Questions Nutrient deposition can modify plant growth and potentially alter the susceptibility of plants to disturbance events, while simultaneously influencing properties of the disturbance regimes themselves. With deposition rates accelerating, ecosystems such as savannas and tropical dry forests that are characterised by frequent disturbances such as fire, might be particularly sensitive to changes in nutrient availability. In these mixed tree-grass ecosystems, tree seedling growth rates strongly influence the ability of seedlings to survive fire, and hence, vegetation structure and tree community composition. However, the effects of nutrient deposition on the susceptibility of recruiting trees to fire remain poorly quantified. Here we ask: (a) how does nutrient deposition influence post-fire survival of tree seedlings, and hence, tree recruitment in savannas; and (b) whether the co-dominant functional groups (N-fixers and non-N-fixers) respond differently, signalling a potential for long-term changes in tree communities.

Location Savannas and tropical dry forests of India.

Methods In a field experiment, seedlings of four N-fixing and four non-N-fixing savanna tree species were grown for 18 months while exposed to a factorial combination of nitrogen (N) and phosphorus (P) addition. We quantified nutrient-mediated changes in mean seedling growth and growth of the fastest growing individuals. Seedlings were then exposed to a fire treatment at the end of 18 months, and post-fire seedling survival recorded after six months.

Results N-fixers showed substantially greater post-fire seedling survival compared to non-N-fixers in unfertilised treatments. Nutrient addition did not alter post-fire survival of N-fixers. However, fertilisation, especially with N, increased post-fire survival in non-N-fixers by increasing the growth of the fastest growing individuals.

Conclusions Nutrient deposition can dilute the relative advantage of N-fixing seedlings in terms of their post-fire survival, potentially leading to increased relative abundances of non-N-fixers in post-fire resprout communities, with long-term consequences for the composition of savanna tree communities.