Climates of vegetated surfaces

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Climates of vegetated surfaces

• It is necessary to consider volume exchanges
• Q QH QE ?QS ?Qp
• where ?QS is the rate of physical heat storage
  and ?Qp is the biochemical heat storage due to
  photosynthesis.

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Oke (1987)
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• In some cases an advection term (?QA) is also
  needed.
• Plant growth is tied to photosynthesis
• CO2 H2O Light ? CH2O O2
• where CH2O denotes carbohydrates.
• Respiration occurs when
• CH2O O2 ? CO2 H2O combustion energy

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• Growth depends on dry matter gained by
  assimilation of CO2 in photosynthesis minus the
  amount lost by respiration.
• ?P P - R
• The rate at which heat is stored by net
  photosynthesis is
• ?Qp f ?P
• where f 1.15 x 107 J kg-1 of CO2 assimilation.
• Note that f represents the heat of carbon
  assimilation.

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• In considering the energy and mass balances of a
  leaf, the resistance analogy is used, especially
  by plant physiologists
• In this analogy, the flow of a property like heat
  is likened to the flow of electricity in an
  electrical circuit following Ohms Law v ir
  (Voltage current x resistance)
• QH Ca (T0 - Ta)/(rb) ( i v/r)
• where rb is the resistance in the laminar
  boundary layer around a leaf.
• E (?v - ?va)/rb

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• Considering stomata too, resistance can be added
• E (?v - ?va)/(rb rst)
• FC (?ca - ?ci)/(rb rst)

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Bailey et al. (1997)
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Oke (1987)
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Oke (1987)
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http//gristmill.grist.org/
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• Radiation extinction in a plant canopy follows
  Beer's Law
• K?(z) K? (0) exp(-aAl(z))
• where K?(0) represents the average incoming
  shortwave radiation above the canopy, a denotes a
  coefficient associated with extinction by leaves,
  and Al(z) is the leaf area accumulated from top
  of canopy down to level z.

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Oke (1987)
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Oke (1987)
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• In forested environments, evapo-transpiration
  potentially dissipates a large portion of the
  daytime radiative surplus.
• Thus QE can become a dominant term in the surface
  energy budget, particularly in situations where
  there is no water stress.
• In these situations, the Bowen ratio (ß) remains
  low and temperatures within the forest are
  relatively lower than in surrounding open areas.

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Oke (1987)
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Oke (1987)
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Bailey et al. (1997)
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• If soil moisture is depleted, water stress can
  induce stomatal closure and shut down the
  transpiration process.
• In such cases, the Bowen ratio increases and can
  lead to rapid warming of the forested
  environments, making it more susceptible to
  fires.

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• The vegetation also plays an important role in
  air flow
• flow behaves as though the surface is somewhere
  in the canopy, not at the ground.
• This level is called the zero plane
  displacement.
• d 2/3 h
• where h is the height of crops or trees.

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Oke (1987)
Oke (1987)
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• Log wind profile then looks like
• u u/? ln((z-d)/z0)
• Other profiles (T, RH, CO2) respond in the same
  manner.
• In reality wind is not 0 at d -- flow is complex
  within canopy.

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• However, u does not approach zero at the canopy
  top at heights less than d, and momentum
  penetrates into the canopy, resulting in wind
  flow within the forest.
• The velocity profile within the forest is
  observed to be exponential in the vicinity of the
  canopy top but again becomes logarithmic near the
  ground.
• Often, a secondary wind maximum is observed deep
  within the forest canopy.

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Bailey et al. (1997)
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Bailey et al. (1997)
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Oke (1987)
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• One of the greatest differences in the
  hydrological balance between forests and short
  vegetation lies in the interception of
  precipitation.
• A much greater fraction of precipitation is
  intercepted by a forest canopy because of the
  large surface area of foliage, the canopy
  structure of forests, and interactions with the
  boundary layer.

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• Precipitation is either intercepted by foliage or
  falls directly to the forest floor as
  throughfall.
• Intercepted precipitation can remain on the
  canopy, evaporate or sublimate, or fall to the
  forest floor.
• Conifers intercept more water (snow and rain)
  than hardwoods, since they maintain their leaves
  throughout the entire year.

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• The amount of intercepted snow depends on canopy
  density, whether the snow is wet or dry, the
  amount already on the canopy, and meteorological
  conditions.
• Large trees in the BC coastal forests intercept
  up to 50 of snowfall, whereas shorter trees
  within the interior tend to intercept less
  snowfall.
• This impacts the amount of snow reaching the
  ground and snowpack evolution in forested
  environments.

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Advective Effects

• As wind flows from one surface type to another,
  air which may have been in equilibrium with the
  original surface must come into balance with the
  new surface
• internal boundary layers are created

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Bailey et al. (1997)
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Most landscapes are a patchwork of different
surface types, here are agricultural types
Bailey et al. (1997)
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Bailey et al. (1997)
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and types of forests
Bailey et al. (1997)
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• Distributions of vegetation
• The forest regions and their boundaries are
  related to macroclimatic (large-scale) controls -
  usually measures of available energy and aridity
  (i.e. temperature/ humidity controls).

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• However, exact cause and effect can seldom be
  proven between a single climatic variable such as
  temperature and the extent of a vegetation type.
• It is often thought that the northern boundary of
  a given species is governed by temperature
  whereas the southern boundary is controlled by
  moisture.
• In western Canada, the northern boundary of
  tundra and open forest - the arctic treeline - is
  correlated strongly with an annual net radiation
  of 750 MJ m-2.

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• The transition from open to closed-crown forest -
  the northern forest line - is about 1170 MJ m-2
  in western Canada.
• The closed-crown forest persists southward until
  annual net radiation reaches 1460 MJ m-2
• At local scales, species occurrence and extent of
  cover are affected by small changes in relief,
  soil type, and soil drainage, which in turn
  affect the microclimate.

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• For instance, in the boreal region, aspen and
  jack pine favour dry, upland sites, and black
  spruce tends to be found in low, poorly drained
  sites.
• Low albedoes are typical of forests because most
  of the radiation is below the top of the canopy
  before it is reflected, and therefore efficient
  trapping ensues.
• Albedo values of 0.15 to 0.18 are typical of
  deciduous forests, whereas coniferous forests
  have lower albedoes.
• Following snowfall, the albedo can increase
  dramatically to about 0.5.