminished in environments with higher temperatures. Synthesis and applications. Our study reveals that bee assemblages in Afrotropical drylands benefit from agricultural intensification in the way it is currently practiced. However, further land‐use intensification, including year‐round irrigated crop monocultures and excessive use of agrochemicals, is likely to exert a negative impact on bee diversity and pollination services, as reported in temperate regions. Moreover, several bee species were restricted to natural savannah habitats. To conserve bee communities and guarantee pollination services in the region, a mixture of savannah and agriculture, with long periods of fallow land should be maintained.

Impacts of insect frass and cadavers on soil surface litter decomposition along a tropical forest temperature gradient.

Insect herbivores play important roles in shaping many ecosystem processes, but how climate change will alter the effects of insect herbivory are poorly understood. To address this knowledge gap, we quantified for the first time how insect frass and cadavers affected leaf litter decomposition rates and nutrient release along a highly constrained 4.3°C mean annual temperature (MAT) gradient in a Hawaiian montane tropical wet forest. We constructed litterbags of standardized locally sourced leaf litter, with some amended with insect frass + cadavers to produce treatments designed to simulate ambient (Control = no amendment), moderate (Amended‐Low = 2 × Control level), or severe (Amended‐High = 11 × Control level) insect outbreak events. Multiple sets of these litterbags were deployed across the MAT gradient, with individual litterbags collected periodically over one year to assess how rising MAT altered the effects of insect deposits on litter decomposition rates and nitrogen (N) release. Increased MAT and insect inputs additively increased litter decomposition rates and N immobilization rates, with effects being stronger for Amended‐High litterbags. However, the apparent temperature sensitivity (Q 10) of litter decomposition was not clearly affected by amendments. The effects of adding insect deposits in this study operated differently than the slower litter decomposition and greater N mobilization rates often observed in experiments which use chemical fertilizers (e.g., urea, ammonium nitrate). Further research is required to understand mechanistic differences between amendment types. Potential increases in outbreak‐related herbivore deposits coupled with climate warming will accelerate litter decomposition and nutrient cycling rates with short‐term consequences for nutrient cycling and carbon storage in tropical montane wet forests.

Carbon, nitrogen, and phosphorus stoichiometry of organic matter in Swedish forest soils and its relationship with climate, tree species, and soil texture

Abstract. While the carbon (C) content of temperate and boreal forest soils is relatively well studied, much less is known about the ratios of C, nitrogen (N), and phosphorus (P) of the soil organic matter, as well as the abiotic and biotic factors that shape them. Therefore, the aim of this study was to explore carbon, nitrogen, and organic phosphorus (OP) contents and element ratios in temperate and boreal forest soils and their relationships with climate, dominant tree species, and soil texture. For this purpose, we studied 309 forest soils located all over Sweden between 56 and 68∘ N. The soils are a representative subsample of Swedish forest soils with a stand age &gt;60 years that were sampled for the Swedish Forest Soil Inventory. We found that the N stock of the organic layer increased by a factor of 7.5 from −2.0 to 7.5 ∘C mean annual temperature (MAT), which is almost twice as much as the increase in the organic layer stock along the MAT gradient. The increase in the N stock went along with an increase in the N:P ratio of the organic layer by a factor of 2.1 from −2.0 to 7.5 ∘C MAT (R2=0.36, p&lt;0.001). Forests dominated by pine had higher C:N ratios in the organic layer and mineral soil down to a depth of 65 cm than forests dominated by spruce. Further, also the C:P ratio was increased in the pine-dominated forests compared to forests dominated by other tree species in the organic layer, while the C:OP ratio in the mineral soil was not elevated in pine forests. C, N, and OP contents in the mineral soil were higher in fine-textured soils than in coarse-textured soils by a factor of 2.3, 3.5, and 4.6, respectively. Thus, the effect of texture was stronger on OP than on N and C likely because OP adsorbs very rigidly to mineral surfaces. Further, we found that the P and K concentrations of the organic layer were inversely related to the organic layer stock, while the N:P ratio was positively related to the organic layer stock. Taken together, the results show that the N:P ratio of the organic layer was most strongly related to MAT. Further, the C:N ratio was most strongly related to dominant tree species even in the mineral subsoil. In contrast, the C:P ratio was only affected by dominant tree species in the organic layer, but the C:OP ratio in the mineral soil was hardly affected by tree species due to the strong effect of soil texture on the OP concentration.

