Climate change in relation to carbon uptake and carbon storage in the Arctic

May 7, 2012, 12:44 pm

This is Section 14.10 of the Arctic Climate Impact Assessment.

Lead Author: Glenn P. Juday; Contributing Authors: Valerie Barber, Paul Duffy, Hans Linderholm, Scott Rupp, Steve Sparrow, Eugene Vaganov, John Yarie; Consulting Authors: Edward Berg, Rosanne D’Arrigo, Olafur Eggertsson,V.V. Furyaev, Edward H. Hogg, Satu Huttunen, Gordon Jacoby, V.Ya. Kaplunov, Seppo Kellomaki, A.V. Kirdyanov, Carol E. Lewis, Sune Linder, M.M. Naurzbaev, F.I. Pleshikov, Ulf T. Runesson,Yu.V. Savva, O.V. Sidorova,V.D. Stakanov, N.M.Tchebakova, E.N.Valendik, E.F.Vedrova, Martin Wilmking.

The role of the boreal forest in the global carbon cycle (14.10.1)

Within the terrestrial biosphere, forests cover 43% of the land area but are potentially responsible for 72% of the annual net primary productivity[1]. The boreal forest covers roughly 1.37 billion hectares (ha) and by itself (not including high-latitude tundra) contains approximately 20% of global reactive soil carbon, an amount similar to that held in the atmosphere[2]. Climate change can affect high-latitude carbon cycling at multiple timescales. The most likely mechanism for significant short-term change in boreal carbon cycling resulting from climate change is a change in rates of organic matter decomposition in the forest floor and mineral soil resulting from major changes in species composition caused by alteration of disturbance regimes. Climate change can also strongly affect rates of carbon cycling through its control of the disturbance regime and the subsequent successional development of ecosystems[3]. Climate-induced shifts in dominant tree species composition within the present boreal forest[4] are likely to have profound impacts on the global carbon budget[5].

Boreal and subarctic peatlands contain approximately 455 Pg (petagrams; 1 petagram = 1015 grams = 1012 kilograms = 121 billion tons) of carbon accumulating at an average rate of 0.096 Pg/yr, and constitute a significant proportion of the total boreal carbon pool[6]. The majority of peat consists of molecules that are highly resistant to degradation (e.g., lignin and cellulose). Species composition in sphagnum bogs is highly resilient (likely to remain the same or recover after change) because the mosses modify the local environment to produce highly acidic conditions, and their resiliency increases with age[7].

The role of disturbance in the carbon cycle of the boreal forest (14.10.2)

Four processes largely control the storage and release of carbon in boreal forests: the rate of plant growth; the rate of decomposition of dead organic matter; the rate of permafrost accretion and degradation; and the frequency and severity of fires[8]. All four processes are affected by landscape-scale disturbance. Differences in carbon cycling between mature and recently disturbed forest ecosystems have been observed in both experimental studies and modeling experiments. Some studies[9] suggest that the annual carbon budget of the mature northern forest is at equilibrium and in some cases losing carbon to the atmosphere. In addition, model results and field experiments show that when ecosystems are disturbed, significant losses of soil carbon and nutrients can occur[10] for a number of years after the disturbance.

The effects of temperature and disturbance (i.e., fire and grazing) on carbon exchange over three years in five undisturbed sites and five disturbed sites in forests of northeast Siberia were measured by Zimov et al.[11] and results show that disturbance increased the seasonal amplitude of net carbon exchange. Disturbance had a larger effect on seasonal amplitude than either interannual or geographic differences in growing season temperature.

Fire affects the storage of carbon in the boreal forest in at least five ways: it releases carbon to the atmosphere; converts relatively decomposable plant material into stable charcoal; re-initiates succession and changes the ratio of forest-stand age classes and age distribution; alters the thermal and moisture regime of the mineral soil and remaining organic matter, which strongly affects rates of decomposition; and increases the availability of soil nutrients through conversion of plant biomass to ash[12]. Each of these effects exerts an influence at different timescales. As a result, the effect of a given climate scenario on carbon storage in the boreal region will be greatly influenced by fire regime (Section 14.9), and represents a complex calculation requiring a great deal of specific spatial and temporal information.

