Relationships among leaf area index, below-canopy light availability and tree diversity along a transect from tropical lowland to montane forests in NE Ecuador

We tested the hypothesis that tree species diversity and stem density in tropical forests are both positively related to leaf area index and negatively to percent diffuse transmittance of photosynthetically active radiation. Eighty old-growth stands were selected in the Sumaco Biosphere Reserve, Ecuador, at 500, 1000, 1500 and 2000 m elevation. Leaf area index decreased significantly with elevation by about 1.1 per 1000 m altitude; from a mean of about 6.6 at 500 m to 5.2 at 2000 m a.s.l., while diffuse transmittance increased significantly with elevation. Diffuse transmittance at the forest floor averaged 2.1 % at 500 m and 4.0 % at 2000 m (relative to incident radiation). There were significant positive relationships between both tree species diversity and stem density on the one hand and leaf area-index on the other, whereas both were negatively related to diffuse transmittance. The observed decrease in leaf area index with elevation appears to be a consequence of an altitudinal decrease in stern density and in tree species diversity, combined with a soil fertility effect.