Response of Fruit Body Assemblage Color Lightness to Macroclimate and Vegetation Cover

Understanding how species relate mechanistically to their environment via traits is a central goal in ecology. Many macroecological rules were found for macroorganisms, however, whether they can explain microorganismal macroecological patterns still requires investigation. Further, whether macroecological rules are also applicable in microclimates is largely unexplored. Here we use fruit body-forming fungi to understand both aspects better. A recent study showed first evidence for the thermal-melanism hypothesis (Bogert’s rule) in fruit body-forming fungi and relied on a continental spatial scale with large grid size. At large spatial extent and grid sizes, other factors like dispersal limitation or local microclimatic variability might influence observed patterns besides the rule of interest. Therefore, we test fungal assemblage fruit body color lightness along a local elevational gradient (mean annual temperature gradient of 7°C) while considering the vegetation cover as a proxy for local variability in microclimate. Using multivariate linear modeling, we found that fungal fruiting assemblages are significantly darker at lower mean annual temperatures supporting the thermal-melanism hypothesis. Further, we found a non-significant trend of assemblage color lightness with vegetation cover. Our results support Bogert’s rule for microorganisms with macroclimate, which was also found for macroorganisms.

Disentangling effects of climate and land use on biodiversity and ecosystem services—A multi‐scale experimental design

Abstract Climate and land‐use change are key drivers of environmental degradation in the Anthropocene, but too little is known about their interactive effects on biodiversity and ecosystem services. Long‐term data on biodiversity trends are currently lacking. Furthermore, previous ecological studies have rarely considered climate and land use in a joint design, did not achieve variable independence or lost statistical power by not covering the full range of environmental gradients. Here, we introduce a multi‐scale space‐for‐time study design to disentangle effects of climate and land use on biodiversity and ecosystem services. The site selection approach coupled extensive GIS‐based exploration (i.e. using a Geographic information system) and correlation heatmaps with a crossed and nested design covering regional, landscape and local scales. Its implementation in Bavaria (Germany) resulted in a set of study plots that maximise the potential range and independence of environmental variables at different spatial scales. Stratifying the state of Bavaria into five climate zones (reference period 1981–2010) and three prevailing land‐use types, that is, near‐natural, agriculture and urban, resulted in 60 study regions (5.8 × 5.8 km quadrants) covering a mean annual temperature gradient of 5.6–9.8°C and a spatial extent of ~310 × 310 km. Within these regions, we nested 180 study plots located in contrasting local land‐use types, that is, forests, grasslands, arable land or settlement (local climate gradient 4.5–10°C). This approach achieved low correlations between climate and land use (proportional cover) at the regional and landscape scale with |r ≤ 0.33| and |r ≤ 0.29| respectively. Furthermore, using correlation heatmaps for local plot selection reduced potentially confounding relationships between landscape composition and configuration for plots located in forests, arable land and settlements. The suggested design expands upon previous research in covering a significant range of environmental gradients and including a diversity of dominant land‐use types at different scales within different climatic contexts. It allows independent assessment of the relative contribution of multi‐scale climate and land use on biodiversity and ecosystem services. Understanding potential interdependencies among global change drivers is essential to develop effective restoration and mitigation strategies against biodiversity decline, especially in expectation of future climatic changes. Importantly, this study also provides a baseline for long‐term ecological monitoring programs.

