Tree ring-based metrics for assessing old-growth forest naturalness

1. Old-growth studies commonly emphasize structural and age conditions, selecting proxy indicators of long-term ecological processes. Transition dynamics from mature to old-growth status reveal how natural legacies are progressively accumulated in forests after major disturbances, including human ones. In late-successional, multi-aged forests, the chronosequential ranking of developmental stages is a difficult task, as stand age provides little information, and time since last stand-replacing disturbance cannot be easily determined. 2. Canopy age features, disturbance/suppression history and growth trajectories were reconstructed from ring-width series of canopy trees in a network of 19 old-growth and managed European beech forests in the eastern Alps and central Apennines. A set of tree-ring metrics able to describe the intensity and time distribution of biological and ecological processes (e.g. understorey suppression, canopy accession age) were used to describe the advancement of old-growth status and compared to established metrics of forest structure. 3. Tree-ring metrics were site dependent, as biogeoclimate affects turnover rates and constrains the onset and recovery rate of old-growth attributes. Under the same environmental conditions (high-mountain, limestone-bedrock beech forests), values of the best indicators (number/duration of growth suppression phases; synchronicity of first release; maximum and range of canopy tree age; canopy accession age of the slowest-growing trees) increased monotonically (two to five times) from managed to secondary and primary old-growth forests. Trees in well-conserved primary old-growth forests experienced several and long suppressions, showing the highest complexity in recruitment history, canopy accession and growth trajectories. The best tree-ring metrics, condensed in a Naturalness Score to provide a synthetic functional ranking of forests, varied coherently with structural complexity, which represented stand dynamics more closely than biomass-related metrics. 4. Synthesis and applications. We propose a synthetic ranking of forest functional naturalness based on the ecological processes experienced by trees. This ranking helps to overcome the limitations associated with the use of arbitrary size-or age-related thresholds of old-growth status and provides a functional approach to establish chronosequences in ecological studies. The quantitative description of complex processes underpinning the unique biological and ecological features (e.g. extreme tree longevity) found in primary old-growth forests enhances their irreplaceable value in nature conservation. The proposed framework of tree-ring indicators describes functional traits tightly related to forest naturalness and may thus become a tool to identify and protect old-growth forests, benchmark the impact of silvicultural practices, prescribe targets or evaluate the effectiveness of restoration programmes.