Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained <0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments). When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea). No significant effect of land-use on early stage litter decomposition was noted within the temperate biome. Our results indicate that multiple drivers are affecting early stage litter mass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed.

Late-spring frosts (LSFs) affect the performance of plants and animals across the world’s temperate and boreal zones, but despite their ecological and economic impact on agriculture and forestry, the geographic distribution and evolutionary impact of these frost events are poorly understood. Here, we analyze LSFs between 1959 and 2017 and the resistance strategies of Northern Hemisphere woody species to infer trees’ adaptations for minimizing frost damage to their leaves and to forecast forest vulnerability under the ongoing changes in frost frequencies. Trait values on leaf-out and leaf-freezing resistance come from up to 1,500 temperate and boreal woody species cultivated in common gardens. We find that areas in which LSFs are common, such as eastern North America, harbor tree species with cautious (late-leafing) leaf-out strategies. Areas in which LSFs used to be unlikely, such as broad-leaved forests and shrublands in Europe and Asia, instead harbor opportunistic tree species (quickly reacting to warming air temperatures). LSFs in the latter regions are currently increasing, and given species’ innate resistance strategies, we estimate that ∼35% of the European and ∼26% of the Asian temperate forest area, but only ∼10% of the North American, will experience increasing late-frost damage in the future. Our findings reveal region-specific changes in the spring-frost risk that can inform decision-making in land management, forestry, agriculture, and insurance policy.

Global patterns of regional (gamma) plant diversity are relatively well known, but whether these patterns hold for local communities, and the dependence on spatial grain, remain controversial. Using data on 170,272 georeferenced local plant assemblages, we created global maps of alpha diversity (local species richness) for vascular plants at three different spatial grains, for forests and non-forests. We show that alpha diversity is consistently high across grains in some regions (for example, Andean-Amazonian foothills), but regional ‘scaling anomalies’ (deviations from the positive correlation) exist elsewhere, particularly in Eurasian temperate forests with disproportionally higher fine-grained richness and many African tropical forests with disproportionally higher coarse-grained richness. The influence of different climatic, topographic and biogeographical variables on alpha diversity also varies across grains. Our multi-grain maps return a nuanced understanding of vascular plant biodiversity patterns that complements classic maps of biodiversity hotspots and will improve predictions of global change effects on biodiversity.

One of the most fundamental questions in ecology is how many species inhabit the Earth. However, due to massive logistical and financial challenges and taxonomic difficulties connected to the species concept definition, the global numbers of species, including those of important and well-studied life forms such as trees, still remain largely unknown. Here, based on global ground-sourced data, we estimate the total tree species richness at global, continental, and biome levels. Our results indicate that there are ∼73,000 tree species globally, among which ∼9,000 tree species are yet to be discovered. Roughly 40% of undiscovered tree species are in South America. Moreover, almost one-third of all tree species to be discovered may be rare, with very low populations and limited spatial distribution (likely in remote tropical lowlands and mountains). These findings highlight the vulnerability of global forest biodiversity to anthropogenic changes in land use and climate, which disproportionately threaten rare species and thus, global tree richness.

Questions: Is there a spatial pattern in the community structure (stem densities, species richness and species composition) of trees and shrubs during more than 100yr of heathland to forest succession? To what extent is community structure influenced by land-use history and distance to seed sources? Do these effects change in time? Location: A 350-ha heathland (NOrholm) in southwest Denmark was abandoned in 1895 and left for free succession. Prior to abandonment the heathland was under traditional management for centuries. Method: Trees and shrubs were recorded and measured in ten surveys spanning 91yr (1921-2012). In the first nine surveys, complete censuses were used, whereas 116 randomly placed plots (10-m radius) were used in the most recent survey. We used mixed models and different multivariate techniques (non-metric multidimensional scaling and permutational multivariate ANOVA) to analyse spatio-temporal patterns in stem densities, species richness and species composition and the effects of land-use history and distance to seed sources. Results: Tree and shrub densities increased exponentially over time and were consistently lower at longer distance from seed sources. Land-use history affected the rate of increase in tree densities, with lower encroachment rates at previously cultivated soils. Initially, species richness increased rapidly but leveled off in later successional stages. Richness was marginally higher at areas closer to seed sources, whereas it was not affected by land-use history. The succession showed a shift from early to mid-successional species over time; but distance to seed source determined which species were colonizing at early stages. Conclusion: Distance to seed source and land-use history can differentially affect the structure and spatial patterns of developing forest communities. Although both factors had long-lasting effects on rates of colonization, the spatial patterns of colonization and species composition were mainly determined by distance to seed sources. The importance of distance to seed sources became less over time, suggesting that dispersal is a stronger driver at early stages. After more than a century since abandonment, forest covers <30% of the area, indicating that lack of heathland management did not strongly promote tree colonization at this site

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Questions: How does heathland vegetation composition change during ca. 100yr of succession in the absence of management? Is succession divergent or mono-directional? Do soil conditions and land-use history explain the variation in vegetation dynamics? Is there evidence for alternative stable states? Location: NOrholm hede, a 350-ha heathland in southwest Denmark that was abandoned in 1895 and left to develop naturally via succession. Methods: Permanent vegetation inventory plots were established and have been revisited 11 times between 1921 and 2012. Soil conditions were recorded in 2012. We used clustering, linear mixed models, linear models, ANOVA and multivariate techniques (redundancy analysis and principal response curves) to investigate changes in the plant community and how differences in plant community composition related to soil conditions and disturbance history. Results: Ericaceous dwarf shrubs dominated most of the heathland initially. A dominance shift from dwarf shrubs to grasses occurred on about half of the plots. The other half of the plots remained dominated by dwarf shrubs, although Empetrum nigrum expanded at the expense of Calluna vulgaris. Lichen cover decreased dramatically across all plots. The divergent successional pattern was not explained by nutrient concentrations. Grasses mainly expanded in areas where they already had a substantial presence, and this initial presence was largely correlated with historical soil disturbance. Plots where dwarf shrubs remained dominant had a relatively thick O-horizon. Conclusions: Vegetation dynamics during heathland succession were not deterministically determined by soil conditions. Grass and tree expansion occurred slowly and dwarf shrub dominance was stable for more than 100yr on large parts of the heath, even in the absence of management. Management actions that disturb stable dwarf shrub vegetation may enhance grass and tree colonization

• Key message A dataset of forest resource projections in 23 European countries to 2040 has been prepared for forestrelated policy analysis and decision-making. Due to applying harmonised definitions, while maintaining country-specific forestry practices, the projections should be usable from national to international levels. The dataset can be accessed at https://doi.org/10.5061/dryad.4t880qh. The associated metadata are available at https://metadata-afs.nancy.inra.fr/ geonetwork/srv/eng/catalog.search#/metadata/8f93e0d6-b524-43bd-bdb8-621ad5ae6fa9.