Relatively few studies have directly examined carbon emissions in the boreal forest resulting from fire. Nonetheless, valuable contributions have been made by studies that used remotely sensed data to estimate direct carbon emissions from combustion as fires are occurring[13]. These studies show that wildland fires have the potential to release a significant amount of carbon directly into the atmosphere. The effects of wildfire on this initial carbon loss are highly variable and strongly influenced by forest type and fire severity[14]. Soil drainage (defined by the water table, moss cover, and permafrost dynamics) is the dominant control of direct fire emissions[15].

Fires cause the release of carbon not only during, but also for a short time after, fires. Tree mortality after surface fires can be extensive, leading to a pulse of carbon released from heterotrophic respiration as fine roots die and aboveground fine fuels (i.e., needles) fall to the ground and decompose rapidly[16]. The non-combustion post-fire release of carbon dioxide (CO2) has the potential to affect global levels of atmospheric CO2 over the short term, representing another mechanism by which the boreal forest can play a significant role in the global carbon cycle. Together, these direct and indirect fire-generated carbon emissions from boreal forests worldwide may exceed 20% of the estimated global emissions from all biomass burning[17].

Wildland fires change the distribution of soil organic carbon pools with respect to their turnover (release to CO2) times. Soil carbon in the form of forest litter that has fallen recently has a relatively rapid turnover rate, but carbon in the form of charcoal is stable (very long turnover rates). If rates of burning increase in the boreal forest, an increasing proportion of soil organic carbon is likely to be converted to stable charcoal. This change in the soil organic carbon pool allocation is difficult to estimate, because forest type and fire severity strongly influence the effects of wildfire on carbon redistribution[18]. Harden et al.[19] show the importance of this shifting distribution of turnover times in soil organic carbon. They developed a system of ordinary differential equations to explore constraints on carbon losses to fire, using modern estimates of carbon production, decomposition, and storage; a model of fire dynamics developed for millennial timescales; and an assessment of the long-term carbon balance of a variety of boreal landscapes in North America. A sensitivity analysis found that their model results were responsive to the rate at which charred plant remains decomposed. Unfortunately, the specific characteristics of fires that result in maximum charcoal production are not well studied. However, it seems reasonable to infer that a moderate-intensity fire, in which combustion is enough to kill trees but not intense enough to consume them completely, would produce the greatest amount of charcoal. In complex mountainous or hilly terrain at high latitudes, north-facing slopes have higher fuel moisture content and as a result generally experience less complete combustion of fuels during fire[20]. This suggests that slope aspect might be an important factor in the conversion of plant biomass to charcoal.

Much of the difference in carbon cycling after disturbance can be linked to shifts in species composition and ecosystem age structure that enhance both peak summer CO2 uptake and winter CO2 efflux. The seasonal amplitude of net ecosystem carbon exchange in northern Siberian ecosystems is greater in disturbed than undisturbed sites, due to increased summer influx and winter efflux[21]. Disturbed sites differ from undisturbed sites during the summer, having 2.1-2.5 times the daytime CO2 influx and 1.8-2.6 times the nighttime CO2 efflux. Winter respiration in disturbed sites is 1.7-4.9 times that in undisturbed sites. Carbon cycling within disturbed ecosystems is more sensitive to interannual temperature variability than older forests, and disturbed sites also experience a greater difference in annual carbon exchange with the atmosphere in warm versus cold years than older forests[22].

