Spatiotemporal variation in aboveground biomass in a tropical grass pasture over a 22-year period

This study monitored spatiotemporal variation in aboveground biomass of a paddock (1.1 ha) in a sown tropical grass pasture under cattle grazing over a 22-year period (1995–2016) by nondestructively estimating biomasses of 182 fixed locations (0.5 × 0.5 m each) in the paddock at about monthly intervals during the annual stocking season (May to October or November). The mean aboveground biomass of the paddock varied seasonally and interannually between 33 and 605 g DM/m² and was explained by a regression model including positive effects of mean air temperature, N fertilizer rate and total precipitation during the preceding 60 days (R² = 0.311, P < 0.001). The coefficient of variation of aboveground biomass, as an indicator of spatial (within-paddock) variability, varied seasonally and interannually between 0.16 and 0.94, and was described by a model incorporating a positive effect of cumulative stocking density during the preceding 40 days and negative effects of mean air temperature, total precipitation during the preceding 20 days and N fertilizer rate during the preceding 60 days (R² = 0.277, P < 0.001). The spatial distribution pattern of aboveground biomass in the paddock was persistent (P < 0.05) or not persistent over the study period, almost without being reversed. In detail, patterns in the mid stocking season tended to maintain mutual similarity in the entire range of the intervals between the measurements (364–7,694 days), whereas those at the beginning and near end of the stocking season lost mutual similarity as the interval increased. There was no indubitable evidence that the changes in botanical composition destabilized the spatial distribution pattern of aboveground biomass. The results indicate the need for assessing how the long-term stability of the within-paddock distribution pattern of aboveground biomass affects agronomic and ecological performance of the grazing system.