Lecture 5 - Disturbance Flashcards
Definition of disturbance
relatively discrete event in time and space that alters the structure of populations, communities, and ecosystems and/or changes resources, substrate availability, or the physical environment
examples of natural causes of disturbance
fire flooding insect outbreaks volcanoes frostheave ocean currents
examples of human cause disturbance
agriculture urbanization logging/resource extraction mining road construction ecotourism
impacts of disturbance are influenced by what 3 general categories?
spatial factors
temporal factors
magnitude
describe the spatial factors influence disturbance
size
shape
connectivity
proximity to unaffected areas
describe the temporal factors influence disturbance
frequency
duration
time of year
what are the 3 most important natural disturbances in the boreal?
fire
insect infestations
severe weather
define succession
changes in equilibrium communities
replacement of one community to another
primary succession
vs
secondary succession
takes place on land where no organic soil exits
(ex glacier scrapes down to bedrock)
takes place after an ecological disturbance
*don’t take away all the soil
what is the most important cause of disturbance?
fire
what are two ways that fire is a primary agent of change in the boreal forest?
- releases nutrients
- reduces litter on forest floor and release nutrients that have been tied up in those materials - allows sunlight
- opens up canopy to sunlight –> stimulates regeneration from seeds and roots
- stimulates growth and reproduction
what causes fire?
lightning!
what percentage of total fires are made up by lightning?
why is it such a big factor?
35% of total fires
BUT
represents 85% of area burned
how will climate change impact lighting fires?
rising temperatures are expected to increase the number of lightning strikes
12% increase for each degree celsius
3 types of fires
surface fires -burn only on ground crown fires -burn only in canopy ground fires - burn along ground or underground
describe surface fires
they only occur when moisture is _____ before and ____ is _____ during
burn fuel s at the ground surface
shrubs, grasses, fallen branches, litter
may kill thin barked trees if it gets hot enough but won’t destroy mature forest
- thins out
- reduces combustible fuels
moderate before
wind moderate
forest with periodic surface fires are ____ flammable than those where light fuels have accumulated for years
why?
LESS
root system usually left alive
describe crown fires
build up enough head to ignite the canopy
more common in boreal forests than deciduous
what is a passive crown fires
burn along ground then spread into each tree
-flame length is enough to ignite the individual tree crowns but canopy fuels and/or rate of spread is insufficient to maintain fire in both surface and crowns
describe ground fires
where are they most common?
burn subsurface organic fuels by moldering combustion
often ignited by surface fires
peatlands!!
relationship between amount of peatland on the landscape and size of fires
more peatlands more fire
define fire regime
characteristics of taiga fires
- pattern, freq, intensity
what is the length of a fire cycle controlled by?
balance between precipitation and evapotranspiration
___ ____ determines many of the plant dynamics of the boreal forest
fire cycle
why can spring be a major fire season?
trees experience water stress (moisture loss bc warm temp, gain little from frozen soil)
low evapotranspirations
how do pines protect themselves from fire?
thick bark
which trees are more likely to crown? why?
conifer trees
depends on tree canopy architecture and understory vegetation
which trees are least fire resistant? most fire resistant?
paper birch
jack pine
what makes jack pine so fire resistant?
self pruning and self shedding
shedding dead branches reduces build of combustible fuels in canopy
self pruning low on trunk removes ladder branches
means of post-fire reproduction
light, wind-dispersed seeds (birches, poplars)
serotinous cones (jack pine, black spruce)
stem sprouting/suckering (aspen/birch)
define serotinous
why is it beneficial?
require heat of fire to open
beneficial time to reproduce bc of lack of other species
also, protects from rain, frost, bacteria, fungus, rodent teeth
how are birch and aspen adapted to regenerate after a fire?
regenerate vegetatively by shoots and suckers arising along lateral roots
surface fires don’t get rid of roots, allowing this regeneration
how do understory plants survive through fire?
survive as seeds buried in the soil for decades
- germinate when canopy and leaf litter are removed by fire
- sun warms soil
example of a boreal plant species highly resistant to fire
cottongrass
how does fire disturbance create a dynamic habitat mosaic?
elevates species diversity as diff species will be successful compared to regions where there hasn’t been fire
why do invasive species pose a bigger threat to ecosystems?
ex?
no predators, diseases, or other natural forces to control their population
brown spruce longhorn beetle
gypsy moth
why are boreal forest so susceptible to large-scale insect disturbance?
low diversity (monoculture) makes it easy for them to spread low productivity
what triggers insect populations to increase?
sustained periods of warm weather
drought
(occurs over multiple years)
what do climate models predict to occur when it comes to insect outbreaks?
increased temp could increase the risk of insect outbreaks
extreme winter weather is detrimental to insect outbreaks
approx what percentage of annual boreal forest damage is weather/blowdown damage compared to harvested area and fire?
44%
how does blowdown related to pathogens?
damage to tree stands increase the likelihood of pathogen outbreaks
what are the top threats to coastal trees?
are susceptible to fire but wind and erosion are concerning
how are boreal mammals adapted to disturbance?
utilizing edge habitats created by small disturbances
moose like disturbance
how does disturbance affect biodiversity?
depends on scale and frequency
how does low freq. disturbance affect biodiversity?
short lived species outcompeted by long lived
how does high freq. disturbance affect biodiversity?
short lived species outcompete. no time to mature.
favours rapid growth rate, short life span, strong ability to colonize disturbed areas
