Allometric equations for hyper-arid desert plant species of AlUla County, Saudi Arabia

BACKGROUND: Estimates of aboveground woody plant biomass in hyper-arid ecosystems have predominantly relied on allometric equations developed in more mesic habitats. However, these equations do not account for local variations in plant morphology, necessitating the development of equations for the hyper-arid context. Here, we present species- and growth-form-specific allometric equations for 11 woody plant species in AlUla County, Kingdom of Saudi Arabia (KSA), based on sample sizes ranging from 8 to 50 individuals per species. RESULTS: Across five nature reserves in AlUla County, individuals of each selected plant species, spanning a range of size classes, were measured for height and crown area. For tree species with suitable structures (i.e. Moringa peregrina and Vachellia gerrardii), basal diameter was also recorded. All sampled plants were then destructively harvested to determine aboveground biomass. For all six shrub species, the best-fitting allometric equations included crown area and height as predictors of aboveground biomass, whereas all five tree species' equations included height (and other predictors, varying by species). The best-fitting general multi-species equations included crown area and height as predictors of aboveground biomass for both shrub and tree growth forms. CONCLUSIONS: The predictors in the best-fitting equations likely reflect the branched, lateral growth forms characteristic of plants in hyper-arid ecosystems, and are expected to improve the accuracy of biomass estimation compared with equations developed in mesic environments. These allometric equations provide a novel foundation for the quantitative monitoring of aboveground plant biomass and carbon stocks in the KSA and hyper-arid regions further afield.