EFFECTS
OF FIRE ON FOREST PLANTS
John
Tappeiner
Table
of Contents:
Introduction
Major
Forest Plant Characteristics Affecting Plant Response to Fire
Predicting What
Plants Will Occur After a Fire
Establishing a New Forest
Introduction
Forest plants have
various characteristics that enable them to withstand or recover from and even
take advantage of effects of fire. These
are genetic traits that have evolved in response to fire and other disturbances
to the forest – like wind and browsing. The
purpose of this section is to provide examples of forest plant adaptation to
fire. The numbers of plant species
is too great to provide an exhaustive list for western forest species.
However, we provide examples of common characteristics of major genera or
species that are widely distributed throughout the west.
Evaluating response to particular fires on specific sites requires local
knowledge of not only fire intensity and plant species, but other environmental
variables like browsing and seed dispersal, soils, and microclimate. In
evaluating the response of forest and forest plants to fire it is important to
note that “indirect effects” are often important. In addition to the direct effect of fire on plant tissue,
environmental variables may affect plants after the fire.
For example, thinned bark species like true firs may have their xylem
tissue exposed by a ground fire. These
fire scars are infected by stem decaying fungi and the trees may be weakened or
fall from wind or heavy snow loads. Also,
the decayed stems may provide habitat for cavity nesting animals; thus the death
of trees does not always have a negative effect, depending upon one’s
viewpoint. Likewise,
fire may weaken trees or reduce their vigor by partially burning the crowns or
damaging the stems. These weakened
trees are often prime habitat for bark beetles that kill them, especially when
their vigor is further reduced by drought.
Insect populations in fire weakened trees may expand and cause mortality
in unburned trees. Also, insect
killed trees (snags) are foraging sites for woodpeckers and the dead trees
(snags) are habitation for cavity nesting animals.
Major
forest plant characteristics affecting plant response to fire:
Size:
Large
plants are generally less prone to damage or mortality than small ones.
Western conifers like Douglas-fir or Ponderosa pine may grow to over
200 feet and, depending upon stand density, their crowns may be 50 to over
100 feet from the ground. Thus,
even relatively intense fires may not damage their crowns.
Also, large trees have thick bark that increases resistance to fire,
and large hardwood trees sprout more vigorously than small trees when their
tops are killed by fire. Large
trees are also more likely to produce seed following fire and, thus, provide
new seedlings to replace trees killed by fire.
Large hardwood trees and shrubs produce larger and more numerous sprouts
than small trees and shrubs. Small
clones may be killed by intense fire, but large ones are not likely to be
killed.
Bark,
Bud Scales:
The
bark of Ponderosa pine, Douglas-fir, sugar pine, and redwood is quite thick,
especially on large trees, and provides insulation from fire at the base of
the tree. If temperatures under
the bark at the cambium reach 150°
F+, the cambium and phloem tissues are killed and the tree will die, because
the ability to transport photosynthate from the crowns to the root is lost
if the phloem is killed around the circumference of the tree.
If the cambium is killed, the tree can no longer produce new
conductive tissues – phloem and xylem.
Bark of lodgepole pine and hardwoods like aspen, alder, and small
conifers is thin; while bark of even large white fir and eastern hemlock is
apparently a poor insulator; these species are susceptible to fire. Shrubs and many hardwoods have thin bark and, therefore,
their tops are likely to be killed by even low intensity fires.
Bud
scales cover the growing points or meristems at the ends of the branches.
Thus, even if the foliage is scorched, the buds may survive.
When the buds swell and grow in the spring, new cohort of foliage
will be produced, enabling the tree to survive.
Since conifers generally maintain three or more cohorts of needles;
tree vigor may be reduced for several years, while a full compliment of
foliage is produced. Ponderosa
Pine has fire resistant bud scales. The
buds of hardwoods and shrubs are not well protected from fire, but these
species can produce new foliate by sprouting from adventitious buds.
Sprouting:
Sprouting
is an adaptation to fire that is a common attribute of hardwoods and shrubs.
The
ability of a plant to sprout is related not only to the species and size of the
parent plant, but also to the environment in which it is growing and the
intensity of the fire. A fire that
kills the overstory trees make many more site resources available to understory
shrubs, and they sprout and grow with much more vigor than if they were in a
relatively dense understory. If a
fire of moderate intensity burns through an understory of small, low vigor
shrubs or hardwoods, many of the, especially the smaller ones, maybe be killed
or sprouting vigor and growth may be quite low.
If fire is withheld for many years (as it has been in many western
forests), then shrub size, vigor, and sprouting potential are likely to
increase. If overstory tree density
decreases from a combination of self-thinning, windthrow, insect or disease
related mortality, shrub sprouting potential will also likely increase. Thus, returning fire to stands of sprouting shrubs from which
fire has been excluded for many years is not likely to have the same effect on
the shrub community as fires that frequently burned areas and kept shrub density and
sprouting potential at a low level. Possibly
very intense fire, manual, or chemical treatments might be needed to reduce
shrub populations to original, pre-fire exclusion levels. Sprouting may
originate from three different locations on a plant:
1.
Epicormic branches:
Epicormic
branches are produced from buds in the phloem and cambium tissues under the
bark. If tops of species like
redwood, big leaf maple or white firs are exposed by fire or other
disturbances, or if branches in the crown are killed, new epicormic branches
are often produced. Hundreds of
new, small branches may occur.
Many of these branches die and large trees often have fewer than 5-10
epicormic branches.
2.
