Juvenile foliage and the scaling of tree proportions, with emphasis on Eucalyptus

Juvenile foliage on seedlings and saplings occurs across a wide range of tr ee species and is particularly notable among eucalypts. Juvenile leaves gen erally have a lower leaf mass per area (LMA) and are often more horizontal in orientation than adult leaves. This paper examines the hypothesis that t he shift from juvenile to adult foliage is related to the scaling of mechan ical support costs. A simple model of stem allometry and production was used to show that low v alues of LMA and leaf area per unit crown area (LAI) should increase the gr owth rates of small, open-grown seedlings, but not larger saplings or trees . This result occurs because relatively little biomass need be invested in stems and branches to spread out the leaf mass of a seedling, favoring the option of increasing light interception by increasing crown area. In contra st, the large support requirements of trees make increasing crown area expe nsive and favor shifts to higher LAI and LMA, thereby increasing within-cro wn light interception and light use efficiency. Reported LMA values for juv eniles are lower than for older trees under similar light levels, and there is evidence for similar trends in LAI. However, juvenile foliage may have a variety of other functions, including the enhancement of shade tolerance, and trade-offs may also occur between efficient light interception and pro tection against herbivores and mechanical damage. Model simulations suggest that the nature and duration of the juvenile phase can have large impacts on early growth, particularly in favorable environments.