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Tundra: Response to Elevated CO2, Nutrients and Warming -- Summary
CO2-induced global warming has long been predicted to turn tundra ecosystems into carbon sources extraordinaire.  Only a few years ago it was nearly universally believed that higher temperatures would lead to the thawing of extensive regions of permafrost and the exposure and decomposition of their vast stores of organic matter, releasing once tightly-held carbon and allowing it to make its way back to the atmosphere as CO2, from whence and in what form it originally came (Oechel et al., 1993, 1995).  Today, however, this long-held belief is being seriously questioned.

A case in point is the article of Weintraub and Schimel (2005), wherein they report that "shrubs are growing in predominance in tundra communities in response to warming," citing the work of Sturm et al. (2001) and Stow et al. (2004) in Alaska and the work of Sturm et al. (2005) in northern Canada and Russia.  Furthermore, since shrubs, in their words, "are the woodiest plants in the tundra," they say this transformation "may increase ecosystem carbon storage, because wood has the highest carbon:nitrogen ratio of any plant tissue and decomposes slowly."  However, they also note that "whether net ecosystem carbon storage increases or decreases will depend on the balance of (a) carbon losses from soil organic matter and (b) carbon storage in plant pools due to higher primary productivity and changes in plant community composition."

In elucidating these ideas via an analysis of the results of a number of recent pertinent studies, Weintraub and Schimel note that "the dominant shrub in the Alaskan arctic tundra, Betula nana, is spreading in response to the changing arctic climate, especially in tussock communities (Hobbie, 1996; Hobbie and Chapin, 1998)."  And because these shrubs "trap and hold snow," they report that "the soil underneath them is better insulated in the winter."  As a result, as they describe it, the consequent enhanced insulation "elevates temperatures in the active layer of the soil enough that there can be dramatic increases in microbial activity over the course of the winter, due to the increase in the unfrozen water content of the soil (Sturm et al., 2005)," which phenomenon "enhances winter N mineralization (Schimel et al., 2004)."  They further note that the consequent higher nutrient availability in the spring has been shown to favor early-season photosynthesis and shrub growth (Bliss and Matveyeva, 1992; Chapin et al., 1995; Schimel et al., 1996; Arft et al., 1999; Michaelson and Ping, 2003; Sturm et al., 2005), which enhances shrub dominance and promotes increased ecosystem carbon storage by increasing the amount of carbon stored per unit of nitrogen and by lowering rates of soil organic matter decomposition in shrub-impacted soils.

Alternatively, the two ecologists note that "faster N cycling in shrub soil, due in part to the high quality of shrub litter, may help to promote higher rates of decomposition of preexisting soil organic matter as shrubs encroach into other tundra communities ... where soil microbes are severely N limited."  In addition, they say that with increasing shrub dominance, "carbon losses from the soil are likely to increase in the winter because of higher rates of carbon mineralization in snow-insulated shrub soils," and that "over time, higher winter soil temperatures under shrubs may also contribute to the mineralization of carbon that is currently frozen in permafrost."

So which set of phenomena predominates?  A review of the pertinent scientific literature provides the likely answer.

In the early to mid-1970s, when the first carbon balance studies of Alaskan Arctic ecosystems were conducted under the aegis of the International Biological Program, 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 (Oechel et al., 1993, 1995).  Then, between 1992 and 1996, in response to further warming and drying - resulting, 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 once again.

How did it happen?  Oechel et al. state that their observations indicate "a previously undemonstrated capacity for ecosystems to metabolically adjust to long-term (decadal or longer) changes in climate."  In other words, they became acclimated to the warmer and drier conditions.  But how did that happen?  Was there help along the way from the contemporaneous rise in the air's CO2 content and its aerial fertilization and anti-transpiration effects?  Although these well-documented impacts of atmospheric CO2 enrichment are known to help plants respond to the environmental challenges of both warming and drying - see Growth Response to CO2 with Other Variables (Temperature and Water Stress in our Subject Index) - these effects are not mentioned.  Instead, the researchers note some other possibilities that are, indeed, quite 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 tundra ecosystems from one of carbon sequestration to one of carbon evolution.  After this initial perturbation, however, the authors 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.  And in this regard, the researchers note "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).

