The contrast in weather was extreme for 2020 and 2021 at our flux sites located in the semiarid shrublands, grasslands, savannas, and forests of southern Arizona. This was especially true at the Santa Rita Mesquite Savanna site (US-SRM in AmeriFlux vernacular) which had the driest summer monsoon growing season on record in 2020 (42 mm vs. 206 mm average 1937-2021) only to be followed by the third wettest monsoon the very next year (380 mm). Fortunately, we now have 18 years of eddy covariance and other ancillary data to be able to quantify how this whiplash in weather affected ecosystem function.
Here we plot a flux “climatology” for gross primary production (GPP) showing the 16 year (2004-2019) daily mean GPP in blue with the shaded regions indicating +/- one standard deviation, and we also show the 2020 and 2021 hydrological years (1 Nov – 31 Oct). We define a hydrological year to begin at the start and end at the start of hydrological winter, when the vegetation is mainly dormant and storage of water in the soil (or snowpack for colder climates) begins to accrue and later contributes to ecosystem productivity when things warm up again in spring. While 16 years is really too short to define a climatology for standard weather variables, it’s still a good start for ecosystem flux data.
We see 2020 GPP was an outlier in multiple ways. First, there was some anomalous carbon uptake in cool season (Jan – Apr) because of the unusually warm winter temperatures that year with no hard freezes to stop the mesquite tree leaf functioning. Second, we also see the effect that the miserable (for those who had to experience it) monsoon had on carbon uptake where the plants had basically shut down after July. The long “dry” continued into the winter and spring of 2021 with the mesquite trees and perennial bunchgrasses at the site showing little signs of life, but they were resurrected in the abundant summer rains of 2021! Carbon uptake was about double the average amount and way north of the normal range of variability.
We hope you’ll come down and visit us sometime and maybe even experience this whiplash for yourself.
Russell Scott, Research Hydrologist, USDA-ARS, Tucson, AZ.