Effective societal responses to rapid climate change in the Arctic rely on an accurate representation of region-specific ecosystem properties and processes. However, this is limited by the scarcity and patchy distribution of field measurements. Here, we use a comprehensive, geo-referenced database of primary field measurements in 1,840 published studies across the Arctic to identify statistically significant spatial biases in field sampling and study citation across this globally important region. We find that 31% of all study citations are derived from sites located within 50 km of just two research sites: Toolik Lake in the USA and Abisko in Sweden. Furthermore, relatively colder, more rapidly warming and sparsely vegetated sites are under-sampled and under-recognized in terms of citations, particularly among microbiology-related studies. The poorly sampled and cited areas, mainly in the Canadian high-Arctic archipelago and the Arctic coastline of Russia, constitute a large fraction of the Arctic ice-free land area. Our results suggest that the current pattern of sampling and citation may bias the scientific consensuses that underpin attempts to accurately predict and effectively mitigate climate change in the region. Further work is required to increase both the quality and quantity of sampling, and incorporate existing literature from poorly cited areas to generate a more representative picture of Arctic climate change and its environmental impacts.

Questions We investigated some commonly held assumptions of community assembly theory needed to provide accurate predictions of changes in plant species assemblages across environmental gradients or following environmental change. Do (1) dominant and subordinate species respond in the same way to changes in environmental variables; (2) plant species assemblages show higher interspecific than intraspecific trait responses; and (3) co‐existing dominant species differ in their responses to the same environmental variables? Location Islands in Lakes Uddjaure and Hornavan, northern Sweden. Methods We explored the responses of forest understorey vegetation assemblages to variation in environmental resources across a chronosequence of 30 lake islands that differ in fire history, above‐ground and below‐ground resource availability and species diversity. For one plot on each island, we measured specific leaf area, leaf dry matter content and foliar N and P of all dominant and subordinate understorey plant species to assess species‐specific and weighted and non‐weighted community‐level trait responses to variation across islands in all major local environmental drivers. Results Consistent with our expectations, we found that species responses to environmental conditions were not homogenous within assemblages, and that responses of dominant and subordinate species differed. Further, intraspecific variation was often an important component of local‐scale plant community‐level responses. Responses were often relatively consistent across species, but dominant species sometimes showed contrasting responses of the same trait to the same environmental factor. Finally, environmental factors that influenced community average trait values also affected functional diversity. Conclusions This study has shown that several common assumptions that underpin community assembly theory do not necessarily hold, and this can cause inaccuracies in predicting plant functional composition responses to changes in environmental variables. Because these assumptions are central to current models that predict vegetation responses to environmental change, it is crucial to further test in which particular environmental context and to what extent these assumptions are critical for model accuracy.

Considering intraspecific trait variability (ITV) in ecological studies has improved our understanding of species persistence and coexistence. These advances are based on the growing number of leaf ITV studies over local gradients, but logistical constraints have prevented a solid examination of ITV in root traits or at scales reflecting species’ geographic ranges. We compared the magnitude of ITV in above‐ and below‐ground plant organs across three spatial scales (biophysical region, locality and plot). We focused on six understorey species (four herbs and two shrubs) that occur both in disturbed and undisturbed habitats across boreal and temperate Canadian forests. We aimed to document ITV structure over broad ecological and geographical scales by asking: (a) What is the breadth of ITV across species range‐scale? (b) What proportion of ITV is captured at different spatial scales, particularly when local scale disturbances are considered? and (c) Is the variance structure consistent between analogous leaf and root traits, and between morphological and chemical traits? Following standardized methods, we sampled 818 populations across 79 forest plots simultaneously, including disturbed and undisturbed stands, spanning four biophysical regions (~5,200 km). Traits measured included specific leaf area (SLA), specific root length (SRL) and leaf and root nutrient concentrations (N, P, K, Mg, Ca). We used variance decomposition techniques to characterize ITV structure across scales. Our results show that an important proportion of ITV occurred at the local scale when sampling included contrasting environmental conditions resulting from local disturbance. A certain proportion of the variability in both leaf and root traits remained unaccounted for by the three sampling scales included in the design (36% on average), with the largest amount for SRL (54%). Substantial differences in magnitude of ITV were found among the six species, and between analogous traits, suggesting that trait distribution was influenced by species strategy and reflects the extent of understorey environment heterogeneity. Even for species with broad geographical distributions, a large proportion of within‐species trait variability can be captured by sampling locally across ecological gradients. This has practical implications for sampling design and trait selection for both local studies and continental‐scale modelling. A free Plain Language Summary can be found within the Supporting Information of this article.

