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

Abstract

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 in situ. 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 3000 m 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 (1000 m) 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 3000 m 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.

Leaf trait variation in species-rich tropical Andean forests

“…Leaf N/P ratios above 16 for most tree species regardless of elevation indicate the limiting role of P for tree growth in the study area 39 . N and P fertilization experiments at the study sites 40 , 41 , the observed large increase in tree root/shoot ratio along the slope 42 , and in situ measurements of nitrogen mineralization rate and P availability in the soil 43 support this assumption. One plausible explanation is that the temperature decrease is hampering soil biological activity, thereby reducing organic matter decomposition and mineralisation rate.…”

Topography as a factor driving small‐scale variation in tree fine root traits and root functional diversity in a species‐rich tropical montane forest

“…Even though several microclimatic, edaphic and hydrologic factors are known to vary along topographical gradients (Takyu et al ., 2003), our PCA of soil variables suggested that variation in the availability of all five plant macronutrients (N, P, Ca, K, Mg) is a major factor differentiating between lower slope and upper slope positions. The results of a fertilisation experiment in the montane forests of the study region revealed a general co‐limitation of tree growth by nitrogen and phosphorus (Homeier et al ., 2012; Cárate‐Tandalla et al ., 2018). Furthermore, nitrogen availability has been shown to decrease greatly towards the upper slope (Wolf et al ., 2011).…”

Moderate phosphorus additions consistently affect community composition of arbuscular mycorrhizal fungi in tropical montane forests in southern Ecuador

“…N and P availability tends to increase in the pre-montane forest relative to the lower and upper montane forests (Wolf et al, 2011;Werner & Homeier, 2015). Despite of this, tree growth at all sites is predominantly limited by P availability (Graefe et al, 2010;C arate-Tandalla et al, 2018).…”