Effects of elevated CO2 and cutting frequency on plant community structure in a temperate grassland

Monoliths of a fertile, N limited, C3 grassland community were subjected (or not) to an atmospheric CO2 enrichment (600 mumol mol(-1) ) using a Mini-FACE system, from August 1998 to June 2001 and were subjected to two contrasting cutting frequencies (3 and 6 cuts per year). We report here the effects of the CO2 and cutting frequency factors on the plant community structure and its diversity. Species-specific responses to elevated CO2 and cutting frequency were observed, which resulted in significant changes in the botanical composition of the grassland monoliths. Elevated CO2 significantly increased the proportion of dicotyledones (forbs + legumes) and reduced that of the monocotyledones (grasses). Management differentiated this response as elevated CO2 increased the proportion of forbs when infrequently and of legumes when frequently defoliated. However, among the two dominant forbs species only one was significantly enhanced by elevated CO2. Moreover, not all grass species responded negatively to high CO2. At a low cutting frequency, the observed decline under ambient CO2 in species diversity (Shannon-Weaver index) and in forb species number was partly alleviated by elevated CO2 . This experiment shows that the botanical composition of temperate grasslands is likely to be affected by the current rise (+ 0.5% per year) in the atmospheric CO2 concentration, and that grassland management guidelines may need to be adapted to a future high CO2 world.