Smithsonian Conservation Biology Institute, Front Royal, VA, USA.
Harvard Forest, Petersham, MA, USA.
Glob Chang Biol. 2017 Oct;23(10):4117-4132. doi: 10.1111/gcb.13704. Epub 2017 Apr 27.
In the context of ongoing climatic warming, certain landscapes could be near a tipping point where relatively small changes to their fire regimes or their postfire forest recovery dynamics could bring about extensive forest loss, with associated effects on biodiversity and carbon-cycle feedbacks to climate change. Such concerns are particularly valid in the Klamath Region of northern California and southwestern Oregon, where severe fire initially converts montane conifer forests to systems dominated by broadleaf trees and shrubs. Conifers eventually overtop the competing vegetation, but until they do, these systems could be perpetuated by a cycle of reburning. To assess the vulnerability of conifer forests to increased fire activity and altered forest recovery dynamics in a warmer, drier climate, we characterized vegetation dynamics following severe fire in nine fire years over the last three decades across the climatic aridity gradient of montane conifer forests. Postfire conifer recruitment was limited to a narrow window, with 89% of recruitment in the first 4 years, and height growth tended to decrease as the lag between the fire year and the recruitment year increased. Growth reductions at longer lags were more pronounced at drier sites, where conifers comprised a smaller portion of live woody biomass. An interaction between seed-source availability and climatic aridity drove substantial variation in the density of regenerating conifers. With increasing climatic water deficit, higher propagule pressure (i.e., smaller patch sizes for high-severity fire) was needed to support a given conifer seedling density, which implies that projected future increases in aridity could limit postfire regeneration across a growing portion of the landscape. Under a more severe prospective warming scenario, by the end of the century more than half of the area currently capable of supporting montane conifer forest could become subject to minimal conifer regeneration in even moderate-sized (10s of ha) high-severity patches.
在持续的气候变暖背景下，某些景观可能处于临界点附近，其火灾发生频率或火灾后森林恢复动态的微小变化，可能导致广泛的森林丧失，从而对生物多样性和碳循环对气候变化的反馈产生相关影响。这种担忧在加利福尼亚州北部和俄勒冈州西南部的克拉马斯地区尤为明显，在那里，严重的火灾最初会将山地针叶林转变为以阔叶树和灌木为主的系统。针叶树最终会取代竞争植被，但在它们这样做之前，这些系统可能会通过重新燃烧的循环得以维持。为了评估在更温暖、更干燥的气候条件下，针叶林对火灾活动增加和森林恢复动态变化的脆弱性，我们在过去三十年的九个火灾年份中，在山地针叶林气候干旱梯度上，对严重火灾后的植被动态进行了特征描述。火灾后针叶树的繁殖受到限制，只有 89%的繁殖发生在最初的 4 年内，而且随着火灾年份和繁殖年份之间的时间间隔增加，其生长趋势会下降。在较干燥的地点，生长减少更为明显，因为那里的针叶树在活体木质生物量中所占比例较小。种子源供应与气候干旱之间的相互作用导致了再生针叶树密度的显著变化。随着气候水分亏缺的增加，需要更高的繁殖体压力（即高严重度火灾的斑块尺寸更小）来支持给定的针叶树幼苗密度，这意味着未来气候干燥度的增加可能会限制景观中越来越大的部分的火灾后再生。在一个更严重的预期变暖情景下，到本世纪末，目前有一半以上的地区能够支持山地针叶林，即使在中等规模（数十公顷）的高严重度斑块中，也可能会出现最小的针叶树再生。
