Microtopographic heterogeneity affects habitat specialization and diversity of understory plants in a northern temperate rainforest

The habitat heterogeneity hypothesis posits that diversity should increase with greater variation in environments. In old growth temperate rainforests, a high degree of habitat heterogeneity is created by an abundance of large nurse logs and woody debris that may influence patterns of diversity. We examined how the structural heterogeneity of the forest understory at a microtopographic scale affects the diversity of understory plants and lichens in the Hoh rainforest, Washington using rugosity as a measure of heterogeneity. We examined habitat associations and community structure of species among substrate types using null models. We found a hump shaped relationship between rugosity and species diversity. This was due to dominance by nonvascular plants in areas with high rugosity created by the specialization of bryophyte species to nurse logs, and dominance by vascular plants in areas with low rugosity created by the specialization of vascular plant species to the forest floor. The redundancy of substrate types where rugosity levels were both low and high caused the mid-levels of rugosity to have the highest habitat heterogeneity and subsequently the highest species diversity. Bryophyte species dominated this system making up 57% of the 54 species surveyed, followed by angiosperms (18%). More species were specialized to nurse logs than the forest floor. Our study revealed that the habitat heterogeneity-diversity relationship is supported in old growth temperate rainforests. We found that nurse logs are keystone structures that provide unique habitats for bryophytes, and shape both the structure of the understory and overall plant community dynamics in these forests.