Climate extremes are on the rise. Impacts of extreme climate and weather events on ecosystem services and ultimately human well-being can be partially attenuated by the organismic, structural, and functional diversity of the affected land surface. However, the ongoing transformation of terrestrial ecosystems through intensified exploitation and management may put this buffering capacity at risk. Here, we summarise the evidence that reductions in biodiversity can destabilise the functioning of ecosystems facing climate extremes. We then explore if impaired ecosystem functioning could, in turn, exacerbate climate extremes. We argue that only a comprehensive approach, incorporating both ecological and hydrometeorological perspectives, enables to understand and predict the entire feedback system between altered biodiversity and climate extremes. This ambition, however, requires a reformulation of current research priorities to emphasise the bidirectional effects that link ecology and atmospheric processes.

INTRODUCTIONCompelling evidence shows that biodiversity enhances essential ecosystem functions, such as productivity and decomposition rates (Loreau & Hector 2001; Hooper et al., 2005; Cardinale et al., 2012). One primary underlying reason may be that individual species or groups of species in different functional groups may have dissimilar niches (niche complementarity effects ) which allow diverse communities to maximize resource utilization and minimize competition (Cardinale et al., 2011; Zuppinger-Dingley et al., 2014). In theory, such niche differences include temporal variation in biological activity (Ebeling et al., 2014), and species in a community can adjust the timing of their biological activity in such a way that they cover the longest possible time and/or use the resources from the largest possible space in the habitat. If phenological niche differences are high enough, they can affect the shape of the phenology at the community level. For instance, if a plant community is composed of species that grow in early spring, the aboveground growing season will be extended, compared with a community lacking those species (Ebeling et al., 2014; Rudolf, 2019). Therefore, species and functional group diversity can affect the timing of community-level productivity (i.e. community phenology) via temporal niche differentiation and/or increasing the probability of species with those traits to occur in the community (selection effect ) (Loreau & Hector 2001). However, variation in phenology is primarily monitored at the species rather than community level. Moreover, phenological variation is typically attributed to changes in climate drivers, such as temperature and water supply (Wright and van Schaik 1994; Staggemeier et al., 2018), and has rarely been quantified as a response to changes in biodiversity (but see Wolf et al., 2017 and Guimarães-Steinike et al., 2019).Most ecosystem processes are soil-related or even soil-dependent (Bardgett & van der Putten, 2014; Soliveres et al., 2016; Schuldt et al., 2018). However, phenology tends to be monitored on easily observed aboveground response variables, and evidence describing soil phenology is mostly lacking (Bonato Asato et al., 2023). This knowledge gap leads to uncertainty about how well soil properties and belowground processes (i.e. root growth and activity of soil organisms) are predicted by aboveground phenological strategies (Eisenhauer, 2012; Blume-Werry et al., 2015; Eisenhauer et al., 2018). Because shoots and roots are interdependent, tight synchrony of their responses to environmental drivers is often expected (Iversen et al., 2015; but see Blume-Werry et al., 2016). However, the role of biotic and abiotic constraints on this synchrony seems to vary significantly among ecosystems and plant types, ultimately affecting which organs grow first, faster, or remain active and alive longer. Moreover, plant (roots and shoots) processes are often assumed to indicate ecosystem functions driven by the activity of organisms at adjacent trophic levels, such as soil fauna, but this may not necessarily be the case. Hot moments (within-year events inducing high activity) in soil organism activity depend, in part, on inputs from root exudates or pulses of detrital inputs from senescent roots (Kuzyakov & Blagodatskaya 2015). However, the limited evidence from the field does not always confirm plant-activity-based assumptions. For example, phenological monitoring of detritivore feeding activity during the growing season in oaks has shown both a negative and no correlation between feeding activity and oak branch production (Eisenhauer et al., 2018). In an experimental grassland, feeding activity rates decreased during the summer, when plant growth is usually high (Siebert et al., 2019; Sünnemann et al., 2021). Evidence suggests