Climate Change: Larch in the Lurchby Bill Chameides | April 6th, 2011
posted by Erica Rowell (Editor)
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The retreat of larch from the Siberian boreal forest could accelerate warming.
As noted in a number of previous posts (such as here, here, here and here), one of the greatest sources of uncertainty in predicting our climate future is the existence and strength of feedbacks — positive feedbacks that amplify warming from greenhouse gases and negative feedbacks that dampen them.
Climate scientists worry about and study a variety of these feedbacks. Now, writing in the journal Global Change Biology, the University of Virginia’s Jacquelyn Kremper Shuman and co-authors raise another one for consideration. This one involving the replacement of the larch tree with conifers in Siberia’s boreal forests. Here’s how it works.
For the Larch, Cold Is Sublime, Warmth is Subprime
The boreal forests of Siberia are an impressive biome — roughly the size of the U.S. lower 48, the region is the “largest continuous forest region on Earth,” according to the authors. And today, much of it is covered by larch — a deciduous tree that typically grows to about 50 to 70 feet and whose needles turn yellow and fall off in autumn. The larch can live for up to 100 years and is adapted to cold climes — hence its abundance in Siberia.
But while larch do well in cold climates, they are less than happy when things warm up. And there’s the climate-feedback rub. Siberian temperatures are on the rise. Over the 20th century, while the average global warming rate was 0.6 degrees Celsius, temperatures in central Siberia increased by 1-2 degrees Celsius on average in both summer and winter. By 2100 some analyses estimate temperatures may increase by as much as 6 degrees Celsius.
A Needling Difference: The Deciduous Conifer
Field studies, such as here and here, show that Siberian forests are on the move. And as temperatures increase, there is little doubt what will happen to the larch — it will hightail it further north. And in its place, we can expect other coniferous trees like the spruce or fir to move in. Most of us might not find that to be much of a deal — larch, spruce … what’s the diff.
A whole lot, it turns out from a climate point of view. And that diff comes about because larches and evergreen conifers interact with sunlight differently. When deciduous larches drop their leaves in the fall (unlike most conifers), they allow more sunlight to be reflected off the snow-covered ground than those conifers that retain their needles do. Because of this, larch-dominated forests tend to be more reflective of sunlight, especially in the winter, than evergreen conifers. So when evergreen species more tolerant of warmer temperatures replace larch populations, the drop in reflectivity from year-round dark vegetation could lead to a feedback that increases local temperatures even more.
To understand what impact the influx of evergreen trees in the Siberian boreal forest would have on temperature, the authors first modeled how changes in temperature (up to 4 degrees Celsius) and precipitation (+/- 10 percent) would alter forest dynamics. Where significant forest succession occurred, the data was further analyzed for the likely changes in surface reflectivity.
Despite Regional Differences, Overall, High Temperatures Meant Lower Larch Numbers
Overall the authors found that as temperature increased, larches suffered and even collapsed abruptly, but there were regional differences across Siberia. In those regions where forest succession occurred, the authors estimate that at maturity these forests would absorb between 2 and 7 percent more radiation over the course of a year and thereby reinforce local warming trends.
The simulations also showed that rising temperatures may change the dynamics of forest succession in the future. The larch’s dominance of the early stages of forest succession were much diminished, leading the authors to speculate that the “inability of larch to maintain dominance can be viewed as a signal of potential collapse for the genus and replacement by evergreen conifers.”
So, the larch — and its surrounding environment — may be in the lurch. Another potential global warming accelerator. Hard not to let such depressing findings fester, but it’s Wednesday, hump day, and time to move on to the next thing.filed under: climate change, faculty, global warming, Planetary Watch, temperatures
and: albedo, climate, climate models, climate science, conifers, deciduous trees, feedback, larch, positive feedback, reflectivity, Siberia, trees