A LEAF-TOUGHNESS METHOD FOR DIRECTLY MEASURING THE PROCESSING OF NATURALLY ENTRAINED LEAF DETRITUS IN STREAMS

The leaf pack method has been used extensively as the standard protocol for tracking the in-stream processing of leaf detritus. However, a major deficiency of the method has been that it does not directly measure the processed condition of natural, multispecies accumulations of leaves. The method reported here allows mass loss of leaf detritus to be related to leaf toughness. Studies of dry-mass loss and leaf toughness of black cherry (Prunus serotina) leaves were conducted in summer 1992 in Linesville Creek and Powdermill Run (Pennsylvania, USA), and during fall/winter 1992-1993 in Powdermill Run. Leaf toughness was measured with a digital force-gauge penetrometer. Penetrometer measurements determined the peak force (nearest 0.001 Newton) necessary to drive a steel rod through the leaf tissue. In all studies, dry mass loss was best explained by a linear regression model and leaf toughness loss by a negative exponential regression model. The natural logarithm of mean toughness per leaf pack predicted percent mass remaining per leaf pack exceptionally well in both streams (r2 = 0.90-0.95). These results suggest that leaf toughness can be used to determine in-stream processing rates of natural leaf detritus if dry mass-toughness relationships are established.