that investments in shoot and root production are commonly not synchronous (e.g. Steinaker & Wilson 2008; Steinaker et al. 2010; Sloan et al. 2016; Blume-Werry et al. 2016), as well as the dynamics of soil organisms (Bonato Asato et al., 2023; Eisenhauer et al., 2018). However, we lack experimental evidence demonstrating whether changes in biodiversity may influence the predictability and synchronization of the dynamics above and below the ground.Presently, two predominant conceptual frameworks delineate the interplay between biodiversity and the synchronization of ecosystem functions. On the one hand, ecosystem stability theory suggests that increasing biodiversity increases temporal asynchrony among populations and functions, which would be one of the primary mechanisms for positive diversity-stability relationships (Cardinale et al., 2013; Loreau & de Mazancourt 2013). In other words, temporal asynchrony is needed for a healthy (stable) ecosystem functioning. On the other hand, ecosystem coupling, as defined by Ochoa-Hueso et al. (2021) as ”the orderly connections between the biotic and abiotic components of ecosystems across spaces and/or time”, suggests the opposite: for more efficiently process, cycle, and transfer of energy and matter, a higher temporal coupling of populations and functions is needed. Under this point of view, temporal synchrony is required for more efficient ecosystem functioning, and monitoring the dynamics of one function or population can be used as an indicator of activity in the other. In both cases, disruptions such as biodiversity change, may affect key aboveground or belowground processes, leading to acceleration or delay of community phenology and desynchronization of ecosystem functions. Despite the potential importance of aboveground-belowground phenological synchrony, the current lack of studies concurrently monitoring shoot, root, and soil fauna dynamics has impeded a thorough understanding of the mechanisms by which changes in biological diversity may influence the responses of these affiliated processes.Here, we examine how experimentally manipulated plant diversity influences the phenological patterns of shoot, root, and soil fauna dynamics (responses). In the framework of a long-term grassland biodiversity experiment (the Jena Experiment; Roscher et al. 2014; Weisser et al. 2017), using well-established methods (LiDAR, phenological cameras, minirhizotrons, bait-lamina strips), we measure ecosystem response variables that are often used to evaluate aboveground-belowground ecosystem functioning and biological activity in annual plant communities (e.g. plant community height, greenness, root production, and detritivore feeding activity) every two to three weeks over four seasons (one full year). We used these data to calculate yearly values for each response variable, phenological patterns, and synchrony between response variables. With this approach, we ask the following questions:1) How does plant diversity affect the yearly accumulated values of aboveground plant traits and belowground activity? We expect that increasing plant diversity throughout the year enhances all response variables (Weisser et al. 2017; Mommer et al., 2015; Eisenhauer et al., 2010).2) Does plant diversity affect intra-annual aboveground and belowground phenological patterns? We predict that plant community shoot dynamics will be concentrated in spring and summer, as usual in temperate regions. Root production should last longer than that of shoots, as found in other studies (Steinaker & Wilson 2008; Blume-Werry et al. 2016), even though it is not clear if this longer activity is driven by an earlier start of the production, a later end, or both. For detritivore feeding activity, we expect a peak in early spring due to high moisture and increased temperature and another peak in autumn, driven by the increased availability of resources by above- and belowground plant-derived inputs and high moisture.3) Do changes in plant diversity affect the synchrony of shoot, root, or soil organism dynamics? We expect plant species richness and functional group richness to enhance aboveground-belowground activity, which could lead to either more or less synchronized patterns. If plant diversity drives enhanced functioning at different time points (e.g. advances plant growth and delays root senescence), we could see a negative diversity effect on synchrony.4) Does the time of year influence the strength/direction/predictability of relationships between aboveground-belowground response variables? Because plant shoots are only active for a restricted period, we expect plant diversity effects to be most pronounced during the growing season (Guimarães-Steinike et al., 2019), while abiotic constraints might mostly drive belowground dynamics out of the growing season.

