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Tree encroachment in fire-maintained woodlands and grasslands is a major management concern, yet little information exists regarding the mechanisms of small tree mortality following prescribed burns. We sought to clarify the relative importance of tree size and fire-induced injury in the post-fire mortality of encroaching Douglas-fir trees and to compare results with an existing mortality model for larger Douglas-fir trees. Crown injury to small Douglasfir trees was a significant explanatory variable in post-fire mortality models, with results suggesting a 20% threshold in crown scorch. Crown injury was strongly related to bole injury, and delayed mortality was important as we documented new mortality 20 months post-burn. Mortality models for large Douglas-fir tend to over-predict small tree mortality, underscoring the need to better understand the mechanisms of fire-caused mortality for small, encroaching trees.

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The ignition and combustion of forest floor duff are poorly understood yet have been linked to soil heating and overstory tree mortality in many temperate coniferous forests. Research to date has focused on the characteristics of duff that facilitate ignition and spread, including fuel moisture, mineral content, and depth. Field observations suggest that the presence of pine cones on and within the forest floor might facilitate ignition of intermixed forest floor fuels. We investigated the effect of cone fuel additions on the ignition of underlying forest floor from fuels collected in long-unburned longleaf pine (Pinus palustris Mill.) forests in northern Florida, USA. Fuels were wetted to threshold gravimetric moisture contents to evaluate the relative effect on ignition. In stark contrast to fuel beds without cones, in which duff ignition only occurred in 17% of samples, those with cones added ignited the underlying duff 94% of the time. Flame heights were 40% taller and flaming duration was 47% longer in fuel beds with cones. Where present, pine cones act as vectors of ignition for forest floor fuels, and their role in fires deserves more attention to enhance our understanding of forest floor combustion.Résumé : Bien qu'on sache peu de choses au sujet de l'allumage et de la combustion de l'humus dans la couverture morte, le réchauffement du sol et la mortalité des arbres de l'étage dominant ont été mis en cause dans plusieurs forêts de conifères tempérées. À ce jour, la recherche s'est concentrée sur les caractéristiques de l'humus qui facilitent l'allumage et la propagation, incluant la teneur en humidité, la teneur en minéraux et l'épaisseur des combustibles. Les observations sur le terrain indiquent que la présence de cônes de pin dans la couverture morte et en surface pourrait faciliter l'allumage du mélange de combustibles dans la couverture morte. Nous avons étudié l'effet de l'ajout de cônes en tant que combustible sur l'allumage de la couverture morte sous-jacente à l'aide de combustibles prélevés dans les forêts de pin des marais (Pinus palustris Mill.) qui n'ont pas brûlé depuis longtemps dans le nord de la Floride, aux États-Unis. Les combustibles ont été humidifiés jusqu'au seuil de saturation selon la teneur en eau gravimétrique pour évaluer l'effet relatif sur l'allumage. Contrastant nettement avec les lits de combustibles exempts de cônes dans lesquels l'allumage de l'humus est survenu dans seulement 17 % des échantillons, l'allumage de l'humus sous-jacent s'est produit 94 % du temps dans ceux qui contenaient des cônes. Les flammes étaient 40 % plus hautes et la durée de l'embrasement 47 % plus longue dans les lits de combustibles contenant des cônes. Où ils sont présents, les cônes de pin agissent comme vecteurs d'allumage des combustibles dans la couverture morte et leur rôle en lien avec les feux mérite d'être davantage étudié afin d'améliorer notre compréhension de la combustion de la couverture morte. [Traduit par la Rédaction]

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Background Within California’s chaparral ecosystems, fuel reduction treatments are commonly used to reduce the negative impacts of wildfire but the durability of fuel treatment changes to fuels and vegetation when exposed to wildfire is less well understood. This study examined the interactive effects of 15-year-old fuel treatments and an extreme wildfire on burn severity, fuel loading, and vegetation in chaparral and oak vegetation types in Whiskeytown National Recreation Area in northern California, USA. Fuel treatment types included hand thinned, mechanical mastication, mechanical mastication + prescribed burning, and prescribed burning only. Results Vegetation and substrate burn severity was characterized as moderate across the study site and did not differ among treatments. Contrasting with higher pre-fire shrub density in the mastication + burning treatment, 2-year post-fire live shrub density did not differ among treatments. Higher pre-fire fine woody fuel loading in the mastication treatment did not correspond to post-fire fuel loading among treatments, while the hand thinned treatment was the only treatment where fine fuel loading was not significantly reduced post-fire. Total plant species richness increased in all treatment types following wildfire, largely driven by an increase in exotic species. Native cover decreased, and exotic cover increased in oak and chaparral types, but greater exotic species cover in the mastication + burning treatment in chaparral was maintained following wildfire. Conclusions Pre-fire differences in fuel and vegetation responses among treatments largely did not persist or were not detectible 1 to 2 years following wildfire. These findings suggest that the extreme wildfire conditions superseded long-term treatment differences in many fuel and vegetation metrics observed prior to wildfire. Despite subtle treatment differences, the hand thinned treatment resulted in the lowest change in fuel loading relative to all other treatments. Lastly, pre-fire differences in exotic species among fuel treatments were retained following wildfire, suggesting some treatments may have greater potential for exotic species expansion or type conversion to exotic grasslands.