Two hypotheses have been advanced to explain why these differences in carbon cycling caused by enhanced CO2 summer uptake and winter efflux occur in disturbed areas. One hypothesis is that the recent increase in March and April temperatures in high-latitude continental regions of North America and Siberia has advanced snowmelt and lengthened the growing season (Section 6.4). The second hypothesis is that temperature-driven increases in summer carbon gain (greater CO2 uptake from greater growth) balanced by increased winter respiration (greater CO2 release from enhanced decomposition and live respiration) could enhance the seasonal amplitude of atmospheric CO2 concentrations without a change in net annual carbon accumulation. Although the mechanism remains somewhat uncertain, experimental studies confirm that these responses do occur under appropriate conditions.

Climate and carbon allocation in the boreal forest (14.10.3)

Changes in species composition modify the way carbon is allocated and stored. Deciduous forests experience greater carbon cycling (production and decomposition) than coniferous stands[23]. Both aboveground net primary production and overall (including belowground) net primary production were roughly two times greater in a boreal deciduous forest than a coniferous forest. The fraction of net primary production allocated to coarse- and fine-root primary production is roughly two times greater for evergreen conifers than deciduous trees[24]. Because of these differential allocation patterns between deciduous and evergreen stands, the amount of carbon in the soil of mature black spruce stands is approximately three times the amount of carbon in the biomass of the trees[25]. Since the rate of decomposition is higher in deciduous than in coniferous forests (because of both litter quality and site conditions), nitrogen is probably more available in deciduous forests, further increasing production[26]. These results make sense physiologically, since deciduous species have higher maximum rates of photosynthesis and productivity than evergreens and produce litter that quickly decomposes.

Aboveground carbon pools are directly related to stand age. Gower et al.[27] found that the effect of stand age (young versus old stands) on net primary production is roughly equivalent to the effect of soils (fertile versus infertile) and annual variation in the environment (favorable versus unfavorable weather). Similarly, Wang et al.[28] found that wildfire exerts a lingering effect on carbon exchange between the boreal forest and the atmosphere via its effect on the age structure of forests and leaf area index (LAI) during succession. They also found a strong inverse linear relationship between aboveground net primary productivity and age that was largely explained by a decline in LAI. Modeling results from Kurz and Apps[29] suggested that forest ecosystems in Canada were a carbon sink from 1920 to 1980 and a source from 1980 to 1989. They suggested that this was a result of a change in the disturbance regime, and this finding is consistent with recent fire statistics (see Section 14.9.2). Sometime around 1977, a regime shift in the climate of the North Pacific Ocean occurred that has been suggested to be part of a low-frequency oscillation[30]. One of the consequences of this shift was that the position of the Aleutian Low associated with El Niño moved even farther eastward than it did in previous El Niño years. This shift is consistent with a more easterly (less southerly) flow component across Interior Alaska, which could exert teleconnective influences on the fire-dominated disturbance regime of Canada[31]. If the key control over fire occurrence was a one-time climate regime shift caused by a change in sea surface temperature in the northeast Pacific Ocean, it suggests that climate changes occurring at low temporal frequencies exert a strong influence on the rate of carbon cycling in regions where the disturbance regime is climatically driven. Kurz and Apps[32] suggested that as stand age increases, the ability to sequester carbon decreases and the susceptibility to disturbance increases.

Modification of soil thermal regime and permafrost degradation as a result of fire have been documented. Warmer and drier (due to reduced cover of saturated mosses) conditions following a forest fire increase decomposition and decrease carbon storage[33]. Simulation results suggest that a 5 °C increase in average annual air temperature results in a 6-20% decrease in the total carbon stored in the soil over a 25-year period[34]. In China, Wang et al.[35] found that soil-surface CO2 flux decreases immediately after wildfire because of a lack of root respiration, which accounts for about 50% of total soil-surface flux. In the time after a fire during which appreciable tree mortality occurred, the majority of respiration shifts from autotrophic to heterotrophic. As a consequence of the increased heterotrophic respiration and low net primary production in the early stages of succession, areas that have recently burned in the boreal forest tend to act as a carbon source for a brief period of time. Rapalee et al.[36] found evidence that fire scars on the landscape are a net carbon source for about 30 years after burning, after which systems become a net carbon sink. Experiments in the Alaskan boreal forest showed that about 20% of the carbon in the soil surface layer is lost through decomposition during the first 20-30 years after a fire due to increased soil temperature[37]. Rapalee et al.[38] asserted that ecosystem changes in net ecosystem production are driven by changes in decomposition and species composition. They further suggested that changes in species composition are driven by fire-induced modification of the active layer. The relative importance of active-layer modification depends to a large extent on aspect and fire severity.

