Climate Change in the Arctic - The Changing Arctic Environment: The Arctic Messenger - page 306

greening shows an increase of 15.5% and 20-26% in different parts of the North American
Arctic and 8.2% in the Eurasian Arctic. This generally reflects shrub growth.
The second of the botanists' expectations is a more complicated story. Although there
are reports of a northward expansion of some boreal zone species into the tundra, it seems
more common at present for tundra species that in the past have kept close to the ground
to now grow much taller, creating a new form of shrubby deciduous woodland. In North
America, one of the last species of conifer to disappear as one passes from boreal forest to
tundra is the black spruce, so you would expect it to be one of the first species to march
into a warmer tundra. However, studies in Alaska show that the new warmer temperature
regime brings growth-limiting difficulties for this species when soils become drier or the
plant tries to grow in the warmer spring air temperatures before there is liquid water avail-
able to the roots. Another species found close to the North American tree line is the white
spruce. Once again, studies tell a complicated story. This tree seems to come in two types,
both of which are present in about the same numbers. One type grows well if summer tem-
peratures are higher than 16°C, but for the other type, growth is reduced. Before 1950, it
was rare to experience 16°C near the tree line, but now it has become quite common. My
reaction on hearing about these studies was to think that probably a genetic condition is
involved, called a polymorphism , that maintains two different varieties in a population. It
enables a quick response to changing conditions. Perhaps this is the case, but more recent
studies showthatinmosttreelinestudyareas, the“warm tolerant” typesarealsobeginning
to show less enthusiasm for the changing conditions. It is quite likely that grazing from
herbivores, especially caribou and reindeer, is also influencing the dynamics of vegetation
change. Another complicating factor is forest fires. Historically, these were rare events on
the tundra, partly because of temperature and the late disappearance of snow but also be-
cause of the scarcity of potential fuel. Now we have accelerating shrub growth and a cor-
responding increase in the frequency and severity of fires, particularly in Alaska.
Confronted with this convoluted picture, some botanists are proposing that the data
fit better with a theory that suggests ecosystems tolerate new conditions until critical