The bottom line of this discussion is that there are several reasons to expect a long-term increase in the carbon-sequestering prowess of Arctic tundra ecosystems in response to the increases in air temperature and CO2 concentration that have occurred 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 when rising temperatures are the cause of the physiological transformation; and we here review the findings of a number of studies that address this aspect of the phenomenon.

Camill et al. (2001) studied (1) changes in peat accumulation across a regional gradient of mean annual temperature in Manitoba, Canada, (2) net aboveground primary production and decomposition for major functional plant groups of the region, and (3) soil cores from several frozen and thawed bog sites that were used to determine long-term changes in organic matter accumulation following the thawing of boreal peatlands.  In direct contradiction of earlier thinking on the subject, but in confirmation of more recent findings (Camill, 1999a,b), the team of five scientists determined that aboveground biomass and decomposition "were more strongly controlled by local succession than regional climate."  In other words, they determined that over a period of several years, natural changes in plant community composition generally "have stronger effects on carbon sequestration than do simple increases in temperature and aridity."  In fact, their core-derived assessments of peat accumulation over the past two centuries demonstrated that rates of biological carbon sequestration can almost double following the melting of permafrost, in harmony with the findings of Robinson and Moore (2000) and Turetsky et al. (2000), who found rates of organic matter accumulation in other recently-thawed peatlands to rise by 60-72% in newly-warmed regions.

Griffis and Rouse (2001) drew upon the findings of a number of experiments conducted over the past quarter-century at a subarctic sedge fen near Churchill, Manitoba, Canada, to develop an empirically-based model of net ecosystem CO2 exchange there.  Grounded in real-world observations, as opposed to purely theoretical considerations, their model teaches us much about what we could expect from northern peatlands in the way of carbon sequestration if the air's CO2 content were ever to double (which could well happen) and produce significant global warming (which is not very likely, in our opinion, but nevertheless worthy of consideration).

The most fundamental of Griffis and Rouse's findings was that "carbon acquisition is greatest during wet and warm conditions," both of which characteristics are predicted for the world as a whole by today's most advanced climate models.  However, since regional predictions are not well defined, they investigated the consequences of a 4°C increase in temperature accompanied by a 30% increase and decrease in precipitation.  And "in all cases," as they put it, "the equilibrium response showed substantial increases in carbon acquisition."

But isn't warming supposed to increase ecosystem respiration and return more carbon to the atmosphere?  Sometimes it does; but in the case of the subarctic sedge fen and other peatland ecosystems studied by Griffis and Rouse and many others, the data suggest, in their words, "that arctic ecosystems photosynthesize below their temperature optimum over the majority of the growing season," so that increasing temperatures enhance plant growth rates considerably more than they increase plant decay rates.

In summing up their findings, Griffis and Rouse reiterate the fact that "warm surface temperatures combined with wet soil conditions in the early growing season increase above ground biomass and carbon acquisition throughout the summer season."  Indeed, they note that "wet spring conditions can lead to greater CO2 acquisition through much of the growing period even when drier conditions persist [our italics]."  They thus conclude that if climate change plays out as described by current climate models, i.e., if the world becomes warmer and wetter - as is also suggested by historical climate analogues - "northern wetlands should therefore become larger sinks for atmospheric CO2."

Marchand et al. (2004) determined the carbon balance of tundra vegetation over the snow-free season in northeast Greenland via measurements of gross photosynthesis, root respiration and canopy respiration in plots exposed to either ambient environmental conditions or to conditions similar in all respects except for air temperatures that were maintained 2.5°C above ambient by infrared radiation emitted from two 1500-W heaters.  In the warmer of the two treatments, gross photosynthesis was enhanced by 24%, while soil respiration was enhanced by 33% and canopy respiration by a smaller insignificant amount; but because absolute rates of gross photosynthesis were about twice as large as root respiration rates, the net carbon balance of the tundra was increased by the warming, rising from an ambient value of 0.86 mol CO2 m-2 to a warming-induced value of 1.24 mol CO2 m-2.  Consequently, the strength of the summer tundra carbon sink rose by fully 44% in response to the 2.5°C increase in temperature.

