Contrasting species responses to continued nitrogen and phosphorus addition in tropical montane forest tree seedlings

Global changes in nutrient deposition rates are likely to have profound effects on plant communities, particularly in the nutrient-limited systems of the tropics. We studied the effects of increased nutrient availability on the seedlings of six tree species in montane forests of southern Ecuador insitu. After five years of continued N, P, or N+P addition, naturally grown seedlings of each of the two most common species at each elevation (1000, 2000, and 3000m asl) were harvested for analyses of leaf morphology, nutrient content, herbivory, and tissue biomass allocation. Most species showed increased foliar N and P concentrations after addition of each respective element. Leaf tissue N:P ratios of >20 in the control plants of all species suggest that P is more growth-limiting in these forests than N. Leaf morphological responses to nutrient addition were species and nutrient specific, with some species (Hedyosmum purparescens, Graffenrieda emarginata) exhibiting increased specific leaf area (SLA), and others (Graffenrieda harlingii) increased leaf area ratios (LAR). Pouteria torta (1000m) had lower SLA and LAR after P addition. Increased herbivory was only evident in G. emarginata (after N and N+P addition). Only the species from 3000m asl modified biomass allocation after nutrient addition. In general, N and N+P addition more strongly affected the species studied at the upper elevations, whereas P addition had a similar range of effects on the species at all elevations. We conclude that the responses of the studied tropical montane forest tree seedlings to chronic N and P addition are highly species-specific and that successful adaptation to increased nutrient availability will depend on species-specific morphological and physiological plasticity.