Questions

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Questions

• How can grazing management affect
  photosynthesis?
  
• What are some differences between C3 and C4
  plants, and how do these differences affect plant
  distribution?
  
• Describe some plant adaptations to cope with low
  water availability
  
• How does carbohydrate storage in grass roots
  change seasonally? After a plant looses leaves
  to herbivory?
  
• How do mycorrhizae affect plant roots, and what
  do they gain in return for their services?

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Erosion, summary

• Biological processes have a large effect on
  erosion and infiltration
  
• Small changes to biological processes can have
  big effects on hydrology in arid and semi-arid
  environments
  
• Multiple interacting factors, and is therefore
  not easy to predict consequences of management
  actions

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Erosion, summary

• Cattle grazing can both increase and decrease
  water infiltration
  
• Cattle grazing most often increases erosivity,
  especially heavy grazing
  
• Situation dependent soils, climate, vegetation,
  timing of grazing, etc. all affect erosion and
  run-off responses to grazing

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In the news...

• Grazing fees will be lowered on March 1st, 2007
• Reduced to 1.35/AUM, which is the lowest fee
  allowable by law (made in 1979)
  
• Fee was 1.79 in 2005, 1.56 in 2006

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Current grazing fee formula

• Fee per AUM 1.23 X (FVI BCPI PPI) / 100

(Animal unit month is amount of forage needed by
one 1000 pound cow and calf for one month)
Prices Paid Index
Beef Cattle Price Index
Forage Value Index
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In 2006

• Fuel costs went up by 10
• Cattle prices went up
• But, grazing fees on private land went up 2.8
• Grazing-fee program runs a 144 million deficit
  every year

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Range Plant Physiology
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Range Plant Physiology

• Photosynthesis
• Water-use
• Below-ground processes

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Simple concepts of range plant management

• Plants are the only source of energy for grazing
  animals
  
• Plants make sugars, starch, and proteins by
  photosynthesis in leaves (seldom stems)
  
• Plants do not get food from soil, they get raw
  materials needed to produced through
  photosynthesis in their leaves. Therefore, they
  are entirely dependent on green leaves for
  survival.
• When leaves are removed from plants,
  food-producing capacity is reduced.

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Photosynthesis
The process by which plants convert energy from
the sun, carbon dioxide from the atmosphere, and
water and minerals from the soil into food they
can use for maintenance and growth
Which processes can we influence??
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Components of photosynthesis

• Water
• Management can impact water availability!

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Components of photosynthesis

• Nitrates and Minerals
• Management can impact soil health (affects
  nutrient holding)
  
• Increase deposition of nitrates in certain areas
  (water sources)

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Components of photosynthesis

• Leaf area
• Grazing reduces leaf area available for
  photosynthesis
  
• The amount of food produced by a plant is
  directly related to leaf area exposed to the sun

How do plants tolerate leaf removal?
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Three photosynthetic pathways

• C3 C4
  CAM
  
• Vary in their efficiency under different
  conditions
  
• Vast majority of plants are C3 species

Cool season
Warm season
Succulents
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Differences between C3 and C4 plants affects

• Global distribution
• Phenology (timing of events)
• Response to changing CO2 levels

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Distribution of C3 and C4 grasses

• Mostly cool-season (C3) species in Great Basin
  because of the timing of rainfall spring and
  summer rain allow C4 grasses to grow in midwest

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Phenology

• Cool-season (C3) are active in the cool season,
  warm-season (C4) are active in the warm season

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Saltgrass (Distichlis spicata)
C4
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Three-awn (Aristida sp.)
C4
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• CO2 levels were much higher- 3.5 times higher- 65
  million years ago, when flowering plants
  diversified

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Recent CO2 fluctuations
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CO2 increases may change communities

• Rising CO2 levels may favor C3 plants over C4
  plants, though there is variability in the way
  species respond
  
• May favor some species over others, and shift
  community composition, even in areas with few C4
  plants

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Photosynthesis summary

• Produces food from sunlight, C02, water, and
  minerals
  
• Stores energy in the form of carbohydrates
• Different photosynthetic pathways C3, C4, and
  CAM. These pathways differ in their water and
  CO2 use.

