Skip to navigation Skip to content

Study shows sheep change tree growth rings

Photograph showing tree growth rings
The study suggests that sheep had a greater impact on growth rings than the temperatures and could help increase the accuracy of the tree ring record as a way of estimating past climatic conditions

Past climatic conditions could be more accurately estimated thanks to work by an Institute researcher.

Climate scientists use growth rings in trees to reconstruct past weather conditions. Each year a tree lays down wood in the trunk in the form of a growth ring. In warmer years, which usually mean better growing conditions, trees grow faster and the rings tend to be wider. Narrow rings are a sign of low temperatures.

However, when animals feed on the leaves and buds, the trees grow less. Fewer leaves means less photosynthesis leading to less growth.

Now a team of researchers, including Research Theme Leader Professor Alison Hester, have for the first time measured how a tree's growth rings can be affected by herbivore grazing. The findings can be used to increase the precision of efforts to estimate past climatic conditions.

The researchers, whose findings are published in the current issue of the journal Functional Ecology, created nine fenced-off areas in the mountains of southern Norway. Some contained no sheep, some had about 10 sheep per square mile and some had about 30 sheep per square mile. Although sheep prefer to eat grass and herbs, in mountain areas supplies may be limited and they sometimes turn to shrubs and small trees to supplement their diet.

After nine years, the researchers sampled cross-sections of more than 200 birch trees in the enclosed areas and measured the trees’ ring widths. “What we found is that the number of sheep had a very large impact,” said James Speed, an ecologist at the Museum of Natural History and Archaeology in Trondheim, Norway, and the study’s lead author. “This shows how important the number of herbivores is".

In areas where there were no sheep, the growth in the tree rings was double that in areas with few sheep. And it was three times the growth seen in areas with the most sheep.

The study suggests that the sheep had a greater impact on growth rings than the temperatures in these pastures high in the mountains and could help increase the accuracy of the tree ring record as a way of estimating past climatic conditions.

The study was funded by the Norwegian Research Council and the Norwegian Directorate for Nature Management.

Notes to editors:

The study was published in the journal of Functional Ecology.
Speed J.D., Austrheim G., Hester A. and Mysterud, A. Browsing interacts with climate to determine tree-ring increment Functional Ecology 2011
DOI: 10.1111/j.1365-2435.2011.01877.x

Professor Alison Hester has over 25 years' research management experience with an international reputation in plant:herbivore interactions and vegetation dynamics particularly in forest/ upland systems, biodiversity, conservation and range management.

Press and media enquiries: 

Bernardo Rodriguez-Salcedo, Media Manager, Tel: +44 (0)1224 395089 (direct line), +44 (0)344 928 5428 (switchboard) or +44 (0)7791 193918 (mobile).


Printed from /news/study-shows-sheep-change-tree-growth-rings on 28/03/24 09:07:43 PM

The James Hutton Research Institute is the result of the merger in April 2011 of MLURI and SCRI. This merger formed a new powerhouse for research into food, land use, and climate change.