Geoscience Reference
In-Depth Information
may be wet due to rainfall interception or dew. This water can evaporate directly,
without stomatal control.
LSMs Based on Plant Physiology (Third Generation)
In the 1990s the advance in plant-physiological knowledge made its way to land-
surface models. Because CO
2
follows - as far as assimilation is concerned - the
same pathway as water vapour a sound mechanistic description of the assimila-
tion processes helps to properly describe transpiration. In particular, the factors
that limit assimilation will limit transpiration as well. Hence, the plant-physiology
approach affects the modelling of the canopy resistance in particular (see
Section
9.2.4
). The advantage of the plant-physiology as compared to the purely empiri-
cal approach is that less empiricism is involved and the number of parameters is
smaller.
In some models, the simulated assimilation (as needed in the modelling of
r
c
) is
used to assign the ixed carbon to particular parts of the plants. In this way some of
the plant-parameters (such as leaf area index) used in LSMs are not prescribed but
predicted by the model itself.
LSMs with Adaptive Vegetation (Fourth Generation)
On the time scale of numerical weather prediction the vegetation (cover and type) can
be assumed to be nearly invariable. However, for long-term simulations with climate
models (or Earth system models in general) the changes in the vegetation need to be
taken into account. For simulations of the current climate, the known seasonal varia-
tion of the vegetation cover can be prescribed. But for simulations of future develop-
ment the vegetation needs to be allowed to respond to a changing simulated climate
(including changing CO
2
concentrations and temperature).
Two concepts are used to model the adaptation of vegetation to changing envi-
ronmental conditions (Levis,
2010
). The irst group of models are so-called equi-
librium vegetation models (EVMs). In these models the occurrence of a certain
biome (combinations of plant types) at a certain location is determined by the local
climate (among others precipitation, temperature, CO
2
concentration) and possi-
bly the soil type. Some EVMs use plant-functional types rather than predeined
combinations of plants. A plant-functional type represents a broad class of plants
that has distinct characteristics (e.g., physiologically and morphologically) from
the other classes: for example, grasses versus tropical rain forest trees. The use of
plant-functional types allows the model to compose its own biomes, depending on
the climate conditions.
The second group of models are dynamic (global) vegetation models (DGVMs).
In these model not only the relationship between local conditions (climate and soil)
are taken into account, but vegetation dynamics as well: for example, succession
between different plant types, competition, disturbances in the form of wild ire.
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