Boreal forest carbon stocks and wood supply: past, present and future responses to changing climate, agriculture and species availability

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AGRICULTURAL AND FOREST METEOROLOGY Agricultural and Forest Meteorology 84 (1997) 137-151

Boreal forest carbon stocks and wood supply: past, present and future responses to changing climate, agriculture and species availability Allen M. Solomon a,*, Rik Leemans b a Western Ecology Division, National Health and Environmental Effects Research Laboratory, US Environmental Protection Agency, 200 S. W. 35th Street, Corvallis, OR 97333, USA b Department of Terrestrial Ecology and Global Change, National Institute of Public Health and Environmental Protection, P.O. Box 1, 3720 BA Bilthoven, Netherlands

Received 30 September 1995; revised 10 March 1996; accepted 1 April 1996

Abstract

The paper assesses the role in boreal forest growth played by environment. It examines past changes in climate coupled with glaciation, and future changes in climate coupled with agricultural land use and tree species availability. The objective was to define and evaluate potential future changes in wood supply and global carbon stocks. Calculations were based on a standard static vegetation model (BIOME 1.I) driven by the most recent climate change scenarios from three coupled ocean-atmosphere general circulation models (GCMs). The results indicated that boreal terrestrial carbon stocks increased greatly following the retreat of continental ice sheets, before which boreal forests covered only about a third the amount of land they cover now. Carbon stocks and wood supplies in boreal forests were also projected to increase if vegetation stabilized under all t ~ , e future climate scenarios (6-15%). However, the opposite response occurred with the addition of expected constraints on forest growth, provided by the lags in immigration of tree species suitable for warmed climate. This transient depauperate condition reduced wood supplies considerably (4-6%). Inclusion of present and future agricultural land uses permitted by a wanning climate forced carbon stocks and wood supplies to decline even more (10-20%). The decline in boreal carbon stocks is the equivalent of 1-2.6 Pg year-l emitted to the atmosphere (rather than the 1-2 Pg year-l global modelers hypothesize is currently being taken up by vegetation from the atmosphere), during the time greenhouse gases are expected to double in concentration. Keywords: Boreal carbon; Boreal wood supply; Paleoecology;Future forests; Future agriculture

1. I n t r o d u c t i o n Current and future climate warming throughout the world is of considerable concern both scientifically and politically (e.g. Houghton et al., 1995).

* Corresponding author,

Much of the scientific concern in the high latitude portions of the earth (above about 50 ° of latitude, primarily the northern or boreal regions) focuses on effects of rapid and intense climate changes on slowly growing forests, in a context o f ecosystems which normally function under extreme environmental conditions. For example, high latitudes are expected to undergo the greatest amount and rate of warming

0168-1923/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. PII S0168- 1 9 2 3 ( 9 6 ) 0 2 3 8 2 - 9

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A.M. Solomon, R. Leemans / Agricultural and Forest Meteorology 84 (1997) 137-151

from increased concentrations of greenhouse gases (GHGs). The average global temperature increase of 0.2-0.3°C per decade, expected during the time required for atmospheric concentrations of GHGs to double (Greco et al., 1994), is closer to 0.6 to 0.7°C decade -1 in winter and 0.1-0.2°C decade -~ in summer at high latitudes (60 to 90°N.). These changes are expected to occur where growing seasons of 2 or 3 months are common, where winter temperatures approach the minimum temperatures trees are known to survive, and where precipitation may not reach 400 mm annually (Hare and Thomas, 1979). In addition to the large magnitude of future warming, high latitudes undergo great spatial and temporal variation in weather and climate (e.g. Briffa et al., 1995). These accompany and modulate other extremely variable and large-scale disturbance agents found at high latitudes. Fires and insect attacks decimate large and continuous areas of forest which must be recolonized a few kilometers at a time, by species which may have a good establishment year only once in several decades (Payette, 1992). At the high-latitude boundary to forest growth, fire may be followed by growth of lichen mats that inhibit establishment of tree seedlings (Bonan, 1992) and that may persist for hundreds of years (Payette et al., 1989). Future variations in these extrinsic forces would lead to a comparable temporal variance in ecosystem structure and function if they become synchronized in either time or space. For example, effects of directional climate change could become stronger than effects of the non-directional extrinsic forces, by severely reducing tree establishment (Kurz and Apps, 1993). Or, climate change could amplify effects of those disturbances. Increased fire frequency and intensity from increased drought (Bessie and Johnson, 1995) and increased insect epidemics (Holling, 1992) are examples of such amplification. These spatially coincident ecological and physical disturbances would simultaneously 'set' the successional 'clocks' of a large portion of boreal forest ecosysterns. The importance to human concerns of such global-scale forest growth and morality waves (Sprugel, 1984) is two-fold. First, decade-scale fluctuations in terrestrial carbon flux could increase, Here, widespread mortality, which generated a pulse

