The trouble with travelling tree rings

How can you trace climate in the past, where there are no historical records? A common solution is to use tree growth rings. Trees don’t just add an extra ring of growth each year. The width of the ring can indicate what growing conditions were like for the tree. This could record whether or not a year had particular drought conditions. The trees sampled act as climate recorders for where they stand. But the samples around the world are not all collected in a way that makes them easy to equate with each other.

An article in Quaternary Science Reviews by Babst and colleagues looks at the problems in tree ring data and how you can go from a local measurement to global data. They identify three problems.

The first is that one core sample is not necessarily an accurate record of the tree’s growth. The rings sampled are a record of tree growth at one particular point on a tree. They advise getting more data from the same tree.

In a similar vein how accurate is a single tree when it comes to climate at a site? The authors state: “The criteria for sampling trees within a site vary according to the aims of a given study. For example, old and dominant individuals are selectively sampled for dendroclimatic reconstructions; plot designs, stratified or random samplings are often preferred for dendroecological studies; and trees with specific characteristics (e.g. scars) are targeted to assess the natural disturbance history of a site. Researchers are also interested in within-site variability that is driven by micro-site conditions and may contain relevant ecological information that is otherwise averaged out when only a mean site chronology is calculated.”. Therefore when comparing sites, it is easy to miss comparing like-with-like.

Finally, they then look at moving from a site to larger scales, like regional data. Again to what extent do the sites selected for a region or continent reflect that region? This might not be perfect coverage for an area. For example, Antarctica is lacking data. Babst and colleagues add: “To represent tree growth across regions or even continents, ideal networks of tree-ring sites densely cover the geographic extent of the study area and reflect, in proportion to the area they occupy, the range of bioclimatic and ecological conditions experienced by species within this area. This ideal has probably rarely been achieved. Instead, traditional sampling for dendroclimatological purposes has often targeted areas with marginal growth conditions, which only occupy a small fraction of the landscape.”

Laying out these problems the authors tackle the issue of connecting data from sites around the world. It’s a task that will have a valuable pay-off for future work. The authors conclude: “An important application of these data will then be to evaluate the ITRDB and ensure that this legacy of decades of tree-ring research can continue to support earth system science.”