Where competition with established vegetation limits plant recruitment, fires create open conditions which can allow invasive species to establish. However, facilitation theory suggests that destruction of vegetation will restrict germination and survival. These hypotheses were tested in a long-term study of establishment by Pinus sylvestris2013 a globally important invasive 2013 on English temperate heaths. Pine seeds were sown at 10 sites which represented replicated stages of heath recovery after fire: from bare soil to dense, tall vegetation. Seedling emergence varied among stages from 3% to 36% and was closely related to vegetation density, increasing with lower shrub height. The same pattern was evident within sites where seedlings emerged mostly at sowing locations with little or no vegetation. Yearly seedling survival showed a quadratic relationship with vegetation density over the first 4 years; with extremely high (> 95%) mortality in both very open and very dense conditions, but low mortality (< 30%) under intermediate cover. Over the subsequent 5 years seedling mortality did not differ among stages or with vegetation density. The initial conditions after fire constrain invasion because high emergence is countered by high mortality. However, the moderate shrub density during the first decade post-fire enhances P. sylvestris seedling survival and so promotes invasion. Invasibility is low in dense mature heath, but becomes high as shrubs collapse in degenerate heath which increases seedling emergence and survival. Synthesis. This study provides evidence that facilitation can occur in temperate regions, and that the plant interactions within a heathland community are a balance of competition and facilitation. This balance is governed by vegetation structure and plant ontogeny. Dominance of competition in more dense vegetation and for seedling emergence changed to facilitative effects in less dense vegetation and for older seedlings.
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