Warming and Intensified Summer Drought Influence Leaf Dark Respiration and Related Plant Traits in Three Dominant Species of the Southern Oak Savanna

The short-term temperature-response of dark respiration may be altered by climate warming through temperature acclimation; however the role of drought in influencing thermal acclimation is not known. We hypothesized that leaf dark respiration in three dominant species of the southern oak savanna in Central Texas, Schizachyrium scoparium, Juniperus virginiana, and Quercus stellata, would respond differently to the effects of warming and intensified summer drought owing to their contrasting photosynthetic pathways, leaf habits, and drought tolerances. Furthermore, changes in respiration were predicted to be linked to alterations in leaf chemistry and structure, including leaf nitrogen and non-structural carbohydrates in response to warming and drought. Monocultures planted in replicated rainfall exclusion shelters were warmed ( 1.5 ?C) and rainfall events were manipulated to intensify summer drought and augment cool season rainfall compared to the long-term mean. Both warming and drought affected the short-term temperature-response functions of dark respiration and species differed in their responses. Evidence of temperature acclimation through adjustment in Q10 (temperature sensitivity) and R10 (base rate at 10 ?C) was found in S. scoparium and Q. stellata but not J. virginiana. All three species showed evidence of reduced temperature acclimation of respiration with progressive summer drought. Redistributed rainfall in J. virginiana increased respiration in midsummer compared to plants receiving the long-term mean rainfall, but differences disappeared in late summer when drought intensified. In response to rainfall events during summer drought, rates in S. scoparium increased, and the effect was greater in unwarmed compared to warmed plants. In both S. scoparium and Q. stellata, Q10 was reduced post-watering. Regression analyses of respiration against leaf N, soluble carbohydrates, and SLA revealed that relationships differed between species and temperature treatments. Respiration rates were uncoupled from changes in soluble carbohydrates in response to drought and rainfall pulses, suggesting that thermal acclimation is diminished by increasing drought stress in drying soils in contrasting tree and grass species. These findings suggest that models of respiratory carbon flux that incorporate temporal changes in respiratory temperature responses with drought and warming and unique species responses will be critical in predicting species and ecosystem-scale responses to climate change.