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1. Existing data show that the surface area to volume ratio (Λ) of leaves generally decreases along both aridity and altitudinal gradients. That results in the relations: Λ decreases as it gets drier and hotter, as well as wetter and colder. Thus variations in rainfall and temperature do not explain the gross trends in Λ in a consistent manner. We hypothesized that Λ should vary in a regular manner with variations in light, water, nutrients and CO2, because recent work has shown regular relationships between Λ and the carbon, nitrogen and water contents of leaves. 2. To test the hypothesis we used existing measurements of leaves from various eucalypt species along aridity, altitudinal and fertility gradients in south-east Australia. The measurements were converted from a dry mass to a volumetric basis using an empirical leaf model known as the RSBS-model leaf. 3. Variation in leaf composition and morphology generally followed that predicted by the RSBS-model leaf. 4. We found that Λ increased with site fertility. Analysis of data from the literature showed that increasing the supply rate of nitrogen also results in an increase in Λ. We conclude that increases in site fertility are associated with increases in Λ. 5. Leaf thickness increased (hence Λ decreased) with light along the aridity and altitudinal gradients, in agreement with theoretical predictions. The increase in light along the aridity gradient is attributed to a decrease in cloud cover, while the increase in light with altitude is due to a decline in the optical thickness of the atmosphere. 6. Along the altitudinal gradient, the soils became increasingly boggy and anoxic with elevation, leading to increased soil acidity and a consequent decline in oxygen and nutrient availability. In contrast, along the aridity gradient decreases in nutrient availability were related to dry soil. 7. Both an excess and shortage of soil water can lead to reductions in the availability of oxygen and/or nutrients. When that relationship was considered, we found that a consistent framework linking Λ with the availability of light, water, nutrients and CO2/O2 could be established. In that framework, leaf area is controlled jointly by the availability of water and nutrients, while leaf thickness is controlled by the availability of carbon (i.e. light, CO2). Λ is the result of interactions between those two factors. The resulting framework should be useful in the development of models linking vegetation with environmental conditions. 8. A method for extending these results to the modelling of vegetation communities is proposed. |
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