Perilous
Times and Climate Change
World's giant trees are dying off rapidly, studies show
Ecological 'kings of the jungle' being toppled by forest
fragmentation, severe drought and new pests and diseases
John Vidal, environment editor
The Guardian, Thursday 26 January 2012
The largest patch of old growth redwood forest remaining in
Humboldt Redwoods State Park, California. Photograph: Michael
Nichols/NG/Getty Images
The biggest trees in the world, known as the true ecological kings
of the jungle, are dying off rapidly as roads, farms and
settlements fragment forests and they come under prolonged attack
from severe droughts and new pests and diseases.
Long-term studies in Amazonia, Africa and central America show
that while these botanical behemoths may have adapted successfully
to centuries of storms, pests and short-term climatic extremes,
they are counterintuitively more vulnerable than other trees to
today's threats.
"Fragmentation of the forests is now disproportionately affecting
the big trees," said William Laurance, a research professor at
James Cook University in Cairns, Australia. "Not only do many more
trees die near forest edges, but a higher proportion of the trees
dying were the big trees."
"Their tall stature and relatively thick, inflexible trunks, may
make them especially prone to uprooting and breakage near forest
edges where wind turbulence is increased," said Laurance in this
week's New Scientist magazine.
Big trees may comprise less than 2% of the trees in any forest but
they can contain 25% of the total biomass and are vital for the
health of whole forests because they seed large areas. "With their
tall canopies basking in the sun, big trees capture vast amounts
of energy. This allows them to produce massive crops of fruits,
flowers and foliage that sustain much of animal life in the
forests. Their canopies help moderate the local forest environment
while their understory creates a unique habitat for other plants
and animals," said Laurance.
"Only a small number of tree species have the genetic capacity to
grow really big. To grow into giants trees needs good growing
conditions, lots of time and the right place to establish their
seedlings. Disrupt any one of these and you lose them."
In some parts of the world, Laurance said, populations of big
trees are dwindling because their seedlings cannot survive or
grow. "In southern India an aggressive shrub is invading the
understorey of many forests, preventing seedlings from dropping on
the floor. With no young trees to replace them, it's only a matter
of time before most of the big trees disappear."
According to Laurance, it is not just the biggest trees in the
world that are suffering, but also the biggest in their
communities. Dutch elm disease killed off many of the stateliest
trees in Britain in the 1960s and 70s, and new exotic organisms
and bacterial infections, often brought in from other continents
via garden centres, are threatening oak, ash and other species.
Longer lasting and more intense droughts, which are becoming more
frequent in many tropical areas with climate change, are also
taking their toll. Studies in Puerto Rico and Costa Rica suggest
that big trees also suffer more in droughts than most other
organisms, said Laurance.
"In rainforests droughts promote surface fires that burn through
leaf litter on the forest floor. Larger trees were initially
thought to survive these fires but in fact many die two to three
years later. In cloud forests, big trees use their branches and
crowns to rake the mist and capture water droplets. Global warming
could push clouds up to higher elevations depriving them of
sources of moisture".
"The danger is that the oldest, largest trees will progressively
die off and not be replaced. Alarmingly this might trigger a
'positive feedback' that could destabilise the climate: as older
trees die, forests would release their stored carbon, prompting a
vicious circle of further warming and forest shrinkage," said
Laurance.
Many of the big trees are the oldest and most ecologically
important inhabitants of the forest. In the Amazon, they are often
400-1,400 years old, in North America giant redwoods can exceed
2,000 years and giant sequoias 3,000 years.