Effects of seasonal fire, bison grazing and climatic variation on tallgrass prairie vegetation

We used univariate and multivariate techniques to evaluate vegetation-envir onment relationships for plant functional groups on a tallgrass prairie sit e in northern Oklahoma, USA burned seasonally and grazed by bison (Bison bi son L.). The objective of the study was to identify important environmental variables associated with variation ill residual aboveground standing crop (phytomass) and abundance of plant functional groups. Phytomass was predic tably linked to season, with highest levels found in the latter portion of the growing season when the warm-season tallgrasses that dominated the site were most actively growing. When the effects of seasonal phenology were re moved, stepwise regression revealed that phytomass variation was best expla ined by year-to-year climatic variation, seasonal burn type, and bison graz ing. Phytomass was negatively related to bison grazing under all conditions . A number of plant functional groups responded to individual environmental variables: relative abundance levels of tallgrasses, little bluestem, annu al grasses, forbs and legumes all varied with burn season; little bluestem, annual grasses, and sedges varied by topoedaphic position, while forbs exh ibited a positive relationship with bison grazing intensity. Canonical Corr espondence Analysis (CCA) was used to directly relate composition patterns of functional groups to environmental variables. CCA bi-plot of the ordinat ion revealed th:lt annual grasses were most closely associated with summer fires, sedges were associated with fall burns and the 1995 sampling year, l egumes associated with a gradient representing the length of time since fir e; while tallgrasses and little bluestem ordinated nearest a gradient repre senting bison grazing. Forbs and perennial grasses did not clearly associat e with any particular environmental gradient, suggesting they were either s imultaneously affected by several parameters or that environmental attribut es important to these groups were not measured. Total phytomass and sedges were strongly influenced by yearly climatic variation. Relative abundance o f some plant functional groups was principally determined by a single facto r, while others were equally influenced by a suite of environmental interac tions.