All Tree Biomass Is Created Equal In Forests Of Equal Size, Whether In The Tropics Or Temperate Climes, Says Cornell Biologist

ITHACA, N.Y. -- Does the Amazon River basin thrive with more tree biomass than that along the shores of Opeongo Lake in Canada's Algonquin Provincial Park? Is the Congo Basin more tree biomass-rich than the Argonne Forest in northeastern France? Conventional wisdom answers yes, believing that equatorial and tropical regions have far more tree biomass than places like North America, Europe and Asia.

Conventional wisdom seems to be wrong.

The amount of tree biomass in any two given similar-sized areas, whether in a tropical or temperate clime, is virtually identical, according to a new study in Nature (April 5, 2001) by Karl J. Niklas, Cornell University's Liberty Hyde Bailey professor of plant biology, and Brian J. Enquist, a biologist from the University of Arizona.

The term "biomass" refers to organic matter that can be converted to energy, from agricultural residues to crops, but most commonly wood.

"Most people believe that tree biomass per unit area increases toward the equator and decreases rapidly toward the higher latitudes. Certainly, I thought that was true," says Niklas. "Now, I believe that the data show there is very little difference in total tree-standing biomass across most closed-canopy forested communities. We hope this study changes a common perception."

The implications of the research are far-reaching in terms of plant ecology and evolution, says Niklas. Most people believe that forest biomass increases as you package more species into the same space. This does not appear to be true. He says, "Since our model applies equally well to ancient forests as well as modern ones, we believe the results seen for living tree communities extend into deep evolutionary history."

To learn if biomass differences exist between tropical and temperate areas, the scientists used forest data collected by the late Alwyn Gentry, a noted biologist who died in a plane crash in Ecuador in 1993. The data included information from 227 sites of tropical and temperate

closed-canopy forests on six continents. The researchers gleaned statistical information from forests between the latitudes of 60 degrees north and 40 degrees south, at elevations ranging from 20 meters (22 yards) to 3,050 meters (3,337 yards). In the Nature report, "Invariant Scaling Relations Across Tree-Dominated Communities," the scientists say that the Gentry data was remarkable for its uniformity, abundance, measurements of plant size and the number of plants across the varying latitudes. A few of these communities consisted of only two tree species, while others, near the equator, had well over 300.

With this information, the researchers determined the size distribution of specific tree diameters from each site. Niklas and Enquist then calculated tree biomass, visible from above the ground, in tropical, dry, moist and wet forests, using a formula based on the Gentry data.

Enquist and Niklas found that the effects of biodiversity on tree biomass are not very important, regardless of how many species of trees exist in a forest. Differences between tree species, says Niklas, are not important because "all trees, regardless of species, are competing for sunlight and physical space in much the same way."

The scientists explain that despite a wide global variation in species diversity, equivalent areas of tree-dominated communities have nearly identical size-frequency distributions and nearly equivalent standing biomass. This comes about because "tree species compete for three-dimensional space and sunlight using the same arsenal of biological weapons, regardless of their other species differences" says Niklas. "In terms of the utilization of space and light, a tree is a tree is a tree."

Niklas and Enquist had predicted virtually everything they observed in the Gentry data with a computerized mathematical model, or simulation. The model assumed that the energy gain by a plant from the sun must be used to construct leaves, stems and reproductive organs in a way "that complies with the simple rules of physics," says Niklas. When the model simulates tree growth, reproduction and competition for space and sunlight, "you see them grow and die before your eyes, and you see their community expand and contract until it reaches equilibrium," says Niklas. Once the virtual forest reaches a stable state of death and birth, it exhibits the same properties as observed for real tree-dominated plant communities around the globe, says Niklas.

Adapted from materials provided by Cornell University.

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Global Limits Of Biomass Energy

Biomass energy—energy generated from agricultural waste or specially grown energy crops—has been widely touted as a clean, renewable alternative to fossil fuels. Research is booming to improve energy crops and methods of converting crops to fuel. Already, Brazil gets 30% of its automotive fuel from ethanol distilled from sugar cane. But critics warn that “energy farming” will gobble up land needed to grow food or will impinge on natural ecosystems, possibly even worsening the climate crisis.

In the February Trends in Ecology and Evolution, Global Ecology director Chris Field, with postdoctoral fellow Elliott Campbell and a colleague, took a sober look at the prospects for biomass energy. They found that while biomass has many benefits—in principle it can be carbon neutral—there are limits to the extent that it can sustainably contribute to global energy needs. For example, the total mass of carbon fixed by all croplands worldwide each year (about 7 billion tons) is still less than that released by fossil fuel emissions (7.7 billion tons). This fact, the authors write, “highlights the challenge of replacing a substantial part of the fossil fuel system with a system based on biomass.”

The researchers used a combination of historical data, satellite imagery, and productivity models to determine best-case estimates of potential yields and of how much biomass could sustainably contribute to the world’s energy needs while also mitigating global warming.

“The area with the greatest potential for yielding biomass energy that reduces net warming and avoids competition with food production is land that was previously used for pasture but that has been abandoned and not converted to forest or urban areas,” they write.

Globally, suitable abandoned cropland and pastureland amounts to approximately 1.5 million square miles. Realistically, energy crops raised on this land could be expected to yield about 27 exajoules of energy each year. This is a huge amount of energy—an exajoule is a billion billion joules, equivalent to 172 million barrels of oil. Yet the biomass yield could still satisfy only about 5% of global primary energy consumption by humans, which in 2005 was 483 exajoules.

The study concludes that at a proper, sustainable scale, biomass energy presents exciting opportunities for increasing energy independence, sustaining farm economies, and decreasing the forcing of climate change. But deployed at a larger scale, it could threaten food security and exacerbate climate change.

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