Natural Climate Gradient Research Articles

Temperature sensitivity of soil organic carbon respiration along a forested elevation gradient in the Rwenzori Mountains, Uganda

Abstract. Tropical montane forests store high amounts of soil organic carbon (SOC). However, global warming may affect these stocks via enhanced soil respiration. Improved insight into the temperature response of SOC respiration can be obtained from in and ex situ warming studies. In situ warming via the translocation of intact soil mesocosms was carried out along an elevation gradient ranging between ca. 1250 m in the Kibale Forest to ca. 3000 m in the Rwenzori Mountains in Uganda. Samples from the same transect were also warmed ex situ. Ex situ results revealed that, following the elevation gradient, which represents a natural climate gradient, specific heterotrophic CO2 respiration decreased linearly by 1.01 ± 0.12 µgCh-1g-1 of SOC per 100 m of elevation increase. The coefficient of temperature sensitivity increased from 1.50 ± 0.13 in the lowest- to 2.68 ± 0.25 in the highest-elevation cluster, showing a linear increase of 0.09 ± 0.03 per 100 m of elevation increase. Additionally, respired CO2 was more depleted in 13C in the warmer lower elevations as compared to colder higher elevations, with a linear decrease of 0.23 ‰ ± 0.04 ‰ per 100 m of elevation increase. Furthermore, the microbial community structure indicated a weak trend along the elevation gradient, with higher elevations more dominated by fungi relative to bacteria. The results indicate an increased recalcitrance and decreased mineralisation of SOC, with elevation likely driven by decreasing soil temperature and pH. Subsequently, after 2 years of in situ warming (0.9 to 2.8 ∘C), specific heterotrophic SOC respiration tended to be lower for warmed soil compared to control soil. Furthermore, in warmed soils, δ13C values and SOC content tended to increase and decrease, respectively. Collectively, this points towards the increased mineralisation and depletion of readily available C during 2 years of warming. In conclusion, our results suggest that climate warming may trigger enhanced losses of SOC from tropical montane forests due to a combination of a higher temperature sensitivity of mineralisation and higher SOC content at higher elevations.

Variation in seed number/mass trade-off for Pinus yunnanensis along a natural climate gradient in the mountains of Southwest China

Variation in seed number/mass trade-off for Pinus yunnanensis along a natural climate gradient in the mountains of Southwest China

Response of Afromontane soil organic carbon, nitrogen, and phosphorus to in situ experimental warming along an elevational gradient

Tropical montane forests store large amounts of carbon (C), nitrogen (N), and phosphorus (P) in soil. These soil C, N, and P pools are vulnerable to increased losses due to the increasing local temperatures. To gain better insight into the effects of climate warming on biogeochemistry in montane forests in Africa, we established study plots along a natural climate gradient in Uganda between 1,250 and 3,000 m in the Rwenzori Mountains. We studied soil C, N, and P contents as well as 13C and 15N isotopic compositions and leaf nutrient contents. Further, we simulated climate warming by 0.9°C–2.8°C for 2 years by conducting in situ soil mesocosms translocation downslope. The results revealed that, along the elevational gradient, soil organic C increased six-fold from 2.6 ± 1.0% at 1,250–1,300 m to 16.0 ± 1.9% at 2,700–3,000 m, with a linear increase of 0.94% per 100 m of elevation increase. Similarly, soil total N increased five-fold, from 0.3 ± 0.1% to 1.3 ± 0.1%, with a linear increase of 0.07% per 100 m of elevation increase. Further, soil bio-available P increased three-fold, from 9.6 ± 5.2 mg kg−1 to 29.5 ± 3.0 mg kg−1, with a linear increase of 1.4 mg kg−1 per 100 m of elevation increase. Soil δ15N decreased linearly by 0.39‰ per 100 m of elevation increase, ranging from 8.9 ± 0.2‰ to 2.9 ± 0.7‰ at 1,250–1,300 m and 2,700–3,000 m, respectively. The accumulation of soil organic C and total N with elevation is due to slow microbial activity under lower temperature. Indeed, the soil δ15N indicated a more closed N cycling with increasing elevation. However, despite the increasing trend in soil C and nutrient status with elevation, leaf N and P contents decreased linearly with elevation. This is likely due to the impairment of nutrient uptake under low temperature and low pH. In addition, following 2 years of in situ soil warming, we found that the soil δ13C and δ15N values relatively increased. Generally, the results imply that warming triggered accelerated transformation processes of accrued soil organic matter.

