Graduate Thesis Or Dissertation
 

Plant colonization of gopher mounds in adjacent pasture and prairie communities

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  • I used field experiments to study how plants in two grassland communities colonized soil mounds made by the Camas pocket gopher, Thomomys bulbivorus (Richardson). I identified potential mound colonizers in each source of colonization (buried propagule bank, seed rain, and established vegetation) and then measured species specific rates of colonization on mounds built by T. bulbivorus. By selectively eliminating different avenues of colonization on artificial mounds, I estimated the relative and combined effects of colonization from (1) germination and growth of buried viable seeds and growth of root fragments in the soil; (2) germination of seeds raining onto the mounds; (3) emergence of buried vegetation and, (4) encroachment and establishment of adjacent vegetation. Artificial mounds were good mimics of mounds built by T. bulbivorus judged by their similarity in colonization rates and composition of colonizing species. I repeated the investigation in adjacent pasture and prairie communities differing in species composition and abundances to compare the effects of these differences on the colonization process. Composition and abundance of species in the expressed and potential vegetation varied considerably between pasture and prairie as did the two communities' response to identical gopher disturbances. Percent cover of vegetation on mounds increased 3 times faster in the Composition and abundance of species in the expressed and potential vegetation varied considerably between pasture and prairie as did the two communities' response to identical gopher disturbances. Percent cover of vegetation on mounds increased 3 times faster in the pasture than the prairie; and vegetation on and off mounds in the pasture was more alike (71% Similarity) than vegetation on and off mounds in the prairie (50% Similarity). Despite these differences, the relative contribution of each source of colonization was strikingly similar in the two communities. Vegetative encroachment and emergence contributed more to overall colonization rates (76% in the pasture; 75% in the prairie) than did establishment from seeds or buried root fragments. Emergence from underneath the mounds was favored by the shallow depth of mounds, minimal alteration of the substrate associated with mound building, and dominance of perennial species with erect growth forms. The small area and high perimeter to surface area ratio resulted in a high percent colonization from encroachment of surrounding vegetation. Colonization from the rain and bank contributed less to mound closure and may have been limited by a low abundance of propagules in those two sources. Successful colonists differed in their patterns of colonization. Festuca rubra, Agoseris heterophylla, Plantago lanceolata and Prune lla vulgaris colonized almost exclusively via emergence. Fragaria virginiana colonized by the extension of stolons both onto (encroachment) and up through mounds (emergence). Colonization from the seed rain was important in many annual species, such as Ranunculus occidentalis, Clarkia quadrivulnera, and Sherardia arvensis and the biennial species, Hypericum perforatum. One annual species, Cynosurus echinatus colonized to some degree from several modes of colonization. Mound disturbances had greater forb and annual species cover in both communities than was represented in the background vegetation, although the difference was much greater in the prairie. Results of this and other studies of gopher disturbance suggest that the relative abundance of perennials and annuals, evenness of species abundance and competitive relationships can help to predict patterns of colonization and effects of gopher mounds on community diversity.
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