Summary Determining the changes in within‐ and between‐species functional diversity in plant communities, and their contribution to overall species trait overlap, can enhance efforts at understanding mechanisms of species coexistence. However, little is known about how variation in species functional diversity influences variation in species trait overlap among contrasting environments. Here, we studied the understorey vegetation in a well‐characterized 5000‐year‐old chronosequence involving 30 forested islands that differ greatly in size, soil fertility and species diversity. Across this chronosequence, we expected consistent changes in both within‐ and between‐species functional diversity that would lead to decreasing overall species trait overlap with increasing successional age, species richness, understorey vegetation density and spatial heterogeneity of soil resources. For each island, we measured specific leaf area (SLA) of each of ten individuals of each plant species present. Using a variance decomposition method, we partitioned the total community functional diversity of SLA on each island into within‐ and between‐species functional diversity. Further, we estimated overall species trait overlap as the ratio of within‐species functional diversity to total functional diversity. Using regression analyses, we then explored relationships of within‐ and between‐species functional diversity, and of overall species trait overlap, with several environmental variables across the 30 islands. Consistent with our hypotheses, overall species trait overlap decreased with successional age due to a statistically significant decrease in within‐species functional diversity, and decreased with species richness due to a simultaneous decrease in within‐species functional diversity and increase in between‐species functional diversity. Against our predictions, overall species trait overlap increased in more competitive environments and did not change with increasing spatial heterogeneity of soil N or P. Synthesis. Our study suggests niche packing as a key mechanism for species coexistence in plant communities. Using SLA as an integrator of plant ecological strategy, we show that community successional age and species richness are significantly linked to trait space distribution of plant individuals of boreal forest understorey vegetation and therefore to local species coexistence. Our results also suggest that the trait space of dominant and subordinate species may respond differently to local environmental variables.

Premise of research. African grass communities are dominated by two distinct functional types: tall, caespitose bunch grasses and short, spreading lawn grasses. Functional type coexistence has been explained by differences in defoliation tolerance, because lawn grasses occur in intensively grazed areas while bunch grasses are less associated with heavy grazing. If different responses to tissue loss explain their distribution, expectations are that biomass production and leaf-level physiology will be negatively impacted in bunch relative to lawn grasses.Methodology. We tested the influence of defoliation on three lawn and three bunch grasses from Tanzania and South Africa by quantifying growth and measuring physiological response of these grasses to simulated herbivory in a glasshouse experiment. Specifically, we measured photosynthesis, transpiration, stomatal conductance, leaf dry matter content (LDMC), specific leaf area (SLA), leaf nitrogen, and leaf pigment concentrations in leaves of bunch and lawn grasses that were clipped or unclipped.Pivotal results. In contrast to our expectations, clipped lawn and bunch grasses did not differ in photosynthesis, leaf nitrogen, or biomass production, and both lawn and bunch grasses upregulated photosynthesis in response to clipping. However, defoliated bunch grasses had higher rates of stomatal conductance and transpiration compared with defoliated lawn grasses. Also, leaf carotenoid concentrations increased in response to clipping for both functional types but much more in bunch than in lawn grasses. An analysis of leaf-level physiological relationships with structural equation modeling showed that lawn and bunch grasses exert control over carbon gain in different ways. In bunch grasses, net carbon gain was associated with leaf-level structural properties (LDMC and SLA) that varied in response to defoliation, while in lawn grasses, increased carbon gain was the result of increased leaf [N] subsequent to defoliation.Conclusions. The varied responses of lawn and bunch grasses to defoliation appear to arise from their different investments in defense and carbon assimilation subsequent to defoliation. Bunch grasses invest relatively more in carotenoid production, likely as a mechanism to enhance regrowth and protect costly leaves from photodamage. Moreover, bunch grasses maintain efficient carbon assimilation by structural adjustments in leaves (decreasing LDMC subsequent to defoliation), while lawn grasses maintain efficient water use by increasing leaf [N] subsequent to defoliation. Thus, we conclude that a key difference between lawn and bunch grasses is not defoliation tolerance per se but physiological adaptations that constrain them to environments with different moisture availability subsequent to defoliation.