Transpiration Rates in the Luquillo Mountains of Puerto Rico

Abstract

Transpiration in elfin woodland on a peak at 1000 m and in montane rain forest at 550 m was found to be extremely low when compared to most other rates reported in the literature, regardless of habitat. Apparently, transpiration differences between the two habitats which we studied depended upon weather conditions. The extremely low saturation deficits of the air in these habitats, frequent cloud moisture deposition, and low insolation reaching the leaves because of frequent fog and closed canopy appear to be the major causes of the slow transpiration; inefficient xylem and effective scarcity of certain minerals may also be involved. Transpiration of sclerophyllous-leaved species characteristic of elfin forest was similar to that of membranaceous-leaved species from the montane rain forest. Sclerophylly appears to resist defoliation by strong mountain winds, rather than reduce transpiration as in some habitats. Inefficient base pumping due to slow transpiration, combined with mechanical wind pruning, is proposed as a major cause of tree height reduction in elfin forest. The proportionately greater stomatal area of elfin forest leaves as compared to their montane rain forest counterparts suggests that plants adapt to counteract this reduced transpiration.