Decreased water limitation under elevated CO ₂ amplifies potential for forest carbon sinks

Abstract

Increasing atmospheric CO 2 concentrations and changing rainfall regimes are creating novel environments for plant communities around the world. The resulting changes in plant productivity and allocation among tissues will have significant impacts on forest carbon storage and the global carbon cycle, yet these effects may depend on mechanisms not included in global models. Here we focus on the role of individual-level competition for water and light in forest carbon allocation and storage across rainfall regimes. We find that the complexity of plant responses to rainfall regimes in experiments can be explained by individual-based competition for water and light within a continuously varying soil moisture environment. Further, we find that elevated CO 2 leads to large amplifications of carbon storage when it alleviates competition for water by incentivizing competitive plants to divert carbon from short-lived fine roots to long-lived woody biomass. Overall, we find that plant dependence on rainfall regimes and plant responses to added CO 2 are complex, but understandable. The insights developed here will serve as an important foundation as we work to predict the responses of plants to the full, multidimensional reality of climate change, which involves not only changes in rainfall and CO 2 but also changes in temperature, nutrient availability, and disturbance rates, among others.rainfall | forest dynamics | plant allocation | carbon storage | evolutionarily stable strategy T he fate of the terrestrial carbon sink hinges on the role of limitation by other resources (1, 2). If additional atmospheric CO 2 causes forests to run up against limitation by other resources, it is possible that a forest carbon sink caused by CO 2 fertilization could diminish or reverse. The fate of this service by plants, currently estimated to mitigate 30% of anthropogenic emissions per year (3), is one of the most uncertain components of global climate predictions (4). Despite this importance, however, the role of resource limitation in carbon sinks is poorly understood and poorly incorporated into global models (1, 2, 5, 6).Here we investigate the effect of water limitation of photosynthesis on forest carbon storage and sinks. With additional CO 2 in the atmosphere, more CO 2 diffuses into leaves, whereas approximately the same amount of water escapes. This increase in water use efficiency at the leaf level has been well documented in experiments (7, 8) and observed in biomes around the world (9, 10). However, fossil CO 2 is not the only factor altering water relations in plant communities. Rising temperatures (11), changing rainfall regimes (12), and nitrogen deposition (13) can also have effects on plant water balance. A complete understanding of forest carbon storage and carbon sinks thus requires understanding a truly complex system.To build the mechanistic models we need to predict forest carbon storage in novel circumstances, we favor bringing together model components whose behavior we can understand and test with controlled exp...

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

“…S4B) supports the idea that the drier sites are where the most significant effects of eCO 2 on NPP should be expected. This finding agrees with modeling studies based on optimality principles (71,72) and is supported by global patterns of positive response of semiarid ecosystems to CO 2 fertilization (73,74), forcing the reevaluation of the role of semiarid ecosystems in the land carbon sink (75,76). All this evidence corroborates our results and suggests that projections of eCO 2 effects at local and global scales are substantially affected by mechanisms and feedbacks contributing to indirect effects, which are inherently more challenging to model than the direct effect on carbon assimilation.…”

On underestimation of global vulnerability to tree mortality and forest die‐off from hotter drought in the Anthropocene

“…Most process-based global vegetation models simulate the combined changes in [CO 2 ], temperature, humidity, and growing season length (among other factors) within earth system models (e.g., dynamic global vegetation models [DGVMs]), generally projecting enhanced growth of terrestrial vegetation as this century proceeds (e.g., Huntingford et al 2013). Such model predictions of overall increasing forest growth are consistent with various observations, theory, and model experiments (e.g., Farrior et al 2015), ranging from work in tropical moist forests (e.g., Lloyd and Farquhar 2008) and CO 2 fertilization experiments in non-water limited situations (e.g., Keenan et al 2013) to evidence of recent expansions of woody plant dominance and biomass in semiarid savannas and shrublands (e.g., Buitenwerf et al 2012, Liu et al 2015a). Indeed, elevated water-use efficiency typically is observed under elevated [CO], even in waterlimited conditions (Leakey 2009, Frank et al 2015a).…”

“…Indeed, elevated water-use efficiency typically is observed under elevated [CO], even in waterlimited conditions (Leakey 2009, Frank et al 2015a). Findings of enhanced tree growth rates in many regions (Phillips et al 2008, McMahon et al 2010, Fang et al 2014, Frank et al 2015a, Xia et al 2015 further support model projections of future increases in global forest growth (Zscheischler et al 2014, Farrior et al 2015, Sitch et al 2015. Even widespread observations of increased background tree mortality rates could largely reflect that rising [CO 2 ] is promoting greater vegetation productivity and thus more competition and higher mortality rates (Phillips et al 2008, Doughty et al 2015.…”

Tragedies and Crops: Understanding Natural Selection To Improve Cropping Systems

“…However, a population of plants with such an optimal allocation to support structures can be invaded by a taller individual because this individual acquires more of the shared resource light [4]. A suite of other traits have been similarly implicated, ranging from above and below ground traits to the interaction with other organisms (Table 1).Occurrence of TOCs in vegetation stands will have important implications for functioning of managed and natural plant communities [7][8][9]. Despite these potential implications, EGT analyses of TOCs in plant communities have been mainly limited to theoretical analyses, and implications for real plant-based systems have been poorly quantified (but see [10]).…”