Multidecadal trends and interactions in land disturbance regimes across California from time series remote sensing

Abstract:

California experiences a variety of disturbances, such as fires, timber harvest, and drought-induced forest die-off, that threaten its natural resources. Because California climate change policy includes the protection of forests and shrublands, it is essential to understand how disturbance regimes have changed in response to climate change, human intervention, or disturbance history. However, current records of disturbance can be coarse and incomplete, making it challenging to assess these feedbacks. To address this, we developed a state-wide map of land disturbances using time series of Landsat surface reflectance data. We applied the Continuous Change Detection and Classification (CCDC) algorithm to all Landsat pixels to build annual (1984 – 2019) maps of potential disturbances at 30 m spatial resolution. We then trained a Random Forest algorithm on the intersection of a sample of 122,533 CCDC-derived breaks with existing disturbance databases. With this algorithm, we classified CCDC-derived breaks as fires, harvests, or forest die-off with an overall accuracy of 85.6%. Preliminary results show that 23,126 km² (17.8%) of the 130,478 km² mapped forest area experienced at least one disturbance between 1984 and 2019. Fires afflicted the largest area (15,272 km²), followed by harvest (12,723 km²) and die-off (1,223 km²). While harvests showed little trend, burned area increased over time, with large burns mostly occurring post-2000. Forest die-off occurs almost exclusively after 2012, despite earlier droughts in the record. We map 1,433 km² less area of forest harvest than is reported by state and federal agencies over 1984-2019. We additionally explore feedbacks and interactions between climate and disturbance history, including how past disturbances influence the probability and severity of subsequent disturbances. These data improve our understanding of the efficacy of forest management programs on the prevention of future catastrophic forest fires and potential feedbacks between disturbance regime, climate, and the vulnerability of California’s natural resources.