Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO₂

Abstract

Increasing concentrations of atmospheric carbon dioxide are expected to affect carbon assimilation and evapotranspiration (ET), ultimately driving changes in plant growth, hydrology, and the global carbon balance. Direct leaf biochemical effects have been widely investigated, whereas indirect effects, although documented, elude explicit quantification in experiments. Here, we used a mechanistic model to investigate the relative contributions of direct (through carbon assimilation) and indirect (via soil moisture savings due to stomatal closure, and changes in leaf area index) effects of elevated CO 2 across a variety of ecosystems. We specifically determined which ecosystems and climatic conditions maximize the indirect effects of elevated CO 2 . The simulations suggest that the indirect effects of elevated CO 2 on net primary productivity are large and variable, ranging from less than 10% to more than 100% of the size of direct effects. For ET, indirect effects were, on average, 65% of the size of direct effects. Indirect effects tended to be considerably larger in water-limited ecosystems. As a consequence, the total CO 2 effect had a significant, inverse relationship with the wetness index and was directly related to vapor pressure deficit. These results have major implications for our understanding of the CO 2 response of ecosystems and for global projections of CO 2 fertilization, because, although direct effects are typically understood and easily reproducible in models, simulations of indirect effects are far more challenging and difficult to constrain. Our findings also provide an explanation for the discrepancies between experiments in the total CO 2 effect on net primary productivity.he leaf-level response to elevated CO 2 (eCO 2 ) is well known: at current CO 2 levels, photosynthesis of C 3 plants is not saturated, whereas, for C 4 plants, it is close to saturation (e.g., refs. 1-4). If acclimation is limited, leaf-level carbon assimilation of C 3 plants will increase as the CO 2 concentration increases, as shown by, among others, observations in Free-Air CO 2 Enrichment (FACE) experiments (5-7). Concurrently, stomatal conductance decreases consistently with eCO 2 in most species (8-11). Even though the leaf-level responses are well characterized and quantifiable, the ecosystem response to eCO 2 remains considerably more uncertain and difficult to predict (12-17). This discrepancy is not simply a consequence of the uncertainty in scaling up from leaf to canopy and ecosystem but derives from indirect effects and feedbacks that may lead to an amplification or dampening of the direct leaf-level response to eCO 2 .Indirect effects may be related to (i) modifications of plant water status through changes in soil moisture within the root zone, which occur as a consequence of stomatal closure; (ii) changes in Leaf Area Index (LAI), root biomass and depth, and canopy structure; (iii) limitations due to soil nutrient scarcity or plant incapability to take up nutrients at a rate sufficient to support enhanced c...

Climate Change Impacts on Net Ecosystem Productivity in a Subtropical Shrubland of Northwestern México

“…While the higher ET under CO 2 fertilization would be expected to deplete soil water, we found no appreciable changes in SWC of the top 10 cm of soil (+0.0013, 0.0007, and 0.0019 m 3 /m 3 ), even for cases where summertime precipitation decreased or remained similar (CNRM‐CM5 and AVE). These results are consistent with Fatichi, Leuzinger, et al () who showed that increased WUE supports a higher LAI through soil water savings but leads to a more rapid consumption of SWC due to the increased vegetation. The effects of increased CO 2 on gains in NEP despite a similar SWC when compared to scenarios without fertilization is attributed to ecosystem alterations in water use efficiency (WUE = GPP/ET, Figure d), which increased substantially (+68%, 42%, and 69%) at the expense of higher summertime interannual variability.…”

Dry‐Season Greening and Water Stress in Amazonia: The Role of Modeling Leaf Phenology

“…Trait‐based approaches typically offer a better representation of ecosystem functioning than models grouping plant traits into broad categories (Pappas et al, ). T&C has been successfully applied to simulate water and carbon fluxes in various ecosystems worldwide (Fatichi & Ivanov, ; Fatichi et al, ; Fatichi, Leuzinger, et al, ; Manoli et al, ; Paschalis et al, , ; Pappas et al, ) and is applied here in a revised form to the Amazon rainforests. Consistently with other DGVMs (Restrepo‐Coupe et al, ), in the case of evergreen biomes the original formulation of T&C does not simulate a phenologic cycle of photosynthetic efficiency, which is maintained fixed throughout the year.…”

Modelling carbon sources and sinks in terrestrial vegetation

“…Le Bauer & Treseder, ). While indirect effects through water savings (due to reduced stomatal conductance under increasing [CO 2 ]) could contribute to increased NPP (Donohue et al ., ; Fatichi et al ., ), the C sink of intact vegetation must show a progressive attenuation in the long term according to results of TBMs that account for N and phosphorus cycles (Zaehle et al ., ; Wieder et al ., ).…”