| Summary: | Summary:<ol><li>Plant-plant interactions are known to vary with changing environmental conditions; however, we have little empirical knowledge of the impact of abrupt environmental changes on millennial scale plant-plant interaction outcomes for long-lived plant species. Here, we used palaeoecological data (13-7.6 k years BP) and a novel statistical modelling approach to determine the impact of multiple environmental drivers on predicted tree-grass population interaction outcomes from our study site in eastern England.</li><li>Changes from high to low herbivore density shortly preceeded changes to low fire levels and a shift to warmer summers. These transitions occurred during a period of increasingly nitrogen (N) availability. Shortly thereafter, there was a shift in landscape dominance from grasses to oaks and then a change to decreasing N availability.</li><li>Model predictions of tree-grass interaction outcomes varied over time with respect to all environmental changes. During the time of high disturbances and cool summers, grasses were predicted to out-compete oaks. After climate warming and the loss of regular disturbances, the predicted outcome was stable coexistence. However, changes in the N cycle corresponded with different predicted outcomes: unstable competition under increasing N availability and facilitation of oaks by grasses when N availability was declining.</li><li>Akaike Information Criterion weights indicate that climate warming and fewer fires were consistent with the best-fitting model of oak-grass interactions for the entire time series (i.e. competitive exclusion to stable coexistence). However, reconciling the conflicting model predictions with the observed population dynamics suggests that a temporary period of unstable competition preceded the predicted shift to stable coexistence. This dynamic behaviour is consistent with known patterns of shifts between alternative stable states.</li><li>Synthesis. We show that abrupt changes in environmental conditions over time lead to similarly abrupt changes in tree-grass interaction outcomes, which were shown to vary in contrasting directions with respect to resource verus non-resource variables. The approach described here allows plant ecologists to test hypotheses of plant-plant interactions over successional time scales for long-lived species and thus can lead to new knowledge about the structural role of these interactions in community dynamics.</li><ol></ol></ol>
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