In May 2025, during an environmental impact assessment, I observed that in the Ponderosa Pine (Pinus ponderosa) dominant zone near Okanagan Lake, Douglas Firs (Pseudotsuga menziesii) had grown into dense forested areas. After further review, I realized that Douglas Fir largely dominates many locations mapped as Ponderosa Pine-dominant. Hope et al. (1991) describe the Ponderosa Pine zone as open forestland characterized by bunch grasses, scattered Ponderosa Pines, and small patches of Douglas Fir or Trembling Aspen (Populus tremuloides) in moist areas.
The transformation of forest ecosystems, particularly in regions dominated by Ponderosa Pine, results from a complex interplay of factors. One key factor is fire suppression. While this practice aims to prevent catastrophic forest fires, it unintentionally increases fuel density in these areas. Historically, regular, low-intensity surface fires kept the understory in Ponderosa Pine forests open. However, a century of fire exclusion has allowed shade-tolerant Douglas Fir to establish and thrive in these areas (Holling, 1986). Compounding this problem is climate change, which modifies ecological dynamics in regions where these two tree species grow. Research by Davis et al. (2020) indicates that rising temperatures and altered precipitation patterns are extending fire seasons and intensifying drought stress, ultimately impacting regeneration and competitive interactions among tree species.
My observation, therefore, raises the question: why are historically open, pine-dominant systems shifting toward Douglas Fir dominance?
References:
Davis, K. T., Higuera, P. E., Dobrowski, S. Z., Parks, S. A., Abatzoglou, J. T., Rother, M. T., & Veblen, T. T. (2020). Fire-catalyzed vegetation shifts in ponderosa pine and Douglas-fir forests of the western United States. Environmental Research Letters, 15(10), 1040b8. https://doi.org/10.1088/1748-9326/abb9df
Holling, C. S. (1986). The resilience of terrestrial ecosystems: local surprise and global change. In W. C. Clark & Munn, R. E. (Eds.), Sustainable development of the biosphere. Cambridge, UK: Cambridge University Press, 292-317.
Hope, G. D., Lloyd, D. A., Mitchell, W. R., Erickson, W. R., Harper, W. L., & Wikeem, B. M. (1991). Ponderosa pine zone. Ecosystems of British Columbia. Special Report Series, 6, 139-152.
AI Comment from GPT 5:
Thank you for documenting this shift in stand structure near Okanagan Lake. Your field observation aligns with multiple lines of evidence showing that historically open Ponderosa Pine systems across the Interior can infill with shade-tolerant conifers—especially where surface fire has been excluded for decades and climate conditions are changing.
A few points to consider in the Kelowna context:
Stand history and fire regime: Where frequent, low-intensity fire was interrupted, dense Douglas-fir cohorts can establish under pine canopies and eventually dominate the mid- and upper canopy. This mechanism is consistent with the fire-exclusion pathway discussed in the post and is widely reported in similar dry forests.
Site moisture gradients: Douglas-fir recruitment and survival are favored on slightly cooler/moister microsites (north aspects, draws, deeper soils). Mapping units labeled “Ponderosa Pine zone” often include embedded mesic inclusions where Douglas-fir can gain a foothold and gradually expand.
Drought and competition: Extended warm-season drought can reduce Ponderosa Pine seedling survival, slow growth of residual pines, and increase mortality under competition, which may reinforce Douglas-fir dominance where it is already established.
Summary of related posts
Regeneration under changing climate: The observation from Castlegar, BC highlights how reduced summer precipitation and extreme heat events complicate regeneration choices for dry forests, noting the relative drought resilience of Ponderosa Pine and the need to adapt reforestation practices to site-specific climate pressures Douglas Fir and Ponderosa Pine Regeneration in a Changing Climate. In the context of your post, it underscores that regeneration outcomes and species competition are shifting with climate, potentially altering the balance between pine and Douglas-fir following disturbance.
Signs of physiological stress in pine: An observation from Spokane describes unusual growth forms and apparent stress symptoms in Ponderosa Pine, with a hypothesis linking changes to elevated CO2 and resulting resource demands Irregular Ponderosa Pine Growth. While causation is not established in that post, the reported stress patterns are consistent with conditions that could disadvantage pine where competition is high.
Landscape context and legacy of harvest: A BC-wide post on the rarity of remaining old-growth points to a long history of industrial activity and conversion across forest types B.C.’s old-growth forest nearly eliminated. In your area, past management can interact with fire exclusion to produce denser, more homogeneous stands where Douglas-fir persists and expands.
Treeline and infilling trends: A Canadian Rockies photo-repeat study shows greater tree density and upslope expansion in roughly 90% of revisited sites, linked to warmer temperatures Century-old photos show effects of climate change in Rocky Mountain forests. This supports a regional tendency toward infilling that can also occur within dry forest zones when establishment windows widen.
Disturbance agents affecting pine: Mountain pine beetle impacts in Alberta expanded with warmer winters, affecting large areas of pine forest Mountain pine beetle takes over Jasper National Park forests. Where beetle-caused mortality reduces pine overstory, post-disturbance pathways may favor Douglas-fir on many interior sites, depending on seed sources and microclimate.
Range shifts elsewhere: Eastern US analyses document tree species’ geographic shifts under changing climate Eastern US Trees Shift North, West With Climate Change. While a different region, it illustrates that competitive balances and establishment patterns are changing broadly.
Taken together, the related posts reinforce that multiple drivers—fire regime change, climate-driven stress and regeneration windows, disturbance history, and site heterogeneity—are interacting across the Interior West and BC. In the Ponderosa Pine zone around Okanagan Lake, these dynamics can plausibly produce the Douglas-fir dominance you observed in areas mapped as pine-dominant, especially where low-intensity fire has been absent for many decades and mesic microsites are common.
Observations From Members