172 The impactsof climate change
Figure 7.12The pattern of world biome types related to mean annual
temperature and precipitation. Other factors, especially the seasonal variations
of these quantities, affect the detailed distribution patterns (after Gates).conditions (e.g. temperature and precipitation) under which a species can
exist and reproduce are known as its niche. Climate niches for some typ-
ical tree species are illustrated in Figure 7.13; under some conditions
a change as small as 1◦C in annual average temperature can make a
substantial difference to a tree’s productivity. For the likely changes in
climate in the twenty-first century, a substantial proportion of existing
trees will be subject to unsuitable climate conditions. This will be partic-
ularly the case in the boreal forests of the Northern Hemisphere where,
as trees become less healthy, they will be more prone to pests, die-back
and forest fires. One estimate projects that, under a doubled CO 2 scen-
ario, up to sixty-five per cent of the current boreal forested area could
be affected.^43
A decline in the health of many forests in recent years has received
considerable attention, especially in Europe and North America where
much of it has been attributed to acid rain and other pollution originating