The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 149

southern Norway about 1000 years ago. Its range migration is one of the most
recent and best constrained invasions of a main tree species in northern Europe
and it allowed Seppa and colleagues to assess using pollen analysis (see section
2.1.5) colonisation patterns and the associated competitive replacement processes
with other species. There was an average spreading rate of 0.2 km year − 1 from
eastern to western Finland. They discovered that the P. a b i e s population in northern
and western Scandinavia (Norway, Sweden and Finland) increased in size from the
time of the initial expansion to levels comparable with the modern situation in just
100-550 years. At each site P. a b i e s invaded a dense, intact Scots pine-birch-alder-
lime-hazel ( Pinus sylvestri - Betula - Alnus - Tilia - Corylus ) forest. The resident mixed
forest provided no or weak resistance to the colonisation of Norway spruce. The
tree species that most clearly declined was small-leaved lime ( Tilia cordata ), the
ecological niche of which considerably overlaps with that of P. a b i e s . T. cordata in
Sweden was also responding to climate change and was already in decline before
the arrival of P. a b i e s (and probably less likely due to human interference 1000 years
ago as suggested by some hypotheses, although as we shall see human interference
does often affect a species' response to climate change). Other species coped better
with the climate change and the invasion of P. a b i e s . Scots pine ( P. sylvestris )showed
no or little change during the P. a b i e s colonisation and population growth, probably
because P. sylvestris predominantly grows in drier and more nutrient-poor soils than
P. a b i e s . Alnus also seemed resilient. Birch ( Betula ), hazel ( Corylus avellana ) and
elm ( Ulmus ) did decline in central Sweden as a result of P. a b i e s invading.
This work demonstrates that ecosystems are transient, and subject to change when
new species attain dominance and replace one (or several) key components of the
previous ecosystem. It shows how the former species-rich mixed ecosystem was
invaded by P. a b i e s and gradually changed to a modern boreal forest, dominated
by Norway spruce and Scots pine ( P. a b i e s and P. sylvestris ). This ecosystem change
happened gradually from east to west, as the spread of P. a b i e s forest caused a decrease
in the distribution of the mixed conifer/deciduous forest.
Today, it is likely that future warming will promote a major northward spread of
temperate deciduous tree species including small-leaved lime ( T. cordata ). By the
year 2100 T. cordata will be a major component of - although Scots pine is likely to
dominate - the forest in central Sweden, its range reaching along the Swedish east
coast up to 68 ◦ N (north of the Baltic Sea coast where Sweden meets Finland).
Biological change did not just occur at the time of glacial-interglacial transition,
it also took place with climatic change within glacials themselves. For example,
the Italian palaeorecords mentioned in section 4.5.1 show that the Lago Grande di
Monticchio region saw switches between three broad biome types. Oak ( Quercus
spp.) and elm ( Ulmus spp.) temperate woodland dominated both the current and
previous Eemian (or Sangamon as it is sometimes known) interglacial. Conversely,
pine ( Pinus spp.) and juniper ( Juniperis spp.) dominated the LGM and other cooler
parts of the last glacial, whereas warmer interstadial climatic events during the glacial
saw wooded steppe dominate.
The genetic consequences of all these biome transitions both within and between
glacial and interglacial conditions were profound for many plant species. For tropical
forests, they were not only reduced during glacials but their present distribution areas