Vertical profiles in leaf mass per unit leaf area (M A ), foliar 13 C composition (δ 13 C) 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, 5 allowing a separation of gradients within individual tree canopies (intra-tree gradients) as opposed to stand level gradients occurring because of

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... ematic 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 M A and [C] DW (the subscript denoting on a dry weight basis) with 10 negative intra-tree gradients observed for δ 13 C, [Mg] DW and [K] DW . No significant intra-tree gradients were observed for [N] DW , [P] DW or [Ca] DW . Although the magnitudes of inter-tree gradients were not significantly different for M A , δ 13 C, [C] DW , [K] DW , [N] DW , [P] DW and [Ca] DW , for [Mg] DW there no systematic difference observed between trees of different heights, this being in contrast to the strongly negative intra-tree 15 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 intraand inter-tree gradients in M A 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 20 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 M A , δ 13 C and [P] (area basis). This was best associated with the overall average area based [P], this also being con-25 sidered to be a surrogate for a leaf's 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-4641 Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion existent 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 gradients in A max 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. 5 20 for simulating rates of canopy photosynthesis and the associated light response (Lloyd et al., 1995; Haxeltine and Prentice, 1996; de Pury and Farquhar, 1997) as well as for simulations of canopy leaf areas (themselves affecting predicted rates of photosynthetic carbon gain) in dynamic vegetation models (Sitch et al., 2003; Woodward and Loomis, 2004). Within tropical forest canopies, this variation may be expected to be 25 especially complicated due to the very high number of species present in any one forest with an associated high tree-to-tree variation, at least some of which can be related 4642