Shrub aboveground biomass estimation considering bulk volume geometry: general and specific models for 14 species in the Mediterranean central Chile

Abstract

Shrubland ecosystems cover a large part of the five Mediterranean regions of the world, and monitoring their biomass is crucial for assessing fire risk and carbon sequestration. However, biomass models for multi-stemmed shrubs remain scarce, making the quantitative assessment of shrubland biomass imprecise and often unreliable. Here, we measured, harvested, and weighed 411 specimens at two representative sites to quantify aboveground biomass (AGB) in 14 shrub species. To develop species-specific and general models of AGB, we used observations on total height (HT), diameter at collar height (DCH), number of stems (NS), and crown size as well as different geometric shapes representing crown area (CA) and bulk volume (BV). General models including all species were fit, including species identity as a random effect to take variation across species into account. A k-fold cross-validation was used to assess and compare the ability of the models to predict independent data. Individual AGB varied markedly both within and among the 14 species, with on average higher values at the site characterized by lower shrub density and species richness. Two biomass components, woody and leaf + twig, were distinguished, and species-specific means of the woody and leaf + twig proportion varied between 30 and 60%. We found that crown BV assessed on different geometric shapes was suitable to predict the biomass for different shrub species and that additional variables were rarely beneficial. The best general model included BV represented as an inverted truncated cone, derived from crown diameter (CD), HT and the DCH of the longest stem. This study provides novel allometric equations essential for assessing AGB shrublands in central Chile. Our general multi-species models based on BV geometry could prove very useful for future studies in Mediterranean shrublands, allowing us to estimate biomass through indirect, non-destructive methods.