Drought and warming alter gross primary production allocation and reduce productivity in a widespread pasture grass

Carbon 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 ℃ in
controlled environment chambers and subjected to drought (-65% relative to
well-watered controls). Leaf, root and canopy carbon fluxes were measured and
linked to growth prior to 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.