Leaf fall in ancient polar forests still a mystery

Explorers in the 1800s discovered through fossils that deciduous forests once covered the poles, but researchers still do not know why leaf-dropping trees were preferred over evergreens.

“The dominant idea since the 1940s was that because of the polar light regime of continuous darkness and warmth, leafless branches had an advantage over evergreen canopies in the polar forests,” says Dana Royer, research associate in geosciences, Penn State.

This carbon loss hypothesis states that the amount of carbon lost when a canopy of leaves is shed annually is less than the total carbon lost by canopy respiration during the warm, dark winter months and the small amount of leaf loss in evergreens. This would give deciduous trees an advantage during long, very dark winters.

During much of the past 250 million years, the Earth’s poles were devoid of ice, and nearly 40 percent of the area was covered by forests consisting mostly of deciduous trees.

“Today, we do not have these types of forests in the polar latitudes so we have no analog,” says Royer. “In most places, the trees we see at the tree line today are evergreens, not deciduous trees.”

Working with Dr. David J Beerling, professor and Dr. Colin P. Osborne in the department of animal sciences, University of Sheffield, Sheffield, UK, the Penn State researcher tested trees considered living fossils to see if the carbon loss hypothesis was correct. They looked at gingko, dawn redwood and bald cypress, all deciduous trees that are considered living fossils because they have existed much as they are since there were polar forests. Also tested were two living fossil evergreens, sequoia and southern beech.

In Sheffield, the trees grew under controlled temperatures, carbon dioxide and light. The researchers monitored the amount of leaf litter and the amounts of carbon lost to respiration for both groups of trees.

The researchers report in this week’s issue of Nature that “the quantity of carbon lost annually by shedding a deciduous canopy is significantly greater than that lost by evergreen trees through winter-time respiration and leaf litter production. We therefore reject the carbon-loss hypothesis as an explanation for the deciduous nature of polar forests.”

The trees studied where young, starting as year-old saplings and monitored for three years. The researchers used mathematical models to extrapolate to full grown mature forests, but found that even then, counter to expectations, the evergreens had an advantage. The modeling suggests that “the cost of producing a deciduous canopy of leaves remains more than twice that incurred by evergreen trees through canopy respiration and turnover.”

The researchers looked at forests that would be at 69 degrees latitude, the mildest regime that is still polar because they initially feared the trees would not survive long periods of complete darkness. They are now looking to simulate even higher latitudes. Two carbon dioxide regimes were used, because it is generally accepted that atmospheric carbon dioxide was higher when these polar forests existed. The temperatures were kept warm, never dipping below freezing.

While the researchers found that the carbon loss hypothesis was not valid, they did not uncover the reasons why deciduous, and not evergreen trees, populated the polar forests.

“What we did find is that while everyone thought that the biggest problems would be during the polar winter — when there is no sunlight — there appears to be a problem with the polar summer, when there is uninterrupted sunlight,” says Royer. “Both types of plants seem to undergo a drop in photosynthesis after long days of unremittant sun.”