Is there evidence for limitations to nitrogen mineralization in upper montane tropical forests?

Abstract

ABSTRACT The structure and function of forest ecosystems often change along altitudinal gradients in the tropics, culminating in short-stature, low-productivity cloud forests at the uppermost elevations. Field data and literature values were used to examine patterns in nitrogen mineralization on tropical mountains and to discuss the potential for nitrogen limitation to net primary productivity. Few trends in net nitrogen mineralization within and across elevation gradients in the tropics were found, and rates were generally comparable to those found in tropical forests at low elevations. Gross nitrogen mineralization rates were much higher than net rates, and in Puerto Rico upper montane forests exhibited higher gross nitrogen mineralization than lower elevation forests. Work from Puerto Rico found no effect of short-term anaerobic conditions. In Hawai'i gross nitrogen mineralization increased with substrate age. Ammonium availability was augmented by dissimilatory nitrate reduction to ammonium in montane forests; nitrogen conservation via this pathway exceeded losses via N 2 O production. Patterns in nitrogen circulation in upper montane forests in Puerto Rico showed that elfin and palm forests had lower nitrogen use efficiency and a higher proportion of nitrogen mineralized from decomposing litter relative to other forest types, further indicating that rates of nitrogen supply in these forests are considerable. In summary, the data reviewed in this chapter suggest that nitrogen limitation alone cannot explain patterns in the structure and function of tropical montane forest vegetation. Alternative factors are offered that warrant further investigation.