Extreme climate events limit northern range expansion of wild turkeys.

For species inhabiting areas at the limit of their environmental tolerance, extreme events often drive population persistence. However, because extreme events are uncommon, their effects on population dynamics of expanding species are poorly known. We examined how extreme climate events in winter and summer affected three populations of wild turkeys occupying a natural climate gradient at the northern edge of their range. First, we examined the mechanism by which vital rates affect the population growth rate. Second, we developed a climate-dependent structured population model. Finally, by linking this population model to IPCC-class climate projections, we projected wild turkey population abundance in response to the frequency of extreme snow events by 2100 for the northernmost population. We showed that the population dynamics of the three populations is driven through different pathways expected from the theory of invading population dynamics; that those populations were mainly limited by heavy snow that decreases winter survival by restraining food access; and that a population of immigrant is projected to decline at the northern species range. This study exemplifies how extreme events affect population dynamics and range expansion of temperate species at the northern edge of the distribution.

Integrative effects of increasing aridity and biotic cover on soil attributes and functioning in coastal dune ecosystems

Climate change-driven increases in aridity will lead to dryland expansion worldwide. In the Atlantic and Mediterranean biogeographic regions, coastal dunes are priority conservation areas highly sensitive to aridification where plants and biological soil crusts may play a key role maintaining ecosystem services. However, we still need further insights into the balance between the effects of increasing aridity and the influence of local vegetation on the soil attributes and functioning of these systems, particularly in transitional areas between humid and arid climatic zones. We sampled 24 dune systems distributed along a natural climate gradient including humid, dry-subhumid and arid environments in the Atlantic coastline of the Iberian Peninsula and investigated (i) how aridity drives the vegetation community structure, soil biotic and abiotic properties and functions in dune ecosystems at a regional scale; and (ii) to what extent soil attributes differ locally under diverse surface microsites (i.e. plants, biocrust and bare soil). Increasing aridity in coastal dunes decreased total biotic cover and favoured the formation of shrub-dominated patches. Vegetation shifts were accompanied by declines in soil microbial diversity, organic matter, nutrient contents and reduced rates of N mineralization. Soil bacterial communities were more sensitive to aridity than fungal communities. Microsites were essential regulators of soil attributes, and their effects were dependent on the degree of aridity. In drylands, shrub microsites were associated to higher soil organic C, microbial (bacterial and fungal) abundance, fungal diversity and potential heterotrophic respiration than bare microsites, likely due to enhanced litter and sediment redistribution. However, at humid and transition systems, microsites controlled soil N pools and microbial diversity, suggesting tight linkages between vegetation patches and soil microorganisms responsible for N availability. These findings provide evidence of the strong negative impacts of aridification on vegetation attributes and nutrient cycling in coastal dunes and highlight the role of the biotic cover in preserving soil fertility, microbial diversity and functioning, particularly in the most arid sites.

Ant foraging strategies vary along a natural resource gradient

Food selection by foragers is sensitive to the availability of resources, which may vary along geographical gradients. Hence, selectivity of food types by foragers is expected to track these resource gradients. Here we addressed this hypothesis and asked if foraging decisions of seed‐eating ants differ along a geographic gradient of habitat productivity. The study was carried out for two years at five sites along a natural climatic gradient, ranging from arid to Mediterranean, where plant productivity varies six‐fold across a short geographic distance of 250 km. We found that in ant colonies of the genus Messor, collective foraging decisions differed along the gradient. Specifically, at the high‐productivity sites, a stronger association was found between plant seed availability and selectivity, suggesting that colonies respond more accurately to within‐patch variation in food amounts. In contrast, colonies in low‐productivity sites foraged in patches with higher concentration of seeds, suggesting that they respond more accurately to among‐patch variation in food amounts. Moreover, at the high‐productivity sites, colonies were more discriminating in their choice of food and preferred bigger seeds, while in the low‐productivity sites, where smaller seeds were relatively more abundant, food collection depended mostly on seed availability. An experiment with artificial seed patches performed along the same climatic gradient, revealed no difference in food selectivity across sites when food type and availability were similar, and a general preference for bigger over medium‐sized seeds. Overall, our findings suggest that resource availability is an important factor explaining food choice along a climatic gradient and imply that in low‐productivity regions small‐seeded species incur high predation pressure, whereas in high‐productivity regions, large‐seeded species suffer higher predation. This could have important consequences for plant species composition, particularly at the face of climate change, which could dramatically alter the foraging decisions of granivores.

