ALBERT

Description: 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 [C]DW (the subscript denoting on a dry weight basis) with negative intra-tree gradients observed for δ13C, [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 MA, δ13C, [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 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.

Description: Soil samples were collected in six South American countries in a total of 71 different 1 ha forest plots across the Amazon Basin as part of the RAINFOR project. They were analysed for total and exchangeable cations, C, N, pH with various P fractions also determined. Physical properties were also examined and an index of soil physical quality proposed. A diverse range of soils was found. For the western areas near the Andean cordillera and the southern and northern fringes, soils tend to be distributed among the lower pedogenetic levels, while the central and eastern areas of Amazonia have more intensely weathered soils. This gives rise to a large variation of soil chemical and physical properties across the Basin, with soil properties varying predictably along a gradient of pedogenic development. Nutrient pools generally increased slightly in concentration from the youngest to the intermediate aged soils after which a gradual decline was observed with the lowest values found in the most weathered soils. Soil physical properties were strongly correlated with soil fertility, with favourable physical properties occurring in highly weathered and nutrient depleted soils and with the least weathered, more fertile soils having higher incidence of limiting physical properties. Soil phosphorus concentrations varied markedly in accordance with weathering extent and appear to exert an important influence on the nitrogen cycle of Amazon forest soils.