Optimisation of photosynthetic carbon gain and within-canopy gradients of associated foliar traits for Amazon forest trees

Vertical profiles in leaf mass per unit leaf area (M-A), foliar C-13 composition (delta C-13), nitrogen (N), phosphorus (P), carbon (C) and major cation 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 (within-tree gradients) as opposed to stand level gradients occurring because of systematic differences occurring between different trees of different heights (between-tree gradients). Significant positive within-tree gradients (i.e. increasing values with increasing sampling height) were observed for M-A and [C](DW) (the subscript denoting on a dry weight basis) with negative within-tree gradients observed for delta C-13, [Mg](DW) and [K](DW). No significant within-tree gradients were observed for [N](DW), [P](DW) or [Ca](DW). The magnitudes of between-tree gradients were not significantly different to the within-tree gradients for M-A, delta C-13, [C](DW), [K](DW), [N](DW), [P](DW) and [Ca](DW). But for [Mg](DW), although there was no systematic difference observed between trees of different heights, strongly negative within-tree gradients were found to occur. When expressed on a leaf area basis (denoted by the subscript 'A'), significant positive gradients were observed for [N](A), [P](A) and [K](A) both within and between trees, these being attributable to the positive intra- and between-tree gradients in M-A mentioned above. No systematic within-tree gradient was observed for either [Ca](A) or [Mg](A), but with a significant positive gradient observed for [Mg](A) between trees (i.e. with taller trees tending to have a higher Mg per unit leaf area). Significant differences in within-tree gradients between individuals were observed only for M-A, delta C-13 and [P] (A). This was best associated with the overall average [P](A) for each tree, this also being considered to be a surrogate for a tree's average leaf area based photosynthetic capacity, A(max). A new model is presented which is in agreement with the above observations. The model predicts that trees characterised by a low upper canopy A(max) should have shallow, or even non-existent, within-canopy gradients in A(max), with optimal intra-canopy gradients becoming sharper as a tree's upper canopy A(max) increases. Nevertheless, in all cases it is predicted that the optimal within-canopy gradient in A(max) should be substantially less than for photon irradiance. Although this is also shown to be consistent with numerous observations as illustrated by a literature survey of gradients in photosynthetic capacity for broadleaf trees, it is also in contrast to previously held notions of optimality. A new equation relating gradients in photosynthetic capacity within broadleaf tree canopies to the photosynthetic capacity of their upper canopy leaves is presented.