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The effect of prescribed fire on sugar pine mortality in Sequoia and Kings Canyon National Parks


Fire is one of the main agents controlling composition, structure, and function of mixed conifer forests in the Sierra Nevada, CA. Over the past century, there has been a dramatic shift in the role of fire in these forests as fire regimes shifted from low intensity, frequent fires prior to European settlement in the region, to extended periods of fire exclusion due to a policy of active fire suppression. This led to many unintended consequences such as increased stand density, shifts in species composition towards more shade tolerant species such as white fir, and an increase in fire severity and extent when fires do occur due to increased fuels and changing climate. In the past several decades, prescribed fire has become one of the most important tools for forest restoration and management, yet its effects are still not fully understood. This uncertainty is especially true for sugar pine, a species that is being affected not only by changing forest conditions, but also by the introduced pathogen white pine blister rust. These multiple factors, and potential interactions among them, have led to population declines of sugar pine in some areas and have raised concerns about potential mortality during prescribed fire. The goal of this research was to better understand what processes were most important in controlling post-fire mortality of sugar pine, use this information to produce more accurate predictive models of post-fire mortality, and to evaluate simple management actions that could ameliorate the risk of mortality following fire.

The first chapter serves as an introduction to the mixed conifer ecosystem and sugar pine in particular. It discusses the main factors that control mortality following prescribed fire and highlights some of the main findings of this research. In the second chapter, structural equation modeling (SEM) was used to assess the direct and indirect effects of multiple variables related to tree health, beetle activity, blister rust, and fire effects and their relative importance in controlling sugar pine mortality following prescribed fire in Sequoia and Kings Canyon National Parks. Multiple factors are known to influence mortality following fire including fuel loads, fire intensity, beetle activity, and tree size, yet little is known about how these factors interact to control post-fire mortality. A total of 436 sugar pine were measured within three separate prescribed fires in Sequoia and Kings Canyon National Parks. SEM was used to evaluate a network of causal relationships between factors that affect post-fire mortality of sugar pine and to assess both direct and indirect effects. Several factors were found to significantly influence post-fire mortality, with crown volume scorch, diameter at breast height (dbh), and post-fire beetle activity showing the strongest effects; though the magnitude of these effects differed among sites. Other factors such as blister rust infection and pre-fire beetle activity had little impact on post-fire mortality. A causal model was developed that considered both indirect and direct effects of multiple factors associated with post-fire mortality while demonstrating the variability in the relative strength of these causal relationships based on specific attributes. This model can be used in forest management to provide a clear understanding of how fire effects interrelate with multiple processes to control post fire sugar pine mortality.

The third chapter examined whether the inclusion of pre-fire tree health (based on tree ring records) in models looking at post-fire sugar pine mortality improved model fit over models based on measures of fire effects alone. This study was conducted within an old-growth mixed-conifer forest in Sequoia National Park that had been prescribed burned during 2001 or 2002. Fire effects measured by percent crown volume scorched and stem char height, and pre-fire tree health measured by multiple indices of growth calculated from tree cores and measures of crown health were assessed for 105 sugar pine. Health status (live or dead) was observed prior to the fire, immediately post-fire and five years post-fire. Logistic regression models were used to evaluate the effects of fire and pre-fire tree health on post-fire mortality. Models based only on tree size and fire effects were compared to models that included fire effects and measures of pre-fire tree health using AICc. Five years following fire, the model that best predicted mortality included dbh, crown volume scorch, 30 year growth trend, and count of sharp declines over a 30 year period. The inclusion of long-term measures of growth markedly improved model fit compared to models based only on fire effects (ΔAICc = 26.4). However, immediately after fire, models that included measures of pre-fire tree health resulted in only marginal improvements over models based only on measures of fire effects (ΔAICc = 2.1). These results imply that multiple processes, in addition to fire, are functioning to influence delayed mortality and that the inclusion of measures of tree health can provide more accurate predictions of post-fire mortality.

Finally, chapter four tests whether raking away duff and litter from the base of the stem can be used as an effective means of reducing sugar pine mortality following prescribed fire. This study was conducted in three prescribed fires in Sequoia and Kings Canyon National Parks and included 457 trees, half of which were raked to mineral soil to 0.5 m away from the stem. Fire effects were assessed and tree mortality was followed for three years after prescribed fires. Overall, raking had no detectable effect on mortality as raked trees averaged 30 % mortality compared to 36 % for unraked trees. There was a significant interaction, however, between raking and average pre-treatment forest floor fuel depth: The predicted probability of survival of a 50 cm dbh tree was 0.94 vs. 0.96 when average pre-treatment fuel depth was 0 cm for a raked and unraked tree, respectively. When average pre-treatment forest floor fuel depth was 30 cm, the predicted probability of survival for a raked 50 cm tree was 0.60 compared to only 0.07 for an unraked tree. Raking did not affect mortality when fire intensity, measured as percent crown volume scorched, was very low (0 % scorch) or very high (>80 % scorch), but the raking treatment significantly increased the proportion of trees that survived by 9.6 % for trees that burned under moderate fire intensity (1 % to 80 % scorch). Raking significantly reduced the likelihood of bole charring and bark beetle activity three years post-fire. This implies that raking can be an effective management tool to reduce tree mortality following prescribed fire under specific fuel and burning conditions.

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