Visual obstruction as a method to quantify herbaceous biomass in southern African semi-arid savannas

Biomass of aboveground vegetation is a useful descriptor for studies of grazing, fire and wildlife habitat use in grassland systems. The traditional method to estimate biomass, hand-clipping, is time intensive and other indices of biomass have been used successfully. In southern Africa, the disc pasture meter has been the tool of choice and the use of visual obstruction has been much less prevalent than in North America. Our goal was to determine if visual obstruction could be used as a correlate to aboveground grass biomass in grassland systems in Namibia. We gathered clipping and visual obstruction samples at three study sites in Highland Savanna and Woodland Savanna in northern Namibia. Dry biomass of grass was correlated with visual obstruction readings when samples from all study sites were pooled (r(2) = 0.64, P < 0.0001), but the strength of the relationship varied among the three study sites. We also evaluated the number of samples needed to characterise biomass at a study site. Variation in the cumulative mean was very low at sample sizes of 15-20 samples. Visual obstruction can be a useful method to evaluate biomass of grassland systems in a quick manner.