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

Abstract

Background: The anthropogenic increase of atmospheric CO₂ concentration (c_a) 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 c_a-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 (δ¹⁸O_cellulose) 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 (c_a - c_i). The two proxies correlated closely (R² = 0.70), in agreement with the hypothesis. In addition, the sensitivity of δ¹⁸O_cellulose 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 δ¹⁸O_cellulose changes for historical reconstruction of stomatal conductance changes at Park Grass. Trends of δ¹⁸O_cellulose 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.

Keywords: 13C discrimination; Grassland; Hay yield; Last-century climate change; N and P nutrition status; Oxygen isotope composition of cellulose; Park Grass Experiment; Plant functional groups; Stomatal conductance; Water use efficiency.