Woody Plant Encroachment and Removal in Mesic Grassland: Production and Composition Responses of Herbaceous Vegetation
Lett, Michelle S., Knapp, Alan K., The American Midland Naturalist
The clonal shrub Cornus drummondii is rapidly displacing mesic grassland in the central U.S. due to fire suppression and changes in land use. Once established, this shrub is not readily eliminated by the return of frequent fire, leading to significant and perhaps irreversible shifts in tallgrass prairie structure and function. We assessed the impacts of C. drummondii encroachment on herbaceous aboveground net primary production (ANPP) and plant community structure in tallgrass prairie and the role this species plays in the conversion of grassland ecosystems to shrub/woodlands. We also removed established C. drummondii clones to assess the potential of the grassland ecosystem to recover after shrub dominance. Aboveground net primary production, vegetative cover and species richness and diversity (exclusive of C. drummondii) were significantly reduced beneath shrub islands relative to open grassland, with reductions in ANPP and richness of up to 94 and 45%, respectively. Forbs were the dominant growth form in the C. drummondii understory, and 10 species primarily associated with woodland habitats occurred only within shrub islands. Upon removal of C. drummondii, ANPP, richness and diversity recovered to grassland values within 2y; however, forbs remained the dominant growth form, comprising 73% of total cover. These results indicate that C. drummondii exerts strong control over the structure and function of unburned mesic grassland ecosystems and that this shrub may be key in the conversion of grasslands to woodlands. Removal of C. drummondii resulted in some aspects of recovery, but the return of graminoid dominance was not attained after 2y. This represents a legacy effect of C. drummondii of unknown duration. Prevention of woody species encroachment through frequent burning is a preferred management option for this ecosystem.
Mesic grasslands (tallgrass prairies) of the central U.S. are fire maintained ecosystems, with recurrent fire ensuring a landscape nearly devoid of woody vegetation (Abrams et al., 1986; Gibson and Hulbert, 1987; Briggs et al, 2002b). Due to changes in land use accompanied by fire suppression, woody species, primarily C^sub 3^ shrubs, have increased in density and cover at the expense of the dominant C^sub 4^ grasses (Bragg and Hulbert, 1976; Abrams, 1986; Knight et al., 1994; Briggs et al., 2002b). This trend is part of a larger phenomenon of woody plant encroachment occurring throughout grassland and savanna ecosystems worldwide (Archer, 1990; Van Auken, 2000; Roques et al., 2001; Silva et al., 2001). Although widely debated, most authors cite changes in climate, CO2 concentration, livestock grazing and fire regimes as primary causes of the displacement of grasslands by woody vegetation (Bahre and Shelton, 1993; Policy et al., 1997; Roques et al., 2001). The shift in dominant growth form that accompanies woody plant expansion in grasslands and savannas can affect resource availability (Belsky et al., 1989; Schlesinger et al., 1990), water flux (Sala et al., 1989; Aguiar et al., 1996), production and decomposition (Norris et al., 2001; Huenneke et al., 2002), carbon dynamics (Gill and Burke, 1999; Jobbágy and Jackson, 2000; Jackson et al., 2002) and community structure (Hobbs and Mooney, 1986; Briggs et al., 2002a). Furthermore, such land cover change represents a significant threat to grassland biodiversity and serves to further fragment the remaining 4% of the original extent of the tallgrass prairie ecosystem in the United States (Samson and Knopf, 1994).
Cornus drummondii C.A. Mey. (rough-leaved dogwood) is one of the most aggressive species expanding into mesic grassland ecosystems of the central U.S. (Briggs et al., 2002b). Cornus drummondii is a clonal species rarely encountered in annually burned prairie, being restricted mainly to ridges and slopes of unburned prairie. Although it is dispersed by birds, its primary mode of reproduction is through vegetative means. …