- Strong, Allan M.
University of Vermont, Rubinstein School of Environment and Natural Resources
Many forests in eastern North America are experiencing declining levels of soil calcium, and these changes may contribute to mortality and decline of sugar maple and other tree species. How changes in soil calcium availability may affect forest herbivores and their predators remains uncertain, however. From 2005 to 2006, we investigated the responses of trees, herbivorous insects, and an insectivorous bird species to the 1999 watershed- scale addition of calcium at Hubbard Brook Experimental Forest, NH, USA designed to replenish soil calcium. Foliage calcium concentrations were 53% greater for sugar maple, 18% greater for American beech, and 38% greater for striped maple in the treatment watershed compared to a nearby reference area. Conversely, manganese and magnesium concentrations in foliage were lower in the treated watershed for these tree species. Despite the strong influence of Ca addition on foliar cation concentrations, we did not find consistent, significant effects of Ca treatment on abundance of leaf-chewing insect herbivores or extent of herbivore damage to foliage. Herbivores were generally more abundant and removed a greater percentage of foliage on sugar maple than beech. In 2006, mean body mass of both wild caterpillars and caterpillars raised in an in situ growth trial on sugar maples were >20% greater in a reference area than in the Ca addition watershed; however, these differences were not statistically significant. We also failed to detect a strong, consistent effect of treatment on foraging behavior by the Black-throated Blue Warbler, a bird that feeds heavily on leaf-chewing herbivores. One measure of foraging behavior, movement rate, was suggestive of decreased food availability in the Ca addition area. Based on our findings, we conclude that to date soil Ca differences between treated and reference areas are not of sufficient magnitude to have caused a cascade of changes in the herbivorous insect and insectivorous bird communities.