Ecophysiological significance of leaf size variation in Proteaceae from the Cape Floristic Region

Summary 1. Small leaves of species endemic to Mediterranean‐type climate areas have been associated with both low rainfall and nutrient availability, but the physiological reasons for this association remain unknown. 2. We postulated that small leaves have thin boundary layers that facilitate transpiration in winter and sensible heat loss in summer. High transpiration rates when water is available may facilitate nutrient acquisition in winter, whereas efficient sensible heat loss reduces the requirement for transpirational leaf cooling in summer. 3. The consequences of varying leaf sizes for water and heat loss in Cape Proteaceae were examined at two scales. At the leaf level, gas exchange and thermoregulatory capacities of 15 Proteaceae species with varying leaf size were assessed under controlled conditions using phylogenetically independent contrasts. At an environmental level, leaf attributes of Proteaceae occurring in the winter‐rainfall area of the Cape Floristic Region were correlated with climatic environments derived from distribution data for each species. 4. Leaf temperature was positively correlated with leaf size when wind speed was negligible. However, transpiration decreased significantly with increasing leaf size when measured on individual leaves, detached branches and when expressed on a per stoma basis. 5. From multiple stepwise regression analysis of climatic variables obtained from distribution data, leaf size was negatively correlated with A‐Pan evaporation, mean annual temperatures and water stress in January. We conclude that leaf size is conservative for survival over relatively rare periods of hot dry conditions with low wind speeds. 6. Narrow leaves enable plants to shed heat through sensible heat loss during summer droughts, without the need for transpirational cooling. Additionally, small leaf dimensions confer a capacity for high transpiration when evaporative demand is low and water is abundant (i.e. winter). This may be a particularly important strategy for driving nutrient mass‐flow to the roots of plants that take up most of their nutrients in the wet winter/spring months from nutrient‐poor soils.