Digital image-derived greenness links deep soil moisture to carbon uptake in a creosotebush-dominated shrubland

Changes in the timing, frequency, and magnitude of precipitation events are projected for semiarid ecosystems worldwide. The ecological consequences associated with these precipitation changes will be better understood if the hydrological triggers of vegetation response can be better identified. Previous research has suggested that soil moisture, likely from large monsoon rainstorms, plays a critical role in triggering the phenological response of semiarid shrublands. Here we propose that the recent emergence of time-lapse repeat digital photography (pheno-cams) can play a role in further explaining the hydrological triggers of phenological response in semiarid shrublands. This study is focused on a creosotebush-dominated ecosystem of the Santa Rita Experimental Range, southeastern Arizona. In addition to typical eddy covariance instrumentation, this site offers continuous measurements of soil moisture in 6 one-meter profiles. Additionally, three pheno-cams have been installed in the footprint of the eddy covariance tower at the site. We demonstrate (1) that the green-up of evergreen creosotebush can be tracked using an average greenness index calculated from multiple pheno-cams within a tower footprint; (2) that the green-up of creosotebush is driven by deep soil moisture (e.g. > 30 cm); and (3) that carbon uptake can be predicted using image-derived green-up of creosotebush.