Plant–plant interactions play a key and complex role in structuring vegetation communities, yet our understanding of their role as drivers of community dynamics is complicated by the tendency of these interactions to vary with respect to prevailing environmental conditions (Bonsall, van der Meijden & Crawley 2003). While there is a large body of research on the effect of changing environmental conditions on plant–plant interaction outcomes (e.g. the stress gradient hypothesis, Brooker et al. 2008) the difficulty in obtaining observational data on population dynamics of long-lived species (e.g. trees) impedes our ability to assess the effect of environmental changes on their interactions. Thus, most work on plant competition and facilitation has been focused on short-lived species (Callaway 2007); however, recent studies have extended the time frame over which direct plant interactions can be assessed up to decades by using dendroecological data (Soliveres et al. 2010). To expand further the length of the observational record beyond the age of living trees (i.e. centuries to millennia) requires the use of the fossil record such as palaeoecological pollen data.
Millennial-scale ecosystem dynamics – as reconstructed by the fossil record – often demonstrate nonlinear shifts in the trajectory of drivers of vegetation change such that discrete disturbance, stress and climatic regimes can be observed on either side of a breakpoint in the time-series data (i.e. a threshold, Willis et al. 2010). Such thresholds can set off cascading changes through ecosystems including shifts between alternative stable states in population and community dynamics (Scheffer & Carpenter 2003), yet they are often difficult to predict (Scheffer et al. 2009). Here, we aimed to assess how plant–plant interaction outcomes respond to abrupt environmental changes: do they demonstrate gradual responses or sharp shifts between alternative stable states?
We achieved this goal by using a model-fitting and model-selection analysis of multiproxy palaeoecological data to generate predicted changes in interaction outcomes between two putative competitors –Quercus sp. (oak) trees and Poaceae (grass) species (Davis et al. 1999) – over time and across abrupt changes in climate, fire patterns, ungulate herbivore density and nitrogen (N) availability. Our study site was a shifting mosaic of chalk grassland and open deciduous woodland in eastern England during the late-glacial/early post-glacial period (i.e. 13–7.6 k years bp). This period is known for numerous abrupt environmental changes including changes in climate (Steffensen et al. 2008), nutrient cycles (Wolfe, Edwards & Aravena 1999), soil types (Willis et al. 1997), mega-herbivore density (Barnosky et al. 2004) and fire level (Marlon et al. 2009). This study introduces a novel approach to the analysis of plant competition and facilitation that greatly extends the time scale over which we can assess variation in plant–plant interaction outcomes with environmental change between long-lived plant populations.