Vegetation-climate interactions in California – an in-depth analysis on the influence of climatic events across different Californian biomes

Sammanfattning: It is widely accepted that climate variability is a key driver of vegetation productivity. Yet, there are discrepancies on the ideal timescales of climatic events and vegetation response. The work herein attempts to clarify how those variables interact in the region of California. The Standard Precipitation Evapotranspiration Index (SPEI), a drought index, was used as an indicator of interannual climate variability in the region. Vegetation productivity was accounted with Normalized Difference Vegetation Index (NDVI) or net growth point data. In this study, four parameters were tested: the length of climate events influencing vegetation, the ideal time to be accounted as vegetation response, the start of the growing season, and the lag between climate and vegetation response.  In total, 594 different scenarios were simulated, with 432 considering the correlation between SPEI and NDVI anomalies and the remaining between SPEI and net growth. The findings shows that the Hot Deserts of California have an early start of the drought season, in March or April, with climate events from 6 months prior influencing vegetation greenness for the next 3 months. In those deserts, the direct correlations between SPEI and NDVI have been the highest, of 0.70 (Mojave) and 0.64 (Sonoran), meaning that, in these locations drier periods will decrease vegetation health. Cold Deserts present a later start of the drought season, in May. Vegetation in these regions will have a delayed response to droughts, with scenarios of 1 to 2 months lags between climate events and vegetation response presenting the highest correlations between SPEI and NDVI. Response that is also longer with climatic events influencing the next 9 months of vegetation greenness. When the correlations were significant, Mediterranean California behaved similarly to cold deserts, with a lag between climate and vegetation, and even longer periods of climatic influence on vegetation, of up to 12 months. In colder regions of California, entailing the entire Western Cordillera, Cold Deserts, and some regions of Mediterranean California an inverse relationship between SPEI and NDVI was found. Drier periods early in the season, in March or April will cause vegetation to be greener during the following months. In cold deserts and Mediterranean California, this climate vegetation relationship happened for short climatic events, as only the previous months will have an impact on vegetation for the following three months. The Cordillera was influenced by longer climatic events, of up to three months, and was the location that showed the best inverse correlations between NDVI and SPEI. In these locations, an early snowmelt and higher temperatures, leading to higher evapotranspiration, could explain the increase in greenness of vegetation by drier periods. However, this observation does not hold when considering a larger scale of climatic events. The correlation between SPEI and Net Growth has showed that when longer periods are considered, with climatic events of 12 or 24 months, a decrease in the net growth of plants will happen for the following season. As Californian climate is predicted to become more extreme it is of great importance understanding the possible consequences for vegetation.