Emmy Lammertsma

Global CO2 rise leads to reduced maximum stomatal conductance in Florida vegetation

A principle response of C3 plants to increasing concentrations of atmospheric CO2 (CO2) is to reduce transpirational water loss by decreasing stomatal conductance (gs) and simultaneously increase assimilation rates. Via this adaptation, vegetation has the ability to alter hydrology and climate. Therefore, it is important to determine the adaptation of vegetation to the expected anthropogenic rise in CO2. Short-term stomatal opening–closing responses of vegetation to increasing CO2 are described by free-air carbon enrichments growth experiments, and evolutionary adaptations are known from the geological record. However, to date the effects of decadal to centennial CO2 perturbations on stomatal conductance are still largely unknown. Here we reconstruct a 34% (±12%) reduction in maximum stomatal conductance (gsmax) per 100 ppm CO2 increase as a result of the adaptation in stomatal density (D) and pore size at maximal stomatal opening (amax) of nine common species from Florida over the past 150 y. The species-specific gsmax values are determined by different evolutionary development, whereby the angiosperms sampled generally have numerous small stomata and high gsmax, and the conifers and fern have few large stomata and lower gsmax. Although angiosperms and conifers use different D and amax adaptation strategies, our data show a coherent response in gsmax to CO2 rise of the past century. Understanding these adaptations of C3 plants to rising CO2 after decadal to centennial environmental changes is essential for quantification of plant physiological forcing at timescales relevant for global warming, and they are likely to continue until the limits of their phenotypic plasticity are reached.

Climate forcing due to optimization of maximal leaf conductance in subtropical vegetation under rising CO2

Plant physiological adaptation to the global rise in atmospheric CO2 concentration (CO2) is identified as a crucial climatic forcing. To optimize functioning under rising CO2, plants reduce the diffusive stomatal conductance of their leaves (gs) dynamically by closing stomata and structurally by growing leaves with altered stomatal densities and pore sizes. The structural adaptations reduce maximal stomatal conductance (gsmax) and constrain the dynamic responses of gs. Here, we develop and validate models that simulate structural stomatal adaptations based on diffusion of CO2 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 gsmax 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 CO2. 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 CO2. Further, our simulations predict that doubling todays CO2 will decrease the annual transpiration flux of subtropical vegetation in Florida by ≈60 W·m−2. We conclude that plant adaptation to rising CO2 is altering the freshwater cycle and climate and will continue to do so throughout this century.

Validation of climate model-inferred regional temperature change for late-glacial Europe

Comparisons of climate model hindcasts with independent proxy data are essential for assessing model performance in non-analogue situations. However, standardized paleoclimate datasets for assessing the spatial pattern of past climatic change across continents are lacking for some of the most dynamic episodes of Earths recent past. Here we present a new chironomid-based paleotemperature dataset designed to assess climate model hindcasts of regional summer temperature change in Europe during the late-glacial and early Holocene. Latitudinal and longitudinal patterns of inferred temperature change are in excellent agreement with simulations by the ECHAM-4 model, implying that atmospheric general circulation models like ECHAM-4 can successfully predict regionally diverging temperature trends in Europe, even when conditions differ significantly from present. However, ECHAM-4 infers larger amplitudes of change and higher temperatures during warm phases than our paleotemperature estimates, suggesting that this and similar models may overestimate past and potentially also future summer temperature changes in Europe.

Reply to Miglietta et al.: Maximal transpiration controlled by plants

We thank Miglietta et al. (1) for their interest in our study (2). Their first and main point arises from the idea that plant transpiration (T) is driven by atmospheric demand, giving plants limited control over the water they lose. Miglietta et al. (1) add that stomatal density (D) will not change in response to increasing atmospheric CO2 concentration ([CO2]), so future changes in T are unlikely. This idea neglects plant physiology and is not supported by recent observations (3). Maximal T is limited by maximal stomatal conductance (gsmax) with stomata fully open. When atmospheric demand exceeds a plants water transport capacity, stomata close in minutes. To long-term gradual [CO2] fluctuations, many perennial C3 plants adjust D and the size of fully opened stomata (amax) to regulate gsmax. Concomitant D and amax adjustments may therefore constrain T also at (multi)decadal time scales.