Thirty-eight years of CO 2 fertilization has outpaced growing aridity to drive greening of Australian woody ecosystems

[摘要] Climate change is projected to increase the imbalance between the supply(precipitation) and atmospheric demand for water (i.e., increasedpotential evapotranspiration), stressing plants in water-limitedenvironments. Plants may be able to offset increasing aridity becauserising CO 2 increases water use efficiency. CO 2 fertilization has also been cited as one of thedrivers of the widespread “greening” phenomenon. However, attributingthe size of this CO 2 fertilization effect iscomplicated, due in part to a lack of long-term vegetation monitoringand interannual- to decadal-scale climate variability. In this study weasked the question of how much CO 2 has contributed towardsgreening. We focused our analysis on a broad aridity gradient spanningeastern Australia's woody ecosystems. Next we analyzed 38 years ofsatellite remote sensing estimates of vegetation greenness (normalizeddifference vegetation index, NDVI) to examine the role of CO 2 in ameliorating climate change impacts. Multiplestatistical techniques were applied to separate the CO 2 -attributable effects on greening from the changes inwater supply and atmospheric aridity. Widespread vegetation greeningoccurred despite a warming climate, increases in vapor pressure deficit,and repeated record-breaking droughts and heat waves. Between 1982–2019we found that NDVI increased (median 11.3 %) across 90.5 % of the woodyregions. After masking disturbance effects (e.g., fire), we statisticallyestimated an 11.7 % increase in NDVI attributable to CO 2 , broadly consistent with a hypothesized theoreticalexpectation of an 8.6 % increase in water use efficiency due to rising CO 2 . In contrast to reports of a weakening CO 2 fertilization effect, we found no consistenttemporal change in the CO 2 effect. We conclude rising CO 2 has mitigated the effects of increasing aridity,repeated record-breaking droughts, and record-breaking heat waves ineastern Australia. However, we were unable to determine whether trees orgrasses were the primary beneficiary of the CO 2 -inducedchange in water use efficiency, which has implications for projectingfuture ecosystem resilience. A more complete understanding of how CO 2 -induced changes in water use efficiency affect treesand non-tree vegetation is needed.

[学科分类] 大气科学