This is a big ouch for the
interpretation of tree rings. We have
relied substantially on tree rings to produce historical climate records. Now we have to factor in another unknown in
the form of browsing behavior.
The work done was with
goats. Yet the most likely browser are
deer and deer populations notoriously swing sharply as predation and disease
cause boom and bust cycles. Thus
historical records are likely to be misleading.
This has thrown in a factor that
does go to the reliability of the data and it will be hard to establish correlations
that will improve that reliability. It
may well obscure actual correlations that we value.
As I said, this is an ouch. Only the clearest trends can be trusted and
apparent missing correlations may well be eliminated by this biological and
plausibly cyclical factor.
Seeing the wood for the trees: New study shows sheep in tree-ring
records
by Staff Writers
Nibbling by herbivores can have a greater impact on the width of tree rings than
climate, new research has found. The study, published this week in the British
Ecological Society's journal Functional Ecology, could help increase the
accuracy of the tree ring record as a way of estimating past climatic
conditions.
Many factors in addition to climate are known to affect the tree ring
record, including attack from parasites and herbivores, but determining how
important these other factors have been in the past is difficult.
Working high in the mountains of southern Norway, midway between Oslo
and Bergen, a team from Norway and Scotland fenced off a large area of
mountainside and divided it into different sections into each of which a set
density of domestic sheep was released every summer.
After nine summers, cross sections of 206 birch trees were taken and
tree ring widths were measured. Comparing these with local temperature and the
numbers of sheep at the location where the tree was growing allowed the team to
disentangle the relationship between temperature and browsing by sheep and the
width of tree rings.
According to lead author Dr James Speed of the NTNU Museum
The impact of large herbivores on tree rings has, until now, been
largely unknown, so these findings could help increase the accuracy of the tree
ring record as a way of estimating past climatic conditions, says Dr Speed:
"Our study highlights that other factors interact with climate to affect
tree rings, and that to increase the accuracy of the tree ring record to
estimate past climatic conditions, you need to take into account the history of
wild and domestic herbivores.
"The good news is that past densities of herbivores can be
estimated from historic records, and from the fossilised remains of spores from
fungi that live on dung."
"This study does not mean that using tree rings to infer past
climate is flawed as we can still see the effect of temperatures on the rings,
and in lowland regions tree rings are less likely to have been affected by
herbivores because they can grow out of reach faster," he explains.
Tree rings give us a window into the past, and have been widely used as
climate recorders since the early 1900s. The growth rings are visible in tree
trunk cross sections, and are formed in seasonal environments as the wood is
laid down faster in summer than winter.
In years with better growing conditions (in cool locations this usually
means warmer) tree rings are wider, and because trees can be very long-lived
and wood is easily preserved, for example in bogs and lakes, this allows very
long time-series to be established, and climatic conditions to be estimated
from the ring widths.
The study was funded by the Norwegian Research Council and the
Norwegian Directorate for Nature Management. James D. M. Speed, Gunnar
Austrheim, Alison J. Hester and Atle Mysterud (2011), 'Browsing interacts with
climate to determine tree ring increment', doi:
10.1111/j.1365-2435.2011.01877.x, is published in Functional Ecology on 27 July
2011.
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