of carbon release for several decades (King and Neilson, 1992; Smith and Shugart, 1993) would alternate with subsequent tree establishment and growth, which would sequester carbon for several more decades (Kauppi et al., 1992) before initiation of another dieback and growth cycle. Indeed, the carbon storage capacity of the boreal forests could decline rather than increase as earlier simulations have suggested (e.g. Leemans, 1989; Smith and Shugart, 1993; Melillo et al., 1993). Considering the necessity of a predicted boreal forest sink for carbon to balance and continue balancing global carbon cycle models (Tans et al., 1990; Denning et al., 1995), such a carbon loss could have serious consequences to understanding future carbon cycle dynamics. Second, potentially large timber supply excesses from salvage fellings would follow the extensive mortality events, and would supply deficits during subsequent prevalence of immature forests. The current large excess of boreal wood supply over demand (Solomon, 1996a; Nilsson and Schopfhauser, 1995) could be reversed for several decades at a time. As a consequence, assessments that indicate simple deficits of temperate and tropical wood supplies in the 21st century, and their mitigation by imports of growing stocks from boreal areas (Solomon et al., 1996), could be missing the occurrence of a much more complex situation. The assessment which follows was executed to examine those carbon cycle and wood supply possibilities. An earlier review on the topic (Solomon, 1992) was limited by lack of past and future boreal vegetation estimates. Quantitative estimates of past, present and future boreal forest distribution and productivity subsequently generated by this and other studies (Prentice et al., 1992, 1993a; Cramer and Solomon, 1993; Solomon et al., 1993) provide a more definitive basis for the current assessment.

2. Methods The most effective approach to assessing both carbon stock and wood supply questions would utilize a succession model which realistically simulates cause and effect of forest responses to climate change and variance (e.g. Bonan, 1992; Leemans, 1992;

A.M. Solomon, R. Leemans / Agricultural and Forest Meteorology 84 (1997) 137-151

Prentice et al., 1993b), to geographic seed source limitations (e.g. Malanson and Armstrong, 1996) and to agricultural land use changes (e.g. Meyer and Turner, 1992; Leemans and Solomon, 1993) which would replace forests. The model would be driven by climate and land use scenarios and applied at a globally comprehensive suite of several thousand (or more) sites, each characterized by local data on soils, climate and land use (e.g. Solomon, 1986; Pastor and Post, 1988; Prentice et al., 1993b). Unfortunately, such a model and its required data bases is not yet available (Solomon et al., 1996). In its place, we used a static model which could narrow the range of possible outcomes of forest responses to changing climate and land use. BIOME 1.1 (Prentice et al., 1992, 1993a) generates spatial distributions of globai biomes from combinations of plant functional type,; (PFTs). The PPTs are independently defined by soils and climate-dictatedparameters based on the unique physiological requirements of each PFT. Biome areas were transformed to carbon and growing stock estimates by application of biome carbon density measurements (Olson et al., 1983) as listed by Prentice et al. (1993a). The model was ,driven with geographically explicit climate scenarios describing past (Lautenschlager and Herterich, 1990), present (Leemans and Cramer, 1991) and future climate conditions. The latter included scenarios of climate at the time of a doubled CO 2, produced by three coupled atmosphere-ocean general circulation models (GCMs), extracted from Greco et al. (1994). These can be ordered by their climate sensitivities: ECHAM-1 (Cubasch et al., 1992) simulates moderate climate differences, responding to GHG concentrations the least, followed by GFDL89 (Manabe et al., 1991) while UKTR (Murphy and Mitchell, 1996) is more sensitive than either ECHAM-1 or GFDL89. All three scenarios are considered by the Intergovernmental Panel on Climate Change (IPCC) to reflect insufficient GHG forcing of climate to realistically portray future climate responses (R. Watson, personal communication, 1995). However, atmospheric particulates (Karl et al., 1995) may provide about the same magnitude of cooling over industrialized temperate regions as the undervaluation of warming, providing a potentially realistic set of climate simulations,