Stomatal density in Pinus sylvestris as an indicator of temperature rather than CO2 : Evidence from a pan-European transect.

The commonly observed negative relationship between stomatal density (SD) and atmospheric CO2 has led to SD being proposed as an indicator of atmospheric CO2 concentration. The use of SD as a proxy for CO2 , however, has been hampered by an insufficient understanding of the intraspecific variation of this trait. We hypothesized that SD in Pinus sylvestris, a widely distributed conifer, varies geographically and that this variation is determined by major climatic variables. By sampling needles from naturally growing trees along a latitudinal range of 32.25°, equivalent to 13.7°C gradientof mean annual temperature (MAT) across Europe, we found that SD decreased from the warmest southern sites to the coldest sites in the north at a rate of 4 stomata per mm2 for each 1°C, with MAT explaining 44% of the variation. Additionally, samples from a provenance trial exhibited a positive relationship between SD and the MAT of the original localities, suggesting that high SD is an adaptation to warm temperature. Our study revealed one of the strongest intraspecific relationships between SD and climate in any woody species, supporting the utility of SD as a temperature, rather than direct CO2 , proxy. In addition, our results predict the response of SD to climate warming.

Productivity does not decrease at the climate extremes of tree ranges in the Japanese archipelago.

As per the abundant-center hypothesis, the cold- and warm-edges of the latitudinal and elevational distributions of vegetation are the result of physiological limitations caused by abiotic stress. The stand-level productivity per leaf mass of plants is an integrated physiological measure of whole-plant carbon gain. The abundant-center hypothesis specifically predicts that the productivity per leaf mass decreases at cold-edges and warm-edges. In the Japanese archipelago, the dominant functional types of trees change from evergreen hardwoods in the south to deciduous hardwoods and evergreen conifers in the north, forming latitudinal ecotones. This study tested the abundant-center hypothesis by analyzing the productivity per leaf mass of each functional type along a gradient of mean annual temperature (MAT), using forest inventory data. Although productivity per leaf mass was variable along the MAT, it neither increased nor decreased with MAT for each functional tree type. The productivity per leaf mass was also noted to not decrease at the cold-edges for evergreen and deciduous hardwoods or at the warm-edges for deciduous hardwoods and evergreen conifers. Productivity per leaf mass was not positively correlated with abundance. Thus, this study did not support the abundant-center hypothesis. Instead, physiological or ecological limitations, particularly at the seedling and sapling stages, may be the important process affecting the distribution edges of these three functional types.

FLEXIBLE SOIL MICROBIAL CARBON METABOLISM ACROSS AN ASIAN ELEVATION GRADIENT

ABSTRACTThe function and change of global soil carbon (C) reserves in natural ecosystems are key regulators of future carbon-climate coupling. Microbes play a critical role in soil carbon cycling and yet there is poor understanding of their roles and C metabolism flexibility in many ecosystems. We wanted to determine whether vegetation type and climate zone influence soil microbial community composition (fungi and bacteria) and carbon resource preference. We used a biomarker (phospholipid fatty acids, PLFAs), natural abundance 13C and 14C probing approach to measure soil microbial composition and C resource use, along a 1900–4167-m elevation gradient on Mount Gongga (7556 m asl), China. Mount Gongga has a vertical mean annual temperature gradient of 1.2–10.1°C and a diversity of typical vegetation zones in the Tibetan Plateau. Soils were sampled at 10 locations along the gradient capturing distinct vegetation types and climate zones from lowland subtropical-forest to alpine-meadow. PLFA results showed that microbial communities were composed of 2.1–51.7% bacteria and 2.0–23.2% fungi across the elevation gradient. Microbial biomass was higher and the ratio of soil fungi to bacteria (F/B) was lower in forest soils compared to meadow soils. δ13C varied between −33‰ to −17‰ with C3 plant carbon sources dominant across the gradient. Soil organic carbon (SOC) turnover did not vary among three soils we measured from three forest types (i.e., evergreen broadleaved subtropical, mixed temperate, coniferous alpine) and dissolved organic carbon (DOC) turnover decreased with soil elevation. Forest soil microbial PLFA 14C and δ13C measurements showed that forest type and climate were related to different microbial C use. The 14C values of microbial PLFAs i15, a15, 16:1, br17 decreased with elevation while those of C16:0, cyC17, and cyC19 did not show much difference among three forest ecosystems. Bacteria and bacillus represented by C16:1 and brC17 showed considerable microbial C metabolism flexibility and tended to use ancient carbon at higher altitudes. Anaerobes represented by cyC17 and cyC19 showed stronger C metabolism selectivity. Our findings reveal specific C source differences between and within soil microbial groups along elevation gradients.

