Shrubs have evolved in response to multiple drivers of disturbance including fire and drought. Many shrubs are clonal with individuals reproducing via vegetative production of stems from below-ground buds and rhizomes. This can give clonal shrubs a competitive advantage over non-clonal species in frequently disturbed environments. Clonal shrubs are encroaching at an accelerated rate compared to non-clonal shrubs in tallgrass prairie ecosystems. This encroachment is broadly driven by changes in climate including increased atmospheric CO2 concentrations and altered precipitation regimes, as well as reduced fire frequency, overgrazing and other management decisions. Given that clonal shrubs are more likely to expand across grasslands and savannas than non-clonal shrubs, understanding how this growth form uses resources and responds to environmental variation will be key for predicting how shrub encroachment may alter carbon and water cycling in the future.
In their new Editor’s Choice study published in AoBP, Wedel et al. assessed intra-shrub leaf-level physiological responses to precipitation and fire in the most dominant encroaching clonal shrub of Kansas tallgrass prairie, Cornus drummondii. C. drummondii, commonly known as the roughleaf dogwood, expands radially via rhizomes and forms discrete clones where the centre of the shrub is older than the periphery. The authors compared leaf gas exchange rates from the periphery to centre within shrub clones during a wet (2015) and extremely dry (2018) year. They also compared leaf physiology between recently burned shrubs (resprouts) with unburned shrubs in 2018. A key aim of the study was to determine whether there was a need to account for intra-clonal differences among stems in response to disturbance to parameterize models more accurately.
The results of the study revealed that leaf physiology does not differ among interconnected stems during a wet or dry year but does differ after fire. Specifically, resprouts after fire had higher gas exchange rates and leaf nitrogen content than unburned shrubs, suggesting that increased rates of carbon gain can contribute to recovery after fire. In areas recently burned, resprouts had higher gas exchange rates in the centre of the shrub than the periphery. In unburned areas, leaf physiology remained constant across the growing season within clonal shrubs.
Wedel et al. conclude that single measurements within shrub clones are sufficient to parameterize models used to understand the effects of shrub encroachment on ecosystem carbon and water cycles. However, these models may require additional complexity when considering the impacts of fire. The authors propose that future work should conduct detailed investigations of intra-clonal physiology for other shrubs that are expanding across grasslands and savannas in order to improve predictions of vegetation cover and ecosystem functioning in a changing climate.