Trampled Vegetation and Floristic Convergence in the Tropics

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Trampled Vegetation and Floristic Convergence in the Tropics Robert E. Frenkel* Ours is an era of cultural convergence, a world-wide sharing of ideas, institutions, methods, artifacts, and also an ecumenical sharing of biota with explosive potentialities.1 Recently Parsons in describing the Africanization of the New World tropical grasslands presented evidence of African grasses rapidly replacing indigenous ones throughout tropical America.2 To contribute to a better understanding of floristic and vegetational convergence, this paper describes habitat modification by human trampling which results in similar patterns of floristic composition in widely distributed places and contributes to the floristic convergence of anthropic vegetation. By foot treading, vegetation is affected directly by abrasion, scuffing and compression; by the dissemination and carriage of diaspores ; and indirectly by soil compaction leading to poor soil aeration , altered soil moisture conditions, and increased resistance to root penetration. Modification of vegetation by treading may be viewed ß Dr. Frenkel is an associate professor of geography at Oregon State University , Corvallis, Oregon 97331. This paper was read at the 33rd annual meeting of the Association. Field work in Costa Rica was undertaken during the summer of 1969 and supported, in part, by the Office of Naval Research under contract Nonr 3656 (03), project NR 388 067 with the Department of Geography , University of California, Berkeley, directed by Professor J. J. Parsons. 1 Charles S. Elton, The Ecology of Invasions by AnimaL· and Phnts (London , 1958). For a more theoretical discussion of biotic invasions, see H. G. Baker and G. L. Stebbins (eds.), The Genetics of Colonizing Species (New York, 1965) ; and Robert H. Mac Arthur and Edward O. Wilson, The Theory of Island Biogeography (Princeton, 1967). 2 James J. Parsons, "The 'Africanization' of the New World Tropical Grasslands ," Tübinger geographische Studien, Vol. 34 (1970), pp. 141-153. 87 88ASSOCIATION OF PACIFIC COAST GEOGRAPHERS as a complex system of interacting factors in which the trampled habitat presents an extreme environment suitable for colonization by only a limited number of taxa. Studies of treaded vegetation have focused on the nature of the trampled habitat, properties of treading resistant plants, and the assemblages or communities of plants occupying these habitats. Lieth demonstrated that many treading plants are capable of growing in soil with little pore space.3 Many species adapted to trampled situations likewise occupy the flooded margins of streams and lakes where soil pore space is also diminished in these seasonally inundated habitats. Soil compaction as measured by a penetrometer correlated with vegetation zonation along trampled pathways in Japan.4 Properties of plants capable of colonizing treaded habitats were studied by Haessler who analyzed the florafrom 223 trampled stands in Western Germany. Some of these properties were diminutiveness protecting the plant by soil surface irregularities, growth flat against the soil surface, small leaves and tissues which by anatomy and morphology are not easily damaged by compression and abrasion, attentuated life span under unfavorable ecologie conditions, rapid nutrient uptake leading to quick growth and quick recovery after damage, superior ability to increase vegetatively, small diaspores often modified for epizoochorous dispersal, and seed germination frequently enhanced by scarification and occurring rapidly at shallow soil depths.5 Considering the specialized habitat of trampled places and the corresponding particular properties demanded of the plants, it is not surprising to find repeated assemblages of species occupying treaded sites. 3 H. Lieth, "Die PorenVolumina der Grünlandböden und ihre Beziehungen zur Bewirtschaftung und zum Pflanzenbestand," Z. Aker-und Pflanzenbau, Bd. 98 (1954), pp. 453-460. *Yoshiwo Horikawa and Akira Miyawaki, habitat Segregation of the Weeds as an Indicator of the Soil Hardness," Yokoiiama National University Science Reports, Section II, No. 3 (1954), pp. 49-62. B Kurt Haessler, "Zur Ökologie der Trittpflanzen," unpublished Ph.D. dissertation , Technische Hochschule, Stuttgart, 1954. YEARBOOK · VOLUME 34 · 197289 Treading communities have been arranged by plant sociologists into a hierarchal system in which the relationship between flooded turf communities and treaded communities is evident. Although associations are commonly accorded clear differentiation as distinct units of vegetation, Segal, in a study of European wall and pavement crack vegetation, regarded the entire constellation of plant communities related to these specialized habitats as an association assembly with no sharp floristic separation between...

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