VARIATION IN LEAF LEVEL ENERGY-BALANCE COMPONENTS OF ENCELIA-CANESCENS ALONG A PRECIPITATION GRADIENT IN NORTH-CENTRAL CHILE

Leaf characteristics of several populations of Encelia canescens, loca ted in north-central Chile, were studied in order to evaluate differen ces in energy balance components along an aridity gradient. In the mos t arid regions, new vegetative growth and plant height were correlated in a 1:1 manner, while in regions where average rainfall is over 25 m m both variables were independent. Leaf absorptance did not change wit h increasing aridity in the year of our studies. The mean value measur ed (70.8+/-1.1%) was between the values reported for desert and medite rranean vegetation types. Leaf angle increased with aridity along the entire gradient (i.e., from 50-degrees at 30-degrees-S to 75-degrees a t 25-degrees30'S), consequently the potential midday solar radiation i ncident on an E. canescens leaf changed from 64% to 26% along the tran sect. The changes in leaf angle observed would result in a reduction i n transpiration of approximately 8% and of 1.2-degrees-C in leaf tempe rature. No differences were found among populations in leaf carbon iso tope ratios (-26.7+/-.27parts-per-thousand), leaf nitrogen content (2. 35+/-0.15%) and carbon/nitrogen ratio (16.42+/-1.40). These results su ggest similar water-use effiency among populations along the latitudin al gradient studied and might also indicate no differences in their po tential rates of photosynthesis. Increases in leaf angle could be a st rategy for optimization of productivity as well as a means of morpholo gical adjustment to the onset of drought.