Photosynthetic capacity and nitrogen nutrition of Ecuadorian montane forest trees

With increasing elevation, the growth conditions of trees in tropical mountains become generally more adverse in terms of decreasing nutrient availabilities, decreasing temperatures and decreasing atmospheric concentration of carbon dioxide (CO2). In tropical montane forests, reduced decomposition rates at higher altitudes lead to thicker organic layers and together with reduced mineralization and nitrification rates to a change in available nitrogen forms and nitrogen has been shown to limit productivity in these forests. How photosynthetic capacity (Amax) of tropical trees on the one hand, and nitrogen uptake capacity and nitrogen form preference on the other hand adapt to variation in environmental conditions along elevation gradients, is not precisely known. The present study was conducted in three tropical montane forest stands along an elevational transect at 1000, 2000 and 3000 m asl in South Ecuador. It aimed (1) to quantify the photosynthetic capacity of adult tropical trees along the elevational transect by means of gas exchange measurements and to analyse the possible controlling effects of temperature, partial pressure of CO2 and nutrient availability on photosynthesis and (2) to investigate altitudinal changes in the use of nitrate, ammonium and organic nitrogen sources by tropical forest trees by means of a stable isotope tracer study with seedlings. Stand-level means of light-saturated net photosynthesis (Asat) were 8.8, 11.3 and 7.2 µmol CO2 m-2 s-1; those of dark respiration (RD) 0.8, 0.6 and 0.7 µmol CO2 m-2 s-1 at 1000, 2000 and 3000 m elevation, respectively, with no significant altitudinal trend. Examining our data in the context of a pan-tropical Asat data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based Asat decreases in tropical mountains by, on average, 1.3 µmol CO2 m-2 s-1 per km altitude increase (or by 0.2 µmol CO2 m-2 s-1 per K temperature decrease). The Asat decrease occurred despite an increase in leaf mass per area with altitude. Lowered Asat together with a reduced stand leaf area decrease canopy carbon gain with elevation in tropical mountains. The P content per leaf mass was the principal factor determining Amax across the altitudinal gradient while the effects of foliar N, temperature and [CO2] were insignificant. Amax was subject to partial, and RD to full homeostatic adjustment to the reductions in temperature and [CO2] at higher elevations, mainly through a large reduction in SLA and the resulting increase in foliar N and P per leaf area, while no altitudinal increase in carboxylation efficiency was detected. We conclude that the altitudinal decrease in both SLA and canopy leaf area are more important determinants of carbon gain in tropical high-elevation forests than adaptive physiological modifications in the photosynthetic apparatus. The seedlings of six tree species differed with respect to their nitrogen form preference but neither the abundance of ammonium and nitrate in the soil nor altitude seemed to influence the preference. Two species (those with highest growth rate) preferred ammonium over nitrate while the other four species took up nitrate and ammonium at similar rates when both nitrogen forms were equally available. After 15N13C-glycine addition, 13C was significantly accumulated in the biomass of three species (two species with arbuscular and one species with ectomycorrhizal symbionts) in addition to a siginficant 15N accumulation indicating that trees in tropical mountain forests can use organic nitrogen sources irrespective of the type of their mycorrhiza.