The capacity of forests to sequester carbon in both above- and belowground compartments is a crucial tool to mitigate rising atmospheric carbon concentrations. Belowground carbon storage in forests is strongly linked to soil microbial communities that are the key drivers of soil heterotrophic respiration, organic matter decomposition, and thus nutrient cycling. However, the relationships between tree diversity and soil microbial properties such as biomass and respiration remain unclear with inconsistent findings among studies. It is unknown so far how the spatial configuration and soil depth affects the relationship of tree richness and microbial properties. Here, we studied the spatial distribution of soil microbial properties in the context of a tree diversity experiment by measuring soil microbial biomass and respiration in subtropical forests (BEF-China experiment). We sampled soil cores at two depths at five locations along a spatial transect between the trees in mono- and heterospecific tree pairs of the native deciduous species Liquidambar formosana and Sapindus saponaria. Our analyses showed decreasing soil microbial biomass and respiration with increasing soil depth and distance from the tree in monospecific tree pairs. We calculated belowground overyielding of soil microbial biomass and respiration - which is a higher microbial biomass or respiration than expected from the monocultures - and analysed the distribution patterns along the transect. We found no general overyielding across all sampling positions and depths. Yet, we encountered a spatial pattern of microbial overyielding with a significant microbial overyielding close to L. formosana trees and microbial underyielding close to S. saponaria trees. We found similar spatial patterns across microbial properties and depths that only differed in their effect size. Our results highlight the importance of small-scale variations of tree-tree interaction effects on soil microbial communities and functions and are calling for better integration of within-plot variability to understand biodiversity-ecosystem functioning relationships.

Anthropogenic global changes are impacting biodiversity, however, many previous meta-analyses investigating the impact of different global changes on biodiversity have omitted soil fauna, or are limited in the scope of the global changes studied. Threats to soil biodiversity by global changes need to be understood to mitigate effects on ecosystem services provided by soils. We conducted a meta-analysis using 3,173 effect sizes from 627 publications focused on six global changes (climate change, land-use intensification, pollution, nutrient enrichment, invasive species, and habitat fragmentation) and their associated environmental stressors on soil fauna. We classified stressors as either pulse (short-term, acute) or press (long-term, chronic) stressors, and expected pulse stressors to have less impact on soil biodiversity due to buffering effects of the soil. Unexpectedly, pollution caused the largest loss in soil fauna communities, which is worrying due to continually increasing levels of pollution, as well as the poor mechanistic understanding of pollution impacts. There was no clear pattern of pulse stressors having a smaller impact on soil biodiversity than press stressors. Overall, this work shows the importance of including soil biodiversity in large-scale global change analyses, as soil organisms often do not show the same responses as organisms above-ground.

Plant and soil biodiversity can have significant effects on herbivore resistance mediated by plant metabolites. Here, we disentangled the independent effects of plant diversity and soil legacy on constitutive and herbivore-induced plant metabolomes of three plant species in two complementary microcosm experiments. First, we grew plants in sterile soil with three different plant diversity levels. Second, single plant species were grown on soil with different plant diversity-induced soil legacies. We infested a subset of all plants with Spodoptera exigua larvae, a generalist leaf-chewing herbivore, and assessed foliar and root metabolomes. Neither plant diversity nor soil legacy had significant effects on overall foliar, root, or herbivore-induced metabolome composition. Herbivore-induced metabolomes, however, differed from those of control plants. We also detected 139 significantly regulated metabolites by comparing plants grown in monocultures with conspecifics growing in plant or soil legacy mixtures. Moreover, plant-plant and plant-soil interactions regulated 141 metabolites in herbivore-induced plants. Taken together, plant diversity and soil legacy independently alter the concentration and induction of plant metabolites, thus affecting the plant's defensive capability. This is a first step towards disentangling plant and soil biodiversity effects on herbivore resistance, thereby improving our understanding of the mechanisms that govern ecosystem functioning.

