Abstract
We measured net ecosystem CO 2 exchange (NEE) in Panamá over C 4 pasture plots that varied in grazing intensity. After adjusting for variation in light, there were noticeable effects of grazing‐related variables on CO 2 exchange that were largely dependent on the developmental stage of the plant canopy. Above‐ground productivity was positively related to grazing intensity ( r 2 =0.30). Two experimentally grazed fields had significantly lower standing crop biomass but no significant difference in CO 2 uptake (24.2 μmol/m 2 /s) compared with two ungrazed fields (20.3 μmol/m 2 /s). Grazed fields had significantly lower ecosystem respiration rates (10.3 μmol/m 2 /s) than did ungrazed fields (17.6 μmol/m 2 /s). These results suggest that, although these pastures were possible sources of CO 2 during the time intervals sampled, the size of the sources tended to be dampened by cattle grazing through reductions in ecosystem respiration. Thus, it appears that disturbance caused by cattle grazing will not always result in an increase in CO 2 release from tropical pastures to the atmosphere.