Grass response to seasonal burns in experimental plantings

A 6-year experiment examined the effects of spring and summer fires on gras ses in southern Wisconsin, Synthetic communities of C-3 and C-4 grasses wer e seeded (100 seeds m(-2) species(-1)) in 1992 and subjected to prescribed burns in May and August of 1995 and 1997, or left unburned, By 1994 all plo ts were virtual monocultures of the C-3 reed canary grass (Phalaris arundin acea L.). By the second post-season sample in 1998, total productivity of p lots burned in May was higher (781 +/- 212 se g m(-2) year(-1)) than those burned in August (362 +/- 28 g m(-2) year(-1)) or left unburned (262 +/- 43 g m(-2) year(-1)) due to the incursions of either the C-4 grasses big blue stem (Andropogon gerardii Vitman), switichgrass (Panicum virgatum L), or bo th. These large late-season grasses are much more productive per area cover ed than P. arundinacea or the other two C-3 grasses present, Elymus virgini cus L, and Poa pratensis L, Even at this early stage of succession, C-4 pro duetion in plots burned in May was 5 to 6 times that in the other 2 treatme nts. August burns produced a mix of C-3 and C-4 grasses but did not strongl y favor the pre-treatment C-3 dominant P. arundinacea. Unburned plots most resembled those burned in August in species composition, but differed in ha ving 4 times the accumulated litter, perhaps foretelling divergence in C-3 and C-4 composition as succession proceeds.