Fire frequencies in the sphagnum peatlands have decreased over the past 7,000 years due to cooler and wetter conditions[39]. Carbon emissions from peat combustion are still an order of magnitude greater than warming-induced oxidation[40]. Fire affects not only the amount of carbon in the forested peat systems but also the subsequent rate of accumulation. Kuhry[41] found that peat accumulation in sphagnum-dominated peatlands of western boreal Canada decreased significantly with increasing fire frequencies. It is estimated that warming and drying would result in relatively rapid decomposition of peat soil organic carbon[42]. Valentini et al.[43] investigated two contrasting land cover types, a regenerating forest and a bog, in the central Siberian region during July 1996, and found that net CO2 uptake was limited by the decreasing soil water content in the regenerating forest. Their results showed substantial differences in both transpiration and carbon assimilation. The bog used the incoming solar energy principally for transpiration and, because of the constant availability of water, transpiration was not sensitive to seasonal changes in moisture conditions. The bog system also maintained high carbon assimilation potential compared to the regenerating forest. This trend was maintained and amplified in dry conditions, when the carbon uptake of regenerating forest decreased significantly. Bogs and peatlands represent a very different land cover type than forests and must be considered separately when assessing the role of disturbance in the boreal forest (see section 8.4.4.4).

Forest cover type, disturbance, and climate change (14.10.4)

Vegetation response to climate change could feed back to cause large changes in regional and global climate through effects on terrestrial carbon storage[44] and on water and energy exchange[45]. The rate and magnitude of this feedback is influenced by transient changes in the distribution of terrestrial ecosystems in response to changes in climate, disturbance regime, and recruitment rates. The long-term direction of ecosystem change is also sensitive to spatial patterns and processes operating at the landscape scale[46].

Current projections of vegetation response to climate change either assume that the disturbance regime does not change[47] or use globally averaged disturbance rates[48]. Conversely, projections of disturbance regimes in a warmer climate[49] generally neglect rates and patterns of vegetation response to climate and disturbance. However, the climate–disturbance–vegetation interactions clearly influence the rate and pattern of changes in vegetation[50] and disturbance[51] through effects on fire probability and spread and pattern of colonization[52].

Landscape-scale interactions between vegetation and disturbance are particularly important in the forest–tundra ecotone[53] where vegetation change is very likely to have large feedbacks to climate[54]. The potential colonization of tundra by forest is very likely to increase terrestrial carbon storage[55], thereby reducing atmospheric carbon, but increase absorption of solar radiation[56] thereby creating a positive feedback to regional temperature increases[57].

Modeling the transient dynamics of vegetation change allows investigation of both the short- and long-term responses of ecosystems to landscape-level disturbance and recruitment, and the subsequent feedbacks between climate and the biosphere. These spatial processes are responsible for long-term changes in vegetation distribution[58] in response to changing climate, and must eventually be incorporated into hemispheric- or global-scale spatio-temporal models of gradual climate change.

Land-use change (14.10.5)

Atmospheric CO2 and oxygen data confirm that the terrestrial biosphere was largely neutral with respect to net carbon exchange during the 1980s but became a net carbon sink in the 1990s[59]. However, the cause of this shift remains unclear. Several studies have indicated that land-use change is responsible for the majority of the terrestrial sink[60]. Calculations of land-use changes can be used to calculate associated carbon fluxes[61]. Since 1850, there has been a 20% decrease in global forest area, and during this period deforestation has been responsible for approximately 90% of the estimated emissions from land-use change[62].

