Improved grasslands are an important part of UK agriculture providing forage for livestock and additional ecosystem services including carbon storage, pollution attenuation and water regulation. In the context of global climate change the IPCC predict greater uncertainty in weather patterns and an increased incidence of extreme weather events, such as heat waves, drought and heavy rains and storms. Subsequently, new areas of grassland are likely to be exposed to such stresses, which may include multiple successive extreme events.
Despite the large risks posed, our understanding of how extreme events will impact on ecosystem functioning remains poor. To address this gap, we established a plot-scale field experiment on an improved lowland sheep-grazed pasture. The experimental design consisted of 16 field plots, subjected to four stress regimes with four replicates as follows: (i) control, (ii) spring flood, (iii) summer drought, (iv) spring flood + summer drought. Each stress was imposed for eight weeks and the recovery was monitored for one year. During each stress event and during recovery, plant and soil indicators of ecosystem function were measured. These included plant biomass, sward composition, soil physical, chemical and biological indicators and greenhouse gas emissions.
Results/Conclusions
Flooding killed all plant species and decreased the annual biomass production in flood and flood + drought treatments by 30 and 51%, respectively. In contrast, the yield following drought quickly recovered after stress removal. Primary productivity in flood-only plots recovered to control levels by 5 months, however, yield remained reduced in flood + drought plots, 1 year following drought stress. Flooding shifted plant species composition, increasing weed content but also increasing the nectar supply potential of the pasture. PLFA analysis indicated a shift in microbial community structure post-flood and post-drought. This shift was short-lived in the drought plots while differences remained in the flood plots 1-year on. A large pulse of N2O and CH4 emissions was observed following floodwater removal but this was off-set by reductions in CO2 emissions. Consequently, there was an overall decrease in total global warming potential of 4.2 CO2 eq ha-1 in the flood + drought plots compared to the control, over the course of the experiment. Results indicate that extreme events can have a significant impact on grassland ecosystem structure with implication for ecosystem function. The lack of recovery in many indicator measurements suggest that flooding may initiate a tipping point whereby ecosystem function is irreversibly altered.