Monday, May 6, 2013

The Aspens that were left behind

When climates change, species move. It’s a fact of life on Earth and probably has been for the past 542 million years, even when species don’t have legs or wings or fins to get them from place to place.

Quaking aspen is one example of a seemingly stationary species that has managed in just the past 20,000 years to expand into the largest range of any native North American tree. These distinctive trees grow across Alaska and Canada all the way south along the Rocky Mountains to Mexico. Since most of their current northern distribution was buried under up to 2 miles of ice just 22,000 years ago, aspens provide a great opportunity to study how species move during climate change.

Researchers at Utah State University were curious whether the aspens that currently live in the mountains of the western United States are stragglers that have occupied this region since the species moved north after the last ice age and have subsequently climbed into the mountains to avoid the warming lowlands, or whether the southern aspens are wayward offspring of the main northern population that found the mountains to be a distant, but suitable, habitat.

In order to find the answer the researchers analyzed short sections of DNA from almost 800 individual trees in 30 different forests spanning the species’ entire range. The genetic variation among these trees showed that forests in the northern part of the aspen’s range are all reproducing with each other across thousands of kilometers, or are at least very recent relatives of one another. In contrast, forests in the south are more isolated and probably represent the remnants of two populations that took refuge on opposite sides of the Great Basin during the last ice age.

This geographic history has interesting consequences for aspen genetic diversity (how much variation there is among members of the same species in their DNA). Southern aspens have lower genetic diversity within a forest compared to aspens in northern forests, mainly because of their isolation and propensity to reproduce by sprouting clones rather than by seeds.  However, the amount of variation between forests in the south is much higher than in the north, so that, taken all together, northern and southern aspens embody about the same level of genetic diversity. This means that losing a single forest in the south will eliminate much more of the genetic diversity within the entire species than losing a single northern forest.

Unfortunately, the aspens with the most unique genes are also the most likely to disappear; climate change in the next century is expected to decrease southern aspen’s habitat by as much as 94%. Genetic diversity holds a species’ potential ability to adapt to new conditions. What incipient adaptability to current climate change is stored in the DNA of the endangered southern aspens? It is a bit ironic that the most vulnerable forests house trees whose genetic material could hold the adaptive key to outrunning the cause of this vulnerability. The question is, will this potential manifest before its basis is lost?

You can find this article at:

ResearchBlogging.orgCallahan, C., Rowe, C., Ryel, R., Shaw, J., Madritch, M., & Mock, K. (2013). Continental-scale assessment of genetic diversity and population structure in quaking aspen (Populus tremuloides) Journal of Biogeography DOI: 10.1111/jbi.12115

Wednesday, May 1, 2013

Sing a song of Sphagnum

For a plant, there is one good thing about being small; it’s a lot easier to get everywhere- for your seeds, that is. Nowhere is this more evident than where all plants are really small- the Arctic tundra.

The northern latitudes are covered by vast expanses of tree-less terrain covered by mosses and lichens that, for the most part, are the same around the world. Biogeographers believe that this similarity is facilitated by the small size of the spores of these organisms. The spores are 20-40 micrometers long (it would take about 500 spores to stretch across a dime) and can easily get picked up by the nearest breeze and transported thousands of kilometers. Upon landing, they germinate and grow into a new baby mosses and lichens. With frequent enough inter-continental exchange of spores, tundra looks like tundra looks like tundra.

Although spores are small and scientists think they travel great distances, testing this is actually quite difficult. How many spores actually do make the trip thousands of kilometers across oceans to new continents?

A scientist at Uppsala University in Sweden went out and measured how many spores of Sphagnum moss could be found at different distances from peat bogs (where Sphagnum and bog mummies live). Although 20 million spores were produced in every m2 of bog, only 6 million made it up into the air and just 4% of these managed to travel 40 meters away. Despite this rapid decline with distance from the bog, some spores are able to travel great distances. On Svalbard, the cloth spore traps showed that 1000 spores are deposited per m2 every growing season. And this is on a barren island 820 km north of the nearest Sphagnum bog (in Norway).

Oddly enough, the spores that made it to Svalbard and other islands were larger than the spores found at sites closer to bogs. Evidently, for Sphagnum, getting smaller is not a recipe for getting farther, and past a certain point, size really doesn’t matter.

You can find this article at:

ResearchBlogging.orgSundberg, S. (2013). Spore rain in relation to regional sources and beyond Ecography, 36 (3), 364-373 DOI: 10.1111/j.1600-0587.2012.07664.x