Forest Canopy Competition Predicted with New Model

The process trees go through as they grow and compete with one another for space in the forest canopy has been mapped by a team of researchers working in Hawaii.

Scientists at Brown University and the Carnegie Institution for Science studied a forest of hardy Metrosideros polymorpha trees native to the windward slopes of Manua Kea on Hawaii's big island.

The project began as an attempt to account for what seemed to be unusually large tree growth on the mountainside.

Writing in the journal Ecology Letters, the researchers report that their tree growth models suggest that "an incumbent tree limb greening up a given square meter would still dominate its position two years later a forbidding 97.9 percent of the time."

To calculate this, the model has to account for tree growth pixel by pixel from aerial images taken of the forest canopy.

"Some ecologists could use that information to learn how much one species is displacing another over a wide area or how quickly gaps in the canopy are filled in," Brown University said in a statement. "Others could see how well a forest is growing overall. Tracking the height of a forest's canopy reveals how tall the trees are and therefore how much carbon they are keeping out of the atmosphere -- that is, as long as scientists know how to interpret the measurements of forest growth."

James Kellner, assistant professor of ecology and evolutionary biology at Brown University, the paper's lead author, said that the canopy growth model will likely work for a variety of different forests because the model interprets what seems to be the forest's normal rate of growth to determine when evidence of vertical growth is more than plausible.

Even in a area of forest where growth is quite uniform, the model is still capable of predicting whether a change in canopy height is due to growth of by takeover from other trees.

Kellner and his team, including Gregory Asner of the Carnegie Institution for Science in Stanford, Calif., found that short trees (less than 11 meters tall) were at a clear disadvantage, but from there tallness of trees was not a factor in how well it would outgrow rival trees.

They found, however, that a tree's proximity to taller neighbors played a role in how successfully the tree grows.

"There's definitely basic ecological interest in understanding what might be called the rules of the game," Kellner said. "If you think of the trees as competing for access to space in the canopy and we can infer what those rules are by analyzing data like these."