Theory suggests that habitats that are structurally complex with a diversity of resources provide more niches for species with specific habitat and resource requirements to partition the habitat so that they may coexist. Thus, habitats that are structurally complex should theoretically have a higher proportion of species that are specialized to particular niches than homogeneous habitats. Small-scale structural heterogeneity, termed rugosity, refers to the variation in the amplitude of a surface (i.e. the forest floor). In temperate rainforests, the rugosity of the forest floor is increased by the presence of nurse logs (fallen trees), woody debris, and the moss species that colonize such substrates.
We examined the effect of the rugosity of the forest floor on nonvascular and vascular plant diversity in the Hoh Rainforest, Washington. We measured rugosity as the ratio of the distance of a 25 m transect to the distance of a transect following the curvature of the forest floor along the 25 m transect. We surveyed forest floor plant communities along the 25 m transects, which varied in rugosity such that we had areas with low rugosity that were dominated by moss-covered forest floor, and areas with high rugosity that had a variety of substrates including woody debris, nurse logs, and the forest floor. We calculated plant diversity using Shannon’s diversity index.
Results/Conclusions
I found that as the rugosity of the forest floor increased, plant diversity decreased significantly. This is likely because although areas with high rugosity are structurally complex due to a large amount of woody debris and nurse logs, such substrates are not suitable for most vascular plant species and cover much of the soil on the forest floor. The plant community therefore has lower diversity because nonvascular bryophyte species (mosses, liverworts and hornworts) found on several types of woody substrates dominate the community. Low rugosity environments have more exposed forest floor soil, little woody debris, and few nurse logs, so such areas are inhabited by several vascular understory species in addition to the bryophyte species found on woody substrates. Although the rugosity of the forest floor had the opposite effect on plant diversity than expected, these results confirm that the structural heterogeneity of forest floor substrates greatly impacts the diversity of the plant communities that inhabit them.