Performance of Seedlings of a Shade-Tolerant Tropical Tree Species after Moderate Addition of N and P

Abstract

Nitrogen deposition to tropical forests is predicted to increase in future in many regions due to agricultural intensification. We conducted a seedling transplantation experiment in a tropical premontane forest in Ecuador with a locally abundant late-successional tree species (Pouteria torta, Sapotaceae) aimed at detecting species-specific responses to moderate N and P addition and to understand how increasing nutrient availability will affect regeneration. From locally collected seeds, 320 seedlings were produced and transplanted to the plots of the Ecuadorian Nutrient Manipulation Experiment (NUMEX) with three treatments (moderate N addition: 50 kg N ha −1 year −1 , moderate P addition: 10 kg P ha −1 year −1 and combined N and P addition) and a control (80 plants per treatment). After 12 months, mortality, relative growth rate, leaf nutrient content and leaf herbivory rate were measured. N and NP addition significantly increased the mortality rate (70 vs. 54% in the control). However, N and P addition also increased the diameter growth rate of the surviving seedlings. N and P addition did not alter foliar nutrient concentrations and leaf N:P ratio, but N addition decreased the leaf C:N ratio and increased SLA. P addition (but not N addition) resulted in higher leaf area loss to herbivore consumption and also shifted carbon allocation to root growth. This fertilization experiment with a common rainforest tree species conducted in old-growth forest shows that already moderate doses of added N and P are affecting seedling performance which most likely will have consequences for the competitive strength in the understory and the recruitment success of P. torta. Simultaneous increases in growth, herbivory and mortality rates make it difficult to assess the species' overall performance and predict how a future increase in nutrient deposition will alter the abundance of this species in the Andean tropical montane forests.

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

“…For seedlings of Pouteria torta that were transplanted for one year to our study plots at 1000 m asl (2011–2012), Cárate‐Tandalla et al . () reported an increase in herbivory in the P‐ and N+P‐treated plots. By contrast, our results show only slightly (but not significantly) higher leaf area loss in this species in the same treatments.…”

“…Likewise, Cárate‐Tandalla et al . () observed increased diameter growth for Pouteria torta seedlings after N or P addition but no change in foliar concentrations of these elements. However, some studies have reported luxury consumption of P following P additions in tropical plant species, which were not accompanied by observed functional responses (Ostertag , Mayor et al .…”

“…In earlier studies, tropical tree seedlings responded to an increase in N and/or P availability with higher specific leaf area (SLA) (e.g., Burslem et al 1995, C arate-Tandalla et al 2015, Wan Juliana et al 2009), higher foliar nutrient concentrations (e.g., Palmiotto et al 2004, Santiago et al 2012, Ostertag & DiManno 2016, and altered N:P ratios (e.g., Schreeg et al 2014, Ostertag & DiManno 2016. In several fertilization experiments, fertilized plants were more attractive to herbivores than control plants due to thinner leaves and higher nutrient contents (e.g., Campo & Dirzo 2003, Andersen et al 2010, Santiago et al 2012, C arate-Tandalla et al 2015. The availability of nutrients is, apart from light, one of the determinants of C allocation patterns in tree seedlings.…”

“…, Cárate‐Tandalla et al . ). The availability of nutrients is, apart from light, one of the determinants of C allocation patterns in tree seedlings.…”

“…, Cárate‐Tandalla et al . , Wan Juliana et al . ), higher foliar nutrient concentrations ( e.g ., Palmiotto et al .…”