Miquelajauregui Yosune, Cumming Steven G, Gauthier Sylvie
Centre d'étude de la forêt, Département des sciences du bois et de la forêt, Faculté de foresterie, de géographie et de géomatique, Université Laval, Québec, Québec, Canada.
Canadian Forest Service, Laurentian Forestry Centre, Québec, Québec, Canada.
PLoS One. 2016 Feb 26;11(2):e0150073. doi: 10.1371/journal.pone.0150073. eCollection 2016.
It is becoming clear that fires in boreal forests are not uniformly stand-replacing. On the contrary, marked variation in fire severity, measured as tree mortality, has been found both within and among individual fires. It is important to understand the conditions under which this variation can arise. We integrated forest sample plot data, tree allometries and historical forest fire records within a diameter class-structured model of 1.0 ha patches of mono-specific black spruce and jack pine stands in northern Québec, Canada. The model accounts for crown fire initiation and vertical spread into the canopy. It uses empirical relations between fire intensity, scorch height, the percent of crown scorched and tree mortality to simulate fire severity, specifically the percent reduction in patch basal area due to fire-caused mortality. A random forest and a regression tree analysis of a large random sample of simulated fires were used to test for an effect of fireline intensity, stand structure, species composition and pyrogeographic regions on resultant severity. Severity increased with intensity and was lower for jack pine stands. The proportion of simulated fires that burned at high severity (e.g. >75% reduction in patch basal area) was 0.80 for black spruce and 0.11 for jack pine. We identified thresholds in intensity below which there was a marked sensitivity of simulated fire severity to stand structure, and to interactions between intensity and structure. We found no evidence for a residual effect of pyrogeographic region on simulated severity, after the effects of stand structure and species composition were accounted for. The model presented here was able to produce variation in fire severity under a range of fire intensity conditions. This suggests that variation in stand structure is one of the factors causing the observed variation in boreal fire severity.
越来越明显的是，北方森林火灾并非一律会造成整片林分的更替。相反，在个体火灾内部和之间，均发现了以树木死亡率衡量的火灾严重程度存在显著差异。了解这种差异产生的条件非常重要。我们将加拿大魁北克省北部单种黑云杉和短叶松1.0公顷斑块的直径级结构模型中的森林样地数据、树木异速生长模型和历史森林火灾记录进行了整合。该模型考虑了树冠火的引发以及垂直蔓延到树冠层的情况。它利用火灾强度、焦高、树冠烧焦百分比与树木死亡率之间的经验关系来模拟火灾严重程度，特别是因火灾致死导致的斑块基部面积减少的百分比。对大量模拟火灾的随机样本进行随机森林和回归树分析，以测试火线强度、林分结构、物种组成和火地理区域对最终严重程度的影响。严重程度随强度增加而增加，短叶松林分的严重程度较低。模拟的高严重程度火灾（例如斑块基部面积减少>75%）的比例，黑云杉为0.80，短叶松为0.11。我们确定了强度阈值，低于该阈值时，模拟火灾严重程度对林分结构以及强度与结构之间的相互作用具有显著敏感性。在考虑了林分结构和物种组成的影响后，我们没有发现火地理区域对模拟严重程度有残留影响的证据。这里提出的模型能够在一系列火灾强度条件下产生火灾严重程度的差异。这表明林分结构的差异是导致北方森林火灾严重程度出现观测到的差异的因素之一。
