Hard times for big trees

Are “Big Trees” on the decline around the world?

That’s the case that William Laurance makes in “Big trees in trouble: how the mighty are fallen” in a recent issue of the New Scientist. He summarizes findings of scientific studies from forests around the world and believes that we are moving toward a time when forests will have fewer big trees because of a variety of environmental changes we are seeing today. This is what he envisions for the future:

“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.”

At first I thought he was talking only about the few species of trees that are capable of getting really, really big: Giant Sequoias, Coast Redwoods, Sitka Spruce, and the like. Who wouldn’t gape when confronted with a towering tree? The first time I set eyes on a Giant Sequoias in Yosemite, I stood still for a while trying to take it in. Then I took a picture.

However, Laurance is really arguing that bigger trees in general are having a harder time surviving than their smaller neighbors. He notes that we are seeing changes in three critical conditions that support the development of big trees in a forest including “the right place to establish its seedling, good growing conditions and lots of time with low adult mortality.”

His reasoning makes sense. People have been cutting down forests for hundreds of years and have done so at an increasingly rapid rate for the past 50 years. Obviously, when you cut down a forest, it takes time for older, bigger trees to return. And it takes just as long (or longer) for the ecological characteristics associated with old-growth forests to develop. In eastern North America, for example, most of our forests are less than 100 years old … so we live in an unprecedented era of small trees and young forests, and have become accustomed to it.

Laurence summarizes findings of a variety of studies indicating that big trees might be getting hit harder than smaller trees even within uncut, intact forest. Here are four examples I found interesting:

  1. His own research demonstrates that the death rate of large trees is three times greater than smaller trees in small forest patches surrounded by pasture (Laurance et al. 2000). The size and structural characteristics of larger trees make them more vulnerable to wind damage and “winds can accelerate over the surrounded cleared lands before slamming into them”.
  2. Scientists studying forest in the Amazon find that larger trees die at twice the rate as smaller trees during drought conditions (Nepstad 2011). Similarly, warmer temperatures have been linked to overall slower growth rates of trees in a forest in Costa Rica (Clark et al. 2010). As the Earth’s climate continues to warm in the future, drought conditions will likely become more frequent and speed up the death of larger trees.
  3. Lianas (woody vines common to many tropical forests) grow up the trunks of larger trees and encourage them to topple over at an earlier age. Lianas have recently increased in abundance in tropical forests (Phillips 2002), perhaps as a result of increasing carbon dioxide levels.
  4. Invasive species are also at play here. For example, an invasive shrub, Lantana camara, has become common beneath forest canopies in India, and the shrub thickets that develop discourage the establishment of the seedlings of trees. If this continues, over time there will be no young trees to replace the older ones when they die.

Laurance’s ideas are not entirely new, it’s clear that we have been reducing the prevalence of older forests on our landscape. However, he does pull together some compelling examples to suggest that there are additional factors currently at play that are discouraging the development of larger trees … and the important ecosystem characteristics that go along with them.

See The Guardian and Mongabay.com for additional summaries.

Clark et al. 2010. Annual wood production in a tropical rain forest in NE Costa Rica linked to climatic variation but not to increasing CO2 Global Change Biology 16(2): 747–759

Laurance. 2012. Big trees in trouble: how the mighty are fallen. New Scientist. January 28.

Laurance et al. 2000 Conservation: Rainforest fragmentation kills big trees. Nature 404: 836.

Nepstad et al. 2007. Mortality of large trees and lianas following experimental drought in an amazon forest. Ecology 88: 2259-2269.

Phillips et al. 2002. Increasing dominance of large lianas in Amazonian forests. Nature 418: 770-774.

Ramaswami and Sukumar. 2011. Woody plant seedling distribution under invasive Lantana camara thickets in a dry-forest plot in Mudumalai, southern India. Journal of Tropical Ecology 27: 365-373.

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