Rapidly declining throughout most of Alaska, results of a study of white spruce in boreal forest along the northeastern Alaskan coast indicates that they’ve been growing well there during the last 60 years despite a warming climate.
For ten years from 1901 to 2001, a research team from Columbia University’s Lamont-Doherty Earth Observatory measured tree ring width and wood density of white spruce in boreal forest along northeastern Alaska’s Firth River, which lies at the edge of the Arctic tundra.
The results of running a local area climate model dating back to 1067 showed a rapid rise in temperatures from 1950.
Matching that data with the tree ring growth data, they found that tree ring growth correlated with climate changes for nearly 1,000 years but broke down around 1950, according to a ClimateWire report.
Tree ring growth continued while temperatures rose, leading them to conclude that tree ring growth was not hindered by the warming climate.
“I was expecting to see trees stressed from the warmer temperatures,” tree ring scientists and study lead author Laia Andreu-Hayles said in a Lamont-Doherty news release. “What we found was a surprise.” The complete study report was published in Environmental Research Letters (pdf).
The Firth River study results are in stark contrast to those of white spruce studies in the rest of Alaska, particularly its interior, where climate change induced changes including insect infestation, fire and other disturbances are taking a heavy toll, the researchers noted.
The climate at the Firth River site is comparatively cooler and may be moister, which may result in a less negative response, at least in terms of wood density, to the changing climate, study co-author Rosanne D’Arrigo posited.
Alaskan White Spruce, CO2 and climate
One of the most genetically diverse species of Alaskan trees, white spruce don’t survive in the state above a mean temperature of 21 degrees Celsius (70 degrees Fahrenheit). Satellite data gathered from a related study revealed that 15 percent of white spruce became greener between 1982 and 2007, while 20 percent became browner.
In addition to measuring tree ring width, the researchers employed the Maximum Lakewood Density (MXD) method in assessing tree ring growth. Using MXD require a lot more labor, as it involves measuring the density of wood as it forms later in the growing season, when trees’ cell walls tend to become thicker.
Using MXD helps separate and distinguish factors affecting tree growth as it’s independent of those that can affect tree-ring width, such as drought.
Both metrics came under attack by climate change skeptics and deniers during what the media dubbed “Climategate“. Use of the MXD method in the Lamont-Doherty team’s study may vindicate its reputation and strengthen it validity, the team members said.
As for the relationship between climate change, increasing levels of CO2 and forest growth in the study area, “[It’s] difficult to tease the two apart,” D’Arrigo stated. “But for now, it appears that climate alone is explaining the trends … that we are observing here, and that warmer temperatures are still favorable to growth.”