139

The differences between modeled 1 × CO 2 climate and 2 × CO e climate were applied to the International Institute for Applied Systems Analysis (IIASA) Climate Data Base (Leemans and Cramer, 1991) as temperature differences and as precipitation differences above zero. The simulation of future climate and its associated vegetation provided an upper limit of forest productivity, by assuming optimum tree growth, species presence and land available in response to the new climate conditions. A lower limit was produced by calculating carbon stocks and growing stock volumes which resulted from assuming certain non-climatic constraints. Agricultural land use permitted by climate was assumed to eliminate half the global above-ground biomass (the percentage of land farmed within the agricultural climate envelope) and one-tenth of below-ground biomass (precisely, 20% of half of the land) within the agricultural climate envelope (Cramer and Solomon, 1993; Solomon et al., 1993). We also projected absence of tree migration during the expected 55 years (e.g. Murphy and Mitchell, 1996) to 70 years (Manabe and Wetherald, 1993) required to reach the climate of a CO 2 doubling. The time period in question appears logically to be inadequate for immigration, establishment and development of new, closed-canopy forests, an observation in agreement with recent evidence from mountain slopes (H~ittenschwiler and K~rner, 1995). Should such forests somehow appear (e.g. through artificial migration and establishment), they would not be detected on the 50-km grid units modeled (Solomon, 1996b), based on forest migration rates measured by paleoecological data (10-40 km per 50 years; Davis, 1983; Gear and Huntley, 1991). The simulation assumed that climate of a doubled CO 2 world, which is inappropriate at any given location for tree PFTs there, eliminates the PFFs by the time wanning takes place, but does not induce the presence of any newly appropriate tree PFTs by that time. We made no attempt to define the negative effects on terrestrial carbon and growing stocks hypothesized from slowed growth and succession induced by chronic climate changes (Solomon, 1986; Solomon and Leemans, 1990) nor the positive effects hypothesized from terrestrial carbon fertilization induced by increased atmospheric CO 2 concentrations (Strain and Cure, 1985; Melillo et al., 1993).

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Separating the signal of change from the high noise of today's environmental variance in boreal regions requires long environmental records, of which few exist. Therefore, prehistoric and recent historical environment data may be inordinately valuable in defining the nature of future stability and change in high latitudes. Our analysis included examination of last glacial maximum (LGM) variations in biome geography based on Prentice et al. (1993b). They simulated vegetation of 21 Ka (18 K 14C year) by employing past climate conditions from the Hamburg GCM (Lautenschlager and Herterich, 1990) and boundary conditions including ocean surface temperatures, land, ocean and ice-sheet geography from CLIMAP Project Members (1976).

3. Results 3.1. Modern climate-dictatedbiome distributions and biomass

The simulated distribution of northern hemisphere biomes dictated by modern climate is shown in Fig. l(a). The polar projection used illustrates a maximum amount of boreal terrestrial area (other hemispheric or global projections show proportionately much more ocean surface). This projection sacrifices little information from the unmapped southern hemisphere which contains no cool or cold evergreen forests, although mixed deciduous and evergreen forests dominate in the vicinity of Tierra del Fuego, the highest latitude land mass in the southern hemisphere, The location and extent of modeled areas compare favorably to those estimated or measured on the ground (Fig. l(b)). Simulated closed boreal forest biomes cover 17.3 million km 2 (Table l(b)). Olson et al. (1983) who appraised the vegetation and land uses actually present, estimated 18 million km 2. This

