@sumac,
NOVEMBER 11, 2011, 11:58 AM
In Warming North, Some Trees Thrive as Others Ail
By ANDREW C. REVKIN
In a new study, a team led by researchers from the tree-ring lab at Columbia University’s Lamont-Doherty Earth Observatory has found that white spruce trees on the edge of the tundra in Alaska’s far north have thrived in the past 100 years, and especially the last 50, in the face of sharp Arctic warming.
Elsewhere, of course, forests are having a much tougher time dealing with a changing climate and other factors. The Arctic climate is prone to big swings and is a region where plants, particularly, have evolved the ability to spread and retreat as conditions change.
Whether the issue is forests or frogs, the response of ecosystems to rising temperatures and carbon dioxide concentrations and changing rainfall remains as complicated, and variegated, as the planet itself. Another case in point is the study in California that found plant communities shifting down slopes in a warming century, against conventional wisdom (apparently because precipitation is the prime driver, not temperature).
The new Arctic forest paper is published in a package of studies in Environmental Research Letters on the greening of Arctic tundra even as boreal forests exhibit signs of stress. In another paper, Russian researchers identify substantial agricultural potential as Siberia warms.
Here’s a release from the news office of the university’s Earth Institute, which has done a particularly good job of filling the void left as conventional science coverage shrinks:
Evergreen trees at the edge of Alaska’s tundra are growing faster, suggesting that at least some forests may be adapting to a rapidly warming climate, says a new study.
While forests elsewhere are thinning from wildfires, insect damage and droughts partially attributed to global warming, some white spruce trees in the far north of Alaska have grown more vigorously in the last hundred years, especially since 1950, the study has found. The health of forests globally is gaining attention, because trees are thought to absorb a third of all industrial carbon emissions, transferring carbon dioxide into soil and wood. The study, in the journal Environmental Research Letters, spans 1,000 years and bolsters the idea that far northern ecosystems may play a future role in the balance of planet-warming carbon dioxide that remains in the air. It also strengthens support for an alternative technique for teasing climate data from trees in the far north, sidestepping recent methodological objections from climate skeptics.
“I was expecting to see trees stressed from the warmer temperatures,” said study lead author Laia Andreu-Hayles, a tree ring scientist at Columbia University’s Lamont-Doherty Earth Observatory. “What we found was a surprise.”
Members of the Lamont Tree-Ring Lab have traveled repeatedly to Alaska, including the Arctic National Wildlife Refuge this past summer. In an area where the northern treeline gives way to open tundra, the scientists removed cores from living white spruces, as well as long-dead partially fossilized trees preserved under the cold conditions. In warm years, trees tend to produce wider, denser rings and in cool years, the rings are typically narrower and less dense. Using this basic idea and samples from a 2002 trip to the refuge, Andreu-Hayles and her colleagues assembled a climate timeline for Alaska’s Firth River region going back to the year 1067. They discovered that both tree-ring width and density shot up starting a hundred years ago, and rose even more after 1950. Their findings match a separate team’s study earlier this year that used satellite imagery and tree rings to also show that trees in this region are growing faster, but that survey extended only to 1982.
The added growth is happening as the arctic faces rapid warming. While global temperatures since the 1950s rose 1.6 degrees F, parts of the northern latitudes warmed 4 to 5 degrees F. “For the moment, warmer temperatures are helping the trees along the tundra,” said study coauthor Kevin Anchukaitis, a tree-ring scientist at Lamont. “It’s a fairly wet, fairly cool, site overall, so those longer growing seasons allow the trees to grow more.”
The outlook may be less favorable for the vast interior forests that ring the Arctic Circle. Satellite images have revealed swaths of brown, dying vegetation and a growing number of catastrophic wildfires in the last decade across parts of interior Alaska, Canada and Russia. Evidence suggests forests elsewhere are struggling, too. In the American West, bark beetles benefitting from milder winters have devastated millions of acres of trees weakened by lack of water. A 2009 study in the journal Science found that mortality rates in once healthy old-growth conifer forests have doubled in the past few decades. Heat and water stress are also affecting some tropical forests already threatened by clear-cutting for farming and development.
Another paper in Science recently estimated that the world’s 10 billion acres of forest are now absorbing about a third of carbon emissions, helping to limit carbon dioxide levels and keep the planet cooler than it would be otherwise.
There are already signs that the treeline is pushing north, and if this continues, northern ecosystems will change. Warming temperatures have benefited not only white spruce, the dominant treeline species in northwestern North America, but also woody deciduous shrubs on the tundra, which have begun shading out other plants as they expand their range. As habitats change, scientists are asking whether insects, migratory songbirds, caribou and other animals that have evolved to exploit the tundra environment will adapt. “Some of these changes will be ecologically beneficial, but others may not,” said Natalie Boelman, an ecologist at Lamont-Doherty who is studying the effects of climate change in the Alaskan tundra.
In another finding, the study strengthens scientists’ ability to use tree rings to measure past climate. Since about 1950, tree ring widths in some northern locations have stopped varying in tandem with temperature, even though modern instruments confirm that temperatures are on a steady rise. As scientists looked for ways to get around the problem, critics of modern climate science dismissed the tree ring data as unreliable and accused scientists of cooking up tricks to support the theory of global warming. The accusations came to a head when stolen mails discussing the discrepancy between tree-ring records and actual temperatures came to light during the so-called “Climategate” episode of 2009-10.
