Climate models predict that altered rainfall distribution and climate warming might be important components of global climate change. Intensified summer drought and warming have the potential to alter net CO₂ exchange in plants. An important aspect of net CO₂ exchange is dark respiration, and understanding how respiration is affected by rainfall distribution and warming will be important in predicting potential feedback effects of plants on future atmospheric CO₂ concentrations. The southern oak savanna in Central Texas is comprised of three dominant species with contrasting photosynthetic pathways and leaf habits and thus may differ in leaf-level responses to these global climate change drivers. Quercus stellata (post oak) is the dominant deciduous tree, Schizachyrium scoparium (little bluestem) is the dominant warm-season C₄ grass, and Juniperus virginiana (eastern redcedar), is an invasive evergreen. We hypothesized that leaf dark respiration rates and temperature response functions of the three study species would respond differently to the effects of rainfall redistribution and warming. 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.

Results/Conclusions

Species differed in their responses of leaf dark respiration to long-term rainfall redistribution and warming treatments. There were little overall long-term effects of rainfall redistribution on respiration rates. Effects were only seen in early summer in eastern redcedar where plants receiving redistributed amounts of rainfall had greater respiration rates than those receiving the long-term mean. However, these differences disappeared in late summer. With regards to warming, we found evidence of temperature acclimation in little bluestem and post oak, while eastern redcedar was unresponsive. Species also differed in their responses of leaf dark respiration to short-term alleviation of water stress following rainfall events during prolonged summer drought. Rates in little bluestem increased post-watering and the effect was greater in the unwarmed plants than in the warmed plants. In both little bluestem and post oak, the temperature sensitivity of respiration was greater pre-watering. Eastern redcedar was unresponsive to short-term alleviation of water stress. Progressive drought in midsummer reduced rates of leaf dark respiration in little bluestem and eastern redcedar and the effects were more pronounced in the unwarmed treatment compared to the warmed treatment. These results indicate that rainfall redistribution and warming differentially affect leaf dark respiration rates in these three species.