included 3 million km 2 of agricultural land in the boreal regions, most of which is derived from previously forested land. About 2 / 3 of the simulated closed forest (10.7 million km 2) is cold mixed forest, middle and southern taiga (cf. 7.2 million km 2 by Olson et al., 1983, who locate most of the boreal agricultural land there). Another large portion of the simulated forest area (6 million km 2) is about equally divided between cold deciduous forest and northern taiga with a small amount of cold mixed forest. These biomes are largely subsumed in the Olson et al. (1983) "northern and maritime taiga" class (4.35 million km2). Wooded tundra (open parkland) occupies about 4.8 million km 2 in simulations, versus 2.6 million km 2 expected from land cover observations (including 0.9 million km 2 of cold mires tabulated by Olson et al., 1983). Simulated tundra and polar desert cover about 11 million km 2 compared to 13 million km 2 estimated by Olson et al. (1983). The spatial distributions of the boreal biomes (Fig. l(a)) produce a clear latitudinal zonation, suggesting the primary importance of warmth to the vegetation classes. The secondary but significant role of maritime versus continental influences is also highly visible in comparisons of coastal and inland biome distributions. The longer growing seasons along seacoasts permit biomes to reach higher latitudes while the more intense winter cold of continental interiors limits the northern distribution of boreal evergreens, most obviously in northcentral Siberia, where only boreal deciduous species are expected to grow. The measured distribution of biomes (Fig. l(b)) mapped by Olson et al. (1983) suggests that the model of spatial distributions is quite valid. The irregularities in geography of boreal forest biomes (southern, middle and northern taiga, cold deciduous forest, cold mixed forest; Fig. l(b)) are replicated very well by the simulated climate-dictated distribution of biomes (Fig. l(a)). The northern limit of

Fig. 1. (a) Polar view of global biomes and land use simulated by the BIOME 1.1 Model (Prentice et al., 1992, 1993a), modified to include Agriculture (Cramer and Solomon, 1993), using modem climate from the CLIMATE data base (Leemans and Cramer, 1991). Boreal biomes include the last 12 biomes in the legend, minus "semi-desert" and "cool grass and shrubs." (b) Polar view of global biomes and land use estimated by Olson et al. (1983), with biome groupings by Prentice et al. (1992, Table 4) and modified by Prentice et al. (1993b). Note the large amount of land dominated by agriculture. Land areas without color contain only substrate-controlled vegetation, primarily bogs and mires.

A.M. Solomon, R. Leemans / Agricultural and Forest Meteorology 84 (1997) 137-151

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A.M. Solomon, R. Leemans / Agricultural and Forest Meteorology 84 (1997) 137-151

Table 1 Boreal biome areas in millions of km 2 under past, present and three future climates Biomes

Full glacial

Present

Instant migration

No migration

ECHAM

GFDL89

UKTR

ECHAM

GFDL89

UKTR

(a) No agriculture Polar desert Non-wooded tundra Wooded tundra Cold deciduous forest Cold mixed forest Northern taiga Middle taiga Southern taiga

20.9 10.4 2.2 1.3 0.4 1.4 3.5 0.6

3.4 7.4 2.6 3.7 0.7 2.3 10.2 3.0

3.2 5.5 2.0 3.6 0.5 2.1 10.5 3.6

2.6 4.2 1.4 3.7 0.7 1.4 10.1 4.1

2.1 3.0 1.4 2.7 0.6 1.4 10.6 3.8

3.6 5.2 4.0 4.0 0.7 2.2 9.2 2.5

3.4 3.4 3.2 4.2 0.8 2.3 9.5 2.7

3.4 1.8 3.1 4.2 1.0 2.2 9.3 2.1

Boreal forested area Boreal non-forested area Boreal vegetation area

7.2 33.5 40.7

19.9 13.4 33.3

20.2 10.7 31.0

20.0 8.1 28.1

19.1 6.6 25.7

18.7 12.8 31.4

19.4 10.1 29.5

18.9 8.4 27.3

3.4 7.6 4.8 2.9 3.0 3.1 5.0 2.9

3.2 5.5 2.0 3.3 0.3 2.1 9.6 2.1

2.4 3.6 1.4 2.8 0.4 1.4 8.4 2.5

2.1 3.0 1.4 2.3 0.3 1.4 8.6 2.3

3.6 5.2 4.0 3.6 0.3 2.2 7.8 1.3

3.4 3.4 3.2 3.3 0.4 2.2 6.1 1.4

3.4 1.8 3.1 3.1 0.5 2.0 5.3 1.1

16.9 15.8 32.7

17.3 10.7 28.1

15.4 7.4 22.8

15.0 6.6 21.5

15.3 12.8 28.1

13.4 10.1 23.5

12.0 8.4 20.4

(b) Agriculture Polar desert Non-wooded tundra Wooded tundra Cold deciduous forest Cold mixed forest Northern taiga Middle taiga Southern taiga Boreal forested area Boreal non-forested area Boreal vegetation area

Full glacial climate is from Lautenschlager and Herterich (1990). Present climate is from Leemans and Cramer (1991). ECHAM scenario is from Cubasch et al. (1992). GFDL89 scenario is from Manabe et al. (1991). UKTR scenario is from Murphy and Mitchell (1996).