The fact that temperatures were rising was never really in dispute among scientists, who had thermometers as well as tree rings to confirm the trend. But still scientists struggled with how to correct for the so-called “divergence problem.’’ The present study adds support for another proxy for tree growth: ring density. Trees tend to produce cells with thicker walls at the end of the growing season, forming a dark band of dense wood. While tree-ring width in some places stops correlating with temperature after 1950, possibly due to moisture stress or changes in seasonality due to warming, tree ring density at the site studied continues to track temperature.
“This is methodologically a big leap forward that will allow scientists to go back to sites sampled in the past and fill in the gaps,” said Glenn Juday, a forest ecologist at University of Alaska, Fairbanks, who was not involved in the study. The researchers plan to return to Alaska and other northern forest locations to improve geographical coverage and get more recent records from some sites. They are also investigating the use of stable isotopes to extract climate information from tree rings.
Other authors of the study include Rosanne D’Arrigo, Lamont-Doherty; Pieter Beck and Scott Goetz, Woods Hole Research Center and David Frank, Swiss Federal Research Institute.
| Addendum |
It’s notable, as described above, that the Columbia University tree researchers, gauging a thousand years of tree growth around Alaska’s Firth River, also found a fresh way to assess relationships of growth rings in tree trunks to temperature. This could help resolve fights related to the controversial “hide the decline” wording in one of the e-mail messages extracted from the University of East Anglia in 2009.
I consulted a few experts in this tree-climate realm. Read on if you’re particularly interested in some of the scientific detail. I prefer to run these contributions unedited. If you have questions about technical details, post them and I’ll ask the researchers to respond.
Julie Brigham-Grette, a researcher at the University of Massachusetts, Amherst, focused on past climate and ecological changes, wrote:
Just a quick note to say that the paleoclimate data for earlier warm periods 125,000 years ago and even 8-10,000 years ago in northern Alaska (paleoclimate warmer than now, [from] different forcings) document the northward advance of the treeline from Nome to Barrow, Alaska, and the Canadian border at different times of change in Earth’s orbital parameters (without a significant change in CO2).
The rate of change in the past was as fast as modern times. However it’s clear that now, modern [changes] forced by higher CO2 are accelerating at rates faster than historical and paleo records would suggest. While we know that treeline can be climatically “elastic” in the space of both latitude and altitude, i.e., moving north and south and up and down with topography, this paper raises important issues about rates of ecological adaption, rates that are being tested by contemporary rates of change forced by human activities.
Here’s Andrea H. Lloyd of Middlebury College, who’s been running studies on the response of woody plants to changes in the Arctic climate:
This is an intriguing paper— I like the diversity of approaches that they bring together. It seems to me that given the complexities of interpreting tree ring data, this kind of multi-faceted approach becomes increasingly important. A few things jump out at me:
1. The analysis of wide rings is interesting and novel (I think— I can’t recall seeing that kind of analysis before). As the authors say, it does start to give some clarity to the causes of divergence between tree-rings and temperature. If drought stress were causing the erosion of that signal, you would expect wide rings to become less common. The fact that they are finding wide rings MORE commonly certainly argues against that interpretation. It would be interesting to re-analyze sites at which divergence has been detected and repeat that analysis — I’m willing to bet that you would find that they group into two general categories: sites at which tree rings are losing sensitivity to temperature but growth is increasing, and those at which tree ring response to temperature has changed (I.e., what were positive correlations have become negative) and growth is declining.
Having done that, I think we could perhaps begin to look at the spatial pattern of those sites, and get some good insights into why temperature is having such different effects on growth in different places. For example, one might predict that the pattern they see (wide rings more common) would be prevalent in cooler, wetter parts of Alaska, whereas the opposite pattern (wide rings less common) might be more prevalent in warmer, drier parts, where warming may have pushed temperature past critical thresholds to the point where warm temps become a limiting factor. It’s a neat approach, in short! And a great reminder that sometimes the coolest insights come from very simple, straightforward approaches to data. When I teach statistics, I always tell my students that they should always always begin by just making a histogram of their data, just because. And that’s exactly what Laia and her colleagues have done— made a simple histogram — and presto, change-o, there’s a pretty nifty pattern in it!
2. Their analysis of wood density seem quite consistent with previous literature, which tends to find that density has a more stable response to temperature. I think these guys do a good job articulating why that may be— as they say, a phenomenon whose phenology is controlled by photo-period ought to exhibit greater stability than other physiological processes. But, not to be a contrarian, I am actually quite fascinated by Figure 4 and the ways in which it diverges from that conclusion. They note, quite correctly, that the tree-ring response to climate changes much more dramatically between the two time periods than the density response. However, the density response is still not quite uniform. From 1900-1950, there is a significant correlation between MXD and june temperature. Between 1951-2001, that disappears entirely. So yes, to some extent they’re right— the response of wood density to climate IS more stable than the response of ring widths. But it’s not entirely stable, and that is really, really interesting to me.