New biomass estimates for chaparral-dominated Southern California landscapes

Abstract

Chaparral shrublands are the dominant wildland vegetation type in Southern California and the most extensive ecosystem in the state. Disturbance by wildfire and climate change have created a dynamic landscape in which biomass mapping is key in tracking the ability of chaparral shrublands to sequester carbon. Despite this importance, most national and regional scale estimates do not account for shrubland biomass. Employing plot data from several sources, we built a random forest model to predict aboveground live biomass in Southern California using remote sensing data (Landsat Normalized Difference Vegetation Index (NDVI)) and a suite of geophysical variables. By substituting the NDVI and precipitation predictors for any given year, we were able to apply the model to each year from 2000 to 2019. Using a total of 980 field plots, our model had a k-fold cross-validation R² of 0.51 and an RMSE of 3.9. Validation by vegetation type ranged from R² = 0.17 (RMSE = 9.7) for Sierran mixed-conifer to R² = 0.91 (RMSE = 2.3) for sagebrush. Our estimates showed an improvement in accuracy over two other biomass estimates that included shrublands, with an R² = 0.82 (RMSE = 4.7) compared to R² = 0.068 (RMSE = 6.7) for a global biomass estimate and R² = 0.29 (RMSE = 5.9) for a regional biomass estimate. Given the importance of accurate biomass estimates for resource managers, we calculated the mean year 2010 shrubland biomasses for the four national forests that ranged from 3.5 kg/m² (Los Padres) to 2.3 kg/m² (Angeles and Cleveland). Finally, we compared our estimates to field-measured biomasses from the literature summarized by shrubland vegetation type and age class. Our model provides a transparent and repeatable method to generate biomass measurements in any year, thereby providing data to track biomass recovery after management actions or disturbances such as fire.

Keywords

Biomass, carbon sequestration, chaparral, NDVI, fire frequency, Mediterranean-type climate regions, national forests.

Citation

Schrader-Patton, Charlie C.; Underwood, Emma C. 2021. New biomass estimates for chaparral-dominated Southern California landscapes. Remote Sensing. 13(8): 1581. https://doi.org/10.3390/rs13081581.