Abstract:
Many herbivores of the Pacific Northwest rely on forest understory shrub leaves for a source of nitrogen, energy, and moisture. I measured nitrogen, protein-binding capacity, and condensed tannin concentration as indicators of available nitrogen; cell wall constituents and lignin as indicators of available energy; and moisture concentration in young and mature leaves of Oregon grape and salal, and in young leaves of sword fern, vine maple and ocean spray. In late summer I estimated lepidopteran larval herbivory of leaves from these species. This research was conducted during the summers of 1995 and 1996 in the Oregon Coast Range. My objectives were to determine whether insect
herbivory and indicators of leaf nutritional quality differed 1) between clear-cut and forest stands (thinned and unthinned second-growth, and old-growth), 2) between thinned second-growth and unthinned second-growth forest, 3) between thinned second-growth and old-growth forest, and 4) between young and mature leaves of Oregon grape and salal. I also measured leaf area, specific leaf mass, and the proportion of available diffuse radiation reaching one meter above the forest floor. Most differences were found between clear-cuts and forest stands with leaves from clear-cuts tending to have less nitrogen concentration, greater protein-binding ability, greater condensed tannin
concentration, lower proportions of cell wall constituents, decreased moisture
concentration, smaller leaf area, and greater specific leaf mass than leaves from
forested stands. Comparisons between the forested stands exhibited few statistically significant differences. Leaf quality differences were likely due to differences in light exposure which was significantly greater in clear-cuts than in forest and differed little among forested stands. Herbivory by lepidopteran larvae tended to be lower in clear-cuts than in forested stands, but differences could not be related to leaf nutritional quality, because environmental variables associated with stand type also may have affected herbivory. Nutritional quality decreased with leaf age, leaves older than one year having more lignin, less nitrogen, greater protein-binding ability, more condensed tannins, and less moisture than young leaves. Herbivory of mature leaves decreased by one-half or more relative to young leaves, and is likely attributable to the low nutritional quality of mature leaves.
