Identifying and modelling the determinants of woody plant invasion of lowland heath

1 The invasion of Betula spp. represents the moment of transition between European lowland heath and scrub ecosystems. We hypothesized that Betula invasion was controlled by a multivariate threshold, comprising factors that may be subdivided into seed and safe-site limitations, and that phosphorus availability was a key determinant of the Betula safe-site. 2 We performed a multifactorial field experiment in which seed rain, P-availability and disturbance were manipulated. All treatments had significant effects on Betula seedling densities, with seed availability proving the single greatest limitation. P addition emerged as having highly significant effects 4 and 12 months after germination. Disturbance had an initially large positive effect that dwindled across time. 3 Detailed descriptions of seedling density data were achieved by converting two factorial variables (seed rain and P availability) to a continuous form, including numerous covariates, and producing 'minimum adequate' generalized linear models (MAMs). These provided accurate descriptions of the data (explained deviance typically > 70%) and delimited site conditions where transition was likely. 4 Seed availability remained the most important factor, but the increased number of parameters in models describing densities of more mature seedlings suggested a temporal accumulation of recruitment limitations. Statistical modelling allowed for the subdivision of disturbance effects into those associated with a reduction in vegetation densities and the remaining 'direct' effects, e.g. soil disturbance, which shifted over time from being positive to negative. The results also support the hypothesis that soil phosphorus sorption capacity (PSC) determines heath-scrub transition as many of the identified determinants are controlled, either directly or indirectly, by PSC. 5 The study demonstrates that a combination of experimental and statistical modelling approaches can provide a detailed description of the factors controlling early stage invasion and may therefore have considerable utility in ecosystem management. Although site specific, the models illuminate the mechanisms by which many larger scale processes, e.g. burning and grazing, regulate the persistence of lowland heath ecosystems.