Canopy gaps and light penetration into a wind-exposed tropical lower montane rain forest

Abstract

Penetration of light into the understory of tropical lower montane rain forests at Monteverde, Costa Rica, depends largely on the nature of canopy disruption by limb fall and tree fall. Midday photosynthetic photon flux densities (PPFD) above the forest canopy are low because of cloud cover (modal values are 400–500 μmol•m−2•s−1; daily integrations are 19 mol•m−2). The proportion of above-canopy PPFD that penetrates to the understory is greater in the elfin forests on wind-exposed ridge crests than in taller cloud forests in protected ravines (39 and 54% of understory locations receive <2% PPFD, respectively). PPFD is higher, and its vertical gradients steeper, in elfin forest gaps than in gaps of the same area in the taller forest. Gaps may or may not influence PPFD in adjacent forest understory, depending on vegetation structure on the gap periphery. In the taller cloud forest, understory PPFD is not correlated with distance to the nearest gap, but in the elfin forest it is (r = −0.467). This variation in the light environment is an integral part of disturbance and regrowth in these forests and seems to have played a major role in the evolution of tree growth strategies.