Does Size Matter? Size-class differences in Cornus drummondii physiology

Abstract:

Woody plants have expanded into grasslands over the last century, altering plant diversity, resource availability, and carbon and water fluxes in these systems. In the tallgrass prairies of northeastern Kansas, Cornus drummondii is a clonal C₃ shrub that forms monospecific “islands” and is currently increasing in abundance and cover. Clonal expansion of C. drummondii may alter site ecohydrology in unpredictable ways, particularly if physiological characteristics vary throughout an island or change as island size increases. Here, we investigated the physiology of C. drummondii within individual islands and across islands of varying sizes in a mesic tallgrass prairie. We selected six C. drummondii islands of varying sizes at the Konza Prairie Long Term Ecological Research site near Manhattan, KS, USA. Within each island, we measured leaf-level gas exchange and leaf water potential at five ramets equidistant from the outer edge of the island to the center, bi-weekly during the 2015 growing season. Additionally, external heat pulse sap flow sensors were installed at three positions within each island that measured stem sap flow every 10 minutes. We found that leaf level gas exchange rates were consistently higher in the small islands than the medium and large islands, and that the outer most sampling location of the islands had higher net photosynthesis compared to the rest of the island. Conversely, leaf-water potential and stem sap flow rates did not differ between island sizes or within individual islands. These results suggest that carbon, not water flux dynamics, may vary as C. drummondii island size increases at the Konza Prairie. Additionally, these results will facilitate scaling water fluxes from individual shrub islands to watersheds that are encroached with C. drummondii and will ultimately improve our ability to predict changes in the water budget between woody encroached grasslands versus unencroached grasslands.