The long-term viability of Los Angeles’ urban forests are at considerable risk due to the current drought and irrigation policy decisions made in response to the drought. These decisions are being made without the historical knowledge of how droughts and irrigation choices effect the long-term health of Los Angeles’ urban forests.
In response, this study investigated the use of Landsat-derived Normalized Difference Vegetation Index (NDVI) to quantify the historical vigor of eight Los Angeles' urban forests since 1985. By combining site visits, forest manager interviews on irrigation history and non-natural events and a dense stack of Landsat imagery (892 images) this study quantified the historical vigor of 408 individual 30 square meter plots of urban forest. By including a local monthly drought index (Palmer Drought Severity Index, PDSI), the impact of varying irrigation policies and drought on the different plots of urban forest became evident.
Our results show a strong correlation in the intra-annual, satellite-derived vigor of non-irrigated forests to the PDSI and a poor correlation of irrigated forests’ vigor to the PDSI. These results also show a strong difference in the inter-annual vigor of non-irrigated and irrigated forests, highlighting non-irrigated forests’ erratic and often stagnant vigor in comparison to irrigated forests. This analysis is intended to help policy-makers understand how changes in irrigation policy will impact the health of LA’s urban forests and what the long-term effects on forest sustainability will likely be.