Variations in leaf physiological properties within Amazon forest canopies

Abstract

Vertical profiles in leaf mass per unit leaf area (MA), foliar 13C composition (δ13C) and leaf nitrogen (N), phosphorus (P), carbon (C), potassium (K), magnesium (Mg) and calcium (Ca) concentrations were estimated for 204 rain forest trees growing in 57 sites across the Amazon Basin. Data was analysed using a multilevel modelling approach, allowing a separation of gradients within individual tree canopies (intra-tree gradients) as opposed to stand level gradients occurring because of systematic differences occurring between different trees of different heights (inter-tree gradients). Significant positive intra-tree gradients (i.e. increasing values with increasing sampling height) were observed for MA and CDW (the subscript denoting on a dry weight basis) with negative intra-tree gradients observed for δ13C, MgDW and KDW. No significant intra-tree gradients were observed for NDW, PDW or CaDW. Although the magnitudes of inter-tree gradients were not significantly different for MA, δ13C, CDW, KDW, NDW, PDW and CaDW, for MgDW there no systematic difference observed between trees of different heights, this being in contrast to the strongly negative intra-tree gradients also found to exist. When expressed on a leaf area basis, significant positive gradients were observed for N, P and K both within and between trees, these being attributable to the positive intra- and inter-tree gradients in MA mentioned above. No systematic intra-tree gradient was observed for either Ca or Mg when expressed on a leaf area basis, but with a significant positive gradient observed for Mg between trees (i.e. with taller trees tending to have a higher Mg per unit area). In contrast to the other variables measured, significant variations in intra-tree gradients for different individuals were found to exist for MA, δ13C and P (area basis). This was best associated with the overall average area based P, this also being considered to be a surrogate for a leaf's photosynthetic capacity, Amax. A new model is presented which is in agreement with the above observations. The model predicts that trees characterised by a low upper canopy Amax should have shallow or even non-existent gradients in Amax, with optimal intra-canopy gradients becoming sharper as a tree's upper canopy Amax increases. Nevertheless, in all cases it is predicted that the optimal within-canopy gradients in Amax should be less than is generally observed for photon irradiance. Although this is consistent with numerous observations, it is also in contrast to previously held notions of optimality.