The majority of land-use change studies have focused on areas outside the boreal region, and a review of the critical issues provides perspective on the impact of land-use change on carbon cycling in the boreal forest. In the tropics, forests contain 20 to 50 times more carbon per unit area than agricultural land, and as a result, tropical deforestation during the early 1990s released 1.2 to 2.3 Pg of carbon annually[63]. Once tropical vegetation is cleared, soil mass is quickly lost through erosion and oxidation. When tropical forest soils are cleared of vegetation and cultivated, surface horizons experience exponential mass loss resulting in roughly a 25% decrease in carbon[64]. Despite the large atmospheric source of CO2 from tropical deforestation, the terrestrial system is acting as a net sink for carbon[65], and in a spatially explicit inversion analysis, Rayner et al.[66] found no evidence of a large net source from the tropics. The spatial inversion analysis allows a more focused examination of carbon fluxes between discrete regions. For example, the lack of a large tropical net source suggests a tropical terrestrial sink of roughly the same magnitude. The exact nature of this tropical terrestrial sink remains a source of debate. Melillo et al.[67] used gas flux studies to show that undisturbed tropical forests in the Brazilian Amazon are responsible for a net carbon uptake, but more work needs to be done to examine and quantify carbon flux from tropical forests experiencing and recovering from deforestation.

Like tropical forests, temperate and boreal regions in the Northern hemisphere have experienced substantial land-use changes in the past several hundred years. Siberian forests account for 20% of global forest area and net primary production; Valentini et al.[68] estimated that approximately 800,000 ha are harvested there annually. Fang et al.[69] found that Chinese forests acted as a carbon source from 1948 to 1980, and as a sink from 1981 to 1998. Subsequent works focused on constraining spatial and temporal aspects of carbon fluxes and therefore, several atmospheric inversion analyses have indicated a large terrestrial carbon sink in the Northern Hemisphere[70]. Estimates of carbon flux in the United States derived from independent forest inventory methods[71] and ecosystem models[72] provide supporting evidence for the presence of a North American sink, although of a lower magnitude than that estimated by Fan et al.[73]. Goodale et al.[74] found that growth rates in unmanaged forests of the eastern United States have changed little over the past several decades, suggesting that nearly all of the carbon accumulation in the region is due to forest regrowth from past disturbance rather than growth stimulated by increased atmospheric CO2, nitrogen deposition, or climate change.

Nitrogen deposition and carbon dioxide fertilization (14.10.6)

A process-based model that simulates the biomass production of Norway spruce in southeastern Norway under both current climate and climate change scenarios was used to project biomass production responses to three climate change scenarios[75]. Net primary production (dry mass) was projected to increase by 7% over the current 10.1 t/ha/yr under a mean annual air temperature elevated by 4°C over present-day levels. Doubling current ambient CO2 concentration was projected to increase net primary production by 36%. The scenario of both elevated temperature and elevated CO2 concentration led to an increase in net primary production of nearly 50%, which was higher than the sum of the two effects alone.

Nitrogen availability is often the limiting factor in net primary productivity. The majority of anthropogenic nitrogen inputs come from combustion (both biomass and fossil fuel) and agricultural fertilizer application[76]. Photosynthetic rate is correlated with the nitrogen content of leaves, since carbon assimilation is driven by the nitrogen-rich enzyme rubisco. Hence, reduced nitrogen availability decreases both leaf nitrogen content and photosynthesis. As a result, the carbon and nitrogen cycles are fundamentally coupled. Due to this coupling, increased anthropogenic nitrogen deposition must be considered in conjunction with elevated CO2 levels. Elevated CO2 levels have multiple direct effects on fundamental biochemical processes such as photosynthesis and respiration, which collectively determine net primary production.

