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The Center for Native Grasslands Management

Title: Comparative Ecology of Warm-Season (C4) versus Cool-Season (C3) Grass Species in Kentucky, with Special Reference to Bluegrass Woodlands
Year: 2005
Author(s): Campbell, J.
Source Title: Proceedings of the 4th Eastern Native Grass Symposium
Source Type: Proceedings
pages: 96-115
Original Publication:  
Abstract: The distributions of C4 and C3 grasses are compared in Kentucky. There is a strong association of C4 grasses with late summer growth in open, nonforested habitats of Kentucky, including sites maintained by fire. Some 86% of C3 grasses typically flower between mid-April and late July, whereas 96% of C4 grasses typically flower between late July and mid-October. Only open habitats that experience much seasonal drying have major concentrations of C4 grasses: (a) xeric pine/cedar-oak woodlands and associated rocky glades; (b) xeric-tending oak woodlands and associated grasslands, especially on gentle uplands; (c) hydric-tending oak woodlands and associated grasslands, especially on high terraces. A minor concentration also occurs on “shrubby/graminoid streambanks” that experience flooding and other seasonal changes in water level sufficient to maintain a distinct zonation of vegetation between forested banks and the low water levels. Subhydric or hydric sites that experience less seasonal drying appear to have virtually no typical C4 grasses, but there are a few locally dominant C3 grasses within these habitats. The few C4 grasses that occur in deeper shade are all perennials in the genus Muhlenbergia. There is no significant overall trend in numbers of C4 versus C3 species along the gradient from acid soils with low fertility to base-rich soils with high fertility. However, locally dominant perennial grasses, especially the few taller species reaching 2 m or more, are mostly C4 species on low to average soil fertility (e.g., Andropogon gerardii), and mostly species C3 on higher soil fertility (e.g., Arundinaria gigantea). Since eutrophic soils support rapid plant growth in general, there is a strong tendency for forest to predominate over the landscape, with rapid recovery from disturbances. Because of this, there may have been relatively little opportunity for selection of grasses that can dominate on sunny, eutrophic soils, especially in phosphatic sections of the Bluegrass region. Moreover, woodlands on eutrophic soils can allow dense grassy ground vegetation with C3 species to develop in the partial shade, especially in spring before trees are fully leafed out. This hypothesis is developed further with reference to the literature on the balance of C4 versus C3 species. It is suggested that frequent spring fires might maintain openings with C4 species but at the expense of some native features in Bluegrass Woodlands. In contrast, seasonally intense foraging of ungulates on the nutritious “herbage” is indicated by some historical accounts, and several characteristic native species are associated with moderate ungulate effects in the vegetation. Ungulates may have helped maintain the high proportion of C3 species due to enhanced nutrient cycling and perhaps overgrazing of incipient C4 grassland patches along intensely used corridors.
Publisher: The University of Kentucky Department of Forestry, Lexington, Kentucky
Editor(s): T. G. Barnes