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This range expansion experiment was installed in the summer of 2021. We experimentally simulated the range expansion of a subalpine buttercup, Trollius albiflorus, by transplanting six adults into the west edge of the ITEX global change experimental plots, for a total of 288 transplants. To parse apart abiotic and biotic drivers on range expansion, we manipulated half the transplants to ‘reduce below-ground biotic interactions’ using PVC pipe vs ‘control’ biotic conditions where below-ground interactions we left intact. To record above-ground interactions, starting in 2022 we recorded neighborhood percent cover at the species-level around each Trollius albiflorus transplant using a 10 cm circular hoop. Starting in 2023, we quantified soil conditions surrounding each transplant by recording soil moisture (%VWC) and soil temperature (Celsius). 

Abstract For decades, community ecologists have examined how diversity varies with ecosystem productivity. Despite this long history, tests of hypothesized mechanisms, namely the interplay between environmental filtering, biotic interactions, and dispersal, are lacking, largely due to the intractability of using traditional approaches. Across a productivity gradient in a serpentine grassland (California, USA), for four annual plant species, we coupled local productivity estimates, occupancy surveys, and measures of persistence tested on transplants under natural conditions and when interactions with neighbors were experimentally reduced. We found a positive effect of productivity on diversity (i.e., the proportion of our focal species occupying a location) despite strong competition limiting species persistence in productive environments. Additionally, across species and for the community, we found a strong mismatch between species occupancy versus persistence, largely due to dispersal excess causing sink populations with negative growth rates. Our results suggest that diversity–productivity relationships can be largely driven by dispersal and its interactive effects with local biotic and abiotic conditions. 

ABSTRACT Global change drivers alter multiple components of community composition, with cascading impacts on ecosystem stability. However, it remains largely unknown how interactions among global change drivers will alter community synchrony, especially across successional timescales. We analysed a 22‐year time series of grassland community data from Cedar Creek, USA, to examine the joint effects of pulse soil disturbance and press nitrogen addition on community synchrony, richness, evenness and stability during transient and post‐transient periods of succession. Using multiple regression and structural equation modelling, we found that nitrogen addition and soil disturbance decreased both synchrony and stability, thereby weakening the negative synchrony–stability relationship. We found evidence of the portfolio effect during transience, but once communities settled on a restructured state post‐transience, diversity no longer influenced the synchrony–stability relationship. Differences between transient and post‐transient drivers of synchrony and stability underscore the need for long‐term data to inform ecosystem management under ongoing global change. 

Abstract Global change is altering patterns of community assembly, with net outcomes dependent on species' responses to the abiotic environment, both directly and mediated through biotic interactions. Here, we assess alpine plant community responses in a 15‐year factorial nitrogen addition, warming and snow manipulation experiment. We used a dynamic competition model to estimate the density‐dependent and ‐independent processes underlying changes in species‐group abundances over time. Density‐dependent shifts in competitive interactions drove long‐term changes in abundance of species‐groups under global change while counteracting environmental drivers limited the growth response of the dominant species through density‐independent mechanisms. Furthermore, competitive interactions shifted with the environment, primarily with nitrogen and drove non‐linear abundance responses across environmental gradients. Our results highlight that global change can either reshuffle species hierarchies or further favour already‐dominant species; predicting which outcome will occur requires incorporating both density‐dependent and ‐independent mechanisms and how they interact across multiple global change factors.