Limited effects of thinning on laminated root rot induced tree mortality in Douglas-fir forests of the Pacific Northwest, USA

Laminated Root Rot caused by the fungal pathogen Coniferiporia sulphurascens is a damaging disease within many Douglas-fir forests of the Pacific Northwest of North America. Management of these forests in a changing climate and fire regime will require changes to silvicultural practices. A long-term study (ca. 30 years) in Oregon, USA provides an opportunity to investigate the effects of thinning on disease dynamics in an area of historically high laminated root rot incidence. The effects of three thinning prescriptions on tree mortality caused by C. sulphurascens were compared on ca. 160 ha within the Siuslaw National Forest. Observations were compared with predictions from the Forest Vegetation Simulator and its Western Root Disease Model extension. No significant effect of thinning treatment on mortality (p = 0.981), or annual basal area increment (p = 0.372) was observed. In contrast to observations, the Forest Vegetation Simulator over estimated growth, while the Western Root Disease Model extension was consistent with field measurements. Thinning treatments appear to have minimal impacts on laminated root rot induced mortality but also do not result in the expected increase in growth rate typically associated with a thinning treatment.