Stomatal conductance limited the CO2 response of grassland in the last century

Abstract

Background The anthropogenic increase of atmospheric CO2 concentration (ca) is impacting carbon (C), water, and nitrogen (N) cycles in grassland and other terrestrial biomes. Plant canopy stomatal conductance is a key player in these coupled cycles: it is a physiological control of vegetation water use efficiency (the ratio of C gain by photosynthesis to water loss by transpiration), and it responds to photosynthetic activity, which is influenced by vegetation N status. It is unknown if the ca-increase and climate change over the last century have already affected canopy stomatal conductance and its links with C and N processes in grassland. Results Here, we assessed two independent proxies of (growing season-integrating canopy-scale) stomatal conductance changes over the last century: trends of δ18O in cellulose (δ18Ocellulose) in archived herbage from a wide range of grassland communities on the Park Grass Experiment at Rothamsted (U.K.) and changes of the ratio of yields to the CO2 concentration gradient between the atmosphere and the leaf internal gas space (ca – ci). The two proxies correlated closely (R2 = 0.70), in agreement with the hypothesis. In addition, the sensitivity of δ18Ocellulose changes to estimated stomatal conductance changes agreed broadly with published sensitivities across a range of contemporary field and controlled environment studies, further supporting the utility of δ18Ocellulose changes for historical reconstruction of stomatal conductance changes at Park Grass. Trends of δ18Ocellulose differed strongly between plots and indicated much greater reductions of stomatal conductance in grass-rich than dicot-rich communities. Reductions of stomatal conductance were connected with reductions of yield trends, nitrogen acquisition, and nitrogen nutrition index. Although all plots were nitrogen-limited or phosphorus- and nitrogen-co-limited to different degrees, long-term reductions of stomatal conductance were largely independent of fertilizer regimes and soil pH, except for nitrogen fertilizer supply which promoted the abundance of grasses. Conclusions Our data indicate that some types of temperate grassland may have attained saturation of C sink activity more than one century ago. Increasing N fertilizer supply may not be an effective climate change mitigation strategy in many grasslands, as it promotes the expansion of grasses at the disadvantage of the more CO2 responsive forbs and N-fixing legumes.

Loss of fungal symbionts at the arid limit of the distribution range in a native Patagonian grass—Resource eco‐physiological relations

“…At this point of the gradient, (a) H. comosum populations were either entirely symbiotic with Epichloë or presented a mixture of symbiotic and non‐symbiotic plants, (b) Epichloë symbiotic plants showed higher nitrogen and phosphorus concentrations than Epichloë ‐free plants and (c) plants with greater AMF colonization associated with decreased Δ 18 O p , which is indicative of enhanced stomatal conductance for comparisons at site level (e.g. Baca Cabrera et al., 2021). Understanding to what extent these eco‐physiological characteristics among H. comosum plants are driven by the symbiotic associations and how they may contribute to niche differentiation (Valladares et al., 2015) could unveil the symbioses‐driven mechanisms explaining the host species abundance (either high or low) along the natural distribution range.…”

“…Interestingly, however, Δ 18 O P also increased towards low MAP along isothermal and iso‐RH gradients (Figures S2 and S3), demonstrating that changes of Δ 18 O P with MAP were not driven by RH or temperature alone (Hirl et al., 2021). Variation of stomatal conductance, independent of RH and temperature, is a likely explanatory factor (Baca Cabrera et al., 2021). In addition, the absence of significant variation in iWUE towards arid and severely grazed sites would indicate that eventual variations of stomatal conductance were matched by proportional variations in photosynthesis (Scheidegger et al., 2000) in that range.…”

“…Variation of stomatal conductance, independent of RH and temperature, is a likely explanatory factor (Baca Cabrera et al, 2021). In addition, the absence of significant variation in iWUE towards arid and severely grazed sites would indicate that eventual variations of stomatal conductance were matched by proportional variations in photosynthesis (Scheidegger et al, 2000) in that range.…”

“…The simultaneous measurement of 13 C‐discrimination (Δ 13 C) and 18 O‐enrichment of cellulose (Δ 18 O; details in Section 2) in C 3 plants, can shed light on the interplay of plant carbon and water relations in the field (Flanagan & Farquhar, 2014). While iWUE can be estimated from Δ 13 C (Farquhar & Richards, 1984; Ma et al., 2021), Δ 18 O has been interpreted as an indicator of stomatal conductance (Barbour, 2007; Moreno‐Gutiérrez et al., 2012; Scheidegger et al., 2000), particularly in comparative studies at site level (Baca Cabrera et al., 2021), and thus, may help separating the (growing season‐integrated, assimilation‐weighted) effects of both net photosynthesis and stomatal conductance on iWUE (Baca Cabrera et al., 2021; Barbour, 2007; Scheidegger et al., 2000). Whether the dual association with Epichloë and AMF affects iWUE or Δ 18 O at site level or across natural aridity ranges of host plants has not been studied so far.…”

Overestimated gains in water‐use efficiency by global forests

“…WUE in the Park Grass Experiment at Rothamsted, and resulted in improved estimations of historical WUE (Baca Cabrera et al, 2021).…”