PLS 147 Lecture 8: Grazing and Restoration
Processes in Plant Communities 1
we have manipulated these processes
● Fire → Fire suppression, ignition
● Flooding → Flood control
● Herbivory → Grazing (new herbivores)
● Timber harvest
These main processes are represented by disturbance regimes
● regime: spatial and temporal pattern of disturbance, of differing severities
○ mosaic of habitat types and successional ages on the landscape
○ eg fire at different times
Grazing
● humans extract things from nature to improve our lives, many of theses activities are
very destructive
● Of all our extractive land uses, grazing has the potential
to be least damaging, because
it may replicate natural processes.
○ has the potential to leave the system the same, just with different herbivore
○ In practice, it often does not turn out that way, particularly west of the Rockies.
■ damaging and degradative
Why are grasslands where they are globally?
● Edaphic (soil) explanations
○intermediate soil dryness
■ grasslands aren't always where it's drier, (deserts can have woody plants
too)
■ not enough woody to suppress them, enough to suppress other woody
(forest)
○edaphic severity
■ shallow, waterlogged
■ harsh soil that can't have trees
■ seasonally inundated but not enough to keep out grasses
● Disturbance explanations
○ fire
■ keep forests from encroaching onto grasslands
■ grass carries a good fire, don't like shade so no fuel into forest's edge
● sharp boundaries
○ grazing
■ great numbers of large herbivores may also maintain open grasslands
■ grass tolerates grazing, woody plants cant
■ some grasslands have been heavily impacted by grazing for a long time,
others have not
○ held in a state of grassy succession ?
○ Grasslands of CA and WA were dominated by perennial bunchgrass
■ now invasive species
■ grasslands of Great Plains, millions of bison
● killed bison, replaced by cattle, converted land to crops
■ Great Basin Grassland, California prairies
What makes grasslands special?
● Great Plains vs western grasslands
○ Grasses tolerate and recover from grazing because
■ ground-level meristems
● growing tip on the bottom, dense, not burned or eaten
● xturf grasses east of mississippi survive better than bunchgrasses
■ rhizomatous growth form
● cattle less damaging in Great Plains
○ dominated by rhizomatous turf grasses
○ here, not as many bison so grass was heavily impacted by cattle
■ more vulnerable
■ western grasslands dominated by bunchgrasses
How is grazing different from historical herbivory?
● Are cattle substitute bison?
○ perhaps in the Great Plains (not California), but ranching is not
● Ways in which ranching livestock is not "natural"
○ managing the animals
○ elimination of problem animals
■ predators, burrowing rodents like prairie dogs
○ loss of carcasses (take away cattle when they die)
■ scavengers, decomposers (California condor)
○ fencing, roads, fragmentation
■ less movement
○ weeds, weed control
■ grazing encources weeds
○ water supply
■ boreholes & riparian areas
■ irrigation
● livestock at much higher densities than bison
History of livestock in CA
● Spanish cattle, free-ranging, mainly for hides and tallow (not ranched)
○ not a high demand for meat, couldn't preserve
Document Summary
Processes in plant communities 1 we have manipulated these processes. These main processes are represented by disturbance regimes. Regime: spatial and temporal pattern of disturbance, of differing severities. Mosaic of habitat types and successional ages on the landscape. Humans extract things from nature to improve our lives, many of theses activities are very destructive. Of all our extractive land uses, grazing has the potential to be least damaging, because it may replicate natural processes. Has the potential to leave the system the same, just with different herbivore. In practice, it often does not turn out that way, particularly west of the rockies. Grasslands aren"t always where it"s drier, (deserts can have woody plants too) Not enough woody to suppress them, enough to suppress other woody (forest) Seasonally inundated but not enough to keep out grasses. Grass carries a good fire, don"t like shade so no fuel into forest"s edge. Great numbers of large herbivores may also maintain open grasslands.
