Boreal Tree Growth Slows as Climate Warms

It appears that climatic warming in boreal forests has pushed many trees beyond the limits of their optimal growing conditions.

In recent decades, the growth rates of northern conifers often slowed down when temperatures got abnormally high.

American scientists examined widths of tree growth rings laid down between 1902 and 2002 at 269 northern sites distributed across North America and Eurasia. They then matched the tree growth data with local historical temperatures and precipitation.

The ten major species of boreal conifers analyzed all showed instances during the 20th century of slower tree growth under higher temperatures. This phenomenon, which the study's authors term "browning", appeared in nearly all areas of Canada, Alaska, Europe and Asia that they sampled.

Notably, browning occurred much more frequently after 1942. Browning has been particularly prevalent in jack pine and four species of spruce: Norway, Siberian, black and white spruces.

Scots pine and Siberian spruce had stalled growth during hot years more frequently on some drier sites with relatively low precipitation compared with moister locations. They, along with white spruce and Siberian larch also experienced browning most often in the warmer portions of their range. For instance, sites where growth rates slowed in Siberian larch were on average 2 °C warmer in summer than sites that never exhibited browning.

One other factor showed up in the intercontinental tree-ring analysis, that of pollution. Tree browning occurred more frequently in eastern Europe and northwestern Russia, regions prone to heavy air pollution, than in undeveloped areas.

The effect of temperature was widespread, though, as even unpolluted forests showed browning. The study's authors conclude that warmer temperatures have dampened tree growth in two ways. Both the direct effect of rising growing-season temperatures and the indirect effect of water stress induced by greater evapotranspiration under warm conditions have hindered boreal tree growth.