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Big trees, like the old-growth forests they inhabit, are declining globally Rhett Butler, mongabay.com January 26, 2012 Already on the decline, demise of giant trees may be hastened by global warming. 
Reviewing research from forests around the world, William F. Laurance, an ecologist at James Cook University in Cairns, Australia, provides evidence of decline among the world's "biggest and most magnificent" trees and details the range of threats they face. He says their demise will have substantial impacts on biodiversity and forest ecology, while worsening climate change. "To persist, big trees need a safe place to live and long periods of stability," he told mongabay.com via email. "But time and stability are becoming very rare commodities in our modern world." Giant trees offer critical habitat and forage for wildlife, while transpiring massive amounts of water through their leaves, contributing to local rainfall. Old trees also lock up massive amounts of carbon — in some forests they can account for up to a quarter of living biomass.
Big trees are also sensitive to fragmentation, which exposes them to stronger winds and drier conditions. Laurance's own work in the Amazon has shown substantial die-off of canopy giants in small forest fragments. Their susceptibility seems counter-intuitive given big trees' life histories, which invariably include periods of drought and other stress. "All around the tropics, big canopy and emergent trees are succumbing to strong droughts," Laurance said. "That's been a surprise to me and many other ecologists, because big, ancient trees would have had to survive many droughts in the past." Forest giants may suffer disproportionately from climate change, writes Laurance in New Scientist, highlighting research in La Selva, Costa Rica by David and Deborah Clark.
The Clarks’ hypothesis, if correct, means tropical forests could shrink over time. The largest, oldest trees would progressively die off and tend not to be replaced. Alarmingly, this might trigger a positive feedback that could destabilize the climate: as older trees die, forests would release some of their stored carbon into the atmosphere, prompting a vicious circle of further warming, forest shrinkage and carbon emissions.
All told, the outlook for big trees is not good, according to Laurance. "The decline of big trees foretells a different world where ancient behemoths are replaced by short-lived pioneers and generalists that can grow anywhere, where forests store less carbon and sustain fewer dependent animals, where giant cathedral-like crowns become a thing of the past."
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(09/14/2011) Old growth rainforests should be a top conservation priority when it comes to protecting wildlife, reports a new comprehensive assessment published in the journal Nature. The research examined 138 scientific studies across 28 tropical countries. It found consistently that biodiversity level were substantially lower in disturbed forests.
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