Among studies that manipulated both CO2 and nitrogen availability, the mean enhancement of photosynthesis by elevated CO2 levels at the lowest level of nitrogen availability was 40%, while the mean enhancement at higher levels of nitrogen availability was 59%[77]. These results indicate that for a fixed increase in CO2 concentration, biomass increases proportionally to increased nitrogen availability. In a review of the literature, McGuire et al.[78] also found that compared to low nitrogen availability and baseline CO2 levels, increased nitrogen availability and elevated CO2 concentrations significantly increased biomass accumulation, sometimes by a multiple of more than two. Differential increases in biomass in response to elevated CO2 concentrations are found in different species: in general, deciduous species exhibit twice the growth response of conifer species to elevated CO2 levels[79].

The carbon and nitrogen cycles are also strongly correlated with evapotranspiration. Arain et al.[80] examined the response of net ecosystem productivity and evaporation to elevated atmospheric CO2 concentrations and found that modeled and measured results showed a linear relationship between CO2 uptake and evaporation. This coupling implies that as nitrogen deposition increases and plant tissue carbon to nitrogen ratios decrease, nitrogen cycling increases at a fixed level of evapotranspiration[81]. This is essentially an example of biological supply and demand. It becomes less efficient for plants to exert energy on translocation before senescence; hence, the quality of litter increases. The scaling of this response from tissue to plant level is seen in the results of modeling and field studies showing that nitrogen fertilization results in increased net primary productivity[82].

At the ecosystem level, studies indicate that conifers and deciduous species differ in their response to elevated CO2 levels. Arain et al.[83] examined the response of boreal net ecosystem productivity to elevated CO2 levels in both a 70-year-old aspen stand and a 115-year-old black spruce stand. They found that the aspen stand was a weak to moderate carbon sink while the black spruce stand was a weak carbon sink in cool years and a weak carbon source in warm years (consistent with Fig. 14.23 and the BOREAS results described in section 14.7.3.2). These results emphasize the practical importance of the strong coupling between water flux and both the carbon and nitrogen cycles. When midsummer temperatures were high, the net ecosystem production of the black spruce stand decreased significantly due to increased respiration. At longer timescales, reduced litter quality resulting from elevated CO2 levels has the potential to cause long-term negative feedbacks that constrain the response of net primary productivity. Litter nitrogen concentration is generally positively correlated with decomposition rates, and Cotrufo et al.[84] found that in deciduous stands, cumulative respiration rates were lower for litter derived from elevated CO2 conditions while rates for Sitka spruce remained relatively unaffected. These results have implications for the response of forests not only to elevated atmospheric CO2 concentrations but also to the warmer temperatures that are very likely to ultimately accompany elevated atmospheric CO2 levels.

Schimel et al.[85] found that the amount of nitrogen lost from an ecosystem is an increasing function of the rate at which nitrogen cycles through the system. For example, nitrogen mineralization is a key index of soil inorganic nitrogen turnover, and is strongly correlated with evapotranspiration. Schimel et al.[86] found evidence that losses of nitrogen trace gases are linked to the rate of mineralization of ammonium (NH4) and nitrate (NO3) from organic matter, a rate that increases as temperature and soil moisture increase. Similarly, the product of water flux and the ratio of NO3 to dissolved organic nitrogen (DON) concentration directly control leaching losses of NO3 and DON[87]. Hence, nitrogen losses are controlled by soil moisture and water flux. This linkage between hydrological and nutrient cycles is of critical importance in assessing the relevance of nitrogen mineralization enhanced by increasing temperature to net primary productivity. Schimel et al.[88] noted that water, energy, and nutrient limitation of net primary productivity and carbon storage tend to equilibrate in near "steady-state" ecosystems. This implies that the greatest potential for discrepancies between carbon, nitrogen, and water cycling exist in recently disturbed ecosystems. In support of this, Schimel et al.[89] noted that in general, carbon accumulation in recovering ecosystems is high and chronosequence studies show lower accumulation in undisturbed landscapes. The decreased net primary productivity observed in "mature" ecosystems is a consequence of the equilibration of the nitrogen and water fluxes. Hence, the impacts of temperature increases on enhanced nitrogen mineralization are likely to be greatest in recently disturbed ecosystems.

