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Forest gains as well as forest dieback and decline are projected, with regional differences in the expected response.
Biogeography models-including a direct physiological CO2 effect under three
of the IPCC's First Assessment Report (FAR) 2xCO2 equilibrium GCM scenarios
(Annex
B)-simulate forest area gains of up to 20% over
the conterminous United States under the cooler (least warming) or wetter scenarios
and forest area losses of as much as 14% under the hottest scenario (VEMAP Members,
1995). The models produced similar forest redistribution patterns, including
some conversion of northWest conifers to broadleaf deciduous under potential
future equilibrium climates. The models have equal skill in simulating potential
natural forest distribution under the present climate; although they diverge
to some extent under future climates, they produce similar spatial responses
and likely bound the range of forest responses to global warming. Extending
these results with the FAR scenarios from the conterminous United States to
all of North America using one of the biogeography models indicates that total
forest area could increase as much as 32%-but that regions of forest decline
or dieback (partial or total loss of trees) could range from 19-96% of the area
of any individual forest type (Tables 8-2 and 8-3).
Under hotter scenarios, forests die back from large areas of the conterminous
United States (expressing declines in U.S. forest area) but expand into northern
Canada and Alaska-so that total North American potential forest area actually
increases. These simulated potential forest distributions do not include current
or possible future land-use patterns, which will affect actual forest distribution.
The models also do not simulate the differential rates of dieback and migration,
which could produce near-term losses in total forest area in North America and
a large pulse of carbon to the atmosphere.
| Table 8-2: Potential future forest area (percentage of current) in North America simulated by the MAPSS and BIOME3 biogeography models under three older (IPCC 1990, WG I) equilibrium 2xCO2 GCM scenarios and under three newer (IPCC 1996, WG I) transient simulations from which 2xCO2 scenarios were extracted (Annex C). The reported ranges include both ecological models under several GCM scenarios. The baseline area estimates are outputs from each model. Because BIOME3 does not differentiate Taiga/Tundra from Boreal Forest, two different aggregations are presented. The Boreal Conifer and Total Forest summaries are MAPSS data only; the "Boreal + Taiga/Tundra" and "Total Forest + Taiga/Tundra" summaries are from both models. Numbers in parentheses are VEMAP results for the conterminous U.S. only, indicating some scenarios with losses in forest area over the U.S., and are based on MAPSS and BIOME2 output (VEMAP Members, 1995). | |||||
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Baseline Area (Mha)
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With CO2 Effect
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Without CO2 Effect
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| Forest Type |
MAPSS
|
BIOME3
|
FAR Scenarios
|
SAR Scenarios
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SAR Scenarios
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|
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| Boreal Forest + Taiga/Tundra |
594
|
620
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65-105%
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64-87%
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28-86%
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| Boreal Conifer Forest |
295
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87-150%
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115-116%
|
110-112%
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|
| Temperate Evergreen Forest |
127 (82)
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110 (86)
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130-180% (58-157%)
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78-182%
|
82-129%
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| Temperate Mixed Forest |
297 (245)
|
383 (260)
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107-141% (88-116%)
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146-198%
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129-159%
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| Total Temperate Forest |
424
|
493
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114-153%
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137-171%
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121-142%
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| Total Forest + Taiga/Tundra |
1,019
|
1,113
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102-116%
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107-118%
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99-105%
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| Total Forest |
719 (327)
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(346)
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125-132% (86-123%)
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142-144%
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121-124%
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Note: FAR = First Assessment Report (IPCC 1990, WG I); SAR = Second Assessment Report (IPCC 1996, WG I). Sources: Mitchell and Warrilow, 1987; Schlesinger et al., 1989; IPCC, 1990; Bengtsson et al., 1995; Mitchell et al., 1995; Bengtsson et al., 1996; IPCC, 1996, WG I, Chapters 5 and 6; Johns et al., 1997. |
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| Table 8-3: Percentage area of current forests that could undergo a loss of leaf area (i.e., biomass decrease) as a consequence of global warming under various older (IPCC 1990, WG I) and newer (IPCC 1996, WG II) GCM scenarios, with or without direct CO2 effect (see Table 8-2 for details), as simulated by the MAPSS and BIOME3 biogeography models (ranges include both models). Losses in leaf area generally indicate a less favorable water balance (drought). | |||
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With CO2 Effect
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Without CO2 Effect
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| Forest Type |
FAR Scenarios
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SAR Scenarios
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SAR Scenarios
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| Boreal Forest + Taiga/Tundra | 19-40% | 0-9% | 4-45% |
| Boreal Conifer Forest | 37-80% | 14-19% | 79-89% |
| Temperate Evergreen Forest | 20-70% | 2-14% | 41-69% |
| Temperate Mixed Forest | 42-96% | 0-7% | 12-76% |
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| Sources: Mitchell and Warrilow, 1987; Schlesinger et al., 1989; IPCC, 1990; Bengtsson et al., 1995; Mitchell et al., 1995; Bengtsson et al., 1996; IPCC, 1996, WG I, Chapters 5 and 6; Johns et al., 1997. | |||
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