closed forest and the distribution of wooded tundra (Fig. l(b)) are particularly well replicated by the simulations (Fig. l(a)), 3.2. LGM climate-dictated biome distributions and biomass

The mapped distribution of the simulated boreal biomes during the LGM is distinctly different (Fig. 2(a)) from either modem distribution (Fig. l(a) and (b)). Most prominent is the large area covered by continental ice sheets. However, the LGM world also contained a boreal forest which was strongly fragmented into several regional forests (eastern North America, Alaska Peninsula, eastern Asia, western Siberia, eastern Europe, southwestern Europe) in

contrast to its almost continuous circumpolar nature today. In North America, competition with other vegetation or water and ice barriers defined southern boundaries of boreal forests. In Asia and Europe, drought defined southern boundaries. Considering that full glacial conditions have prevailed for about 90% of the last 2 million years (Davis, 1983), the apparent lack of boreal tree speciation within these genetically isolated fragments is somewhat unexpected (e.g. Solomon and Tharp, 1985). Although it may be counter-intuitive, the LGM boreal forests are simulated to cover only about a third of the area that they do today (Table l(a)). Southern and middle taiga were reduced the most severely, but cold deciduous forest also was strongly affected. The location and reduced area of LGM

143

A.M. Solomon, R. Leemans/ Agricultural and Forest Meteorology 84 (1997) 137-151

boreal forest is confirmed in the few places fossil pollen evidence is awailable (Prentice et al., 1993a). Biomass storage (above and below ground) in boreal forests of the L G M was also much smaller than in modern boreal forests (Table 2(a)). Note that the category " p o l a r d e s e r t / i c e " is excluded from consideration as it is assumed to possess no organic carbon. Although carbon in all boreal forest biomes increased by modern times, greatest differences in carbon storage by modern times were in southern taiga, which increased over five-fold, and middle taiga, which increased almost three-fold. The nonforested biomes, in contrast, decreased considerably in area and carbon storage, with only wooded tundra undergoing increase, 3.3. Future climate-dictated biome distributions and biomass

An important difference between prehistoric environmental changes and those of the future is the

expected rapid rate of future climate change. Presently, natural tree population migrations are thought to have been constrained by relatively slow rates o f postglacial warming (Prentice et al., 1993b) despite earlier hypotheses to the contrary (Davis, 1983). In contrast, future climate changes are expected to be at least an order of magnitude more rapid than the ability o f tree species to migrate (Solomon et al., 1984; Davis and Zabinski, 1992), leaving populations free to migrate at their intrinsically m a x i m u m rates. The inability of tree populations to ' k e e p up' with future climate changes could produce a temporary absence of tree species from regions in which they are otherwise appropriate. This interval between loss of local tree species by imposition of inappropriate climate and establishment and spread of new tree species, has been termed the "transient response of forests" (Solomon, 1986; Solomon and Bartlein, 1992) to climate change. During the 70 or so years expected to pass before climate characteristic of a

Table 2 Boreal biome carbon storage in petagrams under past, present and three future climates Biomes

Full glacial

Present

Instantmigration

No migration

ECHAM

GFDL89

UKTR

ECHAM

GFDL89

UKTR

140 49 66 10 42 258 94

98 36 46 8 40 306 98

90 26 64 11 28 269 132

101 33 63 7 33 254 119

91 78 69 11 41 248 81

77 63 73 12 40 244 86

91 71 67 10 41 250 88

470 188 658

499 134 632

504 116 621

477 134 611

451 169 619

455 140 596

457 163 620

(b) Agriculture Tundra Wooded tunda Cold deciduous forest Cold mixed forest Northern taiga Middle taiga Southern taiga

140 49 65 10 42 251 75

105 38 62 7 37 254 84

79 25 64 10 26 235 97

58 26 45 9 25 244 90

98 75 69 9 40 217 55

65 60 68 11 40 199 58

35 58 67 15 38 187 47

Boreal forests Boreal non-forest biomes Total boreal biomes

442 188 630

444 143 587

431 105 536

414 84 498

390 173 563

377 125 502

353 93 447

(a) No agriculture Tundra Wooded tundra Cold deciduous forest Cold mixed forest Northern taiga Middle taiga Southern taiga

197 42 23 6 26 89 18

Boreal forests Boreal non-forest biomes Total boreal biomes

161 239 400

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A.M. Solomon, R. Leemans / Agricultural and Forest Meteorology 84 (1997) 137-151

