Impacts of the CZU Lightning Complex Fire of August 2020 on the forests of Big Basin Redwoods State Park

Abstract

The CZU Lightning Complex Fire started on 16 August 2020 and burned across more than 35,000 ha (80,000 acres) of forest lands in Santa Cruz County, California. In this study, Landsat satellite images of pre- and post-fire vegetation cover from 2020 were used to first map burn severity (low, moderate, high fraction) patterns on the CZU Fire landscape in and around Big Basin Redwoods State Park (BBRSP). For mapping of live regrowing versus currently dead forest stands, changes in the normalized difference vegetation index (NDVI) derived from 10-m resolution Sentinel satellite imagery (post-CZU Fire) were transformed into a new assessment metric called the Recovery-Regrowth-Green-Index (RRGI). The RRGI result derived from Sentinel NDVI change from October 2020 to July 2022 showed that just 24% of the burned forest cover in BBRSP was still alive and regrowing to a moderate level of new green canopy cover. Field surveys in BBRSP in July 2022 showed that trees not having attained a RRGI class level of 3 or greater, with sprouting of new green foliage on most of their horizontal limbs two years after the CZU Fire, were no longer alive and growing back. The unprecedented intensity of the CZU Fires together with two successive years (2021 and 2022) of extreme drought and summer heat has inflicted severe damage on the majority of old-growth trees in BBRSP.