Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 261

although computer models have improved they are still a long way from reflecting
climate change in a detailed way in both space and time, or in recreating past major
change with anything other than an approximation.
6.1.2 Currenttropicalrainforestresponse
Tropical rainforests are among the most species-rich tree communities on Earth
and the most productive of terrestrial ecosystems. Consequently tropical rainforests
are affected by global warming and concomitant increases in atmospheric carbon
dioxide. A recent medium-term study, over two decades long in 2004, of 18 tropical
Amazonian rainforest plots that were otherwise undisturbed, has shown a number
of changes (Laurance et al., 2004). In 2001 the Amazonian forest covered about
5.4 million km 2 , approximately 87% of their original extent, with 62% of that which
remains in Brazil. The Amazonian forest is home to perhaps a quarter of the Earth's
terrestrial species, and about 15% of global terrestrial photosynthesis occurs there.
Evaporation and condensation over Amazonia contribute to the global atmospheric
circulation, having downstream effects on precipitation across South America, and
further afield across the northern hemisphere. The Amazon's forests contribute a
significant part of the Earth system's functioning (Malhi et al., 2007).
All trees in each plot of the Laurance et al. study that were more than 10 cm in
diameter at breast height were tagged and mapped. In total nearly 13 700 trees were
recorded. Because rainforests are so biodiverse some 88% of tree species were too
rare for there to be a robust statistical analysis. Furthermore, there were so many
species and subspecies that their analysis was done at the genus level to capture
the changes with statistics. There were 244 genera, of which 115 were sufficiently
abundant for statistical analysis.
They found that within the plots the rates of tree mortality, recruitment and growth
have all increased to some degree or other over the 20 years of the study. Of the
115 relatively abundant tree genera, 27 changed significantly in population density
or basal area to a degree that was nearly 14 times greater than might be expected
from chance alone. This result is similar to other studies. However, contrary to some
predictions there was not an increase in pioneer species. Pioneer species are those
that are commonly found colonising new or disturbed ground, and higher mortality
associated with ecological change would be expected to disturb ground with tree
falls.
The researchers also found an increasing number of faster-growing trees, including
many canopy and canopy-emergent species. These increased in dominance or pop-
ulation density. Conversely, a number of genera of slower-growing trees, including
many sub-canopy species, were declining.
So, what is causing these changes? The researchers noted that it could be that
the plots were recovering from some previous disturbance. However, their plots'
location - away from areas of human land use, in the depth of forests (so avoiding
edge effects) and over an area of 300 km 2 (so avoiding purely local factors) - meant
that previous disturbance was an unlikely cause of the changes seen.
Second, the researchers wondered whether the changes might be due to regional
climate cycles, such as El Ni no-related droughts. These have increased over the last