Photosynthetic parameters, dark respiration and leaf traits in the canopy of a Peruvian tropical montane cloud forest.

Few data are available describing the photosynthetic parameters of the leaves of tropical montane cloud forests (TMCF). Here, we present a study of photosynthetic leaf traits (V(cmax) and J(max)), foliar dark respiration (R(d)), foliar nitrogen (N) and phosphorus (P), and leaf mass per area (LMA) throughout the canopy for five different TMCF species at 3025 m a.s.l. in Andean Peru. All leaf traits showed a significant relationship with canopy height when expressed on an area basis, and V(cmax-area) and J(max-area) almost halved when descending through the TMCF canopy. When corrected to a common temperature, average V(cmax) and J(max) on a leaf area basis were similar to lowland tropical values, but lower when expressed on a mass basis, because of the higher TMCF LMA values. By contrast, R(d) on an area basis was higher than found in tropical lowland forests at a common temperature, and similar to lowland forests on a mass basis. The TMCF J(max)-V(cmax) relationship was steeper than in other tropical biomes, and we propose that this can be explained by either the light conditions or the relatively low VPD in the studied TMCF. Furthermore, V(cmax) had a significant-though relatively weak and shallow-relationship with N on an area basis, but not with P, which is consistent with the general hypothesis that TMCFs are N rather than P limited. Finally, the observed V(cmax)-N relationship (i.e., maximum photosynthetic nitrogen use efficiency) was distinctly different from those in tropical and temperate regions, probably because the TMCF leaves compensate for reduced Rubisco activity in cool environments.