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A.M. Solomon, R. Leemans / Agricultural and Forest Meteorology 84 (1997) 137-151

doubled GHG concentration is established, it is unlikely that tree populations can migrate, become established and then form mature forests across the 50-km spatial units simulated by the vegetation models. Hence, a logical modeling assumption is that trees do not migrate at all between current and doubled-GHG climate, although other plant forms (shrubs, annual and perennial herbs) which can complete a life cycle in a year or two would not be affected. This assumption was implemented in the BIOME 1.1 model (Solomon, 1996b). Areas and biomass values were calculated for all three scenarios with and without the assumption of instant tree migration (Table l(a) and Table 2(a)). We illustrate the results of only one climate scenario (Fig. 2(b) and Fig. 3(a) and (b)). The GFDL89 scenario was selected both because it is intermediate between the other two scenarios, and because its predecessors are the most common GCM output used in assessments by other authors, With instant migration, areas covered by boreal non-forest and by all boreal vegetation decline from modern values while forested areas change little under all three future climate scenarios (Table l(a)). Suppression of arboreal migration increases nonforest area and decreases forested lands under all climate scenarios (Table l(a)) but it does not change the relationships ameng effects by the three climate scenarios: ECHAM-1 affects areal distributions least, UKTR affects distributions most and GFDL89 is intermediate between the two. Biomass stored above and below ground in global boreal vegetation declines from modern values in all three scenarios (Table 2(a)). Like the difference between the LGM and the modern warmed world, the modern and future warmed world differ because biomass dwindles on non-forested boreal land, with tundra declining by 28-36% under instant migration, In contrast, boreal forest biomass increases over current values under instant migration in all three scenarios (1-7%), a result also consistent with the

145

LGM-modern comparison in which total forest area increased with warming. However, should trees not migrate during the future warming, biomass in both forested and non-forested vegetation would decline, a result at odds with the expectation of a carbon sink in the boreal latitudes (e.g. Denning et al., 1995). The difference in non-transient biome distribution between current and future climate (Fig. l(a) versus Fig. 2(b)) is also of interest. Southern and middle taiga are replaced by more southerly biomes (cool mixed and temperate deciduous forests) as one would expect from a general warming. The climate difference results in extirpation of boreal forests from most of northern Europe and central Alaska. In addition, the forest biomes again become very fragmented, much as they did during the LGM (Fig. l(b)), but here the fragments cover considerably more area than during the LGM. Such a fragmentation, continued over a long enough period of time, could considerably alter genetic interchange among populations of trees. Fragmentation combined with the great simulated loss of tundra and rock desert, also could have a myriad of secondary consequences. For example, it could considerably reduce migratory bird populations which depend upon these biomes for nesting and disrupt seasonal migrations of large mammals. When tree migration was suppressed, the pattern changed considerably. Instead of increasing, forest biome carbon storage decreased under all three climate scenarios (Table 2(a)). Trees were eliminated from some areas and others, capable of growing trees, only supported the available grasses and shrubs. As a result, boreal forests were reduced by 3 to 4% instead of increasing by 1 to 7% as under instant migration. Non-forested areas also declined under future climate scenarios, but the decline was much less severe than under instant-migration conditions. Total carbon storage under ECHAM-1 and GFDL89 scenarios was less when migration was suppressed than when it occurred instantaneously. Exception-

Fig. 2. (a) Polar view of global biomes under climate of the last glacial maximum (21 ka ago) based on climate simulations by Lautenschlager and Herterich (1990) and resulting biome distributions by Prentice et al. (1993b). Much of the large area devoted to "ice/polar desert" was covered by continental ice sheets. (b) Polar view of global biomes under GFDL89 climate with instant migration of all plant functional types and without agriculture.

