Master of Science degree with a major in Natural Resources, option Forestry, Watershed, & Wildland Sciences
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HSU Faculty or Staff
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Community Member or Outside Professional
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HSU Faculty or Staff
Interruption of Indigenous stewardship has resulted in hardwood decline along what is now known as the middle Klamath River in northern California related to adverse effects to Tribal food sovereignty and community wellbeing. The survivors of a “legacy” of Indigenous stewardship are highly valued by the Karuk and neighboring Tribes for a myriad of culturally beneficial ecological associations and sources of traditional staple foods. Prescribed fire, following a century of fire exclusion, has resulted in unanticipated mortality of legacy tanoak (Notholithocarpus densiflorus), California black oak (Quercus kelloggii), and madrone (Arbutus menziesii), warranting further investigation. In order to better understand factors contributing to legacy hardwood mortality following prescribed fire, we characterized pre-fire basal fuels and conducted a post-fire mortality inventory for all three species.
For the fuels characterization, we measured litter and duff depth, woody fuels and fine root density around the base of 166 hardwood trees ranging in diameter at breast height [dbh] from 25 to 176 cm (median 51 cm dbh) across six sites in Karuk Aboriginal Territory. Transects accounted for distance from tree base (0-3 m) and direction in relation to the slope (up, down, left, and right facing upslope). Using linear mixed effects models, we determined that fuel depths under the tree crown were 41% and 42% greater than in canopy gaps for litter and duff, respectively. Distance and direction from the tree bole were the best predictive variables for fuel depth and loading. Fuels were largely slope-influenced, with uphill accumulations and downslope fuel “shadows” immediately next to the bole. Tree-level litter and duff depth were negatively related with Douglas-fir (Pseudotsuga menziesii) competition and tree dbh, and positively related with slope. Root density was highly correlated with duff depth, with fine and coarse roots (mm) in 94% of duff samples. Legacy hardwood fuel and root densities were less than previous studies around conifers in the region, but comparable to mixed hardwood-conifer stands in previous studies. High basal fuel accumulation may warrant basal fuels remediation prior to prescribed fire.
For the post-fire mortality inventory, we estimated mortality in six prescribed fire sites (burned 2017-2019) using four to six fixed area transects (1000 m2), and were compared to six representative unburned sites. Legacy hardwood size ranged from 25 to 186 cm diameter at breast height [dbh] (median 39 cm dbh). An estimated 43% of all sampled hardwood trees and 63% of the dead trees had pre-existing fire scars. Generalized mixed effects linear modeling was used for analysis. Stand-scale mortality rates were greater in prescribed burned areas (18%) than controls (13%), but not statistically significant (P = 0.735), and were comparable to observed rates of mortality in the region. Tree-scale mortality due to treatment varied among species (P < 0.0001), where black oaks died at the highest rates and were sensitive to prescribed fire (29% prescribed fire, 13% control). In contrast, tanoak had the same mortality rate between treatments (9% for prescribed fire and control; P < 0.0001). The most informative model predicting mortality was scar width/dbh ratio and species (P < 0.0001), primarily driven by the presence of large pre-existing fire scars among madrones. This metric may account for remaining structural wood, and the likelihood of structural failure, although more research is needed.
These results provide important insights into the factors related to undesired legacy hardwood mortality following prescribed fire with implications into possible mitigation efforts that warrant further Indigenous-led investigation and restoration. This study additionally expands post-fire mortality and fuel accumulation research to western hardwood species of cultural importance to the Karuk and neighboring tribes in the region.
Rickard, Heather D., "Factors contributing to legacy hardwood mortality following prescribed fire" (2023). Cal Poly Humboldt theses and projects. 701.