Biomass partitioning pattern of Rheum tanguticum on the Qinghai–Tibet Plateau was affected by water-related factors

To identify the patterns of aboveground biomass (MA) and belowground biomass (MB) partitioning of a tall perennial herbal plant, Rheum tanguticum Maxim. ex Balf. (R. tanguticum), we determined MA, MB, total biomass (MT), and below- to aboveground biomass ratio (MB/MA) through three consecutive sampling campaigns during 2016–2018 on the Qinghai-Tibetan plateau. We then documented MA, MB, MT, and MB/MA, and their log–log relationship with environmental factors using data from 47 sites. MB/MA showed a significant negative relationship with mean annual precipitation (MAP) and altitude but a positive relationship with mean annual temperature (MAT), soil total nitrogen content (TN), and soil humidity (SH). While MT increased with altitude and decreased with MAT, TN, and SH, but the relationship between MAP and MT was not significant. Reduced major axis analysis revealed a slope of the log–log relationship between MA and MB to be 1.16, supporting an allometric partitioning pattern of R. tanguticum. Furthermore, the scaling exponent revealed different changes to different environmental factors. Specifically, scaling exponent was sensitive to the gradient of MAP and SH, but not to the gradient of altitude, MAT, or TN. This indicated that the scaling exponent was affected by water availability.

Global distribution and bioclimatic characterization of alpine biomes

Although there is a general consensus on the distribution and ecological features of terrestrial biomes, the allocation of alpine ecosystems in the global biogeographic system is still unclear. Here, we delineate a global map of alpine areas above the treeline by modelling regional treeline elevation at 30 m resolution, using global forest cover data and quantile regression. We then used global datasets to 1) assess the climatic characteristics of alpine ecosystems using principal component analysis, 2) define bioclimatic groups by an optimized cluster analysis and 3) evaluate patterns of primary productivity based on the normalized difference vegetation index. As defined here, alpine biomes cover 3.56 Mkm2or 2.64% of land outside Antarctica. Despite temperature differences across latitude, these ecosystems converge below a sharp threshold of 5.9°C and towards the colder end of the global climatic space. Below that temperature threshold, alpine ecosystems are influenced by a latitudinal gradient of mean annual temperature and they are climatically differentiated by seasonality and continentality. This gradient delineates a climatic envelope of global alpine biomes around temperate, boreal and tundra biomes as defined in Whittaker's scheme. Although alpine biomes are similarly dominated by poorly vegetated areas, world ecoregions show strong differences in the productivity of their alpine belt irrespectively of major climate zones. These results suggest that vegetation structure and function of alpine ecosystems are driven by regional and local contingencies in addition to macroclimatic factors.

Soil microbial respiration adapts to ambient temperature in global drylands.

