Modelling Plant Ecology - Environmental Modelling: Finding Simplicity in Complexity - page 201

Z *

Figure 12.2 Representation of the plasticity (angled trunks and flexible canopy shape) represented in the perfect plasticity
assumption of Purves et al . 2007. Z* refers to the canopy height where the bottoms of the canopies intercept, a feature which is
spatially uniform in this manifestation of the model. Trees taller than Z* attain canopy status, but trees shorter than Z* are in the
under storey, leading to a monotonic relationship between growth rate and light availability (Reproduced with permission from
Purves et al . 2007 Plos One. Permission pending Purves, D., Lichstein, J.W. and Pacala, S.W. (2007) Crown plasticity and competition
for canopy space: a new spatially implicit model parameterized for 250 North American tree species. PLoS-One , 2, e870).

by Purves et al . (2008) but has not yet been adopted as a
generic simplification of gap models.

conditions can be successfully estimated from photo-
synthetic capacity parameters measured under controlled
conditions, and from estimated internal leaf CO 2 concen-
tration, as discussed in the next section. Again, this model
uses an 'optimality' assumption - that plants instanta-
neously maximize their photosynthetic rate to match the
limiting conditions - thus removing the need to model
the biochemical processes themselves in any great detail.
When stomata open to acquire carbon dioxide to
assimilate into sugars via photosynthesis, because the
inside of leaves is typically saturated by water vapour, and
the atmosphere is not, it is inevitable that some water is
lost for each molecule of carbon gained. Given this fact,
plants must avoid desiccation to maintain fundamental
biochemical processes, and so they both construct systems
to transport water from soil to leaves, and assert control
over the aperture of the stomata so that desiccation does
not become critical.
The rate of evaporation of water from leaf surfaces is
predictable from basic physical processes. Models of evap-
oration assume that the internal atmosphere of the leaf is
saturated, and then use the incoming solar energy and the
vapour pressure deficit between the leaf and the atmo-
sphere to predict evaporation rate from basic physical
principles, as derived by Monteith and Unsworth (1990).
Vegetation exerts control over the rate of evaporation
via the resistance to the diffusion of gas flow through
the stomatal pores between the inside and outside of the
leaf membrane. However, no comprehensive framework
for estimating stomatal conductance under a given set of
environmental conditions yet exists (Buckley, 2005). Most

Soil-vegetation-atmosphere-transfer models
A second set of models, which are more focused on
questions of the carbon and energy exchange of the land
surface rather than ecological processes, are SVAT (soil-
vegetation-atmosphere-transfer) models. Whereas the
ecophysiological processes used in gap models have tradi-
tionally been less mechanistically detailed, SVAT models
represent a vegetation canopy as a one-dimensional pro-
file of leaf area with no consideration of individual trees
(Sellers, 1986).
SVAT models focus particularly on carbon and water
exchange by leaves. The carbon-assimilation rate via pho-
tosynthesis, given a knowledge of the assimilation capacity
for light and carbon dioxide, the temperature, light con-
ditions, and internal leaf CO 2 , is typically resolved using
the Farquhar model of photosynthesis (Farquhar, 1980,
2001; Prentice et al ., 2007), which simulates the well-
documented observation that photosynthesis is limited
either by the rate of light absorption by photosynthetic
pigments (to produce the energy 'storage' molecule ATP)
or by the rate of use of ATP in the Calvin Cycle to create
sugar compounds from CO 2 and water. The former is
limited by temperature and the abundance of light and
chlorophyll pigment and the latter is principally limited
by abundance of the catalyst enzyme rubisco, temper-
ature and internal leaf CO 2 concentration. Thus, the
rate of carbon assimilation under variable environmental