What if trees could talk? Imagine being able to ask an ancient forest tree about its life. To ask when it was born, about when times were good and bad, its experiences of drought and fire. Imagine being able to ask how the world had changed, throughout its long life.
As it turns out, science can ask trees these questions and many more, through the science of dendrochronology – study of time using trees. Generally, this involves measuring features of the growth rings that are formed in wood as a tree grows. The resulting data provides powerful insights for diverse fields, including ecology, climatology, archaeology, biogeography, ethnobotany, and even carbon accounting programs. Dendrochronology has been used to date patterns of change in forests, assess the progress of climate change, calculate the occurrence rates of volcanic eruptions, cyclones and landslides, and reveal the age of our most iconic ancient trees. The approach is rapidly increasing in popularity.
To get at this important information stored in trees, scientists have two options. They can cut the tree down and examine ring patterns from sectioned disks of the trunk. Or, if the tree is too valuable to sacrifice, they can extract a narrow core of wood using a hollow drill bit, called an increment bore corer. Invented by Hofrath Pressler in 1866, this has been the tool of choice ever since for accessing the treasure trove of dendrochronological information. Hundreds of thousands of trees – including some of the worlds most important for heritage and conservation values, such as the bristlecone pines of the Methuselah stand in California and the massive baobabs in Africa and Madagascar – have been cored using this method.
Most studies assume that coring has no impact on tree health –maintaining reproductive output, trunk strength, growth rate and risk of death. However, there is a small body of research that reports significant, sometimes fatal, outcomes, directly attributable to this sampling technique.
In a recent example, coring a critically endangered Mauritian endemic tree was a strong contributor to the death of the first known tree of that species, leaving just two remaining adult individuals in existence. From a conservation management perspective, there needs to be some resolution between getting the important data that comes from coring potentially priceless trees, and assessing the risks involved in coring rare living specimens, where risking potential impacts on health cannot be tolerated.
The information currently available on tree health outcomes from coring is restricted to a limited set of studies, with minimal replication, that focussed on a suite of temperate climate species. Therefore, researchers looking at coring in new locations and on un-tested species have limited scope to predict if the sampling they do is likely to damage trees they core, or to remedy the wounds they inflict.
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In a recent review we wrote in Biological Reviews, we focus sharp attention on the emerging problem of uncertainty about the impacts of tree coring.
We argue that, although there has been some limited successes in fixing the problem by attending to the core wound after sampling, the results have often been poorly replicated, conducted over very short periods relative to the tree’s lifespan or have made the coring wound much worse. The emerging evidence suggests that we should not sample freely, and apply quick and nasty dressings afterwards (as is often done currently). Instead, best practice techniques require being aware of the specific risks, which vary between different species and environments, and being cautious and sensible about which trees to core. Variations seen in how well trees can recover from a coring wound are likely to be explained by a combination of environmental stresses acting on the tree, tempered by the species’ unique evolved defenses against natural damage.
The observations available from previous coring experiments are not yet clear enough to pinpoint exactly why some trees cope better with coring than others, but the emerging evidence suggests that healthy trees with the capacity to grow quickly are more likely to heal the wound faster (insulating the inner trunk from decay) than stressed individuals. However, even if a wound appears to be healed externally, there may be extensive, ongoing internal damage. To guide the field of dendrochronology toward a best practice application of tree coring technique, we outline a new framework that improves best practice, while working towards maximizing scientific insight from this incredibly useful method.
Ultimately, we have to accept a small risk of damage when gathering data from any natural ecosystem. The trick is to find the optimum threshold between what is acceptable to tolerate, from a conservation perspective, and what is needed to gain important scientific insight to improve conservation management. This is an area where science needs to reflect on the growth and survival of trees at a holistic level, acknowledging that the timespans of most research projects are mere snapshots in the total life of a tree.
Ultimately we all want the trees to talk, but there is little benefit to anyone if we kill them for answering our questions.