Low and contrasting impacts of vegetation CO<sub>2</sub> fertilization on global terrestrial runoff over 1982–2010: accounting for aboveground and belowground vegetation–CO<sub>2</sub> effects

Abstract

Abstract. Elevation in atmospheric carbon dioxide concentration (eCO2) affects vegetation water use, with consequent impacts on terrestrial runoff (Q). However, the sign and magnitude of the eCO2 effect on Q are still contentious. This is partly due to eCO2-induced changes in vegetation water use having opposing responses at the leaf scale (i.e., water-saving effect caused by partially stomatal closure) and the canopy scale (i.e., water-consuming induced by foliage cover increase), leading to highly debated conclusions among existing studies. In addition, none of the existing studies explicitly account for eCO2-induced changes to plant rooting depth that is overwhelmingly found in experimental observations. Here we develop an analytical ecohydrological framework that includes the effects of eCO2 on plant leaf, canopy density, and rooting characteristics to attribute changes in Q and to detect the eCO2 signal on Q via vegetation feedbacks over 1982–2010. Globally, we detect a very small decrease of Q induced by eCO2 during 1982–2010 (−1.7 %). Locally, we find a small positive trend (p < 0.01) in the Q–eCO2 response along a resource availability (β) gradient. Specifically, the Q–eCO2 response is found to be negative (i.e., eCO2 reduces Q) in low-β regions (typically dry and/or cold) and gradually changes to a small positive response (i.e., eCO2 increases Q) in high-β areas (typically warm and humid). Our findings suggest a minor role of eCO2 on changes in global Q over 1982–2010, yet we highlight that a negative Q–eCO2 response in semiarid and arid regions may further reduce the limited water resource there.

Increasing sensitivity of dryland vegetation greenness to precipitation due to rising atmospheric CO2

“…The minimalistic model we employ offers a similar explanation from a hydrological perspective: water savings in dry regions allow for a higher vegetation cover and a greater fraction of precipitation to be transpired by vegetation, while for wetter regions where ecosystems are mostly energy limited, water savings essentially decrease the transpiration fraction and hence the sensitivity of transpiration to precipitation. This contrasting response is also supported by the observed divergent trends in global runoff for drylands and non-drylands 52 , 53 . Other factors including soil texture, rooting depth, water table depth, precipitation seasonality, and stomatal sensitivity to drought also affect ecosystem water availability and precipitation sensitivity, and their interactions with CO 2 needs to be further explored.…”

The Compensatory CO2 Fertilization and Stomatal Closure Effects on Runoff Projection From 2016–2099 in the Western United States

“…In summary, Noah‐MP projected aggravating and alleviating CO 2 impacts on runoff in semi‐arid and semi‐humid regions (Figure 9c), respectively, consistent with previous studies for Australia (Ukkola et al., 2016) and the Jinghe River basin in China (Huang et al., 2019). The water shortage in the semiarid and arid regions is exacerbated partially because elevated CO 2 enhances carbon allocation to roots and results in deeper roots (Nie et al., 2013) and larger root surface area, which benefits root uptake of deeper soil water and increases in transpiration (Trancoso et al., 2017; Y. Yang et al., 2021). These processes are also represented in the Noah‐MP version used in this study.…”

“…Y. Yang et al. (2021) found regional differences in the CO 2 ‐induced historical runoff changes and attributed them to differences in resource availabilities (e.g., water and energy), suggesting a larger CO 2 ‐induced runoff reduction in low resource (semiarid and arid) regions. Although consistent temperature increases and slight precipitation changes are projected for the western US (Easterling et al., 2017; Vose et al., 2017), whether the vegetation response to elevated CO 2 and associated climate change alleviate or aggravate the future water shortage over the already dry western US remains very uncertain.…”

Consistent responses of vegetation gas exchange to elevated atmospheric CO<sub>2</sub> emerge from heuristic and optimization models

“…Despite increasing atmospheric CO2 concentration and VPD, only small changes in canopy-scale evapotranspiration have been observed or predicted (Fatichi et al, 2016;Knauer et al, 2017;Yang et al, 2021). That long-term transpiration is a 'conserved' hydrological quantity had been already noted when comparing forests under current climatic conditions (Roberts, 1983), suggesting that vegetation acclimates in such a way as to maintain stable transpiration under a given climate.…”

“…More generally, canopy transpiration rates are not affected or even increase under elevated atmospheric CO2 when the canopy is relatively open (leaf area index, LAI<5 m 2 m -2 , Donohue et al (2017)). At the catchment scale, evapotranspiration also has not varied significantly with increasing CO2 concentrations, as indicated by minor variations in runoff attributed to trends in atmospheric CO2 (Knauer et al, 2017;Yang et al, 2021). Therefore, the net effect of increasing atmospheric CO2 concentration on canopy transpiration appears lower than the effect at the leaf level.…”