Laminated root rot (LRR; caused by the fungus Coniferiporia sulphurascens) is the most damaging root disease of Douglas-fir (Pseudotsuga menziesii) in the Pacific Northwest, USA and southwestern Canada. LRR’s influences on tree mortality and forest productivity have been well-documented in young commercial Douglas-fir forests (<50 years); however, much less is known about the role of LRR in the dynamics of older forests and how root disease might alter trajectories of forest succession. The objective of this study was to examine the association between LRR, and changes in forest community composition and productivity during succession in Douglas-fir forests of the Pacific Northwest. We used live tree density, basal area, and biomass measurements from 16 long-term permanent plots established in 1910–1940 (5–10 year measurement intervals) and distributed across northwestern Oregon and southwestern Washington State, USA. In 2019 and 2020, each plot was surveyed to assess dead trees for evidence of C. sulphurascens. Dead wood samples from LRR-suspected dead trees were cultured, and fungal isolates were identified using DNA sequencing methods. The LRR pathogen was confirmed in 13 of 16 plots with one or two genets occurring in each plot. The analyses showed that elevated C. sulphurascens incidence (% of tagged dead trees infected) was related to 1) greater decreases in Douglas-fir tree density; 2) increased accumulation of aboveground live tree biomass (AGB) in other species [e.g., western hemlock (Tsuga heterophylla) and western redcedar (Thuja plicata)], with a 20?% reduction in Douglas-fir AGB-weighted dominance; and 3) net primary productivity but not in ecosystem AGB accumulation rate. These results imply that LRR could accelerate late-seral species establishment, growth, and thus replacement of Douglas-fir through forest succession, but may not alter overall AGB dynamics from 50- to 150-year-old stands. This research provides information to natural resource managers about LRR’s influence in second-growth, mature, and old-growth forests of Oregon and Washington, USA, and provides further testament to the need for long-term forest demography plots.
Keywords: Laminated root rot; Forest succession; Net primary productivity