NOAA/WDS Paleoclimatology - O'Connor fire data from Pinalenos_J18, Southeast Arizona - IMPD USJ18001

In recent decades fire size and severity have been increasing in high elevation forests of the American Southwest. Ecological outcomes of these increases are difficult to gauge without an historical context for the role of fire in these systems prior to interruption by Euro-American land uses. Across the gradient of forest types in the Pinaleño Mountains, a Sky Island system in southeast Arizona that experienced two relatively large high-severity fires in the last two decades, we compared fire characteristics and climate associations before and after the onset of fire exclusion to determine the degree of similarity between past and recent fires. We use a gridded fire scar and demography sampling network to reconstruct spatially explicit estimates of fire extent and burn severity, as well as climate associations of fires from individual site to landscape scales from 1640 to 2008 C.E. We found that patterns of fire frequency, size, and severity were relatively stable for at least several centuries prior to 1880. A combination of livestock grazing and active fire suppression after circa 1880 led to (1) a significant reduction in fire spread but not fire ignition, (2) a conversion of more than 80% of the landscape from a frequent, low to mixed-severity fire regime to an infrequent mixed to high-severity fire regime, and (3) an increase in fuel continuity within a mid-elevation zone of dry mixed-conifer forest, resulting in increased opportunities for surface and crown fire spread into higher elevation mesic forests. The two most recent fires affecting mesic forests were associated with drought and temperature conditions that were not exceptional in the historical record but that resulted in a relative proportion of high burn severity up to four times that of previous large fires. The ecological effects of these recent fires appear to be more severe than any fire in the reconstructed period, casting uncertainty upon the recovery of historical species composition in high-severity burn patches. Significant changes to the spatial pattern, frequency, and climate associations of spreading fires after 1880 suggest that limits to fuel loading and fuel connectivity sustained by frequent fire have been removed. Coinciding factors of high fuel continuity and fuel loading, projected lengthening of the fire season, and increased variability in seasonal precipitation suggest that large high-severity fires, especially in mixed-conifer forests, will become the predominant fire type without aggressive actions to reduce fuel continuity and restore fire-resilient forest structure and species composition.

近数十年来，美国西南部（American Southwest）高海拔森林的火灾规模与烈度持续攀升。若缺乏欧美土地利用活动干扰前，火灾在该生态系统中所扮演角色的历史背景，则难以评估此类火灾增长所带来的生态后果。本研究以皮纳莱诺山脉（Pinaleño Mountains）——亚利桑那州东南部的天空群岛（Sky Island）生态系统，过去二十年间曾发生两起规模较大的高烈度火灾——内沿森林类型梯度分布的区域为研究对象，对比了禁火政策实施前后的火灾特征与气候关联模式，以厘清历史火灾与近期火灾的相似程度。本研究采用网格化火灾疤痕与年代学采样网络，重建了公元1640年至2008年间，从单一场点到景观尺度的火灾范围、燃烧烈度的空间显式估算结果，以及不同尺度下火灾的气候关联模式。研究发现，1880年之前的数百年间，火灾发生频率、规模与烈度的变化模式相对稳定。约1880年之后，家畜放牧与主动灭火措施共同引发了三类变化：(1) 火灾蔓延范围显著缩减，但火灾起火点并未减少；(2) 超80%的景观区域从"高频、低-混合烈度"火灾制度（fire regime）转变为"低频、混合-高烈度"火灾制度；(3) 干旱针阔混交林的中海拔区域内可燃物连续性提升，使得地表火与树冠火更容易蔓延至高海拔中生林（mesic forest）区域。近期两起波及中生林的火灾，其发生与干旱及温度条件相关——这类气候条件在历史记录中并非罕见，但其引发的高烈度燃烧占比却达到过往大型火灾的四倍之多。此类近期火灾的生态影响似乎比重建记录中所有火灾都更为严重，这使得高烈度燃烧斑块内历史物种组成的恢复前景充满不确定性。1880年后，蔓延火灾的空间格局、发生频率与气候关联模式均发生显著变化，这表明高频火灾原本维持的可燃物载量与连通性限制已被解除。高可燃物连续性与载量、预计延长的火灾季，以及季节降水变率增大等多重因素叠加表明，若不采取积极措施降低可燃物连续性、恢复具有火灾韧性（fire-resilient）的森林结构与物种组成，大型高烈度火灾（尤其在针阔混交林中）将成为主导性火灾类型。