Recent reports on local extinctions of arthropod species1 and of massive declines in arthropod biomass 2 point to land-use intensification as a major driver of decreasing biodiversity. However, there are no multi-site time-series of arthropod occurrences across land-use intensity gradients to confirm causal relationships. Moreover, it remains unclear which land-use types and arthropod groups are affected and whether the observed declines in biomass and diversity are linked to one another and continue. Here we analyzed arthropod data on more than 1 million individuals and 2,700 species from standardized inventories from 2008 to 2017 at 150 grassland and 140 forest sites in three regions of Germany. Overall gamma diversity in grasslands and forests decreased over time indicating loss of species across sites and regions. In annually sampled grasslands, biomass, abundance and species number of arthropods declined by 67%, 78%, and 34%, respectively. The decline was consistent across trophic levels, mainly affected rare species, and its magnitude was independent of local land-use intensity. However, sites embedded in landscapes with higher cover of agricultural land showed a stronger temporal decline. In 30 forest sites with annual inventories, biomass and species number, but not abundance, decreased by 41% and 36%, respectively. This was supported by analyses of all forest sites sampled in 3year intervals. The decline affected rare and abundant species and trends differed across trophic levels. Our results show that there are widespread declines in arthropods that concern biomass, abundance and diversity across trophic levels. Declines in forests demonstrate that arthropod loss is not restricted to open habitats. Our results 4 suggest that major drivers of arthropod decline act at larger spatial scales, and are, at least for grasslands, associated with agriculture at the landscape level.This implies that land-use relevant policies need to address the landscape scale to mitigate negative effects of land-use practices. Main textMuch of the debate on the human-induced biodiversity crisis has focused on vertebrates 3 , yet population decline and extinctions may be even more substantial in small organisms such as terrestrial arthropods 4 . Recent studies report declines in biomass of flying insects 2 , diversity of insect pollinators 5,6 , butterflies and moths 1,7-10 , hemipterans 11,12 and beetles 7,13,14 . Owing to the associated negative effects on food webs 15 , ecosystem functioning and ecosystem services 16 , the insect loss has spurred an intense public debate. However, time-series data on arthropods are rather limited and studies have so far focused on a small range of taxa 11,13,14 , few land-use and habitat types 12 or even on single sites 1,17 . In addition, many studies lack species information 2 or high temporal resolution 2,12 . Hence, it remains unclear whether reported declines in arthropods are a general phenomenon that is driven by similar mechanisms across land-use types, taxa and functional groups.The ...

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Biodiversity experiments have shown that species loss reduces ecosystem functioning in grassland. To test whether this result can be extrapolated to forests, the main contributors to terrestrial primary productivity, requires large-scale experiments. We manipulated tree species richness by planting more than 150,000 trees in plots with 1 to 16 species. Simulating multiple extinction scenarios, we found that richness strongly increased stand-level productivity. After 8 years, 16-species mixtures had accumulated over twice the amount of carbon found in average monocultures and similar amounts as those of two commercial monocultures. Species richness effects were strongly associated with functional and phylogenetic diversity. A shrub addition treatment reduced tree productivity, but this reduction was smaller at high shrub species richness. Our results encourage multispecies afforestation strategies to restore biodiversity and mitigate climate change.

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Through complementarity interactions, mixedspecies forests can be more productive than monocultures, and this effect can increase with tree-species richness. However, this is not always the case. This review examines the processes and stand structural attributes that can influence diversity-productivity relationships (DPRs); how they influence resource availability, resource uptake, and resource-use efficiency; and also describes some important differences between tree-diversity versus grassland-diversity experiments. The size of the complementarity effects caused by these processes and stand structures varies along spatial and temporal gradients in resource availability and climate. These spatial and temporal dynamics have now been examined in many studies, and the general patterns are summarized using a simple framework; complementarity is predicted to increase as the availability of resource BX^declines (or climatic condition X becomes harsher) if the species interactions improve the availability, uptake, or use efficiency of resource X (or interactions improve climatic condition X). Importantly, this framework differs from the stress-gradient hypothesis to account for a wider range of inter-specific plant interactions (not only facilitation) by considering contrasting methods used to quantify species interactions while accounting for stand structure. In addition, complementarity (as opposed to facilitation) for a given species combination can increase as growing conditions improve in forests, contrary to predictions of the stressgradient hypothesis with regards to facilitation. This review indicates that while the effects of tree-species diversity on growth and other forest functions are now receiving a lot of attention, far less is known about the effects of structural diversity on growth or forest functioning. Direct measurements of the processes, as opposed to focusing mainly on growth responses, could greatly contribute to our understanding of structural diversity effects.

Mixed-species plantations of Eucalyptus with a nitrogen (N 2 ) fixing species have the potential to increase productivity while maintaining soil fertility, compared to Eucalyptus monocultures. However, it is difficult to predict combinations of species and sites that will lead to these benefits. We review the processes and interactions occurring in mixed plantations, and the influence of species or site attributes, to aid the selection of successful combinations of species and sites. Successful mixtures, where productivity is increased over that of monocultures, have often developed stratified canopies, such that the less shade-tolerant species overtops the more shade-tolerant species. Successful mixtures also have significantly higher rates of N and P cycling than Eucalyptus monocultures. It is therefore important to select N 2 -fixing species with readily decomposable litter and high rates of nutrient cycling, as well as high rates of N 2 -fixation. While the dynamics of N 2 -fixation in tree stands are not well understood, it appears as though eucalypts can benefit from fixed N as early as the first or second year following plantation establishment. A meta-analysis of 18 published studies revealed several trials in which mixtures were significantly (P < 0.001) more productive than monocultures, and no instances in which mixtures were less productive than monocultures. Regression analyses of such data were more informative than indices of relative yield, and were more informative in trials that contrasted four or more different species compositions. Thus replacement series examining compositions of 100:0, 67:33, 33:67, and 0:100 were more informative than minimalist 100:0, 50:50 and 0:100 series. #

Wood represents the defining feature of forest systems, and often the carbon in woody debris has a long residence time. Globally, coarse dead wood contains 36-72 Pg C, and understanding what controls the fate of this C is important for predicting C cycle responses to global change. The fate of a piece of wood may include one or more of the following: microbial decomposition, combustion, consumption by insects, and physical degradation. The probability of each fate is a function of both the abiotic environment and the wood traits of the species. The wood produced by different species varies substantially in chemical, micro-and macro-morphological traits; many of these characteristics of living species have 'afterlife' effects on the fate and turnover rate of dead wood. The colonization of dead wood by microbes and their activity depends on a large suite of wood chemical and anatomical traits, as well as whole-plant traits such as stem-diameter distributions. Fire consumption is driven by a slightly narrower range of traits with little dependence on wood anatomy. Wood turnover due to insects mainly depends on wood density and secondary chemistry. Physical degradation is a relatively minor loss pathway for most systems, which depends on wood chemistry and environmental conditions. We conclude that information about the traits of woody plants could be extremely useful for modeling and predicting rates of wood turnover across ecosystems. We demonstrate how this trait-based approach is currently limited by oversimplified treatment of dead wood pools in several leading global C models and by a lack of quantitative empirical data linking woody plant traits with the probability and rate of each turnover pathway. Explicitly including plant traits and woody debris pools in global vegetation climate models would improve predictions of wood turnover and its feedback to climate.