Demise of the World's Forest Giants
Across the planet, the world's biggest and oldest trees are dying out. Why?
In an article just published online, David Lindenmayer of Australian National University and ALERT director Bill Laurance highlight the many reasons that big, old trees are so vulnerable.
Some reasons are obvious, but others are biological mysteries.
Big Trees in Decline
First, the obvious reasons. Big trees tend to favor productive environments -- many of which have been cleared for agriculture. Humans currently farm an area the size of South America and graze an areas the size of Africa.
Big trees are also targeted by loggers. In southern Australia, so many of the giant Mountain Ash trees have been logged out that a group of foresters didn't even know what species they were seeing when they finally encountered a few of the surviving giant old trees.
Where once there were giants (photo (c) Humboldt State University)
In other places, big trees are falling victim to exotic weeds. In large areas of India, Australia, and South Africa, for instance, the tropical weed Lantana grows so densely that the seedlings of the giant trees cannot survive. Unable to reproduce, the giant old trees are slowly dying out.
In northern Australia, native woodlands are being invaded by exotic Gamba Grass, which is native to Africa. The grass, which can grow to 5 meters in height, burns so fiercely that even the biggest trees are killed. One cheeky biologist even suggested that elephants be imported to Australia to help control the noxious weed (he was immediately shouted down, of course).
An inferno of Gamba Grass in northern Australia (photo (c) Samantha Setterfield)
Mysterious Declines
But other declines of giant old trees are more difficult to understand. Surprisingly, big trees seem to be unusually vulnerable to droughts, evidently because they face severe hydraulic stress in trying to get water up to their very tall canopies. But if they're so vulnerable to droughts, how did they get to be giant and old in the first place?
Big trees also appear to be exceptionally vulnerable to habitat fragmentation. No one is sure why. Perhaps they suffer from drier conditions in fragments. Or maybe they are vulnerable to wind turbulence, given that winds can accelerate over the denuded lands surrounding the fragments. One researcher even suggested they might be killed by lightning -- as they're the tallest thing in the landscape.
Big trees are vulnerable in fragmented forests (photo (c) Richard Bierregaard)
A Need for Stability
One point is clear: Big trees are adapted to long periods of stability -- something that is becoming a very rare commodity in our rapidly changing modern world.
It would be a tragedy to lose our giant old trees. They play vital roles in storing carbon, producing water vapor that creates life-giving clouds and rainfall, and they are the supermarkets of the forest -- producing much of the fruit, nectar, and foliage that animals rely on for food. And via their many hollow cavities, big old trees provide homes for a rich array of wildlife species.
Losing our giant trees would be a tragedy for humans as well. What would it be like to live in a world where one can't stare up in wonder at a giant cathedral-like tree crown?
