Gregory A. Greene

University of British Columbia - Vancouver | UBC · Department of Forest and Conservation Sciences

The current state of fire-resilience of dry forests in the southern Rocky Mountain Trench (RMT) of BC remains largely unknown. Specifically, we lack empirical evidence to discern whether the historical fire regime was predominantly (a) lower-severity, with most of the landscape experiencing frequent surface fires; or (b) mixed-severity, with a large portion of the landscape subject to high-severity crown fires. This has direct implications for the management of these ecosystems; it is unclear whether ecological memory and the feedback mechanisms that reinforce fire-resilience have been degraded. We also don’t understand the ecological role of densely-stocked stands within the dry forest matrix. Are they expected or novel components of dry forest ecosystems? Under the veil of a warming climate, the impact of dense stands on large, dominant, fire-tolerant trees is also unknown. Do dense sub-canopies pose a risk to these critical sources of ecological memory? These factors limit our ability to assess the degree to which fire exclusion (i.e., fire suppression and the prevention of Indigenous fire stewardship) and past forest management practices (i.e., historical selective harvesting) are affecting the resilience of these ecosystems. If dry forests were historically under a mixed-severity fire regime, this also questions the legitimacy of where and how ecosystem restoration treatments are being applied. In this dissertation I develop and apply concepts from a fire-resilience framework in three separate studies to investigate and address these knowledge gaps. Utilizing a proportional, stratified-random sampling design, I sampled 20 dense stands (>40% crown closure and >400 trees ha-1) throughout dry forests of the RMT. I targeted dense stands because they are central to the fire-resilience debate between the differing perspectives of lower- and mixed-severity fire regimes. I use information from surveys (i.e., stand structure and composition, stump counts/sizes/species), and samples from live and dead trees to produce tree-ring reconstructions of fire history, harvest history, stand age and size structures, stand composition, and growth rates of canopy and subcanopy trees. In the first study (Chapter 2), I reconstruct the historical fire regime, harvest history, and stand dynamics through time to discern the factors (i.e., disturbance types, sequences, and their interactions) that initiate or otherwise facilitate the establishment and persistence of dense stands, and determine if the fire regime has changed. I do this in the context of fires occurring near pre-historic Ktunaxa settlements, as their influences undoubtedly shaped the historical fire regime. In the second study (Chapter 3), I reconstruct historical stand structures and composition prior to harvesting, and quantify (a) the number, size, and species of trees removed, (b) trends in regeneration and mortality, (c) stand changes through time, and (d) the historical range of variability of pre-harvest stand configurations. I do this with a goal of deriving historical baseline values to determine if contemporary stands reflect degraded components of the dry forest matrix. I focus on the interactive effects of fire exclusion and historical harvesting, how the targeted removal of large, shade-intolerant but fire-tolerant trees affected ecological memory, and the implications of those choices on the trajectory of dry forest resilience. In the third study (Chapter 4), I compare growth rates of trees in different height classes to determine if high stand densities are negatively affecting the growth of large, canopy-dominant, fire-tolerant legacy trees. I do this in the context of stand dynamics and climate change over the past 100 years. Finally (Chapter 5), I conclude with a summary of my findings as they relate to the fire-resilience framework, and I provide guidance for the management of dry, mixed-conifer forests in BC.

The Tano Origins Project was directed by Dr. James E. Snead, and funded by the National Science Foundation.