A.M. Solomon, R. Leemans / Agricultural and Forest Meteorology 84 (1997) 137-151

146

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ally, total biomass was greater in this situation under the UKTR scenario, entirely from greatly enhanced wooded tundra biome biomass (Table l(a)), The difference in forest geography between the instant migration and suppressed migration simulations is considerable (Fig. 3(b)). Compared with instant migration simulations, much of the boreal forest is affected under suppressed arboreal migration. However, the difference in area and amount of carbon in the boreal biomes is less impressive (Table l(a) and Table 2(a)). This occurs because the absence of invasion and loss of southern and middle taiga by temperate forest biomes is balanced by absence of poleward migration and boreal forest expansion onto wooded tundra and tundra, 3.4. Future climate-dictated biome distributions and biomass as modified by non-migration of tree populations and agricultural land use

Another important difference between prehistoric global environmental changes and future ones is that the latter could be accompanied by extensive and intensive land use. Cultivation in particular exerts considerable influence on terrestrial carbon storage capacity. Above-ground biomass of natural vegetation is removed entirely, and below-ground biomass declines considerably from erosion (Eswaran et al., 1993) and from oxidation of soil carbon compounds after cultivation exposes them to the atmosphere. A loss of carbon of 20% for all soils was assumed, based upon the work: of Mann (1986) who analyzed a large globally comprehensive soils data base. Agriculture in boreal regions currently is limited by short growing seasons which are expected to expand significantly under future warming scenarios. The mapped distribution of non-irrigated agriculture permitted by modern climate is superimposed on the modern biomes (Fig. l(a)). Compare this potential agriculture to actual agricultural land (Fig. l(b)) as mapped by Olson et al. (1983). The distributions

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coincide quite well as they should: Cramer and Solomon (1993) used the distribution of agricultural land from Olson et al. (1983) as a template for selecting the matching climate variables hypothesized to limit agriculture. Temperate and southern boreal regions of North America and Europe match very well, whereas potential agriculture in eastern and southeastern Asia and India considerably exceeds that measured. These anomalies may be due to the sparse climate records available (Leemans and Cramer, 1991) to define potential agriculture in the latter, less developed regions. Within this 'agriculture climate envelope' all land is not farmed. Extensive areas may be unsuitable by virtue of slope or substrate (e.g. montane areas), or suitable but unneeded by the population (e.g. temperate industrialized areas), or, populated too sparsely to be farmed (e.g. tropical areas of Latin America). On average, these climatically suitable but unused agricultural areas presently constitute about 1//2 of the land (49.44%) within the agricultural areas defined by Olson et al. (1983). We applied this multiplier to define presence of natural vegetation and its biomass in all biomes which appeared within the agriculture envelope, although we are aware that the proportion of land farmed varies considerably from region to region. The areal extent of the agriculture climate envelope covers 2.9 million km 2 (present climate, Table l(a) minus present climate, Table 2(a)) and increases under the future climate scenarios. Large areas of current middle and southern taiga in North America, Europe and Asia become suitable for non-irrigated agriculture, including a large portio n of Alaska and most of Scandinavia and central and southern Siberia. The magnitude of this result seems counter-intuitive until one considers the great increases in growing season length which result from the large amount of warming in winter. For example, an additional 3 weeks of above-freezing weather at the beginning and end of a 3-month growing season (e.g. Manabe

Fig. 3. (a) Polar view of global biomes under GFDL89 climate, without migration of arboreal plant functional types and without agriculture. The new biome "'depauperate temperate deciduous forest" results from loss in temperate deciduous forest of higher latitude plant functional types, and constitutes a new forest association not found on modem landscapes (Solomon, 1996b). (b) Polar view of difference map between instant arboreal migration and no arboreal migration, both under GFDL89 climate and both without agriculture. Biomes shown are those which would result from the warming without tree migration.

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and Wetherald, 1993) increases the growing season by 50%. The area potentially under cultivation constitutes up to 4.6 million km 2 (GFDL89 scenario) of the areas capable of growing trees (cf. total forest area, Table l(a) and Table 2(a)), when trees are free to migrate. In the absence of tree migration, up to 7 million km 2 (UKTR scenario) of boreal forest land could be cultivated (i.e. over a third of current boreal forest). As a result, simulated future boreal forests decline much more precipitously when climate change is examined with both non-migrating tree populations and land use change than when climate change is considered alone (cf. Table l(a) and (b)). This is particularly true when the focus is on carbon storage because high carbon-density forested land is also the most climatically suitable for cultivation (Cramer and Solomon, 1993). Hence, carbon storage in southern and middle taiga, in which about 3 / 4 of the boreal forest biomass is currently stored, increases under all climate scenarios in which trees are free to migrate (Table 2(a) and (b)). In contrast, it uniformly declined in the absence of migration: by 4 - 6 % in the absence of agriculture (Table 2(a)) and by 17-28% in the presence of agriculture. The latter represents a loss of 54 to 92 Pg of carbon over the space of about 70 simulated years,

