Dead Wood and the Richness of Small Terrestrial Vertebrates in Southwestern Oregon

In southwestern Oregon, 24 mature forest stands were used to test the hypothesis that species richness of small terrestrial vertebrates is positively correlated with dead wood volume, and to compare dead wood loads between capture and non-capture sites for species encountered. Dead wood was separated into two components: coarse woody debris (CWD)—defined as down wood of any length ≥ 10 cm in diameter—and snags, defined as standing dead wood ≥ 0.5 m in height and ≥ 25 cm in diameter. The volume of CWD in stands ranged from 50 to 860 m/ha and snag volumes ranged from 10 to 240 m/ha. Small terrestrial vertebrates numbered between 8 and 20 species per stand based on a pitfall sampling effort of approximately 3,600 trap nights per stand over 2 years. Regression analysis revealed that the species richness of all terrestrial vertebrates taken as a single group increased with increasing volumes of CWD. Viewed as separate taxonomic groups, species richness of small mammals, insectivores, and amphibians all correlated positively with CWD volume; rodent richness showed no significant relationship with CWD. None of the vertebrate groups disclosed significant correlations between species richness and snag volume. Although some individual species at the stand scale did not appear to associate with dead wood, the study results do not preclude the importance of dead wood as a microhabitat feature. The results of this study predict that if all stands are managed to Federal CWD targets in southwestern Oregon, the full component of small terrestrial vertebrates typical of Pacific Northwest forests will not be realized.