Effects of rainfall manipulations versus a natural aridity gradient on plant litter arthropods in desert and Mediterranean ecosystems

Predicting responses of litter arthropods to future variation in precipitation is critical for understanding decomposition processes under global climate change. A field experiment on litter arthropods extracted from litterbags using Berlese-Tullgren funnels was conducted under simulated drought and increasing precipitation treatments at a semiarid site and a Mediterranean site, and along a natural climatic gradient from arid to mesic Mediterranean ecosystems during three consecutive years. Our objective was to examine the effects of variation in water availability on the abundance of litter arthropods along the aridity gradient. The results demonstrated positive responses of total abundances, including Acari abundance, but negative responses of taxonomic richness to experimentally increased precipitation at the semiarid site and the Mediterranean site. Collembolan abundance responded positively while the Shannon index responded negatively to experimental precipitation at the Mediterranean site only. However, total abundance and collembolan abundance were found to decrease across a mean annual precipitation (MAP) gradient from the arid site to the mesic Mediterranean site. Quantile regression for linear mixed-effects models demonstrated a negative effect of MAP on Acari abundance at the medium level of Acari abundance but weak positive effects of MAP on taxonomic richness and the Shannon index. In conclusion, we found that the distribution of abundance was a poor proxy for predicting the response of the studied litter arthropod assembly to spatial variation in precipitation.

Peatland Microbial Community Composition Is Driven by a Natural Climate Gradient.

Peatlands are important players in climate change-biosphere feedbacks via long-term net carbon (C) accumulation in soil organic matter and as potential net C sources including the potent greenhouse gas methane (CH4). Interactions of climate, site-hydrology, plant community, and groundwater chemical factors influence peatland development and functioning, including C dioxide (CO2) and CH4 fluxes, but the role of microbial community composition is not well understood. To assess microbial functional and taxonomic dissimilarities, we used high throughput sequencing of the small subunit ribosomal DNA (SSU rDNA) to determine bacterial and archaeal community composition in soils from twenty North American peatlands. Targeted DNA metabarcoding showed that although Proteobacteria, Acidobacteria, and Actinobacteria were the dominant phyla on average, intermediate and rich fens hosted greater diversity and taxonomic richness, as well as an array of candidate phyla when compared with acidic and nutrient-poor poor fens and bogs. Moreover, pH was revealed to be the strongest predictor of microbial community structure across sites. Predictive metagenome content (PICRUSt) showed increases in specific genes, such as purine/pyrimidine and amino-acid metabolism in mid-latitude peatlands from 38 to 45° N, suggesting a shift toward utilization of microbial biomass over utilization of initial plant biomass in these microbial communities. Overall, there appears to be noticeable differences in community structure between peatland classes, as well as differences in microbial metabolic activity between latitudes. These findings are in line with a predicted increase in the decomposition and accelerated C turnover, and suggest that peatlands north of 37° latitude may be particularly vulnerable to climate change.

Carbon response to changing winter conditions in northern regions: current understanding and emerging research needs

Winter is an important period for ecological processes in northern regions; however, compared to other seasons, the impacts of winter climate on ecosystems are poorly understood. In this review we evaluate the influence of winter climate on carbon dynamics based on the current state of knowledge and highlight emerging topics and future research challenges. Studies that have addressed this topic include plot-scale snow cover manipulation experiments that alter soil temperatures, empirical investigations along natural climatic gradients, laboratory temperature incubation experiments aimed at isolating influential factors in controlled environments, and time series of climate and carbon data that evaluate long-term natural variation and trends. Combined, these studies have demonstrated how winter climate can influence carbon in complex ways that in some cases are consistent across studies and in other cases are difficult to predict. Despite advances in our understanding, there is a great need for studies that further explore: (i) carry-over effects from one season to another, (ii) ecosystem processes in the fall–winter and winter–spring shoulder seasons, (iii) the impacts of extreme events, (iv) novel experimental approaches, and (v) improvements to models to include ecological effects of winter climate. We also call for the establishment of an international winter climate change research network that enhances collaboration and coordination among studies, which could provide a more thorough understanding of how the snow-covered period influences carbon cycling, thereby improving our ability to predict future responses to climate change.