Plant monocultures growing for extended periods face severe losses of productivity. This phenomenon, known as ‘yield decline’, is often caused by the accumulation of above- and belowground plant antagonists. The effectiveness of plant defences against antagonists might help explaining differences in yield decline among species. Using a trait-based approach, we studied the role of 20 physical and chemical defence traits of leaves and fine roots on yield decline of 18-year old monocultures of 27 grassland species. We hypothesized that yield decline is lower for species with high defences, that root defences are better predictors of yield decline than leaf defences, and that in roots, physical defences better predict yield decline than chemical defences, while the reverse is true for leaves. We additionally hypothesized that species increasing the expression of defence traits after long-term monoculture growth would suffer less yield decline. We summarized leaf and fine root defence traits using principal component analysis and analysed the relationship between defence traits mean as a measure of defence strenght and defence traits temporal changes of the most informative components and monoculture yield decline. The only significant predictors of yield decline were the mean and temporal changes of the component related to specific root length and root diameter (e.g. the so called collaboration gradient of the root economics space). The principal component analysis of the remaining traits showed strong trade-offs between defences suggesting that different plant species deploy a variety of strategies to defend themselves. This diversity of strategies could preclude the detection of a generalized correlation between the strength and temporal changes of defence gradients and yield decline. Our results show that yield decline is strongly linked to belowground processes particularly to root traits. Further studies are needed to understand the mechanism driving the effect of the collaboration gradient on yield decline.

Biological invasions have major impacts on a variety of ecosystems and threaten native biodiversity. Earthworms have been absent from northern parts of North America since the last ice age, but non-native earthworms were recently introduced there and are now being spread by human activities. While past work has shown that plant communities in earthworm-invaded areas change towards a lower diversity mainly dominated by grasses, the underlying mechanisms related to changes in the biotic interactions of the plants are not well understood. Here, we used a trait-based approach to study the effect of earthworms on interspecific plant competition and aboveground herbivory. We conducted a microcosm experiment in a growth chamber with a full-factorial design using three plant species native to northern North American deciduous forests, Poa palustris (grass), Symphyotrichum laeve (herb), and Vicia americana (legume), either growing in monoculture or in a mixture of three. These plant community treatments were crossed with earthworm (presence or absence) and herbivore (presence or absence) treatments. Eight out of the eleven above- and belowground plant functional traits studied were significantly affected by earthworms, either by a general effect or in interaction with plant species identity, plant diversity level, and/or herbivore. Earthworms increased the aboveground productivity and the number of inflorescences of the grass P. palustris. Further, earthworms countervailed the increasing effect of herbivores on root tissue density of all species, and earthworms and herbivores individually increased the average root diameter of S. laeve in monoculture, but decreased it in mixture. In this study, earthworm presence gave a competitive advantage to the grass species P. palustris by inducing changes in plant functional traits. Our results suggest that invasive earthworms can alter competitive and multitrophic interactions of plants, shedding light on some of the mechanisms behind invasive earthworm-induced plant community changes in northern North America forests.

Ecosystem management aims at providing many ecosystem services simultaneously. Such ecosystem multifunctionality can be limited by trade-offs and increased by synergies among the underlying ecosystem functions (EF), which need to be understood to develop targeted management. Previous studies found differences in the correlation between EFs. We hypothesised that correlations between EFs are variable even under the controlled conditions of a field experiment and that seasonal and annual variation, plant species richness, and plot identity (identity effects of plant communities such as the presence and absence of functional groups and species) are drivers of these correlations. We used data on 31 EFs related to plants, consumers, and physical soil properties that were measured over 5 to 19 years, up to three times per year, in a temperate grassland experiment with 80 different plots, constituting six sown plant species richness levels (1, 2, 4, 8, 16, 60 species). We found that correlations between pairs of EFs were variable, and correlations between two particular EFs could range from weak to strong correlations or from negative to positive correlations among the repeated measurements. To determine the drivers of pairwise EF correlations, the covariance between EFs was partitioned into contributions from plant species richness, plot identity, and time (including years and seasons). We found that most of the covariance for synergies was explained by species richness (26.5%), whereas for trade-offs, most covariance was explained by plot identity (29.5%). Additionally, some EF pairs were more affected by differences among years and seasons and therefore showed a higher temporal variation. Therefore, correlations between two EFs from single measurements are insufficient to draw conclusions on trade-offs and synergies. Consequently, pairs of EFs need to be measured repeatedly under different conditions to describe their relationships with more certainty and be able to derive recommendations for the management of grasslands.

