Nutrient limitation in rainforests and cloud forests along a 3,000-m elevation gradient in the Peruvian Andes

Abstract

We report results from a large-scale nutrient fertilization experiment along a "megadiverse" (154 unique species were included in the study) 3,000-m elevation transect in the Peruvian Andes and adjacent lowland Amazonia. Our objectives were to test if nitrogen (N) and phosphorus (P) limitation shift along this elevation gradient, and to determine how an alleviation of nutrient limitation would manifest in ecosystem changes. Tree height decreased with increasing elevation, but leaf area index (LAI) and diameter at breast height (DBH) did not vary with elevation. Leaf N:P decreased with increasing elevation (from 24 at 200 m to 11 at 3,000 m), suggesting increased N limitation and decreased P limitation with increasing elevation. After 4 years of fertilization (N, P, N + P), plots at the lowland site (200 m) fertilized with N + P showed greater relative growth rates in DBH than did the control plots; no significant differences were evident at the 1,000 m site, and plots fertilized with N at the highest elevation sites (1,500, 3,000 m) showed greater relative growth rates in DBH than did the control plots, again suggesting increased N constraint with elevation. Across elevations in general N fertilization led to an increase in microbial respiration, while P and N + P addition led to an increase in root respiration and corresponding decrease in hyphal respiration. There was no significant canopy response (LAI, leaf nutrients) to fertilization, suggesting that photosynthetic capacity was not N or P limited in these ecosystems. In sum, our study significantly advances ecological understanding of nutrient cycling and ecosystem response in a region where our collective knowledge and data are sparse: we demonstrate N limitation in high elevation tropical montane forests, N and P co-limitation in lowland Amazonia, and a nutrient limitation response manifested not in canopy changes, but rather in stem and belowground changes.

Large-scale climatic and geophysical controls on the leaf economics spectrum

“…N:P ratios greater than 16 are generally representative of P limitation to productivity, relative to N availability, in humid tropical forests (47). In upper submontane and montane forests, N:P ratios < 16 suggest increasing N limitation relative to P, as recently detected in a field experiment in the Peruvian Andes (26). Low N:P was also found in particular portions of the Amazonian lowlands: extremely anoxic swamp environments are known areas of very low N availability relative to P (asterisks in Fig.…”

“…2, e). Tropical montane forests are generally viewed as N-limited ecosystems, relative to their lowland tropical forest counterparts (24)(25)(26). Particularly low soil N is known to occur on steep portions of hilly or montane tropical terrain, a result of low N residence time in soils (27).…”

Invariant allometric scaling of nitrogen and phosphorus in leaves, stems, and fine roots of woody plants along an altitudinal gradient

“…Some studies on tropical, and subtropical mountains, and subarctic tundra found that leaf N and P declined with altitude (Köhler et al 2006;Macek et al 2009;Soethe et al 2008;Sundqvist et al 2011). By contrast, other studies also showed that the N and P of leaves first increased and then decreased with increasing elevation on Mountain Gongga, China (Shi et al 2012) and in the Peruvian Andes (Fisher et al 2013). Meanwhile, Garkoti (2012) indicated that root N and P declined with elevation in the central Himalayas.…”

Examining variation in the leaf mass per area of dominant species across two contrasting tropical gradients in light of community assembly

“…2015) and P limitation drives N:P variations among lowland Amazonian plots, explaining in particular variations between TAM‐05 and TAM‐06 (Asner and Martin 2011; Fisher et al. 2013). …”