Heterotrophic soil microbial respiration – one of the main processes of carbon loss from soils to the atmosphere – is sensitive to temperature in the short-term. However, how this sensitivity is affected by long-term thermal regimes is uncertain. There is an expectation that soil microbial respiration rates adapt to the ambient thermal regime, but whether this adaptation magnifies or reduces respiration sensitivities to temperature fluctuations remains unresolved. This gap in our understanding is particularly pronounced for drylands as most studies conducted so far have focused on mesic systems. Here, we conducted an incubation study using soils from 110 global drylands encompassing a wide gradient in mean annual temperature. We tested how mean annual temperature affects soil respiration rates at three assay temperatures while controlling for substrate depletion and microbial biomass. Estimated soil respiration rates at the mean microbial biomass were lower in sites with higher mean annual temperatures across the three assayed temperatures. The patterns observed are consistent with expected evolutionary trade-offs in the structure and function of enzymes under different thermal regimes. Our results therefore suggest that soil microbial respiration adapts to the ambient thermal regime in global drylands.

Disentangling temperature effects on leaf wax n-alkane traits and carbon isotopic composition from phylogeny and precipitation

Leaf wax n-alkanes are terrestrial plant biomarkers that have characteristics that are widely employed as proxies for climatic conditions. Understanding the relationship between different environmental factors to the amounts and types of leaf wax n-alkanes in modern plants is crucial for the application of these proxies to paleoenvironmental reconstructions. However, the effects of climate conditions on plant wax characteristics remain complicated due to the interactions among temperature, precipitation and phylogeny. To evaluate the effect of temperature with minimized interfering factors, we collected 106 Artemisia plants across a 15 °C mean annual temperature gradient along the 400 mm isohyet in China. Both total n-alkane concentration (∑alk) and carbon preference index (CPI) varied greatly but did not correlate with temperature. Average chain length (ACL) increased with temperature, especially summer temperature (TJJA, June–August), indicating that ACL could be used as proxy for temperature. The stable carbon isotope compositions of n-C27, n-C29 and n-C31 were very similar in each plant (−38.2‰ to −30.0‰, −39.0‰ to −30.2‰, −38.7‰ to −30.5‰, respectively), which reflects a similar biosynthetic process for all three n-alkane homologues of Artemisia. There was a positive relationship between δ13C of bulk leaf tissues (δ13Cbulk) and of n-alkanes (δ13Calk), and the average offset of δ13C29 relative to δ13Cbulk (εC29/bulk) was −7.1‰. Increasing trends in both δ13Cbulk and δ13C29 were found with temperature. However, correlation of δ13Cbulkwith temperature (R2 = 18%) was much weaker than that of δ13C29 with temperature (R2 = 60%). Therefore, δ13C29 appears to be a better proxy of paleotemperature than δ13Cbulk.

Dominant tree species of the Colorado Rockies have divergent physiological and morphological responses to warming

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 1200m 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.

Ammonia oxidizer populations vary with nitrogen cycling across a tropical montane mean annual temperature gradient.

Functional gene approaches have been used to better understand the roles of microbes in driving forest soil nitrogen (N) cycling rates and bioavailability. Ammonia oxidation is a rate limiting step in nitrification, and is a key area for understanding environmental constraints on N availability in forests. We studied how increasing temperature affects the role of ammonia oxidizing archaea (AOA) and bacteria (AOB) in soil N cycling and availability by using a highly constrained natural mean annual temperature (MAT) elevation gradient in a tropical montane wet forest. We found that net nitrate (NO3- ) bioavailability is positively related to MAT (r2 =0.79, P=0.0033), and AOA DNA abundance is positively related to both NO3- availability (r2 =0.34, P=0.0071) and MAT (r2 =0.34, P<0.001). In contrast, AOB DNA was only detected in some soils across the gradient. We identified three distinct phylotypes within the AOA which differed from one another in abundance and relative gene expression. In addition, one AOA phylotype increased in abundance with MAT, while others did not. We conclude that MAT is the primary driver of ecosystem N availability across this gradient, and AOA population size and structure appear to mediate the relationship between the nitrification and N bioavailability. These findings hold important implications for nutrient limitation in forests and feedbacks to primary production under changing climate.