WATER AND NUTRIENT RELATIONS OF WOODY PERENNIALS FROM TROPICAL DRY FORESTS

Dry forests occupy a larger area in the tropics than rain forests. They grow under a wide range of rainfall conditions; the determining characteristics are the occurrence of a dry season of 2-6 months duration, and the dominance of deciduous woody perennials. The knowledge of the ecophysiological properties of woody perennials from these forests, essential for the development of forest restoration strategies, is still scanty. This paper describes the ecophysiological behaviour of 10 species of woody perennials growing in a secondary dry forest, which has been recovering since 1944 on the hills of the Botanical Garden of Caracas. Four species, Astronium graveolens, Bauhinia megalandra, Sapindus saponaria and Tabebuia chrysantha, were initially planted while the others, Bursera simaruba, Capparis flexuosa, Erythroxylon cumanense, E. densum, E. orinocense and Eugenia casearioides, reproduced from seed sources existing in forest remnants growing nearby. Specific leaf areas measured are relatively high, covering a range from 11 to 34 m2/kg. Nitrogen and phosphorus levels are also high compared to data reported from dry forests elsewhere, therefore no nutrient limitation for photosynthetic productivity is apparent. Osmotic pressure (pi) of leaf sap, extracted from frozen samples taken during the growing season, is correlated with the corresponding content of soluble sugars and the ions Mg and K; Ca-ions do not play a significant role in explaining the variance of pi. Most species maintained a diluted leaf sap during the rainy season, characterized by pi-values between 5-15 bar. The osmotic pressure increased strongly in older leaves and during the dry season. Four species showed more stable pi-values throughout the growing season, with C. flexuosa and B. simaruba characterized by higher and lower pi-values, respectively. Proline was found to be a reliable indicator of water stress in these woody species, the amount of proline measured in leaf sap being logarithmically correlated with the corresponding pi-value. Osmotic pressure of leaf sap and leaf xylem tension was higher during the dry season for all species, while the contrary was true for leaf conductance. Leaf conductance was better correlated with leaf-air vapour pressure deficit than with leaf xylem tension. The most drought-tolerant species were C. flexuosa, E. casearioides and the three Erythroxylon species. Drought resistance of B. simaruba, B. megalandra and A. graveolens was associated with their high sensitivity to leaf-air VPD and lower leaf conductances. The other two species occupied intermediate positions.