The influence of nutrients on growth and photosynthesis of seedlings of two rainforest dipterocarp species

Abstract

This study aims to determine the influence of nutrient additions in the form of nitrogen (N), phosphorus (P) and potassium (K) on the photosynthetic physiology, morphology and growth of Dryobalanops lanceolata Burck and Shorea johorensis Foxw., two co-existing dipterocarp species in the understorey of secondary forest. Analysis of the light environment revealed that the total daily irradiance in the understorey was 4.57 ± 0.70 mol photons m^–2 d^–1, approximately 20% higher than that which could be expected to occur in the primary forest understorey. Around 50% of total irradiance occurred at fluxes over 400 μmol photons m^–2 s^–1, and this was received over 7% of the daylight period. Both D. lanceolata and S. johorensis grew over the 16-month study, but only S. johorensis increased growth in response to nutrient additions. Neither species altered biomass partitioning in response to additional nutrients. Both species also showed increased rates of photosynthetic induction (T_0.5A_sat) in response to nutrient additions, but only S. johorensis showed an increase in light-saturated rates of photosynthetic capacity (A_sat), and the chlorophyll content of leaves. We argue that additional nutrients increased the capacity of S. johorensis to utilise the sporadic burst of high irradiance in sunflecks. Analysis of the nutrient composition of leaves from age cohorts up to > 16 months showed that additional nutrients increased the P and K concentration in D. lanceolata leaves, and the P and N content of S. johorensis leaves. This suggests that neither P nor K were limiting the growth of D. lanceolata in this secondary forest environment, but that P and/or N may limit growth of S. johorensis. Our data highlights differences in the ecology of two co-existing dipterocarp species. We argue that D. lanceolata shows characteristics consistent with a low resource-demanding species that may have a competitive advantage over S. johorensis in low resource environments, while S. johorensis shows traits better suited to higher resource availability. Thus, there are distinct species-specific differences in the ability to respond to nutrients that depend on the irradiance environment, and which may have a role in niche partitioning among the dipterocarps.