Species-rich communities exhibit higher levels of ecosystem functioning compared to species-poor ones, and this positive relationship strengthens over time. One proposed mechanism for this phenomenon is the reduction of niche overlap and thus, competition within plants or consumers. To explore how niche differentiation affects the plant diversity-productivity relationship, we combined bio-energetic population dynamics and food-web assembly models. Our findings reveal that increased niche complementarity of plants can steepen the relationship if it does not increase intraspecific competition, while increasing complementarity among consumers during community assembly can also have a positive effect but with considerable variability. The study highlights the importance of trait variation among and within species, and the interplay between intra- and interspecific competition strength in shaping the functioning of ecosystems over time. These results offer insights into the mechanisms behind the diversity-functioning relationship and have implications for ecosystem management and conservation efforts.

Global change drivers such as anthropogenic nutrient inputs simultaneously alter biodiversity, species composition, and ecosystem functions such as aboveground biomass. These changes are interconnected by complex feedbacks among extinction, colonization, and shifting relative abundance. Here, we use a novel temporal application of the Price equation to quantify the functional contributions of species that are lost, gained, and persist under ambient and experimental nutrient addition in 59 global grasslands. Under ambient conditions, compositional and biomass turnover was high, but species losses (i.e., local extinctions) were balanced by gains (i.e. colonization). There was biomass loss associated with species loss under fertilization. Few species were gained in fertilized conditions over time but those that were, and species that persisted, contributed to net biomass gains, outweighing biomass loss. These components of community change are key to understanding the relationship between change in composition, diversity and functioning.

Disturbances alter the diversity and composition of microbial communities, but whether microbiomes from different environments exhibit similar degrees of resistance or rates of recovery has not been evaluated. Here, we synthesized 86 time series of disturbed mammalian, aquatic, and soil microbiomes to examine how the recovery of microbial richness and community composition differed after disturbance. We found no general patterns in compositional variance (i.e., dispersion) in any microbiomes over time. Only mammalian microbiomes consistently exhibited decreases in richness following disturbance. Importantly, they tended to recover this richness, but not their composition, over time. In contrast, aquatic microbiomes tended to diverge from their pre-disturbance composition following disturbance. By synthesizing microbiome responses across environments, our study aids in the reconciliation of disparate microbial community assembly frameworks, and highlights the role of the environment in microbial community reassembly following disturbance.

Across the globe, ecological communities are confronted with multiple global environmental change drivers, and they are responding in complex ways ranging from behavioural, physiological, and morphological changes within populations to changes in community composition and food web structure with consequences for ecosystem functioning. A better understanding of global change-induced alterations of multitrophic biodiversity and the ecosystem-level responses in terrestrial ecosystems requires holistic and integrative experimental approaches to manipulate and study complex communities and processes above and below the ground. We argue that mesocosm experiments fill a critical gap in this context, especially when based on ecological theory and coupled with microcosm experiments, field experiments, and observational studies of macroecological patterns. We describe the design and specifications of a novel terrestrial mesocosm facility, the iDiv Ecotron. It was developed to allow the setup and maintenance of complex communities and the manipulation of several abiotic factors in a near-natural way, while simultaneously measuring multiple ecosystem functions. To demonstrate the capabilities of the facility, we provide a case study. This study shows that changes in aboveground multitrophic interactions caused by decreased predator densities can have cascading effects on the composition of belowground communities. The iDiv Ecotrons technical features, which allow for the assembly of an endless spectrum of ecosystem components, create the opportunity for collaboration among researchers with an equally broad spectrum of expertise. In the last part, we outline some of such components that will be implemented in future ecological experiments to be realized in the iDiv Ecotron. Key words: food webs, biodiversity and ecosystem functioning, mesocosms, biotic interactions, lysimeters, climate chambers