Forest canopy gap size affects regeneration potential of interior Douglas-fir

There is growing concern about the long term productivity of forests in British Columbia due to changing climatic conditions. Interior Douglas-fir, an economically and culturally valuable conifer species, has recently had inconsistent regeneration success in the dry climatic regions of its distribution due to high summer soil surface temperatures, drought and growing season frost. Seeds of interior Douglas-fir germinate after mixed severity disturbances, but their survival appears to depend on the size of disturbance gaps, environmental resources and conditions, and colonization by mycorrhizal fungal symbionts. In two separate experiments that differed in climate (very dry, hot and dry, cool Interior Douglas-fir (IDF) subzones), and disturbance agent (natural and harvested), I sowed interior Douglas-fir seed into different sized forest canopy gaps. In both experiments, I tested the effects of canopy gap size and access to mycorrhizal networks on seedling performance (establishment, growth, water use efficiency, foliar nutrition, mycorrhizal colonization) and environmental resources and conditions (light, temperature, soil moisture). In the first experiment, regeneration failed in all canopy gap sizes and network treatments due to the harsh climatic conditions. There, neither protection in small gaps nor access to mycorrhizal networks were sufficient to create favourable regeneration conditions. In the second experiment, where the climate was cooler and wetter, seedling survival reached 74% in harvested gaps that were 80 – 300 m² in area, corresponding with greater soil moisture availability. Gaps of 20 – 80 m² were too small to initiate gap-phase regeneration, however, as indicated by low seed emergence and slow height growth rates. Gaps >300 m² resulted in high emergence but low survival (26 %) due to low soil moisture availability. Access to mycorrhizal networks had minor effects on mycorrhizal colonization and water use efficiency. My study shows that regeneration potential of interior Douglas-fir is severely limited by the very dry, hot climate in the low elevation IDF forests, but can be increased in wetter, cooler climates with forest harvesting practices that create moderately sized canopy gaps.Forestry, Faculty ofGraduat