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.

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.

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.

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.

1. Plant-soil feedback (PSF) has gained attention as a mechanism promoting plant growth and coexistence. However, because most PSF research has measured monoculture growth in greenhouse conditions, field-based PSF experiments remain an important frontier for PSF research. 2. Using a four-year, factorial field experiment in Jena, Germany, we measured the growth of nine grassland species on soils conditioned by each of the target species (i.e., PSF). Plant community models were parameterized with or without these PSF effects, and model predictions were compared to plant biomass production in new and existing diversity-productivity experiments. 3. Plants created soils that changed subsequent plant biomass by 36%. However, because they were both positive and negative, the net PSF effect was 14% less growth on ‘home’ than ‘away’ soils. At the species level, seven of nine species realized non-neutral PSFs, but the two dominant species grew only 2% less on home than away soils. At the species*soil type level, 31 of 72 PSFs differed from zero. 4. In current and pre-existing diversity-productivity experiments, nine-species plant communities produced 37 to 29% more biomass than monocultures due primarily to selection effects. Null and PSF models predicted 29 to 28% more biomass for polycultures than monocultures, again due primarily to selection effects. 5. Synthesis: In field conditions, PSFs were large enough to be expected to cause roughly 14% overyielding due to complementarity, however, in plant communities overyielding was caused by selections effects, not complementarity effects. Further, large positive and large negative PSFs were associated with subdominant species, suggesting there may be selective pressure for plants to create neutral PSF. Broadly, results highlighted the importance of testing PSF effects in communities because there are several ways in which PSFs may be more or less important to plant growth in communities than suggested from simple PSF values.