For many years the story out of the Alaskan Arctic tundra was that rising temperatures would change the land from a carbon sink to a carbon source, further exacerbating the cause of the ecosystem change, i.e., regional warming, by adding to the atmosphere's burden of greenhouse gases and hastening that portion of the biosphere's inevitable degradation (Oechel et al., 1993, 1995).  In the early to mid-1970s, for example, when the first carbon balance studies of Alaskan Arctic ecosystems were conducted, both wet-sedge communities and moist-tussock tundra were observed to be net sinks of carbon.  By the mid-1980s and early 1990s, however, following significant increases in air temperature and surface water deficit, both ecosystems had become net sources of carbon.  Then, between 1992 and 1996, in response to further warming and drying that resulted, in the words of Oechel et al. (2000), in "the highest average summer temperature and surface water deficit observed for the entire 39-year period," both ecosystems' net summer releases of CO2 to the atmosphere declined, and they eventually became CO2 sinks.

How did it happen?  In the words of the scientists who conducted the work, their observations simply indicated "a previously undemonstrated capacity for ecosystems to metabolically adjust to long-term (decadal or longer) changes in climate."

But how did that happen?  Was there help along the way from the concomitant rise in the air's CO2 content and its aerial fertilization and anti-transpirant effects?  Although these well-documented consequences of atmospheric CO2 enrichment are known to enable plants to better respond to the environmental challenges of both warming and drying [see the materials posted under our Subject Index headings of Growth Response to CO2 with Other Variables (Temperature, Water Stress)] these effects were downplayed.  Instead, the researchers noted some other possibilities that also seem plausible.

First, there is the likelihood that during the initial stages of warming and soil drying, younger and more labile carbon would be rapidly decomposed, shifting the net summer carbon balance of the ecosystems from one of carbon sequestration to one of carbon evolution.  After this initial perturbation, however, Oechel et al. suggest that "enhanced rates of net nitrogen-mineralization should eventually stimulate rates of gross primary production and atmospheric CO2 sequestration."

Another possibility is a gradual shift in plant species towards more productive types that would further reduce the large initial carbon losses over time.  Writing on this subject, the researchers say "there is evidence that the relative abundance of deciduous shrubs has increased in response to climate change over the past 1-2 decades in Alaskan moist-tusssock tundra ecosystems," which is also something that is expected to occur as a consequence of the ongoing rise in the air's CO2 content [see Trees (Range Expansions) in our Subject Index].

Clearly, there are several reasons to expect a long-term increase in the carbon-sequestering potential of Alaskan Arctic ecosystems in response to the simultaneous increases in that region's air temperature and CO2 concentration that have been experienced over the past few decades.  As with the "no pain, no gain" approach to muscle development in the human body, however, there is the initial pain of ecosystem carbon loss that precedes the ultimate gain of ecosystem carbon acquisition.  The important paper of Oechel et al. (2000) lifts that concept as applied to ecosystems from the level of theory to the pinnacle of reality.

Further south, Baron et al. (2000) used an empirically-based hydro-ecological simulation model to evaluate the consequences of a doubling of the air's CO2 content and 2 to 4°C increases in air temperature on ecosystem performance in a high-elevation Rocky Mountain watershed, finding that "both photosynthesis and transpiration were highly responsive to doubled CO2."  They also determined that the positive effects of the 4°C temperature increase "were additive, so a warmer and carbon-rich environment increased plant growth by 30%."  Hence, they concluded that "forests will expand at the expense of tundra in a warmer, wetter, and enriched CO2 world," and that observed increases in tree height and density in recent decades illustrate "the rapidity with which vegetation can respond to climate change."

With respect to water resources, even though the doubled atmospheric CO2 concentration increased plant water use efficiency, there was little change in basin-wide runoff because of the sparse vegetation cover.  Neither did the 4°C increase in air temperature perturb total runoff significantly.  It did, however, cause snow melt to begin four to five weeks earlier than it currently does, allowing the melt water to infiltrate the soil more gradually and for a longer period of time than at present.  Baron et al. say this phenomenon is particularly beneficial, because the consequent gradual release of nitrates that are retained in the snowpack and otherwise released in a large pulse in the spring relieves some of the ecological pressure caused by high nitrate concentrations in typical springtime flows.

In another study conducted in western North America, Feng (1999) derived variations in plant intrinsic water-use efficiency over the preceding two centuries from 23 carbon isotope tree-ring chronologies.  The results were nearly identical to the historical trend in the air's CO2 content, with plant intrinsic water-use efficiency rising by 10 to 25% from 1750 to 1970, during which time the air's CO2 concentration rose by approximately 16%.  Feng thus concluded that "in arid environments where moisture limits the tree growth, biomass may have increased with increasing transpiration efficiency," noting that the enhanced growth of trees in arid environments may "have operated as a carbon sink for the anthropogenic CO2" emitted over that time span.

Last of all, in a paper entitled "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999," which describes a study that covered essentially the entire continent, Zhou et al. (2001) determined that the satellite-derived normalized difference vegetation index (NDVI) rose by 8.44% over this period.  Noting that the NDVI parameter "can be used to proxy the vegetation's responses to climate changes because it is well correlated with the fraction of photosynthetically active radiation absorbed by plant canopies and thus leaf area, leaf biomass, and potential photosynthesis," Zhou et al. went on to suggest that the increases in plant growth and vitality implied by their NDVI data were primarily driven by concurrent increases in near-surface air temperature.  Since this warming was rather muted in North America, however, and in the United States in particular, where temperatures may have actually declined throughout the eastern part of the country over the period of the study, Ahlbeck (2002) suggested that the observed upward trend in NDVI was primarily driven by the concurrent rise in the air's CO2 content.  In our Editorial of 18 Sep 2002 we indicate that both parameters probably played a role in the observed productivity increase, although the CO2 increase was likely the predominant one.

In summing up these results, it is clear that in the "living laboratory" of the real world of nature, concurrent increases in atmospheric temperature and CO2 concentration have historically elicited significant positive growth responses from the terrestrial vegetation of essentially all of North America.  Furthermore, there are no signs that that response is about to change any time soon.

References
Ahlbeck, J.R.  2002.  Comment on "Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999" by L. Zhou et al.  Journal of Geophysical Research 107: 10.1029/2001389.

Baron, J.S., Hartman, M.D., Band, L.E. and Lammers, R.B.  2000.  Sensitivity of a high-elevation Rocky Mountain watershed to altered climate and CO2.  Water Resources Research 36: 89-99.

Feng, X.  1999.  Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: A response to atmospheric CO2 concentration.  Geochimica et Cosmochimica Acta 63: 1891-1903.

Oechel, W.C., Hastings, S.J., Vourlitis, G., Jenkins, M., Riechers, G. and Grulke, N.  1993.  Recent change of Arctic tundra ecosystems from a net carbon dioxide sink to a source.  Nature 361: 520-523.

Oechel, W.C., Vourlitis, G.L., Hastings, S.J. and Bochkarev, S.A.  1995.  Change in Arctic CO2 flux over two decades: Effects of climate change at Barrow, Alaska.  Ecological Applications 5: 846-855.

Oechel, W.C., Vourlitis, G.L., Hastings, S.J., Zulueta, R.C., Hinzman, L. and Kane, D.  2000.  Acclimation of ecosystem CO2 exchange in the Alaskan Arctic in response to decadal climate warming.  Nature 406: 978-981.

Zhou, L., Tucker, C.J., Kaufmann, R.K., Slayback, D., Shabanov, N.V. and Myneni, R.B.  2001.  Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981-1999.  Journal of Geophysical Research 106: 20,069-20,083.