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plants are very effective at shutting stomata when
under water stress, and therefore limit their water
usage. The water stress occurs when the vapour
pressure deficit is high and there is a high evapo-
rative demand. In this situation the stomata within
a leaf can be likened to a straw. When you suck hard
on a soft straw it creates a pressure differential
between the inside and outside and the sides collapse
in; therefore you cannot draw air easily through the
straw. Stomata can act in a similar manner so that
when the evaporative demand is high (sucking water
vapour through the stomata) the stomata close down
and the transpiration rate decreases. Some plant
species shut their stomata when under evaporative
stress (e.g. conifers) while others continue trans-
piring at high rates when the evaporative demand
is high (e.g. many pasture species). The ability
of plants to shut their stomata can influence the
overall water budget as their overall evaporation is
low. This is illustrated in the case study later in this
chapter on using a lysimeter to measure tussock
evaporation.
It is the role of interception loss (wet leaf evap-
oration) that makes afforested areas greater users of
water than pasture land (see Case Study on p. 42).
This is because the transpiration rates are similar
between pasture and forest but the interception
loss is far greater from a forested area. There are
two influences on the amount of interception loss
from a particular site: canopy structure and
meteorology.
Canopy structural factors include the storage
capacity, the drainage characteristics of the canopy
and the aerodynamic roughness of the canopy. The
morphology of leaf and bark on a tree are important
factors in controlling how quickly water drains
towards the soil. If leaves are pointed upwards then
there tends to be a rapid drainage of water towards
the stem. Sometimes this appears as an evolutionary
strategy by a plant in order to harvest as much
water as possible (e.g. rhubarb and gunnera plants).
Large broadleaved plants, such as oak ( Quercus ), tend
to hold water well on their leaves while needled
plants can hold less per leaf (although they normally
have more leaves). Seasonal changes make a large
difference within deciduous forests, with far greater
interception losses when the trees have leaves than
without. Table 3.2 illustrates the influence of plant
morphology through the variation in interception
found in different forest types and ages. The largest
influence that a canopy has in the evaporation
process is through the aerodynamic roughness of
the top of the canopy. This means that as air passes
over the canopy it creates a turbulent flow that is
very effective at moving evaporated water away
from the surface. The reason that forests have such
high interception losses is because they have a lot of
intercepting surfaces and they have a high aero-
dynamic roughness leading to high rates of diffusion
of the evaporated water away from the leaf (Figure
3.1).
Meteorological factors affecting the amount of
interception loss are the rainfall characteristics.
The rate at which rainfall occurs (intensity) and
storm duration are critical in controlling the inter-
ception loss. The longer water stays on the canopy
the greater the amount of interception loss. Also
important will be the frequency of rainfall. Does
Table 3.2 Interception measurements in differing forest types and ages
Tree type
Age
Interception (mm)
% of annual
precipitation
Deciduous hardwoods
100
254
12
Pinus strobus (White Pine)
10
305
15
Pinus strobus
35
381
19
Pinus strobus
60
533.4
26
Source : From Hewlett & Nutter (1969)
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