Disturbance and ecological change in high-latitude birch woodlands
Wednesday 29 January, 2014
Contribution from Dr. Nick Cutler, former Keble fellow in Geography; currently at University of Cambridge.
Annual growth rings in trees provide an invaluable record of environmental change. Differences in ring width can be used to reconstruct past climatic conditions and to assess the impact of ecological disturbances, whether these disturbances are natural or man-made. Many studies have been conducted using tree rings from temperate locations e.g. Europe and North America. Tree ring records from high-latitude regions are much rarer, mainly because there are fewer trees and low/variable growth mean that tree rings are harder to interpret. This hampers our ability to understand climatic variability in a part of the world that is undergoing very dramatic warming.
My research focuses on long-term (decades to centuries) ecological change in Iceland. Consequently, I am very interested in the ways in which fluctuations in climate and disturbance events (e.g. ash deposition during the eruption of Eyjafjallajökull in 2009) have on the development of vegetation. Tree ring records are an ideal way of doing this and in 2010 I used a Keble Small Project Grant to collect tree ring cores in Iceland. I took the cores from birch trees growing on Mt Hekla, a volcano in Southern Iceland. Hekla is very active: it erupted five times between 1947 and 2000. It is therefore the ideal location to study the ecological impact of both a warming climate and volcanic disturbance.
The results of my research show a dramatic increase in tree ring width over the last decade. In broad terms, ring widths increase as growth conditions get better. In this case, it is likely that climatic warming has lengthened the Icelandic growing season and allowed the birch trees to grow more strongly. This result has implications for the fate of carbon dioxide (an important greenhouse gas): in theory, increased temperatures in the polar regions could result in more carbon being drawn from the atmosphere and tied up in organic matter such as trees, although a corresponding increase in the carbon dioxide released by warming soils is likely to exceed this effect. The rings also revealed that ash deposition had virtually no impact on tree growth: arctic birches are, perhaps unsurprisingly, hardy in the face of disturbance. Finally, and perhaps most significantly, the tree rings showed great variability from tree-to-tree. An understanding this individualistic response is extremely important to studies that seek to generalise from tree ring records, as the recent ‘Climategate’ episode demonstrated.
Dr. Nick CutlerFormer Keble Fellow in Geography
This article was first published on ASC Newsletter HT 2012 (March, 2012)