Climates of vegetated surfaces

1
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)
3

• 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.

5

• 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)

7
Bailey et al. (1997)
8
Oke (1987)
9
Oke (1987)
10
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.

12
Oke (1987)
13
Oke (1987)
14

• 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.

15
Oke (1987)
16
Oke (1987)
17
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.

20
Oke (1987)
Oke (1987)
21

• 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.

23
Bailey et al. (1997)
24
Bailey et al. (1997)
25
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

30
Bailey et al. (1997)
31
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.

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Animal Bioenergetics

• Animal physiology is concerned with the
  interaction of animals and their environment
• Micro climate of animals is a key to their
  physiology
• Animals convert chemical energy from food to
  maintain their temperature
• An animals metabolism (energy output to live) is
  balanced by its gains or losses of energy through
  radiation, convection, conduction, and
  evaporation
• Htot - Hrad - Hconv - Hcond - Hevap (-
  Hstor)

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Radiation

• Longwave
• Lout esT4 (for caribou emissivity, e.99
• Lin aL(.5 L? .5 L?) (absorptivity, aL e)
• L Lin Lout
• Shortwave
• Comprised of direct diffuse reflected
• Depends on colour of coat and hair reflectance
• K as(Ap/A Kp .5 Kd .5 Kr)
• Net solar absorptivity direct
  diffuse reflected from
• correction for beam
  beam ground
• perpendicular

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Convection

• Both Free and Forced (electrical analogy)
• QH ?Cp (Ts Ta) / r (Ts is skin, Ta is air
  T)
• How can animals control this?

41
Convection

• Both Free and Forced (electrical analogy)
• QH ?Cp (Ts Ta) / r (Ts is skin, Ta is air
  T)
• How can animals control this?
• Modifying their exposed surface area
• Modify environment (wind speed)
• Thickness of hair
• Interaction with radiation
• Orientation of body with wind

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Conduction

• Conduction of heat from animal into ground
• Qc kA(Ta Ts)/L
• conductivity area T
  difference distance
• on ground
  btw animal/ground
• Insulation
• Increases with distance/thickness
• Decreases with immersion in water / rain
• Related to animal size

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Evaporation

• Animals can use evaporation to cool off
• Sweating
• E (?vs - ?va) / rv (electricity analog)
• Vapour density skin, exhaled and air
• Panting
• E (?vex - ?va) (per area/time)
• Body size affects strategy
• Larger ? sweat
• Smaller ? pant

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Heat Production
Source http//people.eku.edu/ritchisong/RITCHISO/
thermoneutralzone2.gif

• Animals can minimize their metabolic cost if the
  environment is between the lower and upper
  critical T
• Otherwise they have to expend energy to heat or
  cool themselves
• They will need more food to provide that energy

45
Thermal Cover

• Therefore animals seek thermal cover to
  minimize their metabolic cost
• What strategies could they employ?
• How could land be managed to provide thermal
  cover for wildlife?