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Woody Plants Invading Alaskan Arctic Tundra
Sturm, M., Racine, C. and Tape, K.  2001.  Increasing shrub abundance in the Arctic.  Nature 411: 546-547.

What was done
The authors used repeat photography (1948-50 to 1999-2000) to look for "changes in the three principal deciduous shrubs, dwarf birch (Betula nana), willow (Salix sp.) and green alder (Alnus crispa), and for changes in treeline white spruce (Picea glauca) along the southern edge of the study area," which ran between the Brooks Range and the Arctic coast of Alaska, spanning an area 400 km (east-west) by 150 km (north-south).

What was learned
Over the 50-year period investigated, according to the authors, "increases in the height and diameter of individual shrubs, in-filling of areas that had only had a scattering of shrubs in 1948-50, and expansion of shrubs into previously shrub-free areas" were clearly evident.  At tree sites there was also, in their words, "a marked increase in the extent and density of the spruce forest."

What it means
Noting that their study area was in a location "where human and natural disturbances are minimal," the researchers attributed "much of the increase in the abundance of shrubs to the recent change in climate," which they said has warmed substantially over the last three decades; and they buttressed this claim by noting that the species studied "respond to experimental warming and fertilization in a positive manner."

Latching onto this explanation, media reports of the research were quick to claim that the scientists' findings support the idea that the region is gradually getting warmer, thus adding fuel to the global warming controversy.  But this explanation may not be the whole story; for as the authors correctly note, the woody plants they studied respond positively to both warming and fertilization.  And what is one of the greatest plant fertilizers of all time?  Why, carbon dioxide, of course, which is widely known for its aerial fertilization effect.  But did the CO2 content of the atmosphere rise enough between 1948-50 and 1999-2000 to account for the observed changes in woody plant growth?

From ice core data and direct atmospheric measurements, we know that the air's CO2 concentration rose from a value of approximately 310 ppm in 1949 to a value on the order of 370 ppm in the 1999-2000 timeframe.  This 60 ppm increase in atmospheric CO2 concentration is fully two-tenths of the 300 ppm increase that Idso (1999) determined to be responsible for a mean growth enhancement of 52% in 176 different woody plant experiments conducted by numerous scientists in many different countries around the world.  Hence, in the mean, we could have expected the historical rise in the air's CO2 content over the 50-year period in question to have increased woody plant growth by about 10%.

Although the paper of Sturm et al. does not report any numbers for the increase in growth observed between the initial and final assessments of the repeat photography study, an Associated Press story (Mason, 2001) reports one of the researchers as saying that the largest growth increase they observed was 15%, which suggests that the mean increase could well have been close to the 10% mean increase we calculate above.  In addition, it should be remembered that since atmospheric CO2 enrichment tends to reduce leaf stomatal conductance, plant water use per unit leaf area would be expected to have declined concurrently, increasing plant water use efficiency even more than the 10% calculated for growth.

One of the consequences of these CO2-induced physiological changes in plants, which appear to be more pronounced in woody than in herbaceous species, is that woody plants have a tendency to expand their ranges in response to increases in the air's CO2 content, encroaching upon lands where they could not grow before.  We have reported on several documented occurrences of this phenomenon previously (see Trees -- Range Expansions in our Subject Index); and some of them are even more dramatic than the Alaskan example of Sturm et al.

In view of these several observations and our numerical calculations, it is clear that the growth increases and range expansions of woody plants onto the Arctic tundra over the past 50 years may well have been more a function of the historical rise in atmospheric CO2 concentration than a response to local warming.  In any event, it is clear that the Arctic's "getting greener" (Craig, 2001) can only be considered a plus for a world that is fighting to slow the rate-of-rise of the air's CO2 content; for as Sturm et al. correctly note, the woody plant expansion is "increasing the amount of carbon stored in a region that is believed to be a net source of carbon dioxide."

Craig, A.  30 May 2001.  Arctic, getting greener.  BBC News.

Idso, S.B.  1999.  The long-term response of trees to atmospheric CO2 enrichment.  Global Change Biology 5: 493-495.

Mason, M.  30 May 2001.  Increased shrubbery found in Arctic.  Associated Press.