Gradient analysis of remnant True and Upper Coastal Prairie grasslands of North America

Sixty-three upland True and Upper Coastal Prairie grasslands were sampled for vegetation composition and soil variables. The first axis from principal components analysis produced a south to north arrangement of stands along which temperature and precipitation decrease and soil organic matter increases (P < 0.0001). The second principal components analysis axis was related to a soils gradient, primarily within Texas communities, which had more varied soils than grasslands to the north. Species response curves against the first axis showed a continual replacement from north to south, with Schizachyrium scoparium and Paspalum plicatulum dominants in the south, Andropogon gerardii more important in central and northern communities, and Stipa spartea and Sporobolus heterolepis important in the north. The C3/C4 ratio of grasses increased rapidly northward from Nebraska. Species diversity and richness did not vary greatly and showed nonsignificant correlations with environmental variables across this latitudinal gradient. Stand relationships from cluster analysis corresponded with the results of principal components analysis, and based on these analyses, plus a review of the literature, seven community types were recognized. Five form a continuum, across which Andropogon gerardii increases northward and Schizachyrium scoparium increases southward, while two are limited to high-precipitation areas of north Texas.