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Water

• Water is the medium where all biochemical
  reactions occur in plants
  
• Water carries minerals from the soil into plants
• Plants also rely on water for support of
  non-woody parts and maintaining physical structure

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The problems of growing on land

• First plants were single celled organisms, in
  marine environments, constantly bathed in water
  
• Upon moving to land, first problem to solve was
  stopping the loss of water
  
• Earliest adaptation is the cuticle, which is a
  waxy, non-living layer covering leaves

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Transpiration

• Water always moves from wetter to drier, even
  against gravity
  
• evaporation
• wicking up a paper towel
• Transpiration is the name of the process of water
  moving up the leaves and stems of a plant, and
  evaporating into the atmosphere

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Transpiration
- Water moves from roots up to the leaves
through xylem, which are dead, hollow cells (like
pipes) that water flows through
-Water moves through xylem like a straw, moving
from the wet soil to the dry air
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Plants control transpiration

• Way more water moves through the plant than it
  needs for chemical reactions
  
• Water evaporates through the leaves, through tiny
  pores called stomata
  
• Same place CO2 moves in
• Leaving stomata open in the desert is like
  leaving the faucet dripping- plants can open and
  close stomata, but have to balance their need for
  C02 with loss of water

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Efficient water use

• This is one reason why C4 plants do better in
  warm climates- need less C02, so dont need to
  open stomata as wide
  
• Succulents that use CAM photosynthesis pathway
  only open their stomata at night

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Water Use Efficiency (WUE)

• Review as plants take up C02 through stomata,
  water is lost.
  
• The ratio of the gain of carbon to the loss of
  water is Water Use Efficiency

CAM C4 C3
Or, how much water does it take to make a certain
amount of biomass?
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Adaptations to Low Water Availability

• Drought avoidance
• An annual life form
• Choosing the wettest environments
• Lose all or some leaves during the dry season
  (shrubs and trees), drought-deciduous
  
• Dormancy during the dry season

There is a long time during which the plant
cannot photosynthesize, grow, or reproduce
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How do bunchgrass compensate for dormancy period?

• High photosynthetic rates and growth rates when
  they are active
  
• Large underground root system for storing food
  (carbohydrates) and some water for long periods
  of time
  
• Allows rapid production of leaves and stems when
  conditions become favorable again, while annual
  plants are just germinating

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Desert grasses respond quickly to water
Number of leaves after one week
Count
Population
Squirreltail (Elymus elymoides)
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Other Adaptations to Low Water Availability

• Drought tolerators
• Use a variety of tricks to tough it out in the
  desert (though even these species wait for wetter
  periods for most growth and reproduction)
  
• Tricks like special root structure
• Can be either extensive and shallow (many cacti),
  OR very deep in the soil (sometimes to the water
  table)
  
• Leaf hairs, silver leaves to reduce transpiration

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Local drought tolerators
Sagebrush
Antelope bitterbrush
Shadscale
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Contrast light limited leaves
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Below-ground processes

• All kinds of stuff going on down there- water
  movement, nutrient storage, interaction with
  micro-organsims
  
• Carbohydrate storage
• Mycorrhizae

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Carbohydrate reserves

• Plants store carbohydrates, made during
  photosynthesis, in their roots.
  
• Can be translocated around the plant, i.e. at
  different times of the year, or after grazing
  
• Somewhat contentious- that is, the full story on
  how and when plants use their carbohydrate
  reserves is still being tested, and it varies by
  species

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Seasonal changes in carbohydrate storage
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Effects of herbivory on carbohydrate storage

• Time 0 cow eats grass
• After several hours root function is reduced
• During first few days stored carbohydrate
  reserves are mobilized to support regrowth
  
• After a few days, current photosynthate supports
  growth

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Problems may arise during recovery

• Support for defoliated leaves cannot continue
  forever
  
• If carbohydrates become limited (competition,
  repeated or severe defoliation, lack of water or
  nutrients), tiller and/or plant death results
  
• Particular areas of the plant are more vulnerable
  than others

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Meristems

• Center of plant growth
• Group of undifferentiated cells (like human stem
  cells) that can turn into different structures
  
• Location of meristem on a plant is key to grazing
  tolerance

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Grass meristems
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Grazing the meristem