Carbon Assimilation and Productivity in a North-west Andean Tropical Montane Cloud Forest

The relative importance of the various pedological, climatological and physiological factors contributing to the short stature of tropical montane cloud forest (TMCF) is not well understood. This study endeavours to elucidate the controls on carbon assimilation and productivity, which, in conjunction with differences in carbon allocation, interact synergistically to reduce canopy height relative to lowland rainforest (LRF). Neither soil nutrient status nor soil climate appear to be direct controls on TMCF productivity at CEAT. Of the six soil nutrients (N, K, P, Mg, B and Ca) examined in this study, only Ca and B were found at lower concentration than in LRF. N and P were found at higher concentration. Soil moisture varies only slightly, with pore volume fraction near 0.50 at 0.4 m, except during a rare, extended drought, when it fell to 0.43 at a deforested site. Soil pH is lower than in LRF, ranging from 4.57 ± 0.56 at a depth of 10cm to 5.06 ± 0.26 at 50cm. As a result, Al:Ca ratios are high. Foliar nutrient concentrations are higher than reported for other TMCF's and are similar to levels reported in LRF, on a mass basis. Leaf nutrient status does not vary significantly with forest stage, despite higher soil nutrient levels in primary forest. However, foliar N, K and B decline in concentration with altitude at CEAT and maximum photosynthetic rate is correlated with concentrations of these nutrients. In terms of climatological controls, light limitation and poor light quality are shown to cause a substantial reduction in carbon assimilation. Daily incident