Islands in a sea of grass: size-class differences in Cornus drummondii physiology

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Woody plant encroachment has occurred in tallgrass prairie due to reduced fire frequency, climate change, and urbanization. One woody plant in particular, Cornus drummondii, is a clonal shrub that forms monospecific stands that resemble woody islands in a sea of grass. Increases in C. drummondii abundance and cover alter site ecohydrology in encroached grasslands. Because climate change predictions include hotter and drier conditions, competition for water among growth forms is likely to increase. During clonal expansion of C. drummondii individuals, we hypothesize differences in plant physiology (gas exchange, transpiration and whole-plant water use) will occur, complicating landscape estimates of water flux. Our study site was the Konza Prairie Biological Station within a four-year burned watershed where 6 C. drummondii islands of varying sizes were chosen. Within each island, 5 ramets were chosen equidistant from the outer edge of the island to the center where gas exchange, water potential and LAI were measured bi-weekly during the summer of 2015. During the study period no differences were seen between predawn and midday leaf-water potentials of the varying island sizes or within the islands between ramet sampling points. Net photosynthesis, stomatal conductance, day and nighttime transpiration measurements were always higher in the small islands versus the medium and large islands. Interestingly, we also found that the outer most sampling location of the islands had higher net photosynthesis compared to the rest of the island which showed no difference from the center ramet to the second ramet sampling locations. The small and large islands had higher LAI values than the medium sized islands. These results facilitate scaling water fluxes from individual shrub islands to watersheds that are encroached with C. drummondii. In addition, these results improve our ability to predict changes in the water budget between woody encroached grasslands versus unencroached grasslands.