C3 woody plant expansion in a C4 grassland: are grasses and shrubs functionally distinct?

The expansion of C(3) shrubs into C(4)-dominated tallgrass prairies represents a fundamental shift in growth-form dominance accompanied by changes in resource acquisition and use. We assessed these changes by comparing the ecophysiological traits of the dominant C(4) grass Andropogon gerardii, with traits of three C(3) invasive shrub species, Cornus drummondii, Prunus americana, and Rhus glabra. We tested the hypothesis that ecophysiological traits of the shrubs would be similar within this growth form but distinct from grasses and that these species would conform to the two-layer soil water model. Photosynthetic rates in R. glabra were similar to A. gerardii and higher than in the other two shrubs, while water use efficiency was markedly greater in A. gerardii. Among all species, midday xylem pressure potentials (XPP) were distinctly lower (70%) for P. americana, but were similar among the other species. Predawn XPP was related to soil water at shallow depths for A. gerardii (r(2) = 0.59) and P. americana (r(2) = 0.62), and to deeper soil moisture for R. glabra (r(2) = 0.63); there was no relationship for C. drummondii at any soil depth. Thus, a simple two-layer soil water model for partitioning shrub/grass resource acquisition was not appropriate for this grassland. We conclude that these shrubs could not be considered functional equivalents from an ecophysiological perspective, nor were they, as a group, distinct from A. gerardii in resource acquisition and use.