The physiological importance of small leaf sizes in the mediterranean type ecosystem vegetation of the Cape floristic region
Numerous "Fynbos" species of the Cape Floristic Region (CFR) have particularly fine, narrow leaves. The rates of transpiration and heat loss are partially dependent on boundary layer conductance, which is determined by leaf shape and size, surface modifications and wind speed. We expected fine-leaved species with higher boundary layer conductance to transpire faster than broad-leaved species at low temperatures whereas at higher temperatures we expected transpiration to be limited by stomata! conductance. In contrast, the rate of heat loss may be constrained by thick boundary layers in larger leaves at high temperatures. Leaf gas exchange characteristics at various temperatures were correlated with boundary layer thickness, leaf area and specific leaf area for 14 Proteaceae species using phylogenetically independent contrast species. When the temperatures of individual leaves were altered, while ambient temperature was kept at l 8°C, water loss decreased significantly at both 12°C and 30°C with increased leaf size and thus boundary layer thickness. At 30°C, small leaves with thin boundary layers resulted in leaf temperatures below ambient, while larger leaves with thicker boundary layers had leaf temperatures closer to ambient. However, at 30°C the variation in leaf temperature between the smallest and largest leaves was only 3.4°C. Such a small variation in leaf temperature is unlikely to alter temperature-dependent physiological processes. We conclude that the small boundary layer associated with small leaves enables fine-leaved species to transpire at faster rates when water is plentiful. This may be a particularly important strategy for plants that take up most of their nutrients in the wet winter months from nutrient-poor highly leached soils of the CFR region. We suggest that fine leaves are an adaptation for nutrient uptake during winter, although they may also have the benefit of improved coupling of leaf to ambient temperature during the summer drought period.
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