• Energy Research
• 2016-2025close

Abstract Sustaining grassland production in a changing climate requires an understanding of plant adaptation strategies, including trait plasticity under warmer and drier conditions. However, our knowledge to date disproportionately relies on aboveground responses, despite the importance of belowground traits in maintaining aboveground growth, especially in grazed systems. We subjected a perennial pasture grass, Festuca arundinacea, to year-round warming (+3 °C) and cool-season drought (60% rainfall reduction) in a factorial field experiment to test the hypotheses that: (i) drought and warming increase carbon allocation belowground and shift root traits towards greater resource acquisition and (ii) increased belowground carbon reserves support post-drought aboveground recovery. Drought and warming reduced plant production and biomass allocation belowground. Drought increased specific root length and reduced root diameter in warmed plots but increased root starch concentrations under ambient temperature. Higher diameter and soluble sugar concentrations of roots and starch storage in crowns explained aboveground production under climate extremes. However, the lack of association between post-drought aboveground biomass and belowground carbon and nitrogen reserves contrasted with our predictions. These findings demonstrate that root trait plasticity and belowground carbon reserves play a key role in aboveground production during climate stress, helping predict pasture responses and inform management decisions under future climates.

AbstractCarbon allocation determines plant growth, fitness and reproductive success. However, climate warming and drought impacts on carbon allocation patterns in grasses are not well known, particularly following grazing or clipping. A widespread C3 pasture grass, Festuca arundinacea, was grown at 26 and 30°C in controlled environment chambers and subjected to drought (65% reduction relative to well‐watered controls). Leaf, root and whole‐plant carbon fluxes were measured and linked to growth before and after clipping. Both drought and warming reduced gross primary production and plant biomass. Drought reduced net leaf photosynthesis but increased the leaf respiratory fraction of assimilated carbon. Warming increased root respiration but did not affect either net leaf photosynthesis or leaf respiration. There was no evidence of thermal acclimation. Moreover, root respiratory carbon loss was amplified in the combined drought and warming treatment and, in addition to a negative carbon balance aboveground, explained an enhanced reduction in plant biomass. Plant regrowth following clipping was strongly suppressed by drought, reflecting increased tiller mortality and exacerbated respiratory carbon loss. These findings emphasize the importance of considering carbon allocation patterns in response to grazing or clipping and interactions with climatic factors for sustainable pasture production in a future climate.

Plant biomass tends to increase under nutrient addition and decrease under drought. Biotic and abiotic factors influence responses to both, making the combined impact of nutrient addition and drought difficult to predict. Using a globally distributed network of manipulative field experiments, we assessed grassland aboveground biomass response to both drought and increased nutrient availability at 26 sites across nine countries. Overall, drought reduced biomass by 19% and nutrient addition increased it by 24%, resulting in no net impact under combined drought and nutrient addition. Among the plant functional groups, only graminoids responded positively to nutrients during drought. However, these general responses depended on local conditions, especially aridity. Nutrient effects were stronger in arid grasslands and weaker in humid regions and nitrogen-rich soils, although nutrient addition alleviated drought effects the most in subhumid sites. Biomass responses were weaker with higher precipitation variability. Biomass increased more with increased nutrient availability and declined more with drought at high-diversity sites than at low-diversity sites. Our findings highlight the importance of local abiotic and biotic conditions in predicting grassland responses to anthropogenic nutrient and climate changes.