Open Collections

Southam, Hanno

University of British Columbia

Retention silvicultural systems, defined by leaving mature trees in patches (group retention) or dispersed arrangements (dispersed retention), are used in ~30% of harvested area in coastal British Columbia (BC) to balance timber and ecological objectives. Silvicultural systems (objective-driven pathways of harvesting, regeneration and tending activities in a stand) affect forest pathogens, which creates complex decisions where foresters match associated impacts (positive and negative) of pathogens and silvicultural systems to management objectives. This study measured infection patterns of hemlock dwarf mistletoe (HDM; Arceuthobium tsugense (Rosend.) G.N. Jones subsp. tsugense), a native hemi-parasitic plant on hemlock (Tsuga heterophylla (Raf.) Sarg.), to infer additional infection from mature tree patches in group retention relative to clearcut systems. Infection patterns were measured at 11 sites in coastal BC along 55 m edges separating mature forest harbouring HDM from a regenerating clearcut (23–45 years old). HDM infection was assessed with a six-class rating (DMR) that ranged from 0 (uninfected)–6 (>50% branches infected). Mature forests (infection source) had high incidence (95%) and variable average DMR (range = 1.4–5.7) and infected basal area (range = 18–106 m²/ha). Spread into regenerating forests was variable across infection boundary (3–32 m from edge), incidence (range, trees ≤15 m from edge = 0–100%) and average DMR (range, trees ≤15 m from edge = 0–3.0). Model predictions for a median site indicated light and moderate infection severities were most probable <10 m from the edge; beyond 15 m healthy trees were most probable and only light severity infection plausible. HDM development to severities associated with growth impacts (DMR ≥3) over rotations of ~80 years is anticipated to be restricted to ≤15 m from the edge. Inferred to the stand-level and informed by the regional timber supply where hemlock is a lower-valued species, the potential impact to revenue from HDM in group retention is small. This result, and recognition of HDM’s ecological functions, supports the tentative conclusion that guidelines discouraging group retention in stands with HDM should be reassessed. Future research should predict stand-level infection by scaling edge patterns and better characterize hemlock’s role in timber supply.

Text

2025-05-13

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/91122

Forestry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/