Leaf spectral responses of Poa crymophila to nitrogen deposition and climate change on Qinghai-Tibetan Plateau

Temperature and nitrogen (N) are limited factors on the Qinghai-Tibetan Plateau (QTP). Climate change and increasing N deposition on the QTP have posed threats to the sustainability of alpine ecosystems. To examine plant spectral responses to N deposition and climate change, we measured leaf spectral reflectance of Poa crymophila, a common forage species grown in alpine meadow (AM) and alpine steppe (AS) on the eastern QTP under four N addition treatments (CK, 8, 40, 72 kg N ha −1 yr −1). We used natural climate gradients from colder and drier AS to warmer and wetter AM to simulate the climate change on the QTP. Results showed that both N deposition and climate change had significant effects on entire visible and near-infrared regions (p < 0.05), but the interactive effects were insignificant (p> 0.05). Enhanced visible absorption, increased near-infrared reflection, a red shift for red edge and a blue shift for green peak were observed with increasing N addition levels. The modified red edge simple ratio index and photochemical reflective index increased gradually in response to N addition (p < 0.05). These responses indicated that N deposition may make leaf wider and thicker, increase leaf chlorophyll content and photochemical efficiency, and enhance photosynthetic capacity. Climate warming and wetting had opposite effects from N deposition on leaf reflectance of Poa crymophila. These observations showed the potential to monitor the status of Poa crymophila under future climate change and N deposition with leaf spectral measurements.

Soil organic carbon components in inner Mongolian shrub-encroached grasslands

Shrub encroachment has profound influences on regional carbon cycling. However, few studies have examined the changes in soil organic carbon (SOC) components at the molecular level along a climate gradient. In this study, we aimed to investigate the effects of biotic and abiotic factors on the patterns of SOC components in the shrub patches and the grassy matrix. We analyzed the distribution and controlling factors of SOC components (including free lipids, bound lipids, and lignin-derived phenols) in the topsoil of shrub-encroached grasslands along natural climate gradients in Inner Mongolia, China. We found that the concentrations of bound lipids and lignin-derived phenols were significantly higher and the vanillic acid to vanillin ratio ((Ad/Al)v) was significantly lower in the shrub patches than in the grassy matrix (p < 0.05). After excluding variables exhibiting collinearity, redundancy analysis showed that shrub patch cover and soil pH were the most important variables that influenced SOC composition in the shrub patches, while herb characteristics and shrub density were the most important in the grassy matrix. Structural equation modeling showed that shrub characteristics at the plot scale greatly contributed to the variance in all components in the grassy matrix, whereas soil properties were more important in the shrub patches. Our results highlight that although the topsoil carbon content did not change, shrub encroachment altered the SOC components and their drivers in the Inner Mongolian grasslands.

