Effects of mesophyll conductance on vegetation responses to elevated CO₂ concentrations in a land surface model

Abstract

Mesophyll conductance ( g m ) is known to affect plant photosynthesis. However, g m is rarely explicitly considered in land surface models (LSMs), with the consequence that its role in ecosystem and large‐scale carbon and water fluxes is poorly understood. In particular, the different magnitudes of g m across plant functional types (PFTs) are expected to cause spatially divergent vegetation responses to elevated CO 2 concentrations. Here, an extensive literature compilation of g m across major vegetation types is used to parameterize an empirical model of g m in the LSM JSBACH and to adjust photosynthetic parameters based on simulated A n − C i curves. We demonstrate that an explicit representation of g m changes the response of photosynthesis to environmental factors, which cannot be entirely compensated by adjusting photosynthetic parameters. These altered responses lead to changes in the photosynthetic sensitivity to atmospheric CO 2 concentrations which depend both on the magnitude of g m and the climatic conditions, particularly temperature. We then conducted simulations under ambient and elevated (ambient + 200 μmol/mol) CO 2 concentrations for contrasting ecosystems and for historical and anticipated future climate conditions (representative concentration pathways; RCPs) globally. The g m ‐explicit simulations using the RCP8.5 scenario resulted in significantly higher increases in gross primary productivity (GPP) in high latitudes (+10% to + 25%), intermediate increases in temperate regions (+5% to + 15%), and slightly lower to moderately higher responses in tropical regions (−2% to +5%), which summed up to moderate GPP increases globally. Similar patterns were found for transpiration, but with a lower magnitude. Our results suggest that the effect of an explicit representation of g m is most important for simulated carbon and water fluxes in the boreal zone, where a cold climate coincides with evergreen vegetation.

Mesophyll conductance in land surface models: effects on photosynthesis and transpiration

“…In LSMs, g m has so far been implemented as empirical, multiplicative formulations, which can be written in a generalized form as (Suits et al , ; Sun et al , ; Knauer et al , ):…”

“…It is physiologically meaningful to include a minimum g m value into Equation (6) (e.g. as a minimum function; Knauer et al , ) as it is unlikely that g m will reach values of 0, which would imply a leaf interior that is completely impermeable to CO 2 transfer. The existence of a non‐zero minimum g m has been supported by measurements in which g m did not decrease to 0 even under extreme conditions of water stress (Galmés et al , ; Perez‐Martin et al , ; Limousin et al , ).…”

“…The functions in Equation (6) represent responses of g m to environmental factors on instantaneous (minutes) to intermediate (days to months) time scales that were often reported throughout the literature [Figure ; see Flexas et al () and Flexas et al () for a literature review and Sun et al () and Knauer et al () for a model‐oriented review on these responses]. f L reflects changes of leaf structure and associated decreases of maximum g m within the canopy.…”

“…(a–c) Environmental response functions of g m as shown in Equation (6). (d, e) Additional response functions as implemented in Knauer et al (). L is the leaf area index (LAI), k n is a within‐canopy extinction coefficient (here k n = 0.11), T l is the leaf temperature, T ref is the reference temperature (298.15 K), H a is the activation energy, H d is the deactivation energy, Δ S is the entropy term, R is the universal gas constant, θ is the volumetric soil moisture, θ wilt is the soil moisture at the wilting point (here θ wilt = 0.15 m 3 m −3 ), θ crit is the critical soil moisture (here θ crit = 0.35 m 3 m −3 ), q m is the exponent determining the shape of the curve (here q m = 0.75), C i is the intercellular CO 2 concentration, f min is the minimum g m relative to g m,max25 (here f min = 0.15), and Q a is the absorbed photosynthetic photon flux density.…”

“…g m has often been reported to respond to C i (Flexas et al , ; Hassiotou et al , ; Xiong et al , ), and light (Hassiotou et al , ; Douthe et al , ; Xiong et al , ), but the physiological mechanisms behind these instantaneous responses remain elusive (see also Response of mesophyll conductance to environmental factors). The effects of C i and light on g m in a LSM have so far only been tested once by adding empirical response functions to Equation (6) (Figure d,e; Knauer et al , ). Other factors such as salt stress (Delfine et al , ; Wang et al , ) and ozone (Xu et al , ) have been reported to negatively affect g m instantaneously, but these stressors are not always considered in LSMs and have not yet been investigated with respect to g m .…”