Effects of residual dry matter on net primary production and plant functional groups in Californian annual grasslands

A five-year experiment tested the response of above-ground net primary production (ANPP) and plant functional groups to manipulations in residual dry matter (RDM), reflecting typical grazing practices in Californian annual grasslands. The RDM treatments were 225, 560, 900 and 5000 kg ha(-1) with the latter treatment representing no grazing by livestock. ANPP in autumn, winter and spring showed a strong year x RDM interaction indicating that, in periods with higher herbage mass, the highest RDM treatment, representative of no grazing, was usually more productive. However, the opposite pattern was observed for the autumn period when ANPP was lowest. Random effects models indicated that at most 0.20 of the variation in ANPP for any of the three seasons stemmed from RDM treatments, with the remaining variation partitioned among temporal and spatial dimensions or not explained. Cover of grass species was highest under the highest RDM treatment in four of the five years, and lowest for the lowest RDM treatment compared with the two intermediate RDM treatments in the two years with the highest herbage masses. Cover of forbs and clover was higher on the lowest RDM treatment and lower on the highest RDM treatment than on the intermediate treatments. The results suggest that residual RDM values above 550 kg RDM ha(-1) are appropriate for annual grasslands with a mean annual precipitation of more than 400 mm and offer a compromise between herbage use and promotion of future productivity and diversity, but direct control of ANPP and composition via management is not attainable because the environmental factors are predominant.