Photo 1: 1988 / Historic Yellowstone Fire of 1988
Photo 2: 1989 / One year later
Photo 3: 1999 / Ten years later (All photos taken from nps.gov)
Everyone has seen news footage some time in their life of
massive wildfires burning across thousands of acres. Whether the fire is a high intensity wildfire
or low intensity prescribed fire, the results on the land following fire are
similar. The soil is an ashy black, the entire green thick understory has been
burnt away, and the tall tress remain standing another day but with distinct
black scars around their bases. Following these fires the news trucks pack up
and move on to another "story," but what is to become of the forest
following the fire? How does this natural disturbance effect the environment
after it burned through?
Many effects of fire are similar to those of other natural
disturbances. What makes a disturbance a disturbance is no matter the type of
ecosystem or how old the forest, it is an event that sets back succession. After any natural disturbance, there is a significant
rate of mortality of current plants.
After a fire, this is mainly restricted to the understory of herbaceous
plants and small woody shrubs. Tree species range in fire adaptability, so
certain tree species will die, while others are unaffected or benefited. Old or
diseased trees also have an increased chance of mortality during or after a
fire. With all of the plant mortality comes light. Suddenly light is hitting
the ground in places it may have not hit in a hundred years! With more light
there is more growth. In one study "Direct and Indirect effects of Fire on Shrub Density and
Aboveground Productivity in a Mesic Grassland," the species monitored was Cornus drummondii.
Following fires, C. drummondii's stem
density was increased 600% (Heisler).
With regular fires on the landscape, eventually grasses and herbaceous plants
will dominant the landscape, not because the woody shrubs aren't there, but
because they are regularly suppressed (Heisler).
With the increase in growth, one would assume there would
also be an increase in competition. This is actually not the case immediately
following fires. With limiting factors, such as light, more available following
fires there is no statistically significant data supporting higher competition
after fires (Clarke). Another reason reducing post-fire competition
could be microsite disturbance following fires (Vila). Microsite disturbance is critical for many
seeds.
Fire actually has many positive effects for seeds, other
than just increased light and microsite disturbance. Fire helps disperse seeds in a variety of
ways. The strong winds during fires help
release seeds from the canopies (Lamont).
Other seeds from fire-adapted species actually need a certain high temperature
only reachable from fire to release the seeds. Seeds already in the seed bank
are also benefited due to the soil disturbance and increased light. Although
fire may positively impact seeds, seedlings do not have the same positive
responses. If the seedling manages to survive the fire, the reduction of
nutrients, stress, and disturbance will greatly reduce its growth rate
(sometimes up to 35%), and eventually lead to it being outcompeted by the newly
released seeds that will germinate in optimal conditions and have higher growth
rates (Lamont).
However, the one major disadvantage to being a post-fire seed is the same thing
that benefits, more exposure. The exposure that brings light and space also
brings a higher risk of predation (Clarke).
Besides light, fire also has a large effect on water in the
environment, and often not a positive one.
With removal of many plants, and disturbance to the soil, there is often
an increase in surface runoff, erosion, sediment transport, and deposition
following fires (Ice). Another issue with water following fires is soils becoming
"water-repellent." Normal infiltration of water into the soil is
rapid, with bare soil being moderate to slow. Post-fire water repellent soil
makes infiltration very slow to none (Neary). With incredibly low rates of
infiltration of water into soil, this makes it incredibly difficult for new
seedlings or surviving plants to access water. The reason post-fire soil
becomes water repellent is due to many factors influencing the soil. Fire
removes binding organic matter from the soil and reduces porosity (Neary).
Infiltration
|
|||
Rate
|
Description
|
||
Surface Conditions
|
in/hr
|
mm/hr
|
|
1. Intact forest floor
|
>6.3
|
>160
|
Very Rapid
|
2. Vegatation
|
0.2-2.0
|
5-50
|
Slow to Moderate
|
3. Bare soil
|
0.0-1.0
|
0-25
|
Very to moderately slow
|
4. Water repellent soil
|
0.0-0.04
|
0-10
|
Very slow to none
|
Evapotranspiration is normally a large loss of water from
the hydrologic cycle. With
fire removing many woody plants from the landscape and replacing them with
grasses and herbaceous plants, loss of water from evapotranspiration is greatly
decreased (Neary). Both lower evapotranspiration and lower rates
of infiltration lead to increase stream flow as well as soil water storage. So
while less water can enter the soil, less water is also leaving it. Unlike
light, water has both numerous positive and negative responses from fire, and
is greatly dependent on factors such as fire intensity and topography.
