Niche differentiation is one hypothesis to explain plant species coexistence. Species that coexist often vary in light, water and nutrient requirements, substrate type requirements, or germination requirements. We examined whether nurse logs increase heterogeneity of regeneration sites within old-growth temperate rainforest, which would influence woody plant diversity and species distributions in these forests. Nurse logs are fallen trees that assist seed germination and survival as they provide both biotic and abiotic benefits, including a structure for seedling establishment, higher light relative to the forest floor, and protection from forest soil pathogens. Seedling establishment and survival are higher on nurse logs than on forest floor soils in many forests. Without nurse logs present, seedlings have an expected distribution in a density dependent manner, whereby seedling mortality is highest under the parent tree due to species-specific fungal pathogens. Therefore, it is expected that seedling abundance will be proportionately greater with increasing distance from the parent tree. The presence of nurse logs may disrupt density-dependent factors because they provide safe sites from species-specific fungal pathogens, as well as abiotic factors such as standing water, by serving as a physical barrier between the forest floor soils and seedlings. Here, we examine the role of nurse logs on the distribution patterns of Tsuga heterophylla relative to nurse logs to determine (1) if nurse logs increase the heterogeneity of regeneration sites within old-growth temperate rainforest and (2) whether they disrupt density-dependent factors. Seedling density was assessed using 1 m2 quadrats on nurse logs and the forest floor at varying distances from the parent tree. The parent tree was assumed to be the closest tree of the same species with a dbh (diameter at breast height) > 10 cm.
I found that there was a marginal significant difference in mean seedling abundance between the forest floor and nurse logs, confirming that nurse logs increase the heterogeneity of regeneration sites. I also found that mean seedling abundance did not change significantly with varying distance from the parent tree on nurse logs, but did increase significantly with distance from the parent tree on the forest floor. This confirms that nurse logs disrupt density dependent factors of seedling survival and establishment of Tsuga heterophylla.