Background/Question/Methods   Nitrogen (N) limits net primary productivity in most terrestrial ecosystems globally, but phosphorus (P) is more limiting in many lowland tropical forests such as those in Amazonia. However, tropical montane soils such as those of the Andes may be less P limited than the lowlands. Here we report on results from a large-scale fertilization experiment (N, N+P, P) along a 3000 m elevation transect in the Peruvian Andes and adjacent lowland Amazonia. Our objectives were to determine if N and/or P limits the productivity of these Andean and Amazonian forests, and if there is an elevation gradient in nutrient limitation. We measured leaf and soil nutrients, leaf area index (LAI), diameter at breast height (DBH), and soil respiration partitioning between microbes, roots, mycorrhizae and litter.

Results/Conclusions   Leaf N:P decreased with elevation, crossing 15 at mid-elevation (between 1000–1500 m), suggesting increased P limitation and decreased N limitation with increasing elevation. DBH and tree height decreased with increasing elevation, but LAI did not vary with elevation. LAI did not respond to fertilization, but the DBH response to P fertilization increased with elevation and vice versa for N fertilization. However, root respiration decreased (and mycorrhizal respiration increased) with the DBH increases. This suggests a shift in carbon allocation from roots to wood rather than an overall growth increase to nutrient amendment. With projected changes in the nutrient regime of the Andes, this study provides the first large scale study of nutrient dynamics and limitation in rain and cloud forests in the Peruvian Andes and Amazon.