In deciduous forests of central New York, treefall pits have rich soil, but low plant species richness. To test whether leaf litter limited species distributions, we removed litter in 10 randomly chosen pits in 1983 and compared them with controls over 3 years. Wire cages prevented litter accumulation in experimental plots. By year 1 (1984) litterless pits were significantly warmer and less moist than controls. From years 1 to 3 the average Coefficient of Community (CC) index between experimental mounds and pits increased 10-fold. No changes occurred in controls. Species composition of experimental pits became different from the controls. Species richness increased in litterless pits, with no change in other plots. Experimental and control mounds were similar in species composition all years, indicating no effect of the cage. Species responses to litter removal included: (i) enhanced germination, (ii) establishment, and (or) (iii) no change. All herbs showing increased germination or establishment were species previously restricted to mounds. The species that did not change were those already inhabiting pits. No species declined after litter removal. Litter removal in pits allowed several species to expand their distributions from mounds into litterless pits, without affecting existing pit species. The spatial heterogeneity accentuated by leaf litter accumulation does not facilitate coexistence of species in the forest community, but only limits species population sizes and spatial distributions.

Summary1. Specialist herbivores often become less abundant per unit of host tissue as host density increases (resource dilution). They usually become less abundant when non-host species are mixed with their host plants (associational resistance). Most studies of these trends have involved herbaceous host plants and have not examined both trends for the same herbivores.2. Three hypotheses were tested for the response of insect specialists to host plant density: resource concentration, plant apparency and resource dilution. Two hypotheses were tested for the response of herbivores to non-host plants: associational resistance and plant apparency. 3. From 1992 to 2007, I examined the responses of three monophagous insect herbivores to the densities of their host, Pinus edulis , and of two non-hosts, Pinus ponderosa and Juniperus spp. 4. Herbivore loads increased with host density, though the correlations were weak and often variable between generations. These results were consistent with the resource concentration and plant apparency hypotheses, but not with resource dilution. 5. Herbivore loads decreased as non-host density increased, consistent with the associational resistance hypothesis. This and other studies have shown that associational resistance is important in many types of plant communities. 6. The absence of resource dilution on woodland trees contrasted with studies of herbaceous host plants. Further comparisons of woody and herbaceous host plants are needed to elucidate the reasons for this difference.

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