Working near Alexandra Fiord (79°N) on Ellesmere Island, Nunavut, Canada, Welker et al. (2004) warmed portions of three tundra ecosystems spanning a soil water gradient (dry, mesic and wet) by merely placing small (1.8 m2 surface area, 50-cm tall) open-top chambers on the ground, which passively warmed summertime air and soil surface temperatures by 1-3°C.  After eight full years of this warming treatment, they measured CO2 fluxes from the three types of tundra for a period of two additional years.  Averaged over this two-year period, net carbon capture was positive in the dry and wet tundra ecosystems but negative in the mesic ecosystem.  At the dry site, the passive experimental warming increased annual carbon capture from 23.2 to 26.0 g CO2-C m-2 year-1, while at the wet site it decreased it from 55.3 to 42.1 g CO2-C m-2 year-1.  At the mesic site, on the other hand, where there was a carbon loss on an annual basis, the experimental warming reduced the loss, dropping it from -64.6 to -25.4 g CO2-C m-2 year-1.  Averaged over the three soil water treatments, therefore, the experimental warming increased annual carbon capture from 13.9 to 42.7 g CO2-C m-2 year-1, or by approximately 200%.  This huge percentage increase was driven by a minor increase in carbon capture at the dry site (12%) and a large reduction in carbon loss at the mesic site (61%), which together far outweighed the moderate decline in carbon capture at the wet site (24%).

Turunen et al. (2004) derived recent (0-150 years) and long-term (2,000-10,000 years) carbon accumulation rates for several ombrotrophic peatlands in eastern Canada with the help of 210Pb- and 14C-dating of soil-core materials.  This work revealed that the long-term rate of C accumulation at 15 sites was 19 ± 8 g C m-2 yr-1, which is comparable to long-term rates observed in Finnish bogs by Tolonen and Turunen (1996) and Turunen et al. (2002).  Recent C accumulation rates at 23 sites, on the other hand, were much higher, averaging 73 ± 17 g C m-2 yr-1, which results, in the words of Turunen et al., are also "similar to results from Finland (Tolonen and Turunen, 1996; Pitkanen et al., 1999) and for boreal Sphagnum dominated peat deposits in North America (Tolonen et al., 1988; Wieder et al., 1994; Turetsky et al., 2000)."  Calling these recent rates of C accumulation "strikingly higher" than long-term rates, Turunen et al. suggest that increased nitrogen deposition "leads to larger rates of C and N accumulation in the bogs, as has been found in European forests (Kauppi et al., 1992; Berg and Matzner, 1997), and could account for some of the missing C sink in the global C budget."

In an experiment that looked at other environmental changes in addition to warming, such as the implied nitrogen deposition discussed in the study above, Johnson et al. (2000) studied the long-term effects of warming and nutrient fertilization on the carbon balance of wet sedge tundra ecosystems in Alaska for a period of eight years.  Some of their experimental plots were subjected to enclosure by greenhouses, which elevated their air temperatures by an average of nearly 6°C, while other plots received yearly fertilization with nitrogen and phosphorus.  At the end of the study, it was evident that fertilization had a much greater impact than warming on ecosystem dynamics.  Plant ground cover, for example, increased only slightly in response to warming, but it nearly tripled in response to nutrient fertilization.  Similarly, gross ecosystem photosynthesis was unresponsive to warming, while it increased three-fold in response to soil fertilization.  Thus, in Arctic tundra, the addition of nutrients, by increasing plant photosynthesis and growth, appears to control ecosystem CO2 exchange much more than warming.

In a similar study of wet sedge tundra within the Long-Term Ecological Research site in the northern foothills of Alaska's Brooks Range, Boelman et al. (2003) established the following treatments: control, warming with field greenhouses (GH) and warming together with N + P fertilization (GHNP), which they maintained for a total of 13 years.  During peak growing season measurements made in the 13th year of the study, they found that NEP - defined as "the net gain of CO2 by the whole ecosystem, plants plus soil," - was about 90% greater in the GH and GHNP treatments compared to the control treatment, while aboveground biomass in the GH and GHNP treatments was approximately 40% and 65% greater, respectively.  Noting that "the long-term responses of biomass and C fluxes that were measured in this study were generally consistent with results of similar experiments in a wide range of tundra types (e.g., Henry et al., 1986; Chapin et al., 1995; Jonasson et al., 1996, 1999; Press et al., 1998; Robinson et al., 1998)," Boelman et al. concluded that "global warming and the associated increase in nutrient mineralization have the potential to alter arctic ecosystem processes and states."  In addition, it is clear that the warming-induced change is in the direction of greater carbon sequestration, providing a powerful negative feedback that reduces the rate of rise of the air's CO2 content and the potential for global warming.

