Water Stress and Tree Phenology in a Tropical Dry Forest in the Lowlands of Costa Rica

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Abstract
Phenology and seasonal variations in water stress, as indicated by variations in girth, were monitored for 1 yr in numerous trees of 12 spp. [Albizzia caribaea, Astronium graveolens, Bursera simaruba, Cochlospermum vitifolium, Cordia alliodora, Enterolobium cyclocarpum, Guazuma tomentosa, Licania arborea, Lonchocarpus minimiflorus, Lysiloma seemannii, Spondias purpurea, Tabebuia rosea] growing at dry and moist sites in a tropical lowland deciduous forest in Guanacaste, Costa Rica. At dry sites, trees experienced water stress and shed their leaves early in the dry season. In most species, rehydration, followed by bud break, took place only after heavy rainfalls. In some species, leaf shedding was followed by rehydration and bud break during continuing drought. During shoot extension, which rarely lasted longer than a few weeks, trees experienced water stress in spite of growing in wet soils. At wet sites, trees experienced little or no apparent water stress; they remained evergreen or rapidly exchanged leaves during the dry season. In general, the timing of leaf fall and bud break and, in many species, anthesis was determined to a large extent by changes in tree water status. These phenomena, in turn, were a function of the interaction between the water status of the environment and the structural and functional state of the tree. At times the functional state of the tree would counteract the environmental influences; trees with growing shoots experienced temporary water deficits during the wet season, and bare trees rehydrated during drought. The seasonal pattern of tree development had a high correlation with seasonal variation in tree water status, but only indirectly with environmental water availability. No evidence was found for the control of tree development by seasonal variation in photoperiod or temperature.