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 protoco l for tracking the in-stream processing of leaf detritus. However, a m ajor deficiency of the method has been that it does not directly measu re the processed condition of natural, multispecies accumulations of l eaves. The method reported here allows mass loss of leaf detritus to b e related to leaf toughness. Studies of dry-mass loss and leaf toughne ss of black cherry (Prunus serotina) leaves were conducted in summer 1 992 in Linesville Creek and Powdermill Run (Pennsylvania, USA), and du ring fall/winter 1992-1993 in Powdermill Run. Leaf toughness was measu red 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 mea n toughness per leaf pack predicted percent mass remaining per leaf pa ck exceptionally well in both streams (r2 = 0.90-0.95). These results suggest that leaf toughness can be used to determine in-stream process ing rates of natural leaf detritus if dry mass-toughness relationships are established.