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

Summary 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. 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. 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. 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.