Seasonal controls on ecosystem-scale CO2 and energy exchange in a Sonoran Desert characterized by the saguaro cactus (Carnegiea gigantea).

Episodic precipitation pulses are important for driving biological activity in desert ecosystems. The pattern of precipitation, including the size of rain events and the duration of time between events, can influence ecosystem net CO2 exchange (NEE) by shifting the balance between ecosystem photosynthesis and respiration. Our objective was to measure the response of NEE and its major components, to seasonal variation in precipitation and other environmental conditions. The study was conducted at a site, where 40-60% of annual precipitation comes from the North American Monsoon that typically brings rain in July-September, a time period when temperatures are near the seasonal peak. The results were compared to a model of the expected responses of NEE to seasonal changes in precipitation and temperature. We measured NEE using the eddy covariance technique during September 2015-August 2016. The ecosystem showed large (fivefold) seasonal variation in maximum photosynthesis and ecosystem respiration rate at 10 °C that corresponded to seasonal variation in precipitation and temperature. Ecosystem respiratory activity exceeded photosynthetic activity, so the ecosystem was a net source of CO2 to the atmosphere during June-October, a period that included monsoon rain inputs. Only during the winter months (November-March) did photosynthesis exceed respiration, resulting in net ecosystem carbon sequestration. The ecosystem recorded a net loss of 10 g C m-2 year-1, which was likely caused by below normal annual precipitation during the study. Our results illustrated the important interaction between seasonal variation in precipitation and temperature in controlling the ecosystem carbon budget.