Scrubland ecosystems are increasingly recognised for their biodiversity value and resilience to climatic stress, but their contribution to carbon sequestration remains poorly quantified, particularly in temperate rewilded landscapes. Existing carbon accounting approaches, developed mainly for closed‐canopy forests and agroforestry systems, do not reflect the structural complexity or biomass allocation patterns of scrub, where species commonly form multi‐stemmed stands and invest heavily in belowground biomass (BGB) under herbivory. Using destructive sampling of 290 individuals from five dominant scrub taxa at the rewilded Knepp Estate (UK), we developed taxon‐specific allometric equations incorporating height, canopy area, basal stem diameter and browsing intensity, achieving high predictive accuracy (R2 = 0.63–0.95) for above‐ and BGB. These equations reveal root: shoot ratios that exceed those predicted by standard forest models, particularly in heavily browsed plants, indicating that belowground carbon storage in scrub systems is substantially greater than currently assumed. By improving biomass estimation for rewilded scrub, this study fills a key gap in land‐based carbon accounting and provides practical tools for integrating scrublands into global greenhouse gas inventories and nature‐based climate strategies, supporting recognition of temperate scrub as a significant, currently undervalued carbon sink with direct implications for rewilding policy, restoration planning and carbon offset schemes.