Climate forcing due to optimization of maximal leaf conductance in subtropical vegetation under rising CO ₂

Abstract

Plant physiological adaptation to the global rise in atmospheric CO 2 concentration (CO 2 ) is identified as a crucial climatic forcing. To optimize functioning under rising CO 2 , plants reduce the diffusive stomatal conductance of their leaves (g s ) dynamically by closing stomata and structurally by growing leaves with altered stomatal densities and pore sizes. The structural adaptations reduce maximal stomatal conductance (g smax ) and constrain the dynamic responses of g s . Here, we develop and validate models that simulate structural stomatal adaptations based on diffusion of CO 2 and water vapor through stomata, photosynthesis, and optimization of carbon gain under the constraint of a plant physiological cost of water loss. We propose that the ongoing optimization of g smax is eventually limited by species-specific limits to phenotypic plasticity. Our model reproduces observed structural stomatal adaptations and predicts that adaptation will continue beyond double CO 2 . Owing to their distinct stomatal dimensions, angiosperms reach their phenotypic response limits on average at 740 ppm and conifers on average at 1,250 ppm CO 2 . Further, our simulations predict that doubling today's CO 2 will decrease the annual transpiration flux of subtropical vegetation in Florida by ≈60 W·m −2 . We conclude that plant adaptation to rising CO 2 is altering the freshwater cycle and climate and will continue to do so throughout this century.climate change | physiological forcing | plant evolution P lants respond to the complex of environmental signals they perceive by plastic changes in their phenotype to increase individual fitness (1). The most apparent environmental change that induces phenotypic adaptations in plants is the global increase in atmospheric CO 2 concentration (CO 2 ) (2). In response to this rise in CO 2 , plants reduce the diffusive stomatal conductance of their leaves [g s (mol·m −2 ·s −1 )] to increase drought resistance (3) and reduce physiological costs associated with water transport (4). Plants can reduce g s by dynamically closing their stomata within minutes (5, 6), and structurally within the lifespan of an individual by growing leaves with altered stomatal density [D (number of stomata·m −2 )] and pore size at maximal stomatal opening [a max (m 2 )] (7, 8). Structural adaptations thereby reduce maximal stomatal conductance [g smax (mol·m −2 · s −1 )], which critically reduces actual g s , especially when stomata are fully open during times with ample light and water (9).Reduction of g s via structural adaptation of g smax has the potential to reduce transpiration fluxes and, thus, cause land surface warming in addition to changes in the global hydrological cycle with rising CO 2 (10). This climatic effect is termed the physiological forcing of CO 2 , which acts beside and independent of its radiative forcing. Despite advances to quantify this physiological forcing with global climate models (11, 12), these models rely on semiempirical relations to simulate g s from environmental variables (...

Global CO 2 rise leads to reduced maximum stomatal conductance in Florida vegetation

“…This likely represents the plants' adaptation to increase iWUE by optimizing carbon gain to water loss (11,37). We demonstrate that adaptation of g smax is achieved by species-specific strategies to alter D and/or a max .…”

“…Current increase in CO 2 and the coinciding reduction in plant transpiration already results in increased continental run-off (46), and climate models predict surface temperature increases arising from reduced evaporative cooling (6,7). The mechanisms of optimization of carbon gain to water loss described here could be used to better estimate this physiological forcing for the past and future CO 2 but should be considered within the framework of species-specific phenotypic plasticity (37).…”

“…Both lines of evidence, however, fall beyond or below the timescales of the projected rate of continuing CO 2 increase, which is likely to surpass the time needed for adaptation via natural selection. Consequently, the adaptation within the phenotypic plasticity is likely to constrain epidermis structural adaptation in the near future when phenotypic response limits are reached (35,37). Current increase in CO 2 and the coinciding reduction in plant transpiration already results in increased continental run-off (46), and climate models predict surface temperature increases arising from reduced evaporative cooling (6,7).…”

“…Our results of structural adaptation from ≈280 ppm to 387 ppm CO 2 does not bear any evidence for a diverging response between plant lineages. Whether any g smax off-leveling will occur under continuing CO 2 enrichment, and at what CO 2 concentration this will happen, should be estimated by modeling exercises incorporating adaptation within the speciesspecific phenotypic plasticity (37).…”

Land-use and climate change risks in the Amazon and the need of a novel sustainable development paradigm

“…However, there are large uncertainties about the long-term response of the tropical forests to this effect (127), although modeling of this effect for the Amazon indicates that the impact of high degrees of climate change are attenuated, but not to the extent of avoiding forest loss (128). In a CO 2 -enriched environment, the plants can respond with stomatal closure to capture the same amount of CO 2 required for photosynthesis (129). As a consequence, transpiration rates could decrease, potentially returning a lower amount of vapor to the atmosphere, or, in other words, altering ET rates (130).…”