<p>This study investigated the population ecology of giant reed (<i>Arundo donax</i> L.) species along the environmental gradient in Abha Valley, Asir Mountains, Saudi Arabia. The study involved sampling homogeneous and monospecific stands of <i>A. donax</i> across six sites in the Abha Valley. At each site, three stands were selected, and within each stand, five randomly distributed quadrats (each measuring 1.0 × 1.0&#xa0;m) were used to record the density, morphology, and biomass of the <i>A. donax</i>. The results of this study showed significant variability in the density, morphological, and biomass parameters of the <i>A. donax</i> across the different sampling sites in the Abha Valley. Soil properties, including organic matter, nitrogen, phosphorus, and clay, were found to be strong predictors of <i>A. donax</i> density, morphology, and biomass. The relationship between shoot basal diameter and shoot density suggested density-size effects. The researchers in this study applied regression analysis techniques to develop a predictive equation that can estimate the biomass of <i>A. donax</i> shoots based on shoot height and shoot basal diameter. This method was time-saving and the resulting equation might be useful in evaluating management techniques used for monitoring <i>A. donax</i>. The findings of this study may contribute to the effective management of <i>A. donax</i> and the conservation of native ecosystems in the region.</p>