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.