Nutrients available for plants also go through a variety of
changes post-fire. Many nutrients vital
to plant growth, especially nitrogen, are lost in the upper surface layers
during a fire. Also, soil pH levels increase and mineral elements move to the
ash layer (Boerner).
The increase in erosion following fires actually helps transport nutrients and
microorganisms back to the soil.
With all the negative and positive influencing factors, as
well as variability within nature, it can be difficult to draw a conclusion as
simple as fire being "good" or "bad" for plants,
communities, or ecosystems. Regardless of fire being a positive or negative
there are many plant species and even ecosystems that have evolved over time to
be adapted to better survive fires, and even be benefited from them. Many
species need heat or smoke to help release seeds, re-sprout, and/or germinate.
Other species simply have adapted more flame resistant barks, and higher limbs
(Keeley). Overall, fire is a major natural disturbance
that affects light, water, and nutrient availability for plants in a variety of
complex reactions that often leads to earlier successional communities.
Works Cited
Boerner,
Ralph E. J. "Fire and Nutrient Cycling in Temperate Ecosystems." BioScience.
Oxford
University Press, 01 Mar. 1982. Web. 05 Feb. 2017. <https://academic.oup.com/bioscience/article-
abstract/32/3/187/330723/Fire-and-Nutrient-Cycling-in-Temperate-Ecosystems>.
Clarke,
Peter J., Peter J. Myerscough, and Nicholas J. Skeleton. "Plant
Coexistence in Coastal
Heaths: Between‐ and Within‐habitat Effects of
Competition, Disturbance and Predation in
the
Post‐fire Environment." Austral Ecology. Ecological Society of Australia, Mar. 1996.
Web. 05
Feb. 2017. <http://onlinelibrary.wiley.com/doi/10.1111/j.1442- 9993.1996.tb00585.x/full>.
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Heisler,
J. L., J. M. Briggs, A. K. Knapp, J. M. Blair, and A. Seery. "DIRECT AND INDIRECT
EFFECTS OF FIRE ON SHRUB DENSITY AND ABOVEGROUND PRODUCTIVITY IN A
MESIC
GRASSLAND." Wiley Online Library. Ecological Society of America, 1
Aug. 2004. Web.
05 Feb. 2017. <http://onlinelibrary.wiley.com/doi/10.1890/03-0574/full>.
Ice,
George G., Daniel G. Neary, and Paul W. Adams. "Effects of Wildfire on
Soils and Watershed
Processes." Latest TOC RSS. Society of American
Foresters, Sept. 2004. Web. 05 Feb. 2017.
<http://www.ingentaconnect.com/content/saf/jof/2004/00000102/00000006/art00004>
Keeley,
Jon E., Juli G. Pausas, Phillip W. Rundel, William J. Bond, and Ross A. Bradstock.
"Fire as
an Evolutionary Pressure Shaping Plant Traits." Science Direct.
Cell Press, Aug. 2011. Web. 5
Feb. 2017. <http://www.sciencedirect.com/science/article/pii/S1360138511000835>.
Lamont,
Bryon B., E. T. Witkowski, and N. J. Enright. "Post‐Fire Litter
Microsites: Safe for Seeds,
Unsafe for Seedlings." Wiley Online Library.
Ecological Society of America, 1 Mar. 1993. Web.
05 Feb. 2017. <http://onlinelibrary.wiley.com/doi/10.2307/1939311/full>.
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Maclean,
D. A., S. J. Woodley, M. G. Weber, and R. W. Wein. "Fire and Nutrient Cycling."
CiteULike: Fire and Nutrient Cycling. CiteULike, 1983. Web. 05 Feb. 2017.
<http://www.citeulike.org/group/5855/article/2980817>.
Neary,
Daniel G., Kevin C. Ryan, and Leonard F. DeBano. "Wildland Fire in Ecosystems:
Effects of
Fire on Soils and Water." Wildland Fire in Ecosystems: Effects of Fire on Soils and Water -
Forest
Service Research & Development. U.S. Forest Service, 2005. Web. 05 Feb.
2017.
<https://www.treesearch.fs.fed.us/pubs/20912>.
Vila,
M., and J. Sardens. "Plant Competition in Mediterranean‐type
Vegetation." Journal of
Vegetation Science. Journal of Vegetation
Science, Apr. 1999. Web. 05 Feb. 2017.
<http://onlinelibrary.wiley.com/doi/10.2307/3237150/full>.
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