Wildfire severity data for Sierra Nevada-Southern Cascades from 1984–2020

Although fire is a fundamental ecological process in western North American forests, climate warming and accumulating forest fuels due to fire suppression have led to wildfires that burn at high severity across larger fractions of their footprint than were historically typical. These trends have spiked upwards in recent years and are particularly pronounced in the Sierra Nevada-southern Cascades ecoregion of California, USA and neighboring states. We assessed annual area burned and percentage of area burned at high and low-to-moderate severity for seven major forest types in this region from 1984 to 2020. We compared values for this period against estimates for the pre-Euro-American settlement (EAS) period prior to 1850 and against a previous study of trends from 1984–2009. Our results show that total average annual area burned remained below pre-EAS levels, but that gap is decreasing (i.e., c. 14% of pre-EAS for 1984–2009, but 39% for 2010–2020 [including c. 150% in 2020]). Although average annual area burned has remained low compared to pre-EAS, both the average annual area burned at high severity and the percentage of wildfire area burned at high severity have increased rapidly. The percentage of area burned at high severity – which was already above pre-EAS average for the 1984–2009 period – has continued to rise for five of seven forest types. Notably, between 2010 and 2020, the average annual area burned at high severity exceeded the pre-EAS average for the first time on record. By contrast, percentage of area that burned at low-to-moderate severity decreased, particularly in the lower elevation oak and mixed conifer forest types. These findings underline how forests historically adapted to frequent low-to-moderate-severity fire are being reshaped by novel proportions and extents of high-severity burning. The shift toward a high severity-dominated fire regime is associated with ecological disruptions, including changes in forest structure, species composition, carbon storage, wildlife habitat, ecosystem services, and resilience. Our results underscore the importance of finding a better balance between the current management focus on fire suppression and one that puts greater emphasis on proactive fuel reduction and increased forest resilience to climate change and ecological disturbance.

Methods
The primary source of fire severity data came from the "Vegetation Burn Severity – 1984 to 2017” geospatial data layer (USDA 2018). For the area of analysis, we the same perimeter used by Mallek et al. (2013) and Miller et al. (2009), and is based on the Sierra Nevada ecoregion as defined by the Sierra Nevada Ecosystem Project (SNEP 1996) and the Sierra Nevada Forest Plan Amendment (SNFPA; USDA 2004). We included fires greater than or equal to 80 ha in size (see Methods for specifics). For the 2018–2020 fire years, we estimated burn severity using Google Earth Engine following Parks et al. (2018c, 2021). For the 2018–2020 fires we used the attached zip file of fire perimeters.

Mallek, C., H. Safford, J. Viers, and J. Miller. 2013. Modern departures in fire severity and area vary by forest type, Sierra Nevada and southern Cascades, California, USA. Ecosphere 4.
Parks, S. A., L. M. Holsinger, M. A. Voss, R. A. Loehman, and N. P. Robinson. 2018c. Mean Composite Fire Severity Metrics Computed with Google Earth Engine Offer Improved Accuracy and Expanded Mapping Potential. Remote Sensing 10.
Parks, S. A., L. M. Holsinger, M. A. Voss, R. A. Loehman, and N. P. Robinson. 2021. Correction: Mean Composite Fire Severity Metrics Computed with Google Earth Engine Offer Improved Accuracy and Expanded Mapping Potential. Remote Sens. 10, 879, 2018. Remote Sensing 13.
SNEP. 1996. Sierra Nevada Ecosystem Project: Final report to Congress.in C. f. W. a. W. Resources, editor. University of California, Davis, California, USA.
USDA. 2004. Sierra Nevada Forest Plan Amendment, Record of Decision.in P. S. Region, editor. USDA Forest Service, Vallejo, California, USA.
USDA. 2018. R5 VegBurnSeverity - Metadata.in P. S. R. USDA Forest Service, editor., USDA Forest Service, Pacific Southwest Region, California.

