Patterns and controls of long-term CO2 flux measurements through fire and drought events from three semi-arid chamise-dominated chaparral stands

Chaparral ecosystems (Adenostoma fasciculatum) are the most extensive native plant community in California and are a biodiversity hotspot for native fauna and flora. Undisturbed chaparral ecosystems have been reported to be significant carbon sinks under normal weather conditions (Luo et al., 2007). With climate models predicting an increase in fire and drought frequency in semi-arid regions due to climate change, carbon storage by chaparral ecosystems is likely to be affected by these changes (Storey et al., 2020). While attention to carbon source-sink dynamics in chaparral ecosystems has gained more interest over the recent decade, understanding the controls and patterns of long-term CO2 flux measurements through wildfire and drought events is still poorly understood. Here, I present long-term CO2 flux measurements from three varying-aged chaparral ecosystems to better understand the temporal patterns and controls on CO2 flux and the effects of stand age on carbon sequestration. Through this work, I found that under current climatic conditions, chaparral ecosystems can take up to a decade or more to sequester carbon following a fire and exposure to abnormal rainfall events as the investigated once burned ~20-year-old chaparral ecosystem (US-SO2) acted as a carbon source emitting up to 848 gCm-2yr-1 for eight years post-fire. The stand reverted back to a carbon sink sequestering -69 to -343 gCm-2yr-1 eleven years post-fire. Contrary to the claim that old-growth ecosystems are in a carbon-neutral state, I found that the investigated old-growth 178-year-old chaparral ecosystem (US-SO4) was a source of CO2 to the atmosphere releasing up to 447 gCm-2yr-1 (Odum, 1969; Salati and Vose, 1984; Tan et al., 2011). The meteorological controls of seasonal NEE in a twice burned ~20-year-old chaparral ecosystem (US-SO3) were soil temperature and relative humidity. Monthly NEE was driven by variations in soil temperature, net radiation, and relative humidity. US-SO3 released 45 g