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

Abstract

Vertical profiles in leaf mass per unit leaf area (<i>M</i>_A), foliar ¹³C composition (&delta;¹³C), 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 <i>M</i>_A and [C]_DW (the subscript denoting on a dry weight basis) with negative within-tree gradients observed for &delta;¹³C, [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 <i>M</i>_A, &delta;¹³C, [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. <br><br> 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 <i>M</i>_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). <br><br> Significant differences in within-tree gradients between individuals were observed only for <i>M</i>_A, &delta;¹³C 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, <i>A</i>_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 <i>A</i>_max should have shallow, or even non-existent, within-canopy gradients in <i>A</i>_max, with optimal intra-canopy gradients becoming sharper as a tree's upper canopy <i>A</i>_max increases. Nevertheless, in all cases it is predicted that the optimal within-canopy gradient in <i>A</i>_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.