Focusing finally on the growing predominance of woody plants in tundra communities, we turn our attention to Iceland, where Jonsdottir et al. (2005) studied the effects of 3-5 years of modest surface warming (1-2°C) on two widespread but contrasting tundra communities, one of which was a nutrient-deficient and species-poor moss heath and the other of which was a species-rich dwarf shrub heath.  The elevated temperature treatment elicited no change in community structure in the moss heath; but in the dwarf shrub heath bryophytes decreased by 18%, while the number of deciduous and evergreen dwarf shrubs increased more than 50% and canopy height actually doubled, which changes imply a huge increase in ecosystem carbon sequestration.

Other studies of real-world tundra transformation have observed similar increases in shrub abundance in recent decades. Sturm et al. (2001), for example, 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 treeline white spruce 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).  Over this 50-year period, they documented "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."  At tree sites there was also, in their words, "a marked increase in the extent and density of the spruce forest."

Noting that their study area was conducted 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 had 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 Sturm et al. 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 places 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% 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, which could have also contributed to the increase in tree 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 (again, see Trees (Range Expansions) in our Subject Index); and many 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 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. write, the woody plant expansion is "increasing the amount of carbon stored in a region that [was] believed to be a net source of carbon dioxide."

Based on empirical knowledge gleaned from experiments and real-world observations, various models have been developed to assess the impacts of increasing air temperatures and CO2 concentrations on woody plant invasions of tundra.  In one instance, Baron et al. (2000) employed a regional 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 a high-elevation Rocky Mountain watershed.  They found that "both photosynthesis and transpiration were highly responsive to doubled CO2," and that the positive effects provided by the 4°C temperature increase "were additive, so a warmer and carbon-rich environment increased plant growth by 30%."  They additionally state that their results suggest 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, Baron et al. report that 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 much perturb total runoff.  It did, however, cause seasonal 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.  The researchers 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 a study of vegetation feedbacks on climate via a "new, fully coupled, Global Environmental and Ecological Simulation of Interactive Systems (GENESIS) - Integrated Biosphere Simulator (IBIS) climate-vegetation model with boundary conditions appropriate for 21,000 years before present," Levis et al. (1999) found that under the colder and drier conditions of the Last Glacial Maximum compared to the present, grasslands and tundra largely replace present-day forests in temperate and boreal latitudes.  Hence, if we reverse the direction of change explored in this study and move from the colder and lower atmospheric CO2 concentrations of the Last Glacial Maximum to the warmer and higher CO2 concentrations of the present and beyond, what do we see?  In all parts of the planet we find a proliferation of trees.

Last of all, Esper and Schweingruber (2004) analyzed treeline dynamics over western Siberia during the 20th century by comparing nine undisturbed polar sites located between 59 and 106°E and 61 and 72°N and merging information from them in such a way that, in their words, "larger-scale patterns of treeline changes are demonstrated, and related to decadal-scale temperature variations."  They also related current treeline positions to former treeline locations "by documenting in-situ remnants of relict stumps and logs."  This work revealed two main pulses of northward treeline advance in the mid and late 20th century.

The first of the recruitment phases occurred between 1940 and 1960, while the second period started around 1972 and lasted into the 1980s.  These treeline advances corresponded closely to annual decadal-scale increases in temperature; and Esper and Schweingruber remark that "the lack of germination events prior to the mid 20th century indicates this is an exceptional advance," but that "the relict stumps and logs found at most sites "show that this advance is part of a long-term reforestation process of tundra environments."  They note, for example, 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."

"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)."  Added to their own results from Asia, they conclude that "these findings from Europe and North America support a circumpolar trend, likely related to a global climate warming pattern."  Indeed, the data delineate the most recent phase of the tundra-ward expansion of earth's woody plants that began with the demise of the Little Ice Age and continues to this day.  In addition, they demonstrate the existence of the warmer-than-present period in the vicinity of AD 1000 that we know as the Medieval Warm Period, when much of the world's tundra also gave way to invading shrubs and trees.

In summary, a profusion of scientific evidence indicates that increases in air temperature, CO2 concentration and nutrient deposition all act to enhance tundra productivity, which leads to greater rates of ecosystem carbon sequestration and a slower rate-of-rise in the air's CO2 content, which in turn reduces the potential for CO2-induced global warming.

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Last updated 2 November 2005