Woody structure facilitates invasion of woody plants by providing perches for birds

Abstract

Woody encroachment threatens prairie ecosystems globally, and thus understanding the mechanisms that facilitate woody encroachment is of critical importance. Coastal tallgrass prairies along the Gulf Coast of the US are currently threatened by the spread of several species of woody plants. We studied a coastal tallgrass prairie in Texas, USA, to determine if existing woody structure increased the supply of seeds from woody plants via dispersal by birds. Specifically, we determined if (i) more seedlings of an invasive tree (Tridacia sebifera) are present surrounding a native woody plant (Myrica cerifera); (ii) wooden perches increase the quantity of seeds dispersed to a grassland; and (iii) perches alter the composition of the seed rain seasonally in prairie habitats with differing amounts of native and invasive woody vegetation, both underneath and away from artificial wooden perches. More T. sebifera seedlings were found within M. cerifera patches than in graminoid‐dominated areas. Although perches did not affect the total number of seeds, perches changed the composition of seed rain to be less dominated by grasses and forbs. Specifically, 20–30 times as many seeds of two invasive species of woody plants were found underneath perches independent of background vegetation, especially during months when seed rain was highest. These results suggest that existing woody structure in a grassland can promote further woody encroachment by enhancing seed dispersal by birds. This finding argues for management to reduce woody plant abundance before exotic plants set seeds and argues against the use of artificial perches as a restoration technique in grasslands threatened by woody species.

Interaction of seed dispersal and environmental filtering affects woody encroachment patterns in coastal grassland

“…, Prather et al. ). In coastal barrier systems, dispersal is limited by geographic isolation, making specialized dispersal mechanisms particularly important in shaping communities (Zinnert et al.…”

“…For bird‐dispersed seeds, presence of woody vegetation provides perches, increasing seed rain into the system (Joy and Young , Prather et al. ). Recent studies focusing on long‐distance avian dispersal show that propagules may disperse hundreds of kilometers from source populations (Nathan , Viana et al.…”

“…, Prather et al. ). In coastal systems, environmental filtering is based on position relative to shoreline, elevation above the freshwater lens, and spatial heterogeneity of salinity due to storm overwash (Young et al.…”

“…A series of drivers affect plant community dynamics, including seed dispersal and post-dispersal processes (Lambers et al 2008). Seed arrival is the first important ecological step when considering expanding populations (Kot et al 1996, Turchin 1998, Dennis et al 2007, Schupp et al 2010, Prather et al 2017. In coastal barrier systems, dispersal is limited by geographic isolation, making specialized dispersal mechanisms particularly important in shaping communities .…”

“…Species arriving in systems with high environmental heterogeneity, such as are present in coastal ecosystems, may lack necessary traits to overcome a given stress. In addition, seed recruitment may be low in communities with strong environmental filters that may include heterogeneous resources, frequent disturbances, or limited perches for avian long-distance dispersal (Sales et al 2013, Prather et al 2017. In coastal systems, environmental filtering is based on position relative to shoreline, elevation above the freshwater lens, and spatial heterogeneity of salinity due to storm overwash .…”