Transcript Document

Month Day Topic
Aug.
26
Introduction
28
The ecosystem concept
Sept.
2
Climate/soils
4
Soils II
9
Ecosystem energy
balance
11
Water cycling
16
Carbon
18
GPP/NPP
23
NEP
C,M&M
1
2
3
4
4
5
6
6
Characterizing functional
differences among ecosystems…
• Albdeo
• Bowen ratio
The water cycle
Basic principle of water
balance in ecosystems
• Inputs
– Mainly precipitation, some groundwater, fog
• Outputs
– Evaporation
– Transpiration
– Drainage
Basic principle of water
movement in ecosystems
• Water moves along energy gradient
– From high energy to low energy
• What forces cause water to move?
– Pressures
• Gravity
*
Ecosystems differ in canopy storage
Depends mainly on LAI
Differs among species
Infiltration
• Water movement into soil
• Depends largely on hydraulic
conductivity
– Texture
– Aggregate structure
– Macropores made by animals and roots
Basic principle of water
movement in ecosystems
• Water moves along energy gradient
– From high energy to low energy
• What forces cause water to move?
– Pressures
• Gravity
• Forces created by organisms
– Osmotic gradients
– Matric forces (adsorption)
Water
potential
()
*
Water movement to root
• Moves along water potential gradient*
• Rate depends on hydraulic conductivity
and path length
• Depends on root volume
Soil depth/volume exploited differs among
ecosystems
Transpiration is major driving
force of water movement
• Water moves in continuous column from
film on soil particles to leaf cells
• Moves upward because of strong
cohesive forces among water molecules
Plants have some water storage capacity
Quite limited in most plants (2 hours in this graph)
Most of water must come from soil (not plant storage)
Water loss from leaves
• Driving force is vapor pressure gradient
– Depends on temperature and water vapor
in bulk air
• Stomata are major resistance
– Stomatal conductance depends on
• Soil moisture
• Vapor pressure of air (in some species)
Rate of water loss from leaves
depends on water supply
• Influences water potential gradient
• Plants adjust stomatal conductance to
match water supply
Water-transport capacity of stems is
balanced with LAI
Water inputs to ecosystem
determines water outputs
• P + ∆S = ET + R
– P = Precipitation
– S = Storage
– ET = Evapotranspiration (LE)
– R = Runoff
Controls over ET in moist soils
• Ecosystem boundary layer conductance
– Physical control over water loss
– Depends on vegetation structure,
temperature and wind
– THE major vegetation control when soils
are moist
• Leaf surface conductance
– Depends on stomatal conductance
– Becomes increasingly important as soils
dry
Climatic effects on ET
• Moisture content of air is major control
in rough, well-mixed canopies
• Net radiation is major control in smooth,
poorly coupled canopies
• These environmental controls are the
same in wet and dry canopies
Streamflow is the “leftovers”
after soil storage and ET are
met
• Over long term, runoff depends on ppt
and ET
In moist ecosystems, ET is relatively insensitive to ppt.
Precipitation directly regulates streamflow
Jackson et al. 2005
Lecture ended here
H/LE feedbacks in the Amazon
Positive feedback to grass/fire
 Rooting depth
 ET/LE
H
 Grass/fire
+
 Humidity
 Air T
 ppt
Consequences of deforestation
• 25-40% of ppt in Amazon basin comes from
within-basin ET
• Dry air = decreased transport of ocean air to
basin
• Permanent reduction in ppt
• Favors grasses/savannah vegetation/fire
Albedo feedbacks in the arctic
Positive feedback to migration of arctic
treeline
 Trees
 Albedo
(earlier spring)
Warmer climate
+
 Rnet
H
 Air T
Arctic climate forcing
• Biome shift causes more heating of land
surface
• Comparison of climate forcing factors:
Doubling of CO2: + 4.4 W/m2
Conversion of tundra to shrubland: + 9 W/m2
Conversion of tundra to forest: + 26 W/m2
Chapin et al. 2005