Inferring growth rates from leaf display in tropical forest saplings

In temperate trees, the extension growth history of stems and branches is easily inferred from the positions of the rings of over-wintering bud-scale scars, allowing one to assess environmental responses from surveys of saplings (Beaudet & Messier 1998, Canham 1988, King 2001). But tropical trees rarely show such distinct demarcations of growth flushes. Furthermore, the frequency of flushing mayvarywith tree sizeandenvironment,andanumberof tropical species, often light demanding, show continuous leaf production and extension growth throughout the year (Coley 1983). However, stem extension rates may also be correlated with leaf internode length (King 1993, 1994) and with the total length of the leaf display from the axis tip to the lowest leaf directly attached to it, i.e. the leaved stem length. (Note that ‘leaved stem length’ as used here differs from ‘crown depth’, defined as the vertical distance from the lowest to highest leaf over all axes, though the two measures may be correlated.) For a species with a leaf life span of precisely 1 y, the extension growth over the past yearwill beequal to thecurrent leavedstemlength.Onthe other hand, variation in leaf life span and past extension rates would reduce the correlation between these two variates. This paper examines the relationship between height growth rate and leaved stem length for saplings of three tree species of a wet tropical forest (La Selva, Costa Rica) and one species of a more seasonal tropical forest (Barro Colorado Island, Panama) to assess the utility of this potential measure of growth. Saplings were studied primarily in old-growth forests at the two study sites. The La Selva Biological Station of the Atlantic lowlands of Costa Rica (10◦26′N, 84◦00′W;