4. Discussion The foregoing results describe a future boreal region in which forests either will behave as they did during prehistory or will behave very differently; either will cover more area or less than they do today; and either will sequester more carbon or less carbon than they do today. Each outcome depends upon the assumptions made in the assessment, and on the way in which the assumptions are modeled, The simplest assumption modeled is that climate and other physical environmental variables immediately induce redistribution of species and carbon stocks of all global vegetation (e.g. Emanuel et al., 1985; Leemans, 1989; Sedjo and Solomon, 1989; Smith and Shugart, 1993). This assumption generates boreal tree increases in area (7-9%) and biomass (6-15%) and concomitant declines in non-arboreal, high-latitude vegetation area (20-49%) and biomass

(29-62%). These changes are qualitatively no different from those detected during the last great global warming beginning after the last glacial maximum some 21 000 years ago (Webb and Bartlein, 1992). Models can invoke the more realistic assumption that forest tree populations cannot migrate, establish and develop into mature forests, in the 70-80 years by which future climate-warming scenarios are expected to be in place (Solomon, 1996b). In this case, boreal forest biomes decline some 4-14% in area and 4-6% in carbon stocks. With parallel declines in non-arboreal vegetation, these boreal shifts are qualitatively distinct from those of prehistoric warming as well as from those predicted in the future on the basis of GCM simulations. Yet, the assumption of no tree migration at all, coupled with immediate mortality of currently vulnerable trees, is itself an oversimplification. It excludes the more probable complexity of slow and irregular tree dieback over the next few decades to a century. Use of more realistic dynamic models described in the introduction will provide an important improvement in validity of results based on the no-migration model. The models also become more realistic when they include the assumption that humans now and in the future modify global carbon stocks by cultivating primarily forests (Cramer and Solomon, 1993; Solomon et al., 1993; Solomon, 1996a). Even assuming that forest tree populations instantaneously migrate and mature under new climates, carbon storage either increases less than 1% (ECHAM-1), or, declines 9-12%. If the more realistic assumption is added to exclude tree migration and forest maturation, agriculture and climate change together produce a decline in forest (and total boreal vegetation) biomass of about 10-20%. The agriculture assumption may be the best currently available, yet, may still not be very reliable. It requires the further assumption that agriculture will continue to cover about half of the climatically appropriate lands. Currently, many areas in temperate regions are 90-99% cultivated, while in many boreal and tropical areas, agriculture occupies much less than half the land, and much less land than it could use. In the future, when climate of a GHG doubling occurs, food requirements for a global population three times its current size will probably result in much higher cultivation densities in currently under-

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utilized areas. Despite these obvious flaws, the simulation of cultivation effects on global carbon stocks now, and on boreal carbon stocks in the future, is

probably much more realistic than simulations which lack land use considerations,

The implications of the biome simulations that exclude tree species migration and include agriculture are important. If one equates growing stock volume changes with biomass changes, the simulations suggest as much as a 20% decline in growing stock. This decline from inadvertent effects of elimate and land use change is independent of decreases in growing stock induced by increased harvest to support the demands of the increasing human

population. In addition, the stone simulations suggest a seriOUS difference in carbon storage and flux from that

hypothesized by carbon modelers (Tans et al., 1990; Denning et al., 1995). The 1 to 2.5 Pg year -1 which they propose is being taken up by a high latitude carbon sink is the same magnitude but opposite in sign to the 1-2.6 Pg year -1 ( 6 7 - 1 8 3 Pg per 70 years) of carbon projected to be released from all

boreal biomes under combined warming, land use and stable tree geography. With the documented warming already underway in boreal regions (e.g. Gullett and Skinner, 1992), it would be illogical to believe the shift in carbon storage is not yet underway. Although recent accounting suggests the boreal

biomes are currently a sink for atmospheric carbon (Kolchugina and Vinson, 1993; Kurz and Apps, 1993), the current simulations suggest that boreal biomes soon will be a source of atmospheric carbon which will only intensify with time.

Acknowledgements The manuscript was completed while the senior author was a guest of the Potsdam Institute for

Climate Impacts Research, Potsdam Germany. The research was supported by the US Environmental Protection Agency (USEPA) and the Dutch National Institute of Public Health and Environmental Protection (RIVM). The manuscript has been subject to the

USEPA's review and clearance procedure and has been approved for publication.

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