Effect of tillage, clipping and climate on grass phytomass in a Zambian savanna

Abstract

The effect of tillage, clipping, precipitation and temperature on above-ground grass production was investigated in permanent plots at a Zambian savanna site for 5 y (1996^2001) by the harvest method. Mean species richness was 4.6 species m-2 with a total of 15 species at the study site. Mean end-of-season grass phytomass was 464 g m-2 with no statistically significant differences among years in control quadrats. Grass phytomass recovered within two wet seasons after tillage and a similar trend was observed after cessation of a 2-y monthly harvesting regime. However, in experimental quadrats, plot, treatment and year had significant effects on grass production. Previous- and current-season precipitation had no significant effect on end-of-wet-season grass phytomass but phytomass of the previous season explained 27-53% of the variation in end-of-wet-season phytomass. Grass production peaked in the wet season and declined sharply as the dry season progressed. The interaction between precipitation, temperature, harvesting and duration of the dry season explained 81-91% of the variation in daily grass production but the significance of the interactions varied with season and duration of harvesting. Precipitation-use efficiency (PUE) of grasses declined from 2.25 g m-2 mm-1 at the start of the wet season in December to 0.25 g m-2 mm-1 at the end of the season in March. Monthly harvesting for 1 y reduced PUE to less than 25%. The results indicate very complex relationships between above-ground grass production (dependent variable) and climate and land-use (independent) factors that makes the prediction of grass production in central southern African savannas difficult.