Ant personalities and behavioral plasticity along a climatic gradient

Variable environments can select for behavioral plasticity. Yet, along environmental gradients, personalities (consistent differences in behavior) and behavioral syndromes (associations between behavioral traits) can arise both across and within populations, which point to limits in behavioral plasticity. To date, little is known on how behavioral plasticity and animal personalities are associated along geographical gradients. Hereto, we subjected colonies of the ant Temnothorax longispinosus from populations along a natural climatic gradient to temperature manipulations and behavioral experiments. We asked whether personalities and behavioral syndromes stem from local adaptations or are plastic responses and whether the extent of behavioral plasticity differs between populations from warmer versus colder habitats. Our results reveal that colonies responded to warmer temperatures with increased explorative and brood care behavior. Interestingly, these behavioral traits shifted in the same direction as a syndrome previously found along this climatic gradient, where colonies from warmer environments displayed more exploration and brood care. Additionally, colonies that exhibited lower aggression but more exploration prior to temperature manipulations showed the highest behavioral plasticity. Moreover, only in one behavioral trait, exploration, colonies from colder sites exhibited higher plasticity than those from warmer sites. This suggests that ant behaviors are influenced mostly by prevailing conditions with a limited influence of past experience on behavioral plasticity. Our study proposes that associations between behavioral traits can be modified by climatic conditions, thus providing insights into the complex ways animals respond to current and past environments. Behavioral plasticity and animal personalities could be linked to climate. Yet few studies have investigated behavioral associations along geographical gradients. Hence, we still lack an understanding of how current and past environmental conditions affect such behavioral associations along gradients. Here, we asked whether the extent of behavioral plasticity differs between populations from warmer versus colder sites along a climatic gradient and whether animal personalities stem from local adaptations or plastic responses. We demonstrate in a social insect that associations between behavioral traits can be modified by temperature. Specifically, in response to temperature changes, behavioral traits shifted in a direction consistent with variation in a syndrome across a climatic gradient. This study provides unique empirical support linking animal personalities and phenotypic plasticity across populations along climatic gradients, thus offering new insights into the complex ways animals respond to current conditions and past experience.

Elevation shapes the reassembly of Anthropocene lizard communities.

Human impacts, especially land-use change, are precipitating biodiversity loss. Yet anthropogenic drivers are layered atop natural biogeographic gradients. We ask whether the effects of anthropogenic habitat conversion depend on climatic context. We studied the structure of Anolis lizard communities in intact and human-modified habitats across natural climate gradients in the northern Dominican Republic. Using community-wide mark-resight methods to control for detection bias, we show that the effects of habitat conversion reverse with elevation (and thus macroclimate temperature). Deforestation reduces abundance and biomass in lowland communities but has no such effect at high elevations. In contrast, forest loss results in no compositional change in the lowlands, but complete community turnover between habitats in the highlands. These contrasting community-level patterns emerge from consistent responses of individual species based on their thermal niches. Community reorganization in the highlands stems from thermal niche tracking and habitat switching by abundant lowland species. We find no support for the hypothesis that climate generalists outperform specialists to succeed in anthropogenic habitats. Instead, warm-climate specialists dominate anthropogenic habitats, even in cool macroclimates. Human impacts interact with pre-existing environmental gradients to reorganize biodiversity. Leveraging a biogeographic perspective will provide insight into the future communities of life on Earth.

Divergent Patterns of Selection on Crassulacean Acid Metabolism Photosynthesis in Contrasting Environments

Premise of research. Crassulacean acid metabolism (CAM), a photosynthetic pathway that evolved in response to aridity, is beneficial for plant species in arid and semiarid environments, with positive effects on plant fitness. However, since it implies a higher metabolic cost compared with that for C3 plants, its expression under moist conditions, where it might be less necessary, could have fewer positive or even negative effects on plant fitness. Variations of the effects of CAM expression under different moisture regimes have never been addressed. Our objectives were to determine whether there are such variations and, if so, their possible ecological and evolutionary implications.Methodology. We measured CAM expression (as nocturnal acidification, ∆H+) and moisture-related morphological leaf traits, estimating their impact on reproductive effort in two populations of Puya chilensis living under contrasting moisture regimes in a natural climatic gradient in central Chile. To evaluate the effects of the measured traits on fitness, a phenotypic selection analysis was performed.Pivotal results. Phenotypic selection analysis showed that ∆H+ was the only trait significantly associated with relative fitness in both populations. There is a significant “switch” in the direction of selection between both studied populations: directional selection on CAM was positive in the dry population and negative in the moist population.Conclusions. Results suggest that under moist regimes the expression of CAM by P. chilensis could be removed by natural selection. To our knowledge, this is the first report of a negative effect of CAM on plant fitness. Since CAM is not usually studied in moist environments, we believe this pattern might be more frequent than expected. Further research on this issue is needed, especially in the context of current climate change.

Reduced snow cover alters root-microbe interactions and decreases nitrification rates in a northern hardwood forest.

