Will Plants Move Fast Enough to Keep Up With Contemporary Climate Change
Researchers James Clark and Gabriel Katul Take Different Approaches to Speculate on the Future p.3
By analyzing how inherited genetic sequences now vary from tree to tree, he and his colleagues found they could map prehistoric movements of those sequences from small founding colonies perhaps as near to the ice sheets as what is now Kentucky and Tennessee. At those closer distances, the scientists estimated that trees could have migrated from south to north at less than 100 meters a year.
That rate is consistent with mathematical models Clark has created based on studying the dispersal of seeds and pollen by living trees. But the Ecology paper also notes that, if true, such “past migration rates were substantially lower than the rates that will be needed to track 21st century warming.”
Clark spends much of his time in the woods, some at research sites as close as the Nicholas School-administered Duke Forest and some further west at an experimental forest in North Carolina’s mountains. “For about 14 years we’ve been looking at seed dispersal to hundreds of seed traps in different locations where we’ve mapped all the trees,” he says. “We know where the parents are, and we look at where seeds fall. That gives us a very good estimate of how much seed is being produced by trees of different sizes and different ages, and how far they move.
“We can only say with a certain probability that this seed came from this tree,” he notes. However, “if you are collecting hundreds of thousands of seeds, then those probabilistic statements become very powerful.” But he is quick to caution that “long-distance dispersal is something that nobody can measure. It’s not something that you can see happen.” The movement of a seed over extreme distances, as opposed to the distance between tree and a collection basket, “depends on extreme events like hurricanes and tornadoes, events we can’t predict,” he says.
Other equally unpredictable factors include a bird gobbling up a seed and flying away with it before depositing it on the ground in bodily wastes. “A lot of species produce fruits essentially for that purpose as best we can tell. That would give it a boost in distance,” he says. “Seeds also get stuck on animals, humans and vehicles. That’s another way for seeds to get around rapidly.”
But past patterns “really don’t answer the question of how far seeds will move in the future,” he says. “Will species be able to migrate to areas where they don’t now exist, but where they would have to be able to move because the future climate would require it? Do plants living further north live there because they can tolerate colder winters, or shorter summers, or a shorter growing season? Or is it something very different, like the kinds of soils there? It can be very difficult to say why species live where they do.”
In contemporary North Carolina, he notes that certain tree types like pines will colonize rapidly in abandoned disturbed areas such as old farm fields. But pines tend to be followed by species such as beeches that prefer living in the shade of the trees that grew up earlier. This shade preference introduces a lag factor: pines have to form a canopy before beeches will grow underneath them.
photo captions:1. Jim Clark with a collecting basket at the Duke Forest FACE site. 2. Research tower in Duke Forest. 3. Gaby Katul