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Range Expansion (Woody Plants: Arctic) -- Summary
As the air's CO2 content continues to rise, the vast majority of earth's plants will likely lose less water to the atmosphere via transpiration while producing increasingly greater amounts of biomass, the latter of which phenomena is generally more strongly expressed in woody perennial species than in annual herbaceous plants. Consequently, in concert with future increases in the air's CO2 concentration, bushes, shrubs and trees will likely expand their ranges more than will non-woody species. In this summary, we review the evidence for this phenomenon, focusing on the Arctic. In concentrating on this region, however, we confront the knotty problem of confounding causes, where concomitant post-Little Ice Age warming may be creating an even greater impetus than the ongoing rise in the air's CO2 content for the northward expansion of trees into territories where it was previously too cold for them to survive.

A good example of this complexity is provided by the study of Sturm et al. (2001), who used repeat photography (1948-50 to 1999-2000) to look for changes in the three principal deciduous shrubs - dwarf birch, willow and green alder - and for changes in white spruce treeline location along the southern edge of the study area, which ran between the Brooks Range and the Arctic coast of Alaska, spanning an area of 400 km (east-west) by 150 km (north-south). Over the half-century period investigated, they report that they discovered "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," as well as "a marked increase in the extent and density of the spruce forest."

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 stated 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 Sturm et al.'s findings support the idea that the region has gradually been getting warmer, thus adding fuel to the global warming controversy. But this explanation may not be the whole story; for as the researchers correctly note, the woody plants they studied respond positively to both warming and fertilization. And what is perhaps the greatest plant fertilizer 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 changes Sturm et al. observed?

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 one fifth of the 300-ppm increase that Idso (1999) determined to be responsible for the mean growth enhancement of 52% observed in 176 different woody plant experiments conducted by numerous scientists in many different countries. In the mean, therefore, we could have expected the historical increase in the air's CO2 content over the 50-year period in question to have increased woody plant growth by about 10%.

Although Sturm et al. do not report any numbers for the increase in growth observed between the initial and final assessments of their 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.

In view of these several observations, 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 either case, and acknowledging that the two phenomena were likely jointly responsible for the plant responses, 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. note, the woody plant range expansion and augmented growth are "increasing the amount of carbon stored in a region that is believed to be a net source of carbon dioxide."

Working in western Siberia, Esper and Schweingruber (2004) analyzed treeline dynamics over the 20th century by comparing current treeline locations at nine undisturbed sites with former treeline locations derived from the positions of relict stumps and logs; and in so doing, they discovered two main pulses of northward treeline advance. The first of the recruitment phases occurred between 1940 and 1960, while the second started around 1972 and lasted into the 1980s. These treeline advances corresponded closely with decadal-scale temperature increases; and Esper and Schweingruber remark that "the lack of germination events prior to the mid 20th century indicates this [was] an exceptional advance."

The two researchers temper this statement, however, with the acknowledgement that the relict stumps and logs show that the advance was part of "a long-term reforestation," i.e. a bringing back of the forest to where it had been some time before; and in this regard they note that "stumps and logs of Larix sibirica can be preserved for hundreds of years (Shiyatov, 1992)," and that "above the treeline in the Polar Urals such relict material from large, upright trees were sampled and dated, confirming the existence, around AD 1000, of a forest treeline 30 m above the late 20th-century limit (Shiyatov, 2003)." They also note that "this previous forest limit receded around 1350, perhaps caused by a general cooling trend (Briffa, 2000; Esper et al., 2002)," which cooling constituted the thermal transition from the sustained warmth of the Medieval Warm Period to the unprecedented (within the current interglacial period) coldness of the Little Ice Age.

"Synchronous with the advance shown from the western Siberian network," according to Esper and Schweingruber, a mid-20th-century tree recruitment period was occurring in "central Sweden (Kullmann, 1981), northern Finland (Kallio, 1975), northern Quebec (Morin and Payette, 1984) and the Polar Urals (Shiyatov, 1992)." Based on these widespread observations, as well as their own results from Asia, they thus concluded that "these findings from Europe and North America support a circumpolar trend, likely related to a global climate warming pattern," and, we would add, to the concomitant increase in the air's CO2 concentration.

