Although temperature-controlled vegetation boundaries shift predictably in
all cases, water-controlled boundaries could shift any direction, reflecting
either more or less beneficial water status. Likewise, vegetation change does
not simply consist of shifts in the boundaries between homogeneous blocks of
vegetation. Indeed, changes in vegetation density (via leaf area index, LAI)
may often be more informative, since changes in LAI in water-limited areas generally
indicate a change in the site water status and carrying capacity (Tables
C-2, C-3; Figures C-6,
C-7, C-8 and C-9).
The change in LAI could also be taken as an indication of what could happen
in the near term, since changes in LAI can occur in a matter of a few years
while adjustments of vegetation structure and composition take much longer.
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| Figure C-8: The potential change in vegetation leaf area index (LAI), which can be considered as an index of vegetation density or biomass, as simulated under the HADCM2SUL GCM experiment (Hadley Center, 2 x CO2 greenhouse gas radiative forcing, extracted from transient simulation, plus sulfate aerosols), by (a) MAPSS and (b) BIOME3. Both models have incorporated a direct, physiological CO2 effect. This figure is a companion to Figure C-4. |
 |
| Figure C-9: The potential change in vegetation leaf area index (LAI), which can be considered as an index of vegetation density or biomass, as simulated under the HADCM2SUL GCM experiment (Hadley Center, 2 x CO2 greenhouse gas radiative forcing, extracted from transient simulation, plus sulfate aerosols), by (a) MAPSS and (b) BIOME3. A direct, physiological CO2 effect is not incorporated in either model. This figure is a companion to Figure C-5. |
MAPSS and BIOME3 produce generally similar maps of change in LAI when forced
by the same scenario, except that MAPSS produces a consistently stronger drought
effect (compare Figures C-6 and C-7
and Figures C-8 and C-9).
For example, within the U.S., when not including a direct CO2 effect (Figure
C-9), both models indicate increases in LAI in the SouthWest and either
an increase or no change in LAI over most of the eastern U.S. Both models simulate
a decline or no change (BIOME3) over much of the western U.S. (excluding the
SW). Both models produce increases in LAI over parts of the Sahara/Sahel, either
with or without the direct CO2 effect under all scenarios, both old and new.
Likewise, both models under all scenarios indicate some increases in LAI over
much of the arid interior of Australia. In general, there appear to be increases
in LAI in already low LAI regions, either arid or cold. The increases in cold
regions are due to expansion of forests into non-forested areas. The increases
in arid areas are due to increased rainfall, a consequence of a generally more
vigorous hydrologic cycle. There are many other consistencies between the two
biogeography models with respect to the relative regional or subregional simulated
LAI changes. A more complete discussion of simulated LAI patterns from the VEMAP
models over the conterminous U.S. is in preparation.
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