Barros Ana M G, Pereira José M C
Centro de Estudos Florestais, Instituto Superior de Agronomia, Universidade de Lisboa, Lisboa, Portugal.
PLoS One. 2014 Jan 13;9(1):e84760. doi: 10.1371/journal.pone.0084760. eCollection 2014.
Previous research has shown that fires burn certain land cover types disproportionally to their abundance. We used quantile regression to study land cover proneness to fire as a function of fire size, under the hypothesis that they are inversely related, for all land cover types. Using five years of fire perimeters, we estimated conditional quantile functions for lower (avoidance) and upper (preference) quantiles of fire selectivity for five land cover types - annual crops, evergreen oak woodlands, eucalypt forests, pine forests and shrublands. The slope of significant regression quantiles describes the rate of change in fire selectivity (avoidance or preference) as a function of fire size. We used Monte-Carlo methods to randomly permutate fires in order to obtain a distribution of fire selectivity due to chance. This distribution was used to test the null hypotheses that 1) mean fire selectivity does not differ from that obtained by randomly relocating observed fire perimeters; 2) that land cover proneness to fire does not vary with fire size. Our results show that land cover proneness to fire is higher for shrublands and pine forests than for annual crops and evergreen oak woodlands. As fire size increases, selectivity decreases for all land cover types tested. Moreover, the rate of change in selectivity with fire size is higher for preference than for avoidance. Comparison between observed and randomized data led us to reject both null hypotheses tested ([Formula: see text] = 0.05) and to conclude it is very unlikely the observed values of fire selectivity and change in selectivity with fire size are due to chance.
先前的研究表明，火灾对某些土地覆盖类型的燃烧比例与其丰富程度不成正比。我们使用分位数回归来研究火灾发生时土地覆盖类型的易发性与火灾规模之间的函数关系，假设所有土地覆盖类型的两者呈负相关。利用五年的火灾边界数据，我们估计了一年生作物、常绿橡树林地、桉树林、松树林和灌木丛这五种土地覆盖类型的火灾选择性下限（避免）和上限（偏好）分位数的条件分位数函数。显著回归分位数的斜率描述了火灾选择性（避免或偏好）随火灾规模变化的速率。我们使用蒙特卡洛方法对火灾进行随机排列，以获得由随机因素导致的火灾选择性分布。该分布用于检验以下零假设：1）平均火灾选择性与通过随机重新定位观测到的火灾边界所获得的选择性没有差异；2）土地覆盖类型对火灾的易发性不随火灾规模而变化。我们的结果表明，灌木丛和松树林比一年生作物和常绿橡树林地更容易发生火灾。随着火灾规模的增加，所有测试的土地覆盖类型的选择性均降低。此外，偏好的选择性随火灾规模的变化速率高于避免的变化速率。观测数据与随机数据的比较使我们拒绝了所检验的两个零假设（[公式：见正文] = 0.05），并得出结论，火灾选择性的观测值以及选择性随火灾规模的变化极不可能是由随机因素导致的。
