Abstract
AbstractQuestionsNutrient 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.LocationSavannas and tropical dry forests of India.MethodsIn 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.ResultsN‐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.ConclusionsNutrient 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.
Highlights
IntroductionIncreased atmospheric deposition of nitrogen (N) and phosphorus (P) as a consequence of human activities is an important global change driver (Vitousek 1994; Vitousek et al 1997; Sala et al 2000; Falkowski et al 2000; Galloway et al 2004, 2008; Phoenix et al 2006, 2012; Dentener et al 2006; Mahowald et al 2008; Du and Liu 2014; Vet et al 2014) that has been shown to modify vegetation communities in a number of ecosystems (Elser et al 2007; Xia and Wan 2008; Bobbink et al 2010)
Consistent increases in stem diameter growth rates of the fastest growing individuals of non-N-fixing species allowed these seedlings to attain larger stem diameters, which in turn, increased their survival probabilities following fires
N-fixing species were largely unaffected by either fire or nutrient addition, with virtually all individuals surviving, the increased growth and survival probabilities of the fastest growing non-N-fixers with nutrient addition suggests that non-N-fixers may benefit from increased nutrient deposition in the future
Summary
Increased atmospheric deposition of nitrogen (N) and phosphorus (P) as a consequence of human activities is an important global change driver (Vitousek 1994; Vitousek et al 1997; Sala et al 2000; Falkowski et al 2000; Galloway et al 2004, 2008; Phoenix et al 2006, 2012; Dentener et al 2006; Mahowald et al 2008; Du and Liu 2014; Vet et al 2014) that has been shown to modify vegetation communities in a number of ecosystems (Elser et al 2007; Xia and Wan 2008; Bobbink et al 2010). The availability of resources such as water and nutrients, as well as disturbances, such as fire and herbivory, strongly influence the extent of woody cover and tree species diversity in these communities (Sankaran et al 2004, 2005, 2008; Lehmann et al 2009a, 2014; Staver et al 2009; Dantas et al 2015) They are amongst the most frequently burned biomes in the world (Dwyer et al 1998, 2000; Barbosa et al 1999; Tansey et al 2004; Bond et al 2005), and in the wetter regions of these biomes (e.g. in Africa, where annual precipitation > 650 mm), grass-fuelled fires are the single most important constraint to the recruitment of trees into the adult canopy layer (Higgins et al 2000; Sankaran et al 2005; Bond 2008)
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