Estimating herbaceous plant biomass in mountain grasslands: a comparative study using three different methods

It is a challenge to find effective methods to estimate biomass over a large range of biomass values in diverse plant communities, such as typically found in mountain grasslands. We compared the performance of three non-destructive methods for estimating plant biomass (3D quadrat: a point quadrat method, plate meter: a measure of physical volume, and visual estimation: a component of the BOTANAL method) in mountain grasslands. We tested whether: (1) all methods performed equally in terms of linearity of estimations over a large range of biomass and (2) under and over-estimations of biomass were related to specific plant compositions. We estimated plant biomass in 30 plots, for which real plant biomass was measured by destructive sampling. We accounted for the significant effect of heteroscedasticity (which was significant for all three methods) when testing for the linearity of the relationship between real biomass and biomass estimates. The plate meter displayed non-linearity, being insensitive to variation of biomass at low biomass values. BOTANAL and the 3D quadrat yielded linear relationships, with BOTANAL providing better estimates of real biomass (greater R²). Specific floristic compositions (e.g. presence of Deschampsia cespitosa, Chaerophyllum sp., and abundant small forbs) explained underestimation and overestimation of biomass estimates for the plate meter and 3D quadrat while BOTANAL was insensitive to floristic composition. In heterogeneous grasslands, BOTANAL appeared to be the most appropriate method, given its linear relationship with real biomass over the whole range of biomass and its low residual variation.