首页|Spectral unmixing of a multi-decadal Landsat time sequence to reconstruct herbaceous fractional cover dynamics in wildfire-prone Mediterranean-type ecosystems
Spectral unmixing of a multi-decadal Landsat time sequence to reconstruct herbaceous fractional cover dynamics in wildfire-prone Mediterranean-type ecosystems
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Taylor & Francis
ABSTRACT Portions of Southern California’s native shrubland communities are being invaded and sometimes replaced by herbaceous vegetation that increases the risk of wildfire ignition and spread in a positive feedback loop called the grass-fire cycle. The objective of this study was to assess the extent to which herbaceous cover has expanded and replaced woody vegetation over the last three decades in San Diego County shrublands. To do this, we reconstructed the spatial-temporal distribution of herbaceous growth form cover using spectral mixture analysis (SMA) applied to Landsat multispectral data from 1988 to 2020. The average error in herbaceous cover maps generated from images captured during four single years within the 33-year study period exhibited a mean absolute error (MAE) = 13.30%, root mean square error (RMSE) = 17.62%, and coefficient of determination (R2) = 0.76 relative to reference data derived from orthoimagery. Error estimates for absolute change in herbaceous cover from the earliest (1988) and recent (2020) dates were MAE = 12.17% and RMSE = 15.57% (assessed using 94 reference sampling grids). Between 1988 and 2020, 26.61% of the full study area exhibited an increase in herbaceous cover >20% and 4.98% experienced a decrease in herbaceous cover <−20%, with the greatest concentration of change occurring in wildland-urban interface (WUI) areas. The factors most strongly associated with a substantial increase in herbaceous cover included fire return interval, drought, proximity to development, and elevation. In addition to the overall expansion of herbaceous cover, we also identified locations with evidence of vegetation-type conversion from woody- to herbaceous-dominated fractional cover. These results suggest that a grass-fire cycle has been established in Southern California. The methods from this work can be applied to Mediterranean-type ecosystems around the world to quantify and monitor herbaceous vegetation change over time.
Krista R. Lee West、Douglas A. Stow、Daniel J. Sousa、John F. O’Leary、Dar A. Roberts、Alexandra D. Syphard、Leila M. V. Carvalho、Megan K. Jennings
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San Diego State University||University of California
San Diego State University
University of California
San Diego State University||Conservation Biology Institute
NETL
NSTL
Taylor & Francis
