In this post, Associate Editor Nathalie Butt discusses a recent paper ‘Tree-ring based metrics for assessing the functional naturalness of forests‘ by Alfredo Di Filippo, Franco Biondi, Gianluca Piovesan and Emanuele Ziaco.

Valuable ecosystems

Primeval forest, or ancient woodland in the UK, is an integral part of many epic stories and myths throughout human history, especially in Europe:  just think of all those old tales with bears and wolves! Old-growth forest provides the setting and narrative for societal development. Our interaction with these complex and diverse ecosystems has shaped landscapes across huge scales, spatial and temporal; half of Europe’s primary forest was cleared before the Middle Ages (up to 600 years ago). We consider primary or virgin forests to be old and largely undisturbed climax systems, with long periods of stability, which means that species-rich communities of fauna and flora are often found in these habitats. In many cases, species unable to compete in more disturbed forests act as indicator species for this type of forest, for example, bluebell as an ancient woodland indicator in the UK.

In addition to supporting rare species, old growth forests are important for a range of ecosystem services such as carbon storage (both in the trees themselves and in the soils), provisioning of clean water, air purification and soil maintenance, and climate moderation and control at different scales. Over recent decades, scientific interest in, and understanding of, forest dynamics has highlighted the need to manage and conserve old-growth forest, only 20% of which remain globally. One previously unresolved issue has been how to accurately define and quantify ‘old growth’ itself.

Forest naturalness and old-growth status

Until now, old-growth forest studies have defined ‘old growth’ based on structural and age conditions using proxy indicators of long-term ecological processes (functional dynamics) and forest naturalness (which increases with forest age), such as tree size and stem density of living and dead trees. However, trends (in mean stem diameter or density in forest stands) may be non-linear, and if the selected attributes don’t increase linearly with stand development, the transition from mature stand to primary old-growth status may not be captured, and long-term changes in late successional multi-aged forests may not be tracked. Some kind of naturalness score could be very useful for understanding long-term ecological processes, as it would measure the ecological distance between managed, early and mature old-growth forests. More exact knowledge of the age, naturalness, ecological processes and forest structure is vital for forest monitoring and conservation efforts focussing on these immensely important and valuable ecosystems.

What can we learn from tree-ring dating?

Di Filippo et al. devised a novel method of more accurately estimating forest age and naturalness by using tree-ring data to give information on canopy age, disturbance and growth trajectories.  This allowed them to describe the intensity and temporal distribution of biological and ecological processes, and the development of old-growth status.

They used a network of old-growth European beech Fagus sylvatica forests across Italy and Austria containing the oldest (500-600 years) broadleaf trees in the Northern Hemisphere. The beech forests ranged across several bioclimatic zones: from warm Mediterranean to cool Alpine. Age and functional history indicators vary with local environment, and biogeoclimatic conditions constrain the onset and development rate of old-growth attributes. This means that there is no universal size- or age-related threshold indicator of old-growth status (with the exception of some age structures that clearly indicate a forest is not in an old-growth state, such as tree stumps clearly marking historical management). However, it is possible to develop a framework of benchmark indicators within each bioclimate zone: the indicator of forest naturalness Di Filippo et al. developed varied consistently with forest structural complexity, and thus more accurately describes forest development than current methods.

Managing and conserving our ancient forests

Ideally, perturbations should be managed to maintain natural habitat and landscape conditions, thus protecting ecosystem function, in order to support the full range of seral stages and their associated biodiversity. Old-growth beech forests are characterised by distinctive slow growth processes that underpin extraordinary tree longevity, adding further value to these unique ecosystems. The new approach of Di Filippo et al. provides an improved method for describing and quantifying the legacies of forest management, and is a valuable contribution to our understanding of forest naturalness. This work could inform the identification and protection of old-growth forests, provide a point of reference for the impact of silvicultural practices, and help define conservation targets and and evaluate the effectiveness of restoration projects.