Climate and biological resilience at the northern limit of British Columbia’s Inland Temperate Rainforest

,Climate-mediated shifts in forest productivity hold uncertain impacts for temperate rainforest ecosystems. While considerable focus has been devoted to detecting, monitoring, quantifying, and modeling ecological change, variable tree responses across biogeographic gradients complicate ongoing research. In British Columbia, arid montane forests are dynamically reorganizing to meet evolving environmental conditions, as evidenced by large-scale disturbance processes such as mega-fires and insect outbreaks. Less is known about the more subtle climate responses impacting the Inland Temperate Rainforest. As these forests are among the most productive in the world, shifts in forest growth may have lasting implications for global climate and ecological, economic, and cultural systems. This thesis documents the influence of the recent climate variability on an ITR ecosystem in the northern Rocky Mountains of British Columbia. Findings complement the existing body of limited research in this biologically rich but relatively understudied region by producing multi-species tree-ring chronologies from high- and low-elevation sites. Using multiple analytical techniques, results from tree-ring, biomass, and dendroclimatic analyses highlight the role of biogeography in mediating climate sensitivity and tree growth across species and elevation gradients. Stand-level biomass estimates reveal the significant carbon storage potential of lower-elevation forests, rivaling productive temperate rainforests in the coastal Pacific Northwest. Dendroclimatic analysis highlights the role of temperature and snow in limiting tree growth, with the highest productivity periods occurring during years with slightly above-average temperatures, below-average snowpack, and average precipitation. As expected at this northern latitude site, low growth occurs during cold years with heavy snowpack. Still, trees also display negative growth responses to above-average temperatures and drought, particularly at low elevations. Despite near-normal precipitation in the recent decade, thermal stress during periodic "heat domes" is becoming an overarching driver of reduced biomass accumulation in old-growth western red cedar, an iconic keystone species and dominant carbon pool in the ITR. Intervals of reduced growth throughout the 121-year study period have been followed by notable plasticity across cedar and other co-dominant species, showing potential for increased productivity under projected climate scenarios. The long-term trajectory of the ITR will hinge on species-level adaptations to nonanalog warming conditions projected for the next century and conservation measures that protect the structural and compositional resiliency of these globally significant ecosystems. Remarkable adaptability is evident in study species which thrive from the Northern Rockies (Engelmann spruce and subalpine fir) to the Sierra Madre of central Mexico (Douglas-fir) and along the Pacific Northwest coast to northern California (western hemlock, western red cedar, Douglas-fir). Dedicated research and conservation efforts across these ranges are essential for enabling the forests of the ITR to adapt to changing environmental conditions. This thesis underscores the ITR’s critical role as a globally significant carbon sink and biodiversity reservoir. Building on existing conservation efforts, it calls for the creation of the Great Caribou Rainforest Initiative. Modeled after the Great Bear Rainforest Framework, this initiative aligns scientific evidence with cultural and economic values to create best-practice climate change mitigation strategies safeguarding the ITR’s adaptive capacity, carbon storage, and biodiversity.

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