Contemporary Primary Productivity and Associated Drivers in Southern California Chaparral Communities

Terrestrial carbon exchange is a critical and variable sink for atmospheric carbon dioxide, mitigating approximately 30% of anthropogenic CO¬2 emissions annually. The interannual dynamics of these exchanges are driven by meteorological and disturbance factors that could be exacerbated by changing climate. Semi-arid ecosystems in particular demonstrate the largest contribution to global terrestrial exchange variability. In Southern California, chaparral is an evergreen, semi-arid shrubland community that may have substantial interannual carbon exchange variability. This study addresses gaps in our understanding of chaparral carbon cycling, including the changing roles of various meteorological conditions, the accuracy of current modeling efforts, and the long-term source-sink dynamics of interannual exchange. Experimental design included multiple scales of carbon exchange measurements (from leaf-level to stand-level exchange) and remote sensing (from 3cm to 1km resolutions) to assess these changes over multiple time scales. Overall, vapor pressure deficit (VPD) was found to drive photosynthetic rate of sampled resprouting species, contrasting the historical prioritization of soil moisture. VPD was also found to drive errors in the MOD17 carbon exchange model for chaparral systems, resulting in a 30% underestimation of gross primary productivity (GPP) during the sampling period. Over the past two decades, Net Ecosystem exchange (NEE) in Southern California shrublands strongly correlated with precipitation, mediated by GPP. The average compensation point, where source/sink dynamics shifted was 128 mm precipitation year-1, ranging between 70 – 240 mm year-1 depending on community composition, previous disturbance, and stand age. This study works towards accurate projections and estimates of carbon exchange in chaparral communities, which is cited as a major component of net-zero strategies at global, state, and local levels.