Tom Ovenden
As a discipline, dendroecology has helped to fundamentally re-shape our understanding of how both individual trees and forests function (Amoroso et al 2017). Where the field of ecology aims to understand how organisms of both the same and different species interact with each other and their environment, dendroecology is the use of accurately and precisely dated tree-ring timeseries to address ecological questions and understand the dominant processes that were at play when these tree-rings were being formed. The great power of tree-rings for ecological research lies in their incredible temporal resolution and the fact that trees capture and store information relating to the conditions in which they were growing (see Section 1: Introduction to Dendrochronology), providing us with a window through which we can look back in time.


In an era of unprecedented global environmental change, the attributes of tree-rings and the information contained within them has seen the importance and scope of dendroecological research rapidly increase in recent decades. As we face the combined impact of biodiversity loss and climate change, many species will increasingly be faced with a set of conditions which are outside the normal range of variability to which they are adapted, presenting new challenges for the continuity of ecosystems and the ecosystem services they provide. There is a pressing need to ensure that all forests, from those that are managed to provide a sustainable timber resource to those that are critically important for biodiversity, are resilient to a range of future challenges. Dendroecology enables us to understand how individual organisms and the wider ecological systems they are part of develop and change through time and how they have responded to stressful events in the past, which in turn will allow us to plan more effectively to meet these challenges in the future.
Current state of knowledge
Set against the backdrop of steadily rising temperatures and shifts in patterns of precipitation is an expected increase in the frequency and severity of extreme climate events across the world, including Scotland and the wider UK (Dodd et al 2021; Kirkpatrick et al 2023). As these changes unfold, there is a growing risk that these events may force some species to cross ecological thresholds, resulting in fundamental shifts in forest structure, function, and composition (Cavin et al 2013; Ovenden et al 2021; Suarez and Kitzberger, 2008). Understanding where these thresholds might exist and the tree or forest attributes that confer either resilience or vulnerability to disturbance is important if we are to mitigate some of the negative impacts of these events, making it the focus of much dendroecological research in recent years.

Of particular concern is the rapid and widespread tree mortality caused by extreme climate events. For example, more than 9000ha of forest in Scotland was affected by storm Arwen in 2021 which resulted in extensive tree mortality, whilst 2022 was the hottest year on record in Scotland. Adding to these concerns is the compounding effect of multiple interacting events, such as a storm or drought followed by an outbreak of a damaging insects (eg bark beetles) (Spiecker and Kahle, 2023). Recent dendroecological work has shown how the likelihood of tree mortality is linked to particular tree attributes, such as tree size and how resilient those same trees were to previous, non-lethal events, suggesting that larger trees which were historically less resilient may be more likely to die during subsequent events (DeSoto et al 2020). These results, combined with ecophysiological evidence on the physical limits of trees under water stress (Adams et al 2017) are contributing to efforts to develop early warning signals of tree mortality (Cailleret et al 2019; Camarero et al 2015).

Complementing this work on tree mortality is a growing body of dendroecological research into the relationships between diversity, resilience, and ecosystem functioning which is starting to highlight important differences between the stress tolerance of individual species and between simple, and more species rich communities (Haberstroh and Werner, 2022). In some cases, this work has demonstrated how it is not species diversity per-se that confers greater resilience, but that the response of individual trees to stress partly depends on the characteristics of the other species in a tree’s immediate neighbourhood (Gillerot et al 2021; Vitali et al 2018) in combination with the type of disturbance event (Messier et al 2021). In the European Alps, tree-rings have even been used to reconstruct the population dynamics of defoliating insects such as the larch budmoth (Zeiraphera diniana Gn.) over the past 1200 years (Esper et al 2007). These authors were able to use this reconstruction to demonstrate a recent change in this insect’s otherwise remarkably regular cycles of abundance, corresponding to a current period of exceptional warmth in the study region and highlighting the sensitivity of an otherwise stable ecological system to recent warming. This example demonstrates how dendroecology could be used to understand the impacts and future risks of a growing number of tree pests and diseases in Scotland.
The complex interactions between individual trees, species and the characteristics of a particular event makes reliably operationalising resilience concepts challenging; however, the insights gained through the application of dendroecological techniques to this challenge is a promising area of applied research that continues to develop. Understanding relationships between the aspects of diversity (eg species or structural diversity) and ecosystem functioning will be a critical next step if we are to design, establish, restore, and maintain forests in Scotland that are resilient to the anticipated range of future challenges.