Finally, although it is not always the case that nitrogen limits growth, a review of studies exposing plants to both elevated CO2 levels and increased soil nitrogen concentrations showed significant increases in net primary productivity[90]. Under the assumption that "mature" ecosystems exhibit decreased net primary productivity because of equilibration of the nitrogen and water fluxes, elevated atmospheric CO2 concentrations are likely to stimulate growth. The relatively rapid increase in atmospheric CO2 concentrations makes this scenario even more plausible. Elevated atmospheric CO2 concentrations have been shown to increase the amount of photosynthesis per unit of water transpired, also known as water-use efficiency. Schimel et al.[91] suggested that if CO2 or fertilizer and pollutant nitrogen increase global net primary production over the coming decades, it is possible for soil carbon increases to occur on a commensurate timescale. Despite this prospect of greater carbon storage, the estimated effect of increasing temperatures is sensitive to the feedback between primary production and decomposition via the nitrogen cycle. As soil organic matter is lost through enhanced nitrogen mineralization caused by temperature increases, more nitrogen becomes available for plant growth, which results in the formation of more soil organic matter, thus acting as a negative feedback[92].

Another possible mechanism causing equilibration of the carbon, nitrogen, and water fluxes comes from examination of the tissue-level response to elevated atmospheric CO2 concentrations. Acclimation to elevated CO2 levels can occur through one or more of three processes of leaf-level carbon assimilation: carboxylation, light harvest, and carbohydrate synthesis[93]. Under saturating light conditions at low levels of intercellular CO2, assimilation is limited by the quantity and activity of rubisco, the enzyme that is primarily responsible for capturing atmospheric carbon in the production of sugars. At high levels of intercellular CO2, the enzymatically controlled rate of carbohydrate synthesis, which affects the phosphate regeneration that is necessary for harvesting light energy, may regulate the fixation of carbon[94]. Hence, mechanisms acting at scales from tissue-level biochemistry to ecosystem-level nutrient cycling exert influences to equilibrate carbon, nitrogen, and water fluxes in mature ecosystems. This equilibration has implications for recent patterns of carbon flux observed in terrestrial ecosystems. Schimel et al.[95] noted that the terrestrial carbon sink must eventually become saturated because photosynthesis follows a saturating function with respect to CO2. As the rate of photosynthesis slows, plant and microbial respiration must catch up eventually, reducing incremental carbon storage to zero. Simulations with the Terrestrial Ecosystem Model (Marine Biological Laboratory, Woods Hole) concur, suggesting that soon after atmospheric CO2 concentration stabilizes, heterotrophic respiration comes into balance with net primary production and the CO2-stimulated terrestrial carbon sink disappears[96].

In addition to the effect that anthropogenic modification of carbon and nitrogen cycles has on atmospheric CO2, temperature in boreal and tundra regions affects the intra-annual variability in atmospheric CO2 concentrations. Surface temperature in the north is positively correlated with seasonal amplitude of atmospheric CO2 the following year. Zimov et al.[97] found that 75% of the annual increases in mean annual air temperature between 1974 and 1989 coincided with decreases in CO2 amplitude at the Barrow monitoring station, consistent with observations of net summer CO2 efflux from tundra and boreal forest during warm years.

 

Chapter 14: Forests, Land Management, and Agriculture
14.1. Introduction
14.2. The boreal forest: importance and relationship to climate
14.3. Land tenure and management in the boreal region
14.4. Use and evaluation of the ACIA scenarios
14.5. Agriculture
14.6. Tree rings and past climate
14.7. Direct climate effects on tree growth
14.8. Climate change and insects as a forest disturbance
14.9. Climate change and fire
14.10. Climate change in relation to carbon uptake and carbon storage
14.11. Climate change and forest distribution
14.12. Effects of ultraviolet-B on forest vegetation
14.13. Critical research needs

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Committee, I. (2012). Climate change in relation to carbon uptake and carbon storage in the Arctic. Retrieved from http://www.eoearth.org/view/article/151248

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