February 19, 2009
Tropical Forest Biomass Shrinking Or Growing?

A new paper in Nature argues that in intact African forests the total biomass is increasing. Note the important qualifier. This refers to biomass in those forests which still exist.

Tropical forests hold more living biomass than any other terrestrial ecosystem. A new report in the journal Nature by Lewis et al. shows that not only do trees in intact African tropical forests hold a lot of carbon, they hold more carbon now than they did 40 years ago--a hopeful sign that tropical forests could help to mitigate global warming. In a companion article, Helene Muller-Landau, staff scientist at the Smithsonian Tropical Research Institute, says that understanding the causes of this African forest carbon sink and projecting its future is anything but straightforward.

The paper argues that per acre or hectare of mature forest the amount of carbon held is rising.

Growing trees absorb carbon. Dead, decomposing trees release carbon. Researchers expect growth and death to approximately balance each other out in mature, undisturbed forests, and thus for total tree carbon stocks, the carbon held by the trees, to remain approximately constant. Yet Lewis and colleagues discovered that on average each hectare (100 x 100 meters, or 2.2 acres) of apparently mature, undisturbed African forest was increasing in tree carbon stocks by an amount equal to the weight of a small car each year. Previous studies have shown that Amazonian forests also take up carbon, although at somewhat lower rates.

One possible cause: the rise in atmospheric carbon dioxide (CO2) could basically fertilize the plant life so that it grows more rapidly and densely.

"If you assume that these forests should be in equilibrium, then the best way to explain why trees are growing bigger is anthropogenic global change – the extra carbon dioxide in the atmosphere could essentially be acting as fertilizer." says Muller-Landau, "But it's also possible that tropical forests are still growing back following past clearing or fire or other disturbance. Given increasing evidence that tropical forests have a long history of human occupation, recovery from past disturbance is almost certainly part of the reason these forests are taking up carbon today."

The boost due to higher CO2 won't continue indefinitely. Other nutrients become rate-limiting. Also, more biomass means more food for herbivores and so their numbers grow and they eat more green.

A big chunk of the CO2 from burning fossil fuels is getting absorbed by forests.

Globally, tropical trees in undisturbed forest are absorbing nearly a fifth of the CO2 released by burning fossil fuels.

The researchers show that remaining tropical forests remove a massive 4.8 billion tonnes of CO2 emissions from the atmosphere each year. This includes a previously unknown carbon sink in Africa, mopping up 1.2 billion tonnes of CO2 each year.

Published today in Nature, the 40 year study of African tropical forests–one third of the world's total tropical forest–shows that for at least the last few decades each hectare of intact African forest has trapped an extra 0.6 tonnes of carbon per year.

The scientists then analysed the new African data together with South American and Asian findings to assess the total sink in tropical forests. Analysis of these 250,000 tree records reveals that, on average, remaining undisturbed forests are trapping carbon, showing that they are a globally significant carbon sink.

I am expecting growing populations and industrializing populations to eventually tap a lot of this increased biomass. Cellulosic technologies for making ethanol will eventually decline in cost to the point that wood as a source of car fuel will become economically competitive. Then all that increased biomass will become valuable as an energy source. Also, the demand for land for cattle and other livestock will grow with rising living standards which allow people to eat more meat. Picture a billion Chinese people eating as much meat as Americans.

Some scientists believe migrations of rural populations to cities are returning lots of farm land back to nature.

These new “secondary” forests are emerging in Latin America, Asia and other tropical regions at such a fast pace that the trend has set off a serious debate about whether saving primeval rain forest — an iconic environmental cause — may be less urgent than once thought. By one estimate, for every acre of rain forest cut down each year, more than 50 acres of new forest are growing in the tropics on land that was once farmed, logged or ravaged by natural disaster.

At least in some countries the amount of land returning to forest may be greater than the amount of forest getting cut down.

In Panama by the 1990s, the last decade for which data is available, the rain forest is being destroyed at a rate of 1.3 percent each year. The area of secondary forest is increasing by more than 4 percent yearly, Dr. Wright estimates.

With the heat and rainfall in tropical Panama, new growth is remarkably fast. Within 15 years, abandoned land can contain trees more than 100 feet high. Within 20, a thick rain-forest canopy forms again. Here in the lush, misty hills, it is easy to see rain-forest destruction as part of a centuries-old cycle of human civilization and wilderness, in which each in turn is cleared and replaced by the other. The Mayans first cleared lands here that are now dense forest. The area around Gamboa, cleared when the Panama Canal was built, now looks to the untrained eye like the wildest of jungles.

The new-growth forests can not support as many species as the old growth forests. So this increase in biomass is not a solution to the habitat and species loss problem.

Dr. Wright, looking at a new forest, sees possibility. He says new research suggests that 40 to 90 percent of rain-forest species can survive in new forest.

Dr. Laurance focuses on what will be missing, ticking off species like jaguars, tapirs and a variety of birds and invertebrates.

Share |      Randall Parker, 2009 February 19 11:43 PM  Trends Habitat Loss

Xenophon Hendrix said at February 21, 2009 4:22 AM:

How long does it take new growth forests to become old growth forests?

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