From function to definition: a proposal for addressing the old-growth forests dilemma

Old-growth forests are biodiversity-rich ecosystems and crucial for ecosystem functioning, making their conservation a global priority. Yet, their identification remains challenging due to the absence of a universally accepted definition that integrates both quantitative and qualitative criteria, as well as a consistent assessment method. The 2023 EU guidelines highlight three indicators   native species, substantial deadwood and large, old trees   but these remain strongly site- and species-dependent. For instance, deadwood accumulation varies widely with tree composition, climate and productivity. This study proposes a species- and site-independent benchmark for defining oldgrowth forests, enabling a functional and globally applicable approach. We hypothesize that in late-successional forests, where resources are almost fully utilised, tree size distribution follows the Energetic Equivalence Principle (EEP): declines in tree abundance are compensated for by proportional increases in individual resource use. This principle underpins a universal tool for quantifying “old-growthness” by comparing observed size distributions with those predicted by the EEP. Using basic tree parameters   height, stem diameter, crown length and crown radius   crown volume can be estimated as a proxy for resource use, allowing the prediction of expected size distributions. We tested this framework across temperate and tropical forests under contrasting stand conditions. In Italy, a seminatural stand (Somadida Natural Reserve) and a managed stand (Regola di Villapiccola) were analysed, while in Malaysia, semi-natural (core) and sub-natural (edge) stands were examined within Kabili-Sepilok Forest Reserve. Crown volume-height scaling yielded exponents of ~2.1 (temperate) and ~3 (tropical), corresponding to predicted size-distribution slopes of -2.05 and -2.53. Semi-natural stands matched predictions (-2.1; -2.4), while the managed stand deviated strongly (-1.25) and the sub-natural stand only slightly (-2.17). Based on these findings, we propose a quantitative, broadly applicable definition: old-growth forests are characterized by a tree size distribution aligned with EEP. Combined with biodiversity and deadwood metrics, this approach offers a standardized, functional method to assess and monitor old-growth status across ecosystems.