Download Adapting to Disturbance: Plants Resist, Depend, and Grow with Fire

Adapting to Disturbance: Plants Resist, Depend,
and Grow with Fire
by Elizabeth Fisher
Wildfire in North America is a natural phenomenon that burned millions of acres for millions of years
before European settlement. The relationship between ecosystems and wildfire has evolved in such a
way, that many species of plants and other processes depend on this natural disturbance to regenerate
and maintain a healthy ecosystem. Fire reacts differently according to, environmental factors
(topography, climate, vegetation type) or by the present health of the stand (insect invasion, build up of
fuel loads). Depending on these factors, fire can burn with minimal damage or can scorch an area
severely. Suppressed fires due to agriculture, ranching, or other human development have altered the
environment. Fuel loads have increased, and vegetation types have changed, leaving areas
susceptible to intense fires.
The area of focus is not only on the effects of fire on vegetation types, but more specifically, on our
rangelands. All areas of the world that are not barren desert, farmed, covered by bare soil, rock, ice, or
concrete are defined as rangelands. More specifically, focus will be on rangeland that is uncultivated
land that provides the necessity of life for grazing and browsing animals. There are a variety of
vegetation communities that can be included in these lands. Some of these communities include the
Ponderosa pine, sagebrush grasslands, salt desert shrub, the Great Plains, and more.
Fire Ecology
Plant species are greatly impacted by fire for many reasons. Often there are dominating species that
take over the associating species in the area. With frequent fires, these dominating plants are
substantially reduced and provide room for the less dominating and sometimes more palatable
species. Build up of woody debris or even an abundance of herbaceous ruminants can reduce
germination due to poor light. Burning these areas can provide light for new growth and reduce fuel
loads. With smaller fuel loads, fires are less intense. These frequent low intensity fires are important for
our rangelands. Grasses germinate soon after fire and are abundant, woody species are few due to
unsuccessful seedling survival. Some sites to visit about fire ecology are provided below.
Rangeland Development
Plant communities go through stages of development. Through succession, different communities
occupy the same area. Over time species are lost while others invade and become the dominating
community. These also may die off, and new invaders come in their place. Hundreds or even
thousands of years later a community may develop that is stable or remains at a steady state known as
the climax stage. The climax stage of an ecosystem is not necessarily tree dominant; grasslands are
often the climax community. Grasslands have remained largely treeless because of frequent wildfire, a
natural selective force in the development of grasslands.
Throughout centuries, grasslands have developed a resistance to fires, allowing for further
development and regrowth after burning. Grasses are monocots, these plants have low apical
meristems allowing for fast recovery after grazing or fire. After being trimmed or burned the meristems
send a message to the plant to produce sheaths or stems. The plant produces photosynthetic material
as quickly as possible to begin photosynthesis. Because tall grasses burn so quickly, the heat of the
fire does not linger long enough to do adequate damage. A wildfire can burn these grasslands while
sometimes not even touching the ground level vegetation. Therefore, despite the roaring speeds and
high temperature, lethal damage to basal portions and root systems rarely occurs.
Nutrient Cycling
Nitrogen, phosphorus and carbon are the main nutrients needed for plant production and function.
These nutrients are often limiting because they are not usually directly available to plants. Nitrogen is
used to produce proteins and many other compounds. The large amount of nitrogen that exists in the
atmosphere is not readily available. Plants cannot take up nitrogen unless it is in the form of nitrate.
Fire makes these nutrients available for plant growth by releasing some of the fixed nitrogen from the
atmosphere, back into the soil as nitrate. Fire aids in germination by depositing black ash after a green
fire and provides a bed of these nutrients.
Phosphorus is required for all living cells to function, it is a component of ATP, ADP, Nucleic acids, and
phospholipid development. Plants take phosphorus from the soil in the form of orthophosphate.
Phosphorus is often a limiting nutrient in soils and its low concentration can inhibit the growth of plants
on a site. Fire changes phosphorus into orthophosphate and is released into the soil by fire making it
more available for plant use.
Carbon is not a limiting factor in plant growth like nitrogen and phosphorous but is still very important in
the building block of life. Through photosynthesis, plants use carbon to create compounds necessary
for life. Combustion releases the carbon fixed in the organic matter into the atmosphere in the form of
carbon dioxide, which plants use for photosynthesis, and then returns the carbon into the soil in the
form of elemental carbon.
Soils
Light burns frequently increase soil pH, stimulate nitrification, and improve soils chemically. Black ash
settles on the soil surface after a green fire and because of the dark color, solar energy is conducted at
higher degrees. Higher soil temperature stimulates seed germination for rapid growth. There is
increased availability of phosphorus, potassium, calcium, and magnesium in the ash deposit. It has
been observed in areas under similar conditions, grass in blackened ash verses white ash (resulting
from dryer grasslands) promoted earlier growth.
Soils in burned grasslands usually have slightly higher pH values increasing the nutritional value.
Because of the increasing metals that released by fire the pH levels are higher. In acidic grasslands,
pH increases often improve germination time and accelerates plant production and succession. Mineral
salts of calcium, potassium, sodium, and magnesium are often increased; these minerals are water
soluble and usually are taken up by organisms and plants after burning occurs.
The new growth after a fire is more productive and palatable for herbivores than prior to the burn.
These new sprouting plants are not only nutritious but they are also greener, larger, and higher in water
content. Animals grazing burned areas gain more weight and have less problems with ticks, mites, and
flies.
Fire-Dependent Species
There are grass and shrub species found in these forest regions that are fire dependant for
reproduction. Lupines are nitrogen-fixing plants increase production with the extra nitrogen after a fire.
Spruce does exceptionally well after fire because of the mineral soil and reduced humus layer. These
conditions are ideal for the small, slow-growing roots. There are trees like the Lodgepole pine and Jack
pine that are dependant on fire for seed dispersal. Lodgepole, normally found on higher elevations than
ponderosa pine, is a fairly thin-skinned tree that does not survive fires easily. Even though these trees
cannot resist fires they have serotinous cones, needing heat to open cones for seed dispersal.
Snowbrush (Ceanothus velutinus) species are also dependent on fires, these plants require heat to
germinate. There are many other species that need fire as an aid in reproduction and growth including,
deerbrush, pinegrass, alders, blueberry, ricegrass, and reedgrass species.
Conclusion
Fire is an essential element in our ecosystem as natural management technique to control species
dominance, noxious invasion, healthy plant production, and successful germination. Fires do not have
a natural detrimental effect on grasslands. There are factors however that can influence the change in
vegetative response to fire. Grazing and fuel loading impacts fire behavior and vegetation changes
after fire. Grazing can reduce the amount of biomass and potential fuel, resulting in fire suppression.
Weeds can be introduced, making it difficult for the perennials to compete for space. Introduced
annuals that die in early season make these areas fire prone and in later months less palatable to
grazers.
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