Selective grazing modifies previously anticipated responses of plant community composition to elevatedCO₂in a temperate grassland

Abstract

Our limited understanding of terrestrial ecosystem responses to elevated CO2 is a major constraint on predicting the impacts of climate change. A change in botanical composition has been identified as a key factor in the CO2 response with profound implications for ecosystem services such as plant production and soil carbon storage. In temperate grasslands, there is a strong consensus that elevated CO2 will result in a greater physiological stimulus to growth in legumes and to a lesser extent forbs, compared with C3 grasses, and the presumption this will lead in turn to a greater proportion of these functional groups in the plant community. However, this view is based on data mainly collected in experiments of three or less years in duration and not in experiments where defoliation has been by grazing animals. Grazing is, however, the most common management of grasslands and known in itself to influence botanical composition. In a long-term Free Air Carbon Dioxide Enrichment (FACE) experiment in a temperate grassland managed with grazing animals (sheep), we found the response to elevated CO2 in plant community composition in the first 5 years was consistent with the expectation of increased proportions of legumes and forbs. However, in the longer term, these differences diminished so that the proportions of grasses, legumes and forbs were the same under both ambient and elevated CO2 . Analysis of vegetation before and after each grazing event showed there was a sustained disproportionately greater removal ('apparent selection') of legumes and forbs by the grazing animals. This bias in removal was greater under elevated CO2 than ambient CO2 . This is consistent with sustained faster growth rates of legumes and forbs under elevated CO2 being countered by selective defoliation, and so leading to little difference in community composition.

Plant community feedbacks and long-term ecosystem responses to multi-factored global change

“…No significant effect for C. vulgaris (evergreen dwarf shrub)Less D. flexuosa biomass in warmed + drought plot compared with drought plots but CO 2 counterbalanced this decrease in three-factor plotsKongstad et al (2012)French upland grassland600 ppmTemp3No significant effect on AGBNoProportion of Agrostris capillaris increased in warm + drought + CO 2 vs. warm + drought. Proportion of Trisetum flavescens decreased in warm + drought + CO 2 compared with controlBloor et al (2010)TX grassland200–560 ppm rangeH 2 O435 % averaged over 4 yearsYes, accounted for 21–38 % of CO 2 stimulationAdded precipitation altered community, but was not applied factoriallyPolley et al (2014)New Zealand grassland475 ppmGrazing11Positive, but reported elsewhere?CO 2 decreased proportion of C 3 grasses and increased legumes and forbs driving CO 2 responseGrazing reduced proportion of legumes and forbs, allowing grasses to increase, eliminating CO 2 responseNewton et al (2014)AGB, aboveground biomass; ANPP, aboveground net primary production.…”

“…After 18 years of exposure, the same brackish marsh plots showed a weak response of plant community to elevated CO 2 compared with the community responses to background variability in other abiotic factors (Erickson et al 2007). Similarly, elevated CO 2 initially drove a shift in species dominance in a New Zealand grassland, but after 11 years, an additional factor of grazing negated that shift and the ecosystem CO 2 response (Newton et al 2014). …”

Soil N cycling processes in a pasture after the cessation of grazing and CO 2 enrichment

“…The mean annual temperature at the site since 1945 is about 12.9°C and annual rainfall about 870 mm. The pasture is a mixture of C 3 and C 4 grasses, legume and forbs that was established over 50 years ago (Newton et al, 2014). The experiment has three replicates -three ambient CO 2 (aCO 2 ) and three eCO 2 12 m diameter rings (plots) -which are paired into blocks based on their initial botanical composition.…”

“…Details of the grazing dates and durations can be found in Newton et al (2014). During the grazing period of up to 4 days the sheep were confined within the ring to ensure that nutrients removed from that ring during grazing were returned to the same ring via excreta; however, the excreta was not returned to the whole ring -a consequence of the system design.…”

“…During the grazing period of up to 4 days the sheep were confined within the ring to ensure that nutrients removed from that ring during grazing were returned to the same ring via excreta; however, the excreta was not returned to the whole ring -a consequence of the system design. All rings had a circle of standpipes used to release CO 2 and these were protected by an electric fence which prevented the animals damaging the pipes and allowed them to be confined within the ring during grazing (see Plate 1 of Newton et al, 2014). The grazing sheep exhibited the expected behavior when confronted with an electric fence, that is, they grazed the whole of the area but always faced the electric fence thus leaving an area of about 1 m from the fence which was free from excretal returns and the remaining area (11 m in diameter) which received excreta.…”