Outside of extreme events research, dendroecology has been used to directly compare the annual growth performance of different species and species mixtures, demonstrating how more diverse forests can often also be more productive than monocultures of the same species (Pretzsch et al 2015). Owing to the exact calendar dated annual resolution of tree-ring timeseries, these comparisons also allow a more detailed understanding of when particular species come to dominate forest development, when management prescriptions such as thinning might be appropriate (Chung et al 2017) and at what intensity (Pérez-De-Lis et al 2011). Crucially, the subsequent collection of post-intervention tree-ring data helps us to evaluate the effectiveness of forest management prescriptions, and how long it took for any intervention to achieve the desired result. For example, a recent meta-analysis of 23 studies (the majority of which utilised dendrochronological data) demonstrated that the timing as well as the intensity of thinning mattered for increasing forest resilience to drought (Sohn et al 2016). With the growing recognition of the need to diversify our planted forests, the ability to use dendroecology to directly compare the productivity and developmental histories of different species, species mixtures and silvicultural systems across a range of sites and conditions will be a valuable tool for guiding future decisions under a changing climate, but one that has been relatively underutilised in Scotland to date.

The combination of tree-ring measurements with other diverse sources of data is starting to provide us with a more complete understanding of forest dynamics and how forest systems respond to change than is possible from tree-rings alone (Trotsiuk et al 2020). As remote sensing technologies decrease in cost but increase in accessibility, temporal resolution and spatial coverage, the opportunity to combine these data products with dendrochronologically derived timeseries on tree growth, quantitative wood anatomy, stable isotopes and ecophysiology is an exciting frontier of ecological research, enabling us to link patterns across scales, from leaf to stand level processes (Kannenberg et al 2019). Similarly, tree-ring data can be combined with palaeoecological records from peat and lakes to understand historic, long-term ecological changes (Edvardsson et al 2022) or combined with genetic data to forecast population dynamics, invasion dynamics and species range expansions using individual-based modelling approaches (Lamonica et al 2021). As computational advances such as the use of deep learning to automatically detect and measure tree-rings is likely to bring efficiencies in processing dendrochronological data (Poláček et al 2022), technological progress such as the application of low-cost environmental sensors is enabling the automated, minute by minute collection of tree growth and local environmental data over multiple years (Rebaudo et al 2023). Leveraging these computational and technological developments whilst integrating these diverse datasets with dendroecology will enable us to build a more comprehensive understanding of how Scotland’s forests function and change than ever before, which in turn will help guide conservation and forest management decisions in the face of regional and global change.

The future of dendroecology in Scotland
Like many regions of the world, the impact of climate change in Scotland will show considerable regional variation, be accompanied by an increase in the frequency and severity of a range of extreme climate events (Sniffer, 2021) and depend on our ability to dramatically reduce our greenhouse gas emissions. These changes will pose challenges for the continuity of existing woodland habitats of high conservation value, the establishment of new productive forests that can meet the growing demand for timber, and for Scottish Government to achieve its commitments to improve the condition of native woodlands and increase the total area of forest and woodland cover (Scottish Government, 2019). As we tackle these challenges head on, difficult questions relating to appropriate restoration goals and the effectiveness of forest management techniques, species and provenance choice, climate change adaptation, carbon sequestration and storage, and sustainable timber and fibre production (amongst many others) will require the best available evidence to make informed decisions. Globally, dendroecological research has already demonstrated its ability to generate and contribute much of this evidence; however, the use of dendrochronological techniques to address ecological questions relating to these topics in Scotland has been relatively little explored but presents a significant opportunity for the future.
To realise the potential of Scottish dendroecological research and to help guide the direction of this work, here we highlight three main priority themes to which dendroecology could contribute much needed evidence. This list is not intended to be exhaustive, but to broadly categorise key areas upon which future research could focus. Owing to the inter-related nature of many of the challenges we face, there will often be significant overlap between these three research areas. For example, a key aspect of sustainable forest management necessarily requires our forests to be resilient to the challenges of a changing climate whilst balancing social, ecological, and economic priorities, with productive, species rich and structurally complex forests likely to be a key mechanism of achieving this. Such synergies across these research themes highlights how dendroecological research can simultaneously contribute evidence to a number of current challenges.