It has been suggested that positive biomass responses of grassland to elevated CO result from moisture savings in the soil as opposed to direct photosynthetic stimulation. In order to test this hypothesis for calcareous grassland we subjected experimental communities consisting of two important graminoid components of such grasslands (Carex flacca and Bromus erectus) on natural substrate to a fully factorial treatment of ambient (360 ppm) and elevated (600 ppm) CO concentration and four irrigation regimes (9 mm, 18 mm, 27 mm and 36 mm week). Biomass stimulation under elevated CO was higher the lower the irrigation rate was. Superimposed on the effects of irrigation on soil moisture, elevated CO-induced higher soil water contents in all irrigation treatments via reduced plant water consumption (on average one-third lower stomatal conductance). This led to eight different soil moisture regimes instead of the intended four. When growth parameters were plotted against the effective soil water content rather than irrigation treatment, the "pure" CO effect on total biomass and other traits became much smaller and completely disappeared for biomass per tiller, leaf area per ground area, leaf mass fraction (LMF) and root mass fraction (RMF). We conclude that the CO response observed in this graminoid system consisted of a small primary CO effect and a large secondary, CO-induced, soil moisture effect. Thus, it is difficult to use responses to CO from experiments in which CO-induced soil moisture savings occur to predict CO effects as long as future soil moisture regimes are not defined. We suggest that direct and indirect (moisture driven) CO effects should be strictly separated, which requires data to be tested against soil moisture.
Summary• A field experiment was established at 2000 m above sea level (asl) in the central Swiss Alps with the aim of investigating the effects of elevated ozone (O 3 ) and nitrogen deposition (N), and of their combination, on above-ground productivity and species composition of subalpine grassland.• One hundred and eighty monoliths were extracted from a species-rich GeoMontani-Nardetum pasture and exposed in a free-air O 3 -fumigation system to one of three concentrations of O 3 (ambient, 1.2 × ambient, 1.6 × ambient) and five concentrations of additional N. Above-ground biomass, proportion of functional groups and normalized difference vegetation index (NDVI) were measured annually.• After 3 yr of treatment, the vegetation responded to the N input with an increase in above-ground productivity and altered species composition, but without changes resulting from elevated O 3 . N input > 10 kg N ha -1 yr -1 was sufficient to affect the composition of functional groups, with sedges benefiting over-proportionally. No interaction of O 3 × N was observed, except for NDVI; positive effects of N addition on canopy greenness were counteracted by accelerated leaf senescence in the highest O 3 treatment.• The results suggest that effects of elevated O 3 on the productivity and floristic composition of subalpine grassland may develop slowly, regardless of the sensitive response to increasing N.
The sensitivity of seminatural grasslands to ozone (O 3 ) pollution is not well known, in spite of the important function of these common ecosystems for agriculture and nature conservation. A 5-year field experiment was carried out at a rural, mid-elevation site at Le Mouret (Switzerland) to investigate the effect of elevated O 3 on yield and species composition of a permanent, extensively managed, species-rich old pasture. Using a free-air fumigation system, circular plots of 7 m diameter were exposed to either ambient air (control plots) or to air containing O 3 of approximately 1.5 Â ambient concentrations. The resulting accumulated O 3 exposure over the threshold of 40 ppb for one season ranged from 13.3 to 59.5 ppm h in the elevated O 3 plots, and from 1.0 to 20.7 ppm h in the control plots. Subplots in each ring were harvested three times each year, and harvested biomass was separated into functional groups (FGs) (grasses, legumes, forbs). Productivity in both treatments decreased over time, but the yield of O 3 -exposed plots decreased faster than that of the control plots, with the reduction being twice as large by the end of the fifth season. Compared with the ambient air control, loss in annual dry matter yield was 23% after 5 years. The change in annual biomass production because of O 3 stress was accompanied by a change in the fractions of FGs, with the legume fraction showing a strong negative response. Such long-term effects of O 3 stress could have negative implications for the maintenance of biological diversity in rural landscapes across large areas of Europe. The results from this first long-term experiment show that a moderately elevated O 3 level reduces the productivity of intact grasslands during a 5-year exposure under real field conditions.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.