December 1, 2015

Theory of 'smart' plants may explain the evolution of global ecosystems

Princeton University researchers suggest in a new theory of land-biome evolution
that plants are not passive features of their environments, but may instead actively
behave in ways that determine the productivity and composition of their ecosystems.
The theory was developed to explain why trees known as "nitrogen fixers," which produce
their own fertilizer from atmospheric nitrogen, flourish in nitrogen-rich tropical soils,
but are short-lived in the nitrogen-poor soils of boreal or temperate forests. The aerial photo
above shows a rainforest in Panama in which nitrogen-fixing trees are abundant
(about 10 percent of all trees), diverse, and persist in both young and old forests.
The researchers found that tropical nitrogen fixers evolved to stop producing nitrogen
in order to compete with neighboring trees.
(Photo courtesy of Smithsonian Tropical Research Institute, Panama)

(December 1, 2015)  It's easy to think of plants as passive features of their environments, doing as the land prescribes, serving as a backdrop to the bustling animal kingdom.

But what if the ecosystems of the world take their various forms because plant "decisions" make them that way? A new theory presented by Princeton University researchers in the journal Nature Plants suggests that in some cases that may be exactly what happens. In one of the first global theories of land-biome evolution, the researchers write that plants may actively behave in ways that not only benefit themselves but also determine the productivity and composition of their environs.

"Our theory explains biomes based on the new idea that we must consider plants to be smart and strategic," said senior author Lars Hedin, a Princeton professor of ecology and evolutionary biology and department chair. "This is a global theory that explains why biomes differ in nutrient conditions and in their abilities to respond to disturbances and to absorb carbon dioxide from the atmosphere."

The researchers developed their theory to solve a longstanding mystery in ecology of why trees that can produce their own fertilizer from atmospheric nitrogen grow where they do — they thrive where scientists suppose they shouldn't, and struggle in seemingly ideal conditions.

These plants, known as "nitrogen fixers," use secretions to invite soil bacteria known as rhizobia to infect their roots cells. In exchange for carbohydrates that the plant produces by photosynthesis, rhizobia convert nitrogen in the air into the fertilizer form plants need, with excess nitrogen from the host plant eventually creating a nitrogen cycle that benefits neighboring trees. The majority of nitrogen fixers occur in the diverse legume family that includes beans and peas as well as trees.

journal reference >>