When silver firs and other tree species began showing widespread signs of disease and weakness in the early 1980s, many foresters and researchers were initially at something of a loss. The suspected cause was acid rain, i.e. sulphur-containing emissions that fell on forests as rain, weakening the trees.
While the large-scale 'forest dieback' proclaimed by the media did not ultimately materialise, the public and scientific researchers realised that many trees were in poor health and that as a result there was an urgent need to reduce pollutant emissions and also to carry out long-term monitoring of depositions in forests and the condition of trees. Monitoring programmes, such as the Sanasilva inventory and the Long-term Forest Ecosystem Research programme (LWF), both run by the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), were launched across Europe.
The best indicator of tree health
Various aspects served as characteristics for tree health, depending on the research question: first and foremost, the loss of leaves or defoliation in the treetops, indicated by 'crown transparency', but also tree-ring width or an increase in tree mass. However, the criticism now made by three researchers, led by WSL dendrochronologist Paolo Cherubini, in a background report in the journal Current Forestry Reports is that none of these indicators reliably indicate tree health. "Despite the efforts to monitor forest health that have been ongoing for almost 50 years [so since the 'forest dieback'], there is still no clear and universally agreed concept of the vitality of individual trees or the broader concept of forest health," they write.
They suggest using analyses of the biochemical composition of tree rings as in-situ indicators. "Tree rings not only tell us about past tree growth but also about the physiological processes that have occurred in a tree," explains Cherubini. These differ depending on the environmental conditions in which tree-ring growth took place.
Environmental stress reflected in tree rings
The proposed analyses draw on the fact that variations in carbon (C) and oxygen (O) concentrations occur in the carbon dioxide (CO2) that plants take up from the atmosphere. Differing volumes of these isotopes are deposited in the wood, depending on how effectively the tree can carry out photosynthesis. Severe drought, for example, causes the stomata in the leaves – which a tree uses to 'breathe' – to close up, so that the tree does not lose too much water. An isotope analysis can demonstrate this chemical imbalance.
Isotope ratios vary because of factors such as light, temperature and the availability of water and nutrients, and are also affected by disease, pests, climatic extremes or pollution. Given that in our climate, tree rings indicate the growth of wood, these influences can be dated back a long way. "We can use this technique to reconstruct whether a tree has experienced environmental stress in the past," explains Cherubini. The costs involved in these analyses have fallen dramatically in recent years, he notes. "As a result, we recommend integrating tree-ring isotope analysis into forest-health monitoring programmes in the future."
Those responsible for the monitoring programmes at WSL are open to this suggestion but do have a few reservations. For a quick assessment of how trees are faring, crown transparency and tree mortality serve as a meaningful and cost-effective indicators, explains Peter Brang, who has worked on characterising tree health since the 1990s. Arthur Gessler, a biogeochemist working on the LWF project, adds: "Tree-ring isotopes are a very good, additional measurand for determining tree vitality." However, they are no substitute for other analyses. There are, he says, unanswered questions, such as how well the extraction of tree-ring cores can be standardised across Europe so that the data can be compared, and whether the costs are still too high.
Cherubini sums up by saying that tree-ring isotope studies are promising indicators of tree vitality and allow conclusions to be drawn about why trees are in good or poor health. They also have the considerable advantage of enabling retrospective insights.