COS 185-6 - Who needs old growth forests? Multi-taxa biodiversity surveys of forests in the Pacific Northwest

Abstract: On a global scale, researchers have documented sharp and significant declines in biodiversity over the past century across multiple taxonomic groups including invertebrates, amphibians, songbirds, mammalian carnivores, among others. Forest ecosystems harbor a large portion of the Earth’s terrestrial biodiversity, but timber harvest continues to exert significant pressure on biological communities, especially in the Pacific Northwest. Until recently, biodiversity was difficult to quantify, but new genetic methods allow us to survey across multiple taxonomic groups efficiently and simultaneously. The objectives of this study were to determine the multi-taxa response to disturbance on federal forests in the Oregon Cascade mountains, identify species that are dependent on old-growth forests, and develop a method for conservation planning that maximizes biodiversity conservation while maintaining economic output in the form of timber harvest. During 2018, we conducted multi-taxa biodiversity surveys at 96 sites in the Willamette National Forest where we surveyed for fungi, invertebrates, vegetation, songbirds, and mammals across elevation and disturbance gradients. Our data consisted of a combination of direct observations and identifying species through genetic methods including shotgun sequencing and DNA metabarcoding. We found that fungi had the greatest species diversity (n = 1977) followed by invertebrates (n = 891) and sites that were previously logged had higher species richness. Non-metric multi-dimensional scaling showed that community composition across all taxa was strongly related to elevation and the amount of old growth structural characteristics. For bird communities, differences in community composition were correlated to functional traits related to diet and behavior. Using joint species distribution models and remotely sensed imagery, we produced single species distribution maps and predicted species richness across the landscape. Using these maps, we calculated indices of irreplaceability to highlight areas of high conservation value. Together, these results suggest that communities in multiple taxonomic groups are affected by similar environmental factors. In particular, species found in old-growth forests differ significantly from those in previously harvested areas, suggesting that the loss of old growth forests would result in biodiversity loss.