The Functional Significance of Leaf Angle in Eucalyptus

The genus Eucalyptus L’Herit. contains a large proportion of species with nearly vertical foliage, as well as species with shallower leaf angles. This review uses the observation that species with steeply inclined foliage have concolorous leaves (same colour on both sides) to assess the broad patterns of leaf angle across the natural range of Eucalyptus . Species with shallow-angled leaves are shown to be largely restricted to areas of moderate to high rainfall and mild to warm climates along the margins of Australia where they co-occur with steeper-leaved species. A review of patterns in leaf angle on other continents and the functional significance of this trait supports the commonly held view that vertical leaves have been selected to reduce midday heat loads, thereby increasing water use efficiency and hence carbon gain, in eucalypts of dry interior regions. However, steep-leaved species also dominate the wetter, more productive montane forests of south-eastern Australia and Tasmania. In this case, vertical foliage may increase the competitive ability of individuals through a number of mechanisms, including the more efficient interception of light for the low sun angles prevalent at high latitudes, and by somewhat ameliorating low leaf temperatures during still, clear nights, thus decreasing subsequent cold-associated photoinhibition.