The canopy light extinction coefficient (k) is defined as the exponential decline in the amount of light passing through the leaf layers as a function of leaf area index (LAI). This definition is standard in oil palm breeding trials and models of canopy photosynthesis, where k is sometimes assumed to have a fixed value. The present experiment aims to validate the alleged constancy of k. Therefore, k was inferred from the fractional transmission of photosynthetically active radiation (PAR) and LAI, as obtained from dura x pisifera test crosses of Nigeria and Ghana pisifera origins. The palms were planted at two densities (135 and 160 palms ha−1) in North Sumatra in 2010. At the age of 7.5 years after planting, the area of newly opened leaves approached a maximum. Transmission of PAR remained very low and was only weakly related to k. By contrast, LAI exerted a strong negative effect on k, which generated, under both densities, considerable differences in k between both origins and among pisifera within an origin. The assumption of applying a fixed k value for a certain genotype or palm density, as obtained during leaf expansion at closed canopy, may therefore not be realistic. The present study suggests that the relationship of k with LAI over time merits further investigation, starting just before canopy closure.

Fog interception and rainfall were measured during one year in three elfin cloud forests on small mountains along the Caribbean coast of South America and one in the Venezuelan Andes. (1) While rainfall increases from west to east in the small mountains along the coast, fog interception decreases. In 1985, the total annual rainfall and fog interception were 853 mm and 796 mm in the cloud forest of Serrania de Macuira, 1630 mm and 518 mm in Cerro Santa Ana and, 4461 mm and 480 mm in Cerro Copey. In the Andean forest of El Zum-bador, the 1985 rainfall was 1983 mm and the annual fog interception was only 72 mm. (2) Fog interception seems to be an important source of water to the elfin cloud forests of the small mountains which are surrounded by dry vegetation types and where the rainfall regime is highly seasonal. (3) Fog interception increases with altitude (in the same mountain), exposure (windward slopes) and leaf inclination. These variations of fog interception could partially explain the observed distribution of epiphytic flora in some of these cloud forests.