Linear mixed-effects models for estimating biomass and fuel loads in shrublands

Abstract

Shrubland biomass is important for fire management programmes and for carbon estimates. Aboveground biomass and the combustible portion of biomass, the fuel load, in the past have been measured using destructive techniques. These techniques are detailed, highly labour intensive, and costly; hence, an alternative approach was sought. The new approach used linear mixed-effects models to estimate biomass and fuel loads from easily measured field variables: shrub overstorey height and cover, and understorey height and cover. Site was regarded as a random effect. Sampling sites were located throughout New Zealand and included a range of shrubland vegetation types: manuka ( Leptospermum scoparium J.R. Forst. et G. Forst.) and kanuka ( Kunzea ericoides (A. Rich.) J. Thomps.) scrub and heath, pakihi (mixed low heath, fern, and rushes), and gorse ( Ulex europaeus L.). The approach was extended and confidence intervals were constructed for the regression models. Statistical analysis showed that understorey height and overstorey cover were significant (at the 5% level) in some cases. Overstorey height was highly significant in all cases (p < 0.0001), allowing development of models useful to the operational user. The models allow rapid estimation of average fuel loads or biomass on new sites, and double sampling theory can be applied to calculate the error in the resultant biomass estimate.