How Much Forest Persists Through Fire? High-Resolution Mapping of Tree Cover to Characterize the Abundance and Spatial Pattern of Fire Refugia Across Mosaics of Burn Severity
How Much Forest Persists Through Fire? High-Resolution Mapping of Tree Cover to Characterize the Abundance and Spatial Pattern of Fire Refugia Across Mosaics of Burn Severity
复制标题
有多少森林在大火中幸存下来？
作者：
Ryan B. Walker;Jonathan D. Coop;William M. Downing;Meg A. Krawchuk;Sparkle L. Malone;Garrett W. Meigs
森林生态系统中的野火产生景观马赛克，包括相对未受影响的地区，称为火灾避难所。这些持续的森林覆盖可以支持火灾敏感的物种和生物遗产重要的火灾后森林恢复，但很少有人知道他们的丰度和分布在火灾周边地区。容易访问的30米分辨率的卫星图像和衍生的烧伤严重程度的产品通常用于表征火灾后的景观，然而，粗糙的图像分辨率，广义烧伤严重程度阈值，和其他限制，可以限制火灾避难所的准确表示。本研究量化了2000年至2003年期间发生在黄松和干针叶混交林的10起火灾中火灾避难所的丰度和模式。我们开发了高分辨率的地图火灾后的景观使用半自动化，基于对象的分类1米航空影像，进行图像和基于字段的准确性评估，并将这些与Landsat衍生的烧伤严重程度指标。烧伤范围内的火灾避难所面积为20%~ 57%。避难所的比例一般随着Landsat导出的烧伤严重程度的增加而下降，但仍占高严重程度地区的3-12%。补丁大小范围从1平方米孤立的树木，近8000公顷，中位补丁大小为0.01公顷，大大小于30米的Landsat像素。补丁大小呈负相关，烧伤的严重程度，距离火灾避难所从开放地区呈正相关。最后，当与1 m分辨率地图进行验证时，30 m火灾后标准化燃烧比（NBR）和相对差异标准化燃烧比（RdNBR）的优化阈值以77%的准确度描绘了火灾避难所。估计火灾避难所丰度的基础上，Landsat衍生的烧伤严重程度指标是不太可能检测到小，孤立的火灾避难所补丁。更高分辨率的地图可以提高对森林遗产分布的了解，并为火灾后的管理活动提供信息，包括重新造林和处理。
Wildfires in forest ecosystems produce landscape mosaics that include relatively unaffected areas, termed fire refugia. These patches of persistent forest cover can support fire-sensitive species and the biotic legacies important for post-fire forest recovery, yet little is known about their abundance and distribution within fire perimeters. Readily accessible 30-m resolution satellite imagery and derived burn severity products are commonly employed to characterize post-fire landscapes; however, coarse image resolution, generalized burn severity thresholds, and other limitations can constrain accurate representation of fire refugia. This study quantifies the abundance and pattern of fire refugia within 10 fires occurring in ponderosa pine and dry mixed-conifer forests between 2000 and 2003. We developed high-resolution maps of post-fire landscapes using semi-automated, object-based classification of 1-m aerial imagery, conducted imagery- and field-based accuracy assessments, and contrasted these with Landsat-derived burn severity metrics. Fire refugia area within burn perimeters ranged from 20% to 57%. Refugia proportion generally decreased with increasing Landsat-derived burn severity, but still accounted for 3–12% of areas classified as high severity. Patch size ranged from 1-m2 isolated trees to nearly 8000 ha, and median patch size was 0.01 ha—substantially smaller than a 30-m Landsat pixel. Patch size was negatively related to burn severity; distance to fire refugia from open areas was positively related to burn severity. Finally, optimized thresholds of 30-m post-fire normalized burn ratio (NBR) and relative differenced normalized burn ratio (RdNBR) delineated fire refugia with an accuracy of 77% when validated against the 1-m resolution maps. Estimations of fire refugia abundance based on Landsat-derived burn severity metrics are unlikely to detect small, isolated fire refugia patches. Finer-resolution maps can improve understanding of the distribution of forest legacies and inform post-fire management activities including reforestation and treatments.