Questions: Trait differentiation among species occurs at different spatial scales within a region. How does the partitioning of functional diversity help to identify different community assembly mechanisms?Location: Northeastern Spain.Methods: Functional diversity can be partitioned into within-community (a) and among-communities (b) components, in analogy to Whittaker's classical a and b species diversity concept. In light of ecological null models, we test and discuss two algorithms as a framework to measure a and b functional diversity (the Rao quadratic entropy index and the variance of trait values). Species and trait (specific leaf area) data from pastures under different climatic conditions in NE Spain are used as a case study.Results: The proposed indices show different mathematical properties but similarly account for the spatial components of functional diversity. For all vegetation types along the climatic gradient, the observed a functional diversity was lower than expected at random, an observation consistent with the hypothesis of trait convergence resulting from habitat filtering. On the other hand, our data exhibited a remarkably higher functional diversity within communities compared to among communities (a ) b). In contrast to the high species turnover, there was a limited functional diversity turnover among communities, and a large part of the trait divergence occurred among coexisting species.Conclusions: Partitioning functional diversity within and among communities revealed that both trait convergence and divergence occur in the formation of assemblages from the local species pool. A considerable trait convergence exists at the regional scale in spite of changes in species composition, suggesting the existence of ecological redundancy among communities.

Motivation The Tundra Trait Team (TTT) database includes field‐based measurements of key traits related to plant form and function at multiple sites across the tundra biome. This dataset can be used to address theoretical questions about plant strategy and trade‐offs, trait–environment relationships and environmental filtering, and trait variation across spatial scales, to validate satellite data, and to inform Earth system model parameters. Main types of variable contained The database contains 91,970 measurements of 18 plant traits. The most frequently measured traits (> 1,000 observations each) include plant height, leaf area, specific leaf area, leaf fresh and dry mass, leaf dry matter content, leaf nitrogen, carbon and phosphorus content, leaf C:N and N:P, seed mass, and stem specific density. Spatial location and grain Measurements were collected in tundra habitats in both the Northern and Southern Hemispheres, including Arctic sites in Alaska, Canada, Greenland, Fennoscandia and Siberia, alpine sites in the European Alps, Colorado Rockies, Caucasus, Ural Mountains, Pyrenees, Australian Alps, and Central Otago Mountains (New Zealand), and sub‐Antarctic Marion Island. More than 99% of observations are georeferenced. Time period and grain All data were collected between 1964 and 2018. A small number of sites have repeated trait measurements at two or more time periods. Major taxa and level of measurement Trait measurements were made on 978 terrestrial vascular plant species growing in tundra habitats. Most observations are on individuals (86%), while the remainder represent plot or site means or maximums per species. Software format csv file and GitHub repository with data cleaning scripts in R; contribution to TRY plant trait database (www.try-db.org) to be included in the next version release.

Summary1. Nutrient additions often result in species dominance/compositional changes in wetland ecosystems, but the impact of nutrients may be constrained by different salinity levels. Wetlands of northern Belize, distributed along a salinity gradient, are strongly phosphorus-limited and dominated largely by three species of emergent macrophytes: Eleocharis cellulosa Torr., Cladium jamaicense Crantz and Typha domingensis Pers. 2. We conducted a mesocosm experiment to assess changes in growth characteristics [biomass allocation, plant height, relative growth rate (RGR), rhizome length] and nutrient uptake of these three species in response to simultaneous changes in levels of nutrients (nitrogen and phosphorus) and salinity. 3. The growth characteristics of Typha and Eleocharis responded positively to N and especially P addition, whereas the growth response of Cladium was largely insignificant. The RGR of Typha increased under P addition, while RGR of Eleocharis increased with N and decreased with salinity addition. Nutrient addition increased the rhizome number of both Typha and Eleocharis . However, plasticity in rhizome length was observed only in Typha , which showed increased rhizome length at medium and high P. 4. Salinity decreased plant height and shoot and root biomass of Cladium and Eleocharis , while in Typha it reduced only height. Rhizome number and length were decreased only in Eleocharis . 5. Both medium and high P additions increased tissue P content in all three species, but Eleocharis accumulated significantly more P than Cladium and Typha . Nitrogen additions increased tissue N content in Cladium and Eleocharis , but not in Typha . 6. Cladium exhibited strong morphological constraint and behaved as a stress-tolerator that was well adapted to low nutrients. Typha -characterized by its plastic, opportunistic guerrilla growth strategy, fast and efficient space occupancy, and rather wasteful nutrient management -behaved as a typical competitor. Eleocharis responded rapidly to nutrients but displayed limited rhizome plasticity, and its growth was affected at higher salinity. 7. According to recorded traits, we hypothesize that P input into wetlands will result in expansion of Typha , leading to competitive exclusion of both co-occurring species. The only conditions allowing coexistence of all three species are those limiting vertical and horizontal growth of Typha : low P and higher salinity. To ensure the stability of Belizean wetlands, the maintenance of oligotrophic status is therefore crucial.

Ungulate trampling modifies soils and interlinked ecosystem functions across biomes. Until today, most research has focused on temperate ecosystems and mineral soils while trampling effects on cold and organic matter‐rich tundra soils remain largely unknown. We aimed to develop a general model of trampling effects on soil structure, biota, microclimate and biogeochemical processes, with a particular focus on polar tundra soils. To reach this goal, we reviewed literature about the effects of trampling and physical disturbances on soils across biomes and used this to discuss the knowns and unknowns of trampling effects on tundra soils. We identified the following four pathways through which trampling affects soils: (a) soil compaction; (b) reductions in soil fauna and fungi; (c) rapid losses in vegetation biomass and cover; and (d) longer term shifts in vegetation community composition. We found that, in polar tundra, soil responses to trampling pathways 1 and 3 could be characterized by nonlinear dynamics and tundra‐specific context dependencies that we formulated into testable hypotheses. In conclusion, trampling may affect tundra soil significantly but many direct, interacting and cascading responses remain unknown. We call for research to advance the understanding of trampling effects on soils to support informed efforts to manage and predict the functioning of tundra systems under global changes. A free Plain Language Summary can be found within the Supporting Information of this article.

Snow is an important driver of ecosystem processes in cold biomes. Snow accumulation determines ground temperature, light conditions and moisture availability during winter. It also affects the growing season’s start and end, and plant access to moisture and nutrients. Here, we review the current knowledge of the snow cover’s role for vegetation, plant-animal interactions, permafrost conditions, microbial processes and biogeochemical cycling. We also compare studies of natural snow gradients with snow manipulation studies, altering snow depth and duration, to assess time scale difference of these approaches. The number of studies on snow in tundra ecosystems has increased considerably in recent years, yet we still lack a comprehensive overview of how altered snow conditions will affect these ecosystems. In specific, we found a mismatch in the timing of snowmelt when comparing studies of natural snow gradients with snow manipulations. We found that snowmelt timing achieved by manipulative studies (average 7.9 days advance, 5.5 days delay) were substantially lower than those observed over spatial gradients (mean range of 56 days) or due to interannual variation (mean range of 32 days). Differences between snow study approaches need to be accounted for when projecting snow dynamics and their impact on ecosystems in future climates.

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