In total, these data demonstrate the positive response of the biosphere to the warming that accompanied the demise of the Little Ice Age and to the extra CO2 that was added to the air by the Industrial Revolution. Last of all, they demonstrate that the circumpolar warming of the Northern Hemisphere over the 20th century has yet to return the region to the degree and duration of warmth characteristic of the Medieval Warm Period, when there was much less CO2 in the air than there is today.

Last of all, in an expansion of the work initiated by Sturm et al. (2001), Tape et al. (2006) analyzed northern Alaska photographs taken between 1945 and 1953, together with photos of the same sites taken between 1999 and 2002, to further elucidate the nature of shrub expansion in that region over the past half-century. Then, for Canada, Scandinavia and parts of Russia, as well as the same region of northern Alaska, they analyzed both previously published plot studies and satellite remote sensing data for evidence of shrub expansion over the broader pan-Arctic region.

The repeat photography from northern Alaska, in Tape et al.'s words, "shows that large shrubs have increased in size and abundance over the past 50 years, colonizing areas where previously there were no large shrubs." In addition, they say their review of plot and remote sensing studies confirms that "shrubs in Alaska have expanded their range and grown in size" and that "a population of smaller, intertussock shrubs not generally sampled by the repeat photography, is also expanding and growing." Taken together, they say these three lines of evidence "allow us to infer a general increase in tundra shrubs across northern Alaska." Likewise, the plot and remote sensing studies outside of Alaska also indicate, in the researchers' words, that shrubs are "expanding across much of arctic Canada and in Scandinavia, and possibly Russia and Siberia." Based on these results, they conclude that "a pan-Arctic expansion of shrubs is underway," providing still further support for the circumpolar trend identified by Esper and Schweingruber.

So what is the cause of the Arctic-wide shrub expansion? ... and when did it begin? Tape et al. attribute it to large-scale pan-Arctic warming; and from analyses of logistic growth curves, they estimate that the expansion began about 1900, "well before the current warming in Alaska (which started about 1970)." Hence, they conclude that "the expansion predates the most recent warming trend and is perhaps associated with the general warming since the Little Ice Age."

This inference is well justified, although we would add that the increase in the air's CO2 concentration since 1900 (a gain of some 80 ppm) has likely played a significant role in the shrub expansion as well; and if this trend continues, the three researchers say the transition "will alter the fundamental architecture and function of this ecosystem with important ramifications," the great bulk of which, in our opinion, will be positive. In fact, it would appear that nature is currently in the process of restoring what the Little Ice Age destroyed, as it returns the southern fringes of the Arctic to conditions more conducive to supporting the biological bounty that was theirs during the Medieval Warm Period, or as we like to call it, the good old days.

Briffa, K.R. 2000. Annual climate variability in the Holocene: Interpreting the message of ancient trees. Quaternary Science Reviews 19: 87-105.

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

Esper, J., Cook, E.R. and Schweingruber, F.H. 2002. Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability. Science 295: 2250-2253.

Esper, J. and Schweingruber, F.H. 2004. Large-scale treeline changes recorded in Siberia. Geophysical Research Letters 31: 10.1029/2003GL019178.

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

Kallio, P. 1975. Reflections on the adaptations of organisms to the northern forest limit in Fennoscandia. Paper Presented at the Circumpolar Conference on Northern Ecology, National Research Council, Ottawa, Canada.

Kullmann, L. 1981. Pattern and process of present tree-limits in the Tarna region, southern Swedish Lapland. Fennia 169: 25-38.

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

Morin, A. and Payette, S. 1984. Expansion recente du meleze a la limite des forets. (Quebec nordique). Canadian Journal of Botany 62: 1404-1408.

Shiyatov, S.G. 1992. The upper timberline dynamics during the last 1100 years in the Polar Ural mountains. In: Frenzel, B. (Ed.) Oscillations of the Alpine and Polar Tree Limits in the Holocene. Fischer, Stuttgart, Germany, pp. 195-203.

Shiyatov, S.G. 2003. Rates of in the upper treeline ecotone in the Polar Ural Mountains. Pages Newsletter 11: 8-10.

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

Tape, K., Sturm, M. and Racine, C. 2006. The evidence for shrub expansion in Northern Alaska and the Pan-Arctic. Global Change Biology 12: 686-702.

Last updated 21 March 2007