Northern Ireland Trees Provide Clues to Climate Change

The results of a new study "Climate signal in tree-ring chronologies in a temperate climate: a multi-species approach" (Full paper - PDF) involving researchers Ana García-Suárez and John Butler at Armagh Observatory and Mike Baillie at Queen's University Belfast have recently been published in the scientific journal Dendrochronologia. Tree-ring widths and densities have been used as indicators of climate change for several decades, but the question of which aspects of climate, for example average temperature, rainfall, drought or sunshine, the trees really respond to has remained open.

The background to this study is that trees grown close to one of their geographical limits, for example at their upper altitude limit, may be particularly susceptible to changes in temperature, and such stressed trees from high altitudes or high latitudes have most often been used to estimate mean atmospheric temperatures for the period before thermometers came into general use in the 18th and 19th centuries. For example, tree rings have sometimes been used to estimate how warm the world is now compared to, say, the late medieval period or the time when the Romans ruled Britain.

The new work attempts to establish which climate parameters on a monthly or seasonal scale are most important for the growth of four common species currently widespread in the British Isles, namely Oak, Ash, Beech and Pine. The authors use trees grown close to one of the longest running meteorological stations in Europe, namely that at Armagh Observatory, Northern Ireland, in an attempt to link the tree-ring widths to an array of climate variables. Here the trees have grown in relatively benign conditions with adequate rainfall and moderate temperatures, and have mostly grown well within their geographical limits.

The study draws three main conclusions of interest to the climate-change community. First, none of the four species allows reconstruction of any Annual climate variable, though they can allow reconstruction of specific seasonal climate parameters. These seasonal reconstructions can sometimes be unstable in time. Secondly, they find that Ash and Beech are more sensitive to climate changes than Oak and that these species respond more clearly to rainfall and drought conditions than to mean temperature. This could provide a way to estimate changes in rainfall over the past few centuries in parts of the British Isles where a reliable instrumental record does not exist. Thirdly, they find that combinations of tree-ring widths from several species that have grown together are more successful in reconstructing climate than those of a single species.

These are promising results, but it will be difficult to extrapolate them back further than the last few centuries owing to the requirement to date the specimen trees. Currently, there is only the long Northern Ireland Oak chronology (Queen's University Belfast) and it is unlikely that parallel chronologies for other species can be constructed because of the lack of suitable sources. Also, the growth of trees is affected by the local environmental conditions (i.e. whether they have grown in forests, in open country, or on bogs), which may not have been the same in the past as now. Tree-ring widths nevertheless may provide an important proxy to climate change as long as these causes and effects can be unravelled.

FOR FURTHER INFORMATION PLEASE CONTACT: John Butler at the Armagh Observatory, Tel.: 028-3752-2928; E-mail: cjbat signarm.ac.uk; or Mike Baillie at Queen's University Belfast, Tel: 028-9097-3977; E-mail: m.baillieat signqub.ac.uk.

Last Revised: 2009 December 16th