Folliage growth rhythm and logevity of the tree layer in the A-Brenes Biological Reserve, Costa Rica

During one and a half years (February 1991 - July 1992) the growth rhythm a nd the longevity of leaves of 22 abundant understory species in the tropica l premontane rain forest of Reserva Biologica Alberto Ml. Brenes, Alajuela, Costa Rica, were investigated. Three types of growth rhythm were identifie d. Most species, especially the monocotyledones, produce new leaves in defi ned intervals throughout the year. Only some have a slower leaf production in the dry season. Other species sprout periodically or only once a year. T his interval, which is the time period between the beginning of the shootin g of one leaf and the next, varied among the investigated species between 1 7-22 days (Malvaviscus arboreus, Urera elata, Myriocarpa longipes) and more than half a year (Cyclanthus bipartitus, Iriartea deltoidea, Ocotea gomezi i). For some species this interval was longer at dry and dark sites (M. arb oreus), but in many cases microclimatical factors such as temperature, rela tive light intensity, and evaporation varied to such a small extent within the forest that the differences did not lead to significant influences on g rowth factors of the plants. The mean leaf longevity varied between a half and four years. The maximum leaf age of the palms Cryosophila warscewiczii, I. deltoidea and Calyptrogyne trichostachys was estimated to be more than five years. In some cases leaf age was significantly correlated with microc limatical factors. Leaves of Begonia cooperi growing at higher light intens ities in gaps fell earlier than leaves of plants in darker sites in the und erstory. Generally, the longevity of leaves was lower in plants growing at moister sites, but in most cases this relation was not significant. The dat a show that even in a rather constant climate in the understory of a very m oist premontane rain forest, there is some seasonality in growth rhythm and that plant growth varies between different sites within the forest althoug h climate variations are very small.