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Review
, 90 (4), 537-44

Impacts of Climate Change on the Tree Line

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Review

Impacts of Climate Change on the Tree Line

John Grace et al. Ann Bot.

Abstract

The possible effects of climate change on the advance of the tree line are considered. As temperature, elevated CO(2) and nitrogen deposition co-vary, it is impossible to disentangle their impacts without performing experiments. However, it does seem very unlikely that photosynthesis per se and, by implication, factors that directly influence photosynthesis, such as elevated CO(2), will be as important as those factors which influence the capacity of the tree to use the products of photosynthesis, such as temperature. Moreover, temperature limits growth more severely than it limits photosynthesis over the temperature range 5-20 degrees C. If it is assumed that growth and reproduction are controlled by temperature, a rapid advance of the tree line would be predicted. Indeed, some authors have provided photographic evidence and remotely sensed data that suggest this is, in fact, occurring. In regions inhabited by grazing animals, the advance of the tree line will be curtailed, although growth of trees below the tree line will of course increase substantially.

Figures

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Fig. 1. Mean diurnal trends of temperature in June at four sites near the tree line in the Cairngorm mountains of Scotland. Solid lines are air temperatures, broken lines are the temperatures measured within terminal meristems (Wilson et al., 1987).
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Fig. 2. Effect of temperature on photosynthesis (triangles) and growth (circles), expressed relative to the value at 20 °C. The photosynthetic data are the mean of eight C3 species plotted in Grace (1977) and the growth data are from Pinus sylvestris (Junttila and Nilsen, 1993).
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Fig. 3. Rates of photosynthesis near to the northern limit of trees at Värriö, northern Finland (67°46′N, 29°35′E, 390 m a.s.l.) in contrasting years: 1999 (closed circles) and 2001 (open circles). Graphs show daily totals of photosynthesis, measured with in situ branch chambers (A), annual patterns of temperature (B) and cumulative photosynthesis over the first half of the growing season (C). Data kindly provided by Professor P. Hari.
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Fig. 4. Changes in ring width on approaching the tree line in the Swiss and Austrian Alps (Paulsen et al., 2000).
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Fig. 5. Photographs of Pinus sylvestris at the tree line in the Cairngorms Scotland, taken from the same point in 1980 and 2000. Note the height of the crown relative to the dead main stem.
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Fig. 6. Potential increase in the elevation of the tree line, assuming a 4·5 °C rise in temperature over 100 years.

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