Last-Century Increases in Intrinsic Water-Use Efficiency of Grassland Communities Have Occurred over a Wide Range of Vegetation Composition, Nutrient Inputs, and Soil pH  

Abstract

Last-century climate change has led to variable increases of the intrinsic water-use efficiency (W i ; the ratio of net CO 2 assimilation to stomatal conductance for water vapor) of trees and C 3 grassland ecosystems, but the causes of the variability are not well understood. Here, we address putative drivers underlying variable W i responses in a wide range of grassland communities. W i was estimated from carbon isotope discrimination in archived herbage samples from 16 contrasting fertilizer treatments in the Park Grass Experiment, Rothamsted, England, for the 1915 to 1929 and 1995 to 2009 periods. Changes in W i were analyzed in relation to nitrogen input, soil pH, species richness, and functional group composition. Treatments included liming as well as phosphorus and potassium additions with or without ammonium or nitrate fertilizer applications at three levels. W i increased between 11% and 25% (P , 0.001) in the different treatments between the two periods. None of the fertilizers had a direct effect on the change of W i (DW i ). However, soil pH (P , 0.05), species richness (P , 0.01), and percentage grass content (P , 0.01) were significantly related to DW i . Grass-dominated, species-poor plots on acidic soils showed the largest DW i (+14.7 mmol mol 21 ). The DW i response of these acidic plots was probably related to drought effects resulting from aluminum toxicity on root growth. Our results from the Park Grass Experiment show that W i in grassland communities consistently increased over a wide range of nutrient inputs, soil pH, and plant community compositions during the last century.

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

“…2). The modest observed variation of c i /c a across the century agrees with other 13 C-based observations over geological timescales with greatly varying c a [11] and with plant responses from seasonal-to century-scale changes of climatic, edaphic, and nutritional stresses in forests and grasslands, including at Park Grass [5,23,32]. Modest variation of c i /c a is an expected result of coordinated reciprocal adjustments of photosynthesis and stomatal conductance that serve Lime application is represented with the letters "L" (limed) or "U" (unlimed) in the treatment name.…”

“…As a field resource, it enables not only exploration of the relationships between grassland community composition, nutrient status, and herbage yields, but also canopy stomatal conductance, water use efficiency, and N acquisition by ways of elemental and isotopic analysis of samples stored in the Rothamsted Sample Archive (the "Methods" section). In this analysis, we also expand on the temporal scope of earlier studies on yield trends [22] and intrinsic water use efficiency [23], by investigating changes during the last 100 years by using two periods in the early twentieth (1917)(1918)(1919)(1920)(1921)(1922)(1923)(1924)(1925)(1926)(1927)(1928)(1929)(1930)(1931) and twenty-first century (2004-2018), each of 15 years. Over that time span (1917 to 2018), atmospheric CO 2 concentration (c a ) increased by c. 30% or 100 μmol mol −1 [25,26].…”

“…Understanding relationships between stomatal conductance, photosynthesis, transpiration and N acquisition are fundamental for explaining why for some temperate humid grasslands the CO 2 fertilization responses in aboveground biomass production have been small or absent [ 4 , 21 ]. That such interactions may have already operated in the last century is indicated by an investigation of herbage yields in 1891–1992 on several unlimed plots of the Park Grass Continuous Hay Experiment (Rothamsted, U.K.), which found no significant trend [ 22 ], although water use efficiency has increased [ 23 ]. It has been hypothesized from elevated CO 2 studies in the field that the limited fertilization effect of elevated c a in temperate grassland communities is connected with stronger reduction of stomatal conductance in the grasses, and greater photosynthetic acclimation, in comparison with forbs and legumes, or by nutrient limitation, especially for N [ 4 , 7 , 21 ].…”

Quantifying Light Response of Leaf-Scale Water-Use Efficiency and Its Interrelationships With Photosynthesis and Stomatal Conductance in C3 and C4 Species

“…WUE can thus indicate the natural selection on the balance between these fluxes (Hetherington and Woodward, 2003). Characterizing the environmental impacts on WUE among plant species and/or plant function types can advance our knowledge on differential plant adaptation strategies, and improve our prediction on consequences of environmental challenges (Avola et al, 2008;Egea et al, 2011;Zhou et al, 2014Zhou et al, , 2016De Kauwe et al, 2015;Köhler et al, 2016;Ahrar et al, 2017). For instance, plant species with the highest WUE would show the greatest fitness in dry habitats (Dudley, 1996;Zhou et al, 2019).…”

An oxygen isotope chronometer for cellulose deposition: the successive leaves formed by tillers of a C4 perennial grass

“…Short-term isotopic chronologies of cellulose-18 O presented by the successive leaves formed along tillers of C. squarrosa (or other C 3 and C 4 grasses) could be combined with analyses of 18 O in phytoliths (Webb & Longstaffe 2006) and 13 C in biomass or cellulose of the same tissue (Köhler et al 2016) for comprehensive reconstructions of short-term variation of environmental conditions and physiological adaptations (Barbour 2007, Franks et al 2013Gong et al 2011Gong et al , 2017. Such works could take advantage of plant tissue from herbaria (e.g.…”