尽管林火是北美西部森林的核心生态过程，但气候变暖以及因林火抑制（fire suppression）累积的森林可燃物，已导致林火以远超历史常规的高烈度、大范围蔓延。近年来这一趋势愈发加剧并显著攀升，尤其在美国加利福尼亚州内华达山脉-南喀斯喀特生态区（Sierra Nevada-southern Cascades ecoregion）及邻近州表现尤为突出。

本研究针对该区域7种主要森林类型，评估了1984年至2020年间的年均过火面积，以及高烈度（high severity）、中低烈度（low-to-moderate severity）过火面积的占比。将该时段的数据与1850年欧洲裔定居（Euro-American settlement, EAS）前的史前基准数据，以及2009年之前的一项1984–2009年林火趋势研究结果进行了对比。

研究结果显示，年均总过火面积仍低于欧洲裔定居前水平，但二者的差距正在逐步缩小：1984–2009年间，该数值约为史前水平的14%，而2010–2020年间已升至39%（2020年甚至达到约150%）。尽管年均总过火面积相较史前水平仍偏低，但年均高烈度过火面积以及高烈度过火面积占林火总过火面积的比例均快速攀升。1984–2009年间，高烈度过火占比已高于史前平均水平，此后7种森林类型中有5种的该占比持续上升。值得注意的是，2010–2020年间，年均高烈度过火面积首次突破史前平均记录。与之形成对比的是，中低烈度过火面积占比出现下降，在低海拔橡木林与混合针叶林类型中尤为明显。

这些研究结果凸显出，原本适应高频次中低烈度林火的森林，正被新型高烈度林火的规模与范围重塑。以高烈度为主导的林火制度（fire regime）转变，会引发一系列生态扰动，包括森林结构、物种组成、碳储量、野生动物栖息地、生态系统服务以及生态恢复力的改变。本研究结果强调，当前过度侧重林火抑制的管理模式，亟需与更强调主动可燃物减量管理以及提升森林对气候变化与生态扰动的恢复力的管理模式找到更好的平衡。

## 研究方法
林火烈度数据的主要来源为“植被火烧烈度——1984–2017”地理空间数据图层（美国农业部[USDA]，2018）。本研究沿用了Mallek等（2013）与Miller等（2009）所使用的分析边界，该边界基于内华达山脉生态系统项目（Sierra Nevada Ecosystem Project, SNEP，1996）以及《内华达山脉森林计划修正案》（Sierra Nevada Forest Plan Amendment, SNFPA；USDA，2004）所定义的内华达山脉生态区。研究纳入了面积≥80公顷的林火，具体细节参见研究方法部分。对于2018–2020年的林火，本研究遵循Parks等（2018c、2021）的方法，通过谷歌地球引擎（Google Earth Engine, GEE）估算火烧烈度。2018–2020年的林火边界数据采用随附的压缩包文件。

## 参考文献
Mallek, C., H. Safford, J. Viers, and J. Miller. 2013. Modern departures in fire severity and area vary by forest type, Sierra Nevada and southern Cascades, California, USA. *Ecosphere* 4.
Parks, S. A., L. M. Holsinger, M. A. Voss, R. A. Loehman, and N. P. Robinson. 2018c. Mean Composite Fire Severity Metrics Computed with Google Earth Engine Offer Improved Accuracy and Expanded Mapping Potential. *Remote Sensing* 10.
Parks, S. A., L. M. Holsinger, M. A. Voss, R. A. Loehman, and N. P. Robinson. 2021. Correction: Mean Composite Fire Severity Metrics Computed with Google Earth Engine Offer Improved Accuracy and Expanded Mapping Potential. *Remote Sens.* 10, 879, 2018. *Remote Sensing* 13.
SNEP. 1996. Sierra Nevada Ecosystem Project: Final report to Congress. in C. f. W. a. W. Resources, editor. University of California, Davis, California, USA.
USDA. 2004. Sierra Nevada Forest Plan Amendment, Record of Decision. in P. S. Region, editor. USDA Forest Service, Vallejo, California, USA.
USDA. 2018. R5 VegBurnSeverity - Metadata. in P. S. R. USDA Forest Service, editor., USDA Forest Service, Pacific Southwest Region, California.