Net ecosystem CO ₂ exchange in Mojave Desert shrublands during the eighth year of exposure to elevated CO ₂

Abstract

Abstract Arid ecosystems, which occupy about 35% of the Earth's terrestrial surface area, are believed to be among the most responsive to elevated [CO 2 ]. Net ecosystem CO 2 exchange (NEE) was measured in the eighth year of CO 2 enrichment at the Nevada Desert Free‐Air CO 2 Enrichment (FACE) Facility between the months of December 2003–December 2004. On most dates mean daily NEE (24 h) (μmol CO 2 m −2 s −1 ) of ecosystems exposed to elevated atmospheric CO 2 were similar to those maintained at current ambient CO 2 levels. However, on sampling dates following rains, mean daily NEEs of ecosystems exposed to elevated [CO 2 ] averaged 23 to 56% lower than mean daily NEEs of ecosystems maintained at ambient [CO 2 ]. Mean daily NEE varied seasonally across both CO 2 treatments, increasing from about 0.1 μmol CO 2 m −2 s −1 in December to a maximum of 0.5–0.6 μmol CO 2 m −2 s −1 in early spring. Maximum NEE in ecosystems exposed to elevated CO 2 occurred 1 month earlier than it did in ecosystems exposed to ambient CO 2 , with declines in both treatments to lowest seasonal levels by early October (0.09±0.03 μmol CO 2 m −2 s −1 ), but then increasing to near peak levels in late October (0.36±0.08 μmol CO 2 m −2 s −1 ), November (0.28±0.03 μmol CO 2 m −2 s −1 ), and December (0.54±0.06 μmol CO 2 m −2 s −1 ). Seasonal patterns of mean daily NEE primarily resulted from larger seasonal fluctuations in rates of daytime net ecosystem CO 2 uptake which were closely tied to plant community phenology and precipitation. Photosynthesis in the autotrophic crust community (lichens, mosses, and free‐living cyanobacteria) following rains were probably responsible for the high NEEs observed in January, February, and late October 2004 when vascular plant photosynthesis was low. Both CO 2 treatments were net CO 2 sinks in 2004, but exposure to elevated CO 2 reduced CO 2 sink strength by 30% (positive net ecosystem productivity=127±17 g C m −2 yr −1 ambient CO 2 and 90±11 g C m −2 yr −1 elevated CO 2 , P =0.011). This level of net C uptake rivals or exceeds levels observed in some forested and grassland ecosystems. Thus, the decrease in C sequestration seen in our study under elevated CO 2 – along with the extensive coverage of arid and semi‐arid ecosystems globally – points to a significant drop in global C sequestration potential in the next several decades because of responses of heretofore overlooked dryland ecosystems.