Snow cover is projected to decline during the next century in many ecosystems that currently experience a seasonal snowpack. Because snow insulates soils from frigid winter air temperatures, soils are expected to become colder and experience more winter soil freeze-thaw cycles as snow cover continues to decline. Tree roots are adversely affected by snowpack reduction, but whether loss of snow will affect root-microbe interactions remains largely unknown. The objective of this study was to distinguish and attribute direct (e.g., winter snow- and/or soil frost-mediated) vs. indirect (e.g., root-mediated) effects of winter climate change on microbial biomass, the potential activity of microbial exoenzymes, and net N mineralization and nitrification rates. Soil cores were incubated in situ in nylon mesh that either allowed roots to grow into the soil core (2mm pore size) or excluded root ingrowth (50μm pore size) for up to 29months along a natural winter climate gradient at Hubbard Brook Experimental Forest, NH (USA). Microbial biomass did not differ among ingrowth or exclusion cores. Across sampling dates, the potential activities of cellobiohydrolase, phenol oxidase, and peroxidase, and net N mineralization rates were more strongly related to soil volumetric water content (P<0.05; R2 =0.25-0.46) than to root biomass, snow or soil frost, or winter soil temperature (R2 <0.10). Root ingrowth was positively related to soil frost (P<0.01; R2 =0.28), suggesting that trees compensate for overwinter root mortality caused by soil freezing by re-allocating resources towards root production. At the sites with the deepest snow cover, root ingrowth reduced nitrification rates by 30% (P<0.01), showing that tree roots exert significant influence over nitrification, which declines with reduced snow cover. If soil freezing intensifies over time, then greater compensatory root growth may reduce nitrification rates directly via plant-microbe N competition and indirectly through a negative feedback on soil moisture, resulting in lower N availability to trees in northern hardwood forests.

Water balance creates a threshold in soil pH at the global scale.

Soil pH regulates the capacity of soils to store and supply nutrients, and thus contributes substantially to controlling productivity in terrestrial ecosystems. However, soil pH is not an independent regulator of soil fertility-rather, it is ultimately controlled by environmental forcing. In particular, small changes in water balance cause a steep transition from alkaline to acid soils across natural climate gradients. Although the processes governing this threshold in soil pH are well understood, the threshold has not been quantified at the global scale, where the influence of climate may be confounded by the effects of topography and mineralogy. Here we evaluate the global relationship between water balance and soil pH by extracting a spatially random sample (n = 20,000) from an extensive compilation of 60,291 soil pH measurements. We show that there is an abrupt transition from alkaline to acid soil pH that occurs at the point where mean annual precipitation begins to exceed mean annual potential evapotranspiration. We evaluate deviations from this global pattern, showing that they may result from seasonality, climate history, erosion and mineralogy. These results demonstrate that climate creates a nonlinear pattern in soil solution chemistry at the global scale; they also reveal conditions under which soils maintain pH out of equilibrium with modern climate.

Nitrate and dissolved organic carbon mobilization in response to soil freezing variability

Reduced snowpack and associated increases in soil freezing severity resulting from winter climate change have the potential to disrupt carbon (C) and nitrogen (N) cycling in soils. We used a natural winter climate gradient based on elevation and aspect in a northern hardwood forest to examine the effects of variability in soil freezing depth, duration, and frequency on the mobilization of dissolved organic carbon (DOC) and nitrate (NO3 −) in soils over the course of 2 years. During a winter with a relatively thin snowpack, soils at lower elevation sites experienced greater freezing and especially variable freeze/thaw cycles, which in turn led to greater leaching of DOC from the organic horizon during the following growing season. In contrast to several previous field manipulation studies, we did not find changes in soil solution NO3 − concentrations related to soil freezing variables. Our results are consistent with a soil matrix disturbance from freezing and thawing which increases leachable C. These results build upon previous laboratory experiments and field manipulations that found differing responses of DOC and NO3 − following soil freezing, suggesting that mobilization of labile C may suppress NO3 − losses through microbial immobilization of N. This research highlights the importance of studying natural variation in winter climate and soil freezing and how they impact soil C and N retention, with implications for surface water runoff quality.

