Newberry Brooklynn M, Power Collin R, Abreu Rodolfo C R, Durigan Giselda, Rossatto Davi R, Hoffmann William A
Department of Plant and Microbial Biology, North Carolina State University, Campus Box 7612, Raleigh, NC, 27695, USA.
Department of Fisheries, Wildlife, and Conservation Biology, North Carolina State University, Raleigh, NC, 27695, USA.
New Phytol. 2020 Nov;228(3):910-921. doi: 10.1111/nph.16742. Epub 2020 Jul 15.
Vegetation-fire feedbacks are important for determining the distribution of forest and savanna. To understand how vegetation structure controls these feedbacks, we quantified flammability across gradients of tree density from grassland to forest in the Brazilian Cerrado. We experimentally burned 102 plots, for which we measured vegetation structure, fuels, microclimate, ignition success and fire behavior. Tree density had strong negative effects on ignition success, rate of spread, fire-line intensity and flame height. Declining grass biomass was the principal cause of this decline in flammability as tree density increased, but increasing fuel moisture contributed. Although the response of flammability to tree cover often is portrayed as an abrupt, largely invariant threshold, we found the response to be gradual, with considerable variability driven largely by temporal changes in atmospheric humidity. Even when accounting for humidity, flammability at intermediate tree densities cannot be predicted reliably. Fire spread in savanna-forest mosaics is not as deterministic as often assumed, but may appear so where vegetation boundaries are already sharp. Where transitions are diffuse, fire spread is difficult to predict, but should become increasingly predictable over multiple fire cycles, as boundaries are progressively sharpened until flammability appears to respond in a threshold-like manner.
植被-火灾反馈对于确定森林和稀树草原的分布至关重要。为了了解植被结构如何控制这些反馈，我们在巴西塞拉多地区，对从草地到森林的树木密度梯度上的可燃性进行了量化。我们对102个样地进行了实验性燃烧，测量了植被结构、燃料、小气候、点火成功率和火灾行为。树木密度对点火成功率、蔓延速度、火线强度和火焰高度有强烈的负面影响。随着树木密度增加，草地生物量下降是可燃性下降的主要原因，但燃料湿度增加也有一定作用。尽管可燃性对树木覆盖的响应通常被描述为一个突然的、基本不变的阈值，但我们发现这种响应是渐进的，很大程度上由大气湿度的时间变化驱动，存在相当大的变异性。即使考虑到湿度，中等树木密度下的可燃性也无法可靠预测。稀树草原-森林镶嵌体中的火灾蔓延并不像通常假设的那样具有确定性，但在植被边界已经清晰的地方可能看起来是这样。在过渡区域不明显的地方，火灾蔓延很难预测，但在多个火灾周期中应该会越来越容易预测，因为边界会逐渐清晰，直到可燃性似乎以阈值般的方式做出响应。
