A cost-benefit analysis of leaves of four Australian savanna species.

Abstract

We conducted a cost-benefit analysis of the contrasting phenologies of two evergreen and two deciduous species of the savannas of north Australia. Stomatal conductance, rates of light-saturated assimilation (A(max)) and dark respiration were measured for six leaves from each of five or six trees. These leaves were then analyzed for total nitrogen, ether-soluble lipids, ash content, and heat of combustion. Construction and maintenance costs, nitrogen-use efficiencies and instantaneous transpiration efficiencies were then calculated from these data. Evergreen species had significantly lower specific leaf area, leaf nitrogen and leaf ash content than deciduous species. Evergreen species also had significantly higher heat of combustion and lipid content of crude extracts than deciduous species. Light-saturated assimilation rates were higher in evergreen species on a leaf area basis, but were higher in deciduous species on a leaf dry weight basis. In both evergreen and deciduous species, A(max) and total Kjeldahl nitrogen were linearly related. Similarly, nitrogen-use efficiency did not differ among species. Leaf construction costs were significantly higher for evergreen species than for deciduous species, but maintenance costs did not differ among species. Evergreen species had a higher cost:benefit ratio than deciduous species but because of their longer-lived leaves, the payback interval was longer in evergreen species than in deciduous species. These results support the hypotheses that: (1) longer-lived leaves are more expensive to construct than shorter-lived leaves, and (2) there is a higher investment of nitrogen into short-lived leaves to support a higher A(max) over a shorter payback interval. We conclude that deciduous and evergreen species partition resources both temporally and spatially, thereby reducing interspecies competition.