Physiological integration of Cornus drummondii and the mechanisms driving woody encroachment in the tallgrass prairie

Background/Question/Methods

Clonal woody species appear to be one of the greatest drivers of woody encroachment in graminoid-dominated mesic grasslands. Part of the high success rate may be due to the ability of clonal species to circumvent recruitment filters. Acropetal translocation of nutrients and water from mother to daughter ramets may reduce the effects of environmental heterogeneity and provide a buffer against competitive interactions. The physiological integration of intraclonal ramets of Cornus drummondii make this species well adapted to respond to resource limitations and disturbances, imparting a competitive advantage that facilitates the displacement of native graminoids and forbs. To assess the physiological integration hypothesis as an adaptive trait for clonal expansion of C. drummondii and to determine the degree of physiological interdependency among intraclonal ramets, we implemented a rhizome severing experiment on 384 clonal ramets across six watersheds at the Konza Prairie Biological Station. Rhizomes were severed close to the nearest neighbor with a sharpened steel blade inserted into the soil in a full circle to insure that all connections to parent ramets were severed while minimizing damage to the root system. Severing occurred in early June 2011 and each ramet was permanently tagged and monitored throughout the growing season.

Results/Conclusions

We found that over 58% of severed ramets died throughout the growing season while the intact C. drummondii ramets were less water stressed, had higher photosynthetic rates, and greater above-ground net primary productivity. Photosynthesis at ambient, sub-ambient and super-ambient CO₂ levels was largely inhibited in ramets that had their connective rhizome severed, with a strong influence from burning (F = 4.99, p = 0.0132). Water potential was also more negative for ramets with severed rhizomes (F = 19.76, p < 0.0001). Leaf demography, as defined by nodal populations of leaves, also showed a decline in the demographic parameter lambda value when rhizomes were severed, interacting again with the burning treatments. Our results indicate that daughter ramets of C. drummondii are physiologically reliant on the connective vasculature in the rhizomes to support growth and respond favorably to disturbances such as fire. Physiological connection to parent ramets that have deeper roots and access more reliant water tables allows daughter ramets to survive mid-summer drought and increases the likelihood of permanent establishment and further clonal reproduction. This may explain how clonal woody plant species are able to establish new ramets and persist in suboptimal conditions.