Variation in plant functional traits across and within four species of Western Australian Banksia (Proteaceae) along a natural climate gradient

AbstractPlant traits are fundamental components of the ecological strategies of plants, relating to how plants acquire and use resources. Their study provides insight into the dynamics of species geographical ranges in changing environments. Here, we assessed the variation in trait values at contrasting points along an environmental gradient to provide insight into the flexibility of species response to environmental heterogeneity. Firstly, we identified how commonly measured functional traits of four congeneric species (Banksia baxteri, B. coccinea, B. media and B. quercifolia) varied along a longitudinal gradient in the South Western Australian Floristic Region. This regional gradient provides significant variation in moisture, temperature and soil nutrients: soil nitrogen content decreases with declining rainfall and increasing temperature. We hypothesized that (i) the regional pattern in trait–environment associations across the species would match those observed on a global scale and (ii) that the direction and slopes of the within‐species relationships would be similar to those across species for each of the measured traits. Along the regional gradient we observed strong shifts in trait values, and cross‐species relationships followed the expected trend: specific leaf area was significantly lower, and leaf Narea and seed dry mass significantly higher, at the drier end of the rainfall gradient. However, traits within species were generally not well correlated with habitat factors: we found weak patterns among populations, either due to the small ecological gradient or perhaps because fine‐scale structuring among populations (at a micro‐evolutionary scale) was low due to high gene flow within species. Understanding how species traits shift as a result of climatic influences, both at the regional (across species) and local (within species) scale, provides insight into plant adaptation to the environment. Such studies have important applications for conservation biology and population management in the face of global change.

Seasonal and Regional Controls of Phytoplankton Production along a Climate Gradient in South-West Greenland During Ice-Cover and Ice-Free Conditions

ABSTRACTAcross a small geographic area (<180 km), the region of South-West Greenland covers a natural climate gradient. Variation in temperature and precipitation result in marked differences in limnology at three discrete locations: ice sheet margin, inland, and the coast. Replicate lakes from each location were sampled for physical (temperature, light), chemical (dissolved oxygen, pH, conductivity, nutrients), and biological (chlorophyll a [Chl a], photosynthetic pigments) variables on three occasions within a 12-month period: July–August 2010, April–May 2011, and June–July 2011 spanning ice cover. Variation in ice phenology was linked to the climate gradient; however, phytoplankton production and community composition did not differ regionally. Large-scale seasonal fluctuations in temperature and nutrient availability were the strongest predictors of phytoplankton production, with a shift from nitrate to phosphorus controlled production between ice-cover and ice-free conditions. Underlying seasonal drivers, variables predicting production were unique to each location—ice sheet margin (soluble reactive phosphorus), inland (temperature), and coast (silicate)—and reflect local differences in nutrient availability. Results from the current study have important consequences when controls over phytoplankton production in Arctic lakes are inferred from a limited number of sites, but up-scaled to represent pan-Arctic trends.

Temperature, precipitation and biotic interactions as determinants of tree seedling recruitment across the tree line ecotone.

Seedling recruitment is a critical life history stage for trees, and successful recruitment is tightly linked to both abiotic factors and biotic interactions. In order to better understand how tree species' distributions may change in response to anticipated climate change, more knowledge of the effects of complex climate and biotic interactions is needed. We conducted a seed-sowing experiment to investigate how temperature, precipitation and biotic interactions impact recruitment of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) seedlings in southern Norway. Seeds were sown into intact vegetation and experimentally created gaps. To study the combined effects of temperature and precipitation, the experiment was replicated across 12 sites, spanning a natural climate gradient from boreal to alpine and from sub-continental to oceanic. Seedling emergence and survival were assessed 12 and 16 months after sowing, respectively, and above-ground biomass and height were determined at the end of the experiment. Interestingly, very few seedlings were detected in the boreal sites, and the highest number of seedlings emerged and established in the alpine sites, indicating that low temperature did not limit seedling recruitment. Site precipitation had an overall positive effect on seedling recruitment, especially at intermediate precipitation levels. Seedling emergence, establishment and biomass were higher in gap plots compared to intact vegetation at all temperature levels. These results suggest that biotic interactions in the form of competition may be more important than temperature as a limiting factor for tree seedling recruitment in the sub- and low-alpine zone of southern Norway.