Basal Sprouts:
Basal
sprouts occur at the base of the stems killed by fire (cutting, girdling, as
well). Often, many sprouts
(>10-100 total) will occur immediately after fire. (Figures 1a
and 1b)
As the sprouts grow, their numbers decrease and, after 30 to 40
years, a sprouting tree or shrub may have only 3 or 4 main stems… or
fewer. Some species like
Green-leaf Manzanita and tanoak produce ligno tubers or burls below the soil
surface that are the source of sprouts.
Other species like Ceanothus or big-leaf Maple sprout from buds in
the main stems. Redwood
and Yew are sprouting conifers. The
vigor of the sprouts, their rate of growth and density, varies with species
and size of tree or shrub. For
example, Ceanothus species are generally vigorous sprouters, with the
exception of Ceanothus cordulatus, which is a weak sprouter.
Big-leaf maple, madrone and tanoak sprout vigorously, while red alder
is a much less vigorous sprouter. Large
trees and shrubs nearly always produce more vigorous sprouts when their tops
are killed than smaller individuals of the same species.
Because the buds of a sprouting species occur at the base of the
stems below ground, they are generally well insulated from fire.
Only a very intense fire could raise soil temperatures sufficiently
to kill the tissues containing these buds.
3.
Root Sprouts:
Aspen
is the major root sprouter among western species.
After the top is killed, hundreds of sprouts (called root suckers)
are produced along the roots. (Figure 1c)
Thus, within several years, an acre that formerly had 2-5 large aspen
trees might be stocked with thousands of new aspen saplings several feet
tall.
Rhizomes:
Many
shrubs, commonly produce rhizomes, or underground stems.
As shrubs grow, these rhizomes extend and produce new aerial stems
and new rhizomes. A clone of salal, for example, may extend for over 10 to 20
meters. When a fire kills
aerial stems, buds throughout the rhizomes are released and a dense
population of new aerial stems results.
A population of large, vigorous clones may support several miles of
rhizomes/acre, with a bud for every inch or two of rhizome.
Thus, the sprouting potential of rhizomatous shrubs is often quite
high. Rhizomes are generally
insulated from fire, since they grow several inches below the soil surface.
(Figures 2a and 2b) However, on some
sites, they grow in the forest floor or at the or at soil source.
In these cases, fire may damage the rhizomes sufficiently to
substantially reduce sprouting.
Seeding:
There
are two major seed sources from which forest plants are established after fire
or other disturbances: Stored seed and transported seed.
Seed may be stored on site either in:
a)
serotinous cones or,
b)
in the upper soil horizons.
Conifers
like lodgepole pine and knobcone pine produce cones that persist on the parent
tree for decades. The seed in these
cones is released when the cone scales open after exposure to high
temperatures—from fire or radiant heat from the ground.
Some shrubs and trees produce seed with hard seed coats, and this seed
remains viable in the forest floor apparently for hundreds of years.
Fire aids in overcoming seed coat dormancy… that is, breaking its coat
and making it permeable to water so that the seed can germinate.
For example, the forest floor in western stands of conifers may contain
100,000’s of seeds per acre of ceanothus or manzanita that was produced by
former shrub communities. (Figures 3a and 3b) These
communities are often overtopped and “shaded out” by conifers. There maybe no evidence of the former shrubs save for an
occasional clump of dead stems. Following
fire, 1000’s of shrub seedlings may germinate from the stored seed.
Very intense fires, however, may consume some of the upper soil horizons
the seed stored there.
Seed may be transported to burned sites primarily by wind, but also by animals.
Wind dispersal is largely dependent on seed weight.
A variety of light seed species like thistle, fox glove, semecio, as well
as various grasses, along with seed bank species, are commonly found on forest
sites after wild or prescribed fires. Light
seeded tree species like red alder, western hemlock, and redwood can be expected
to be more readily dispersed over larger areas than species with heavy
seed---like sugar pine, California black oak and tanoak. These heavy seed
species and species like Pacific madrone, whose seed is borne in berries are
dependent on animals for dispersal, as are small seed shrub species like bitter
brush and bearclover, whose seed is borne close to the ground.
Frequency and size of seed crops are also an important characteristic in
determining effectiveness of a species to reproduce by seed.
For example, ponderosa pine and Douglas-fir seed are relatively light and
readily dispersed by wind. However,
they produce large seed crops infrequently—every 4-8 years.
During this period, other plants that inhibit establishment of pine and
Douglas-fir might occupy burned areas. Also,
intense fire that kills or damages potential seed trees will affect post burn
seed supply and, consequently, species composition of the new plant community.
Predicting
what plants will occur after a fire:
Currently,
there are no models that can accurately predict the species composition
following a wild or prescribed forest fire.
However, with an understanding of how plans reproduce, and evaluation of
older burns, one can make a reasonable estimate of how succession will proceed
after a fire. (Figure) The most
certain prediction involves sprouting species.
If hardwoods like tanoak, Pacific madrone or black oak, or shrubs like
green leaf manzanita, Ceanothus, or bearclover are present, it’s nearly
certain that they will sprout and, furthermore, the larger they are or the
denser their cover, the more likely they are to rapidly reestablish a dense
cover after a fire. The occurrence
of grasses and forbes following a fire is almost certain.
These species are easily dispersed by wind and readily germinate on
exposed soil among the sprouting shrubs and hardwoods.
They are often particularly abundant the first several years after fire
and then decrease as woody plants cover decreases.
Seed bank species like Ceanothus and manzanita frequently germinate
following fire and many persist as a dense cover for several decades.
However, their presence is not readily known, as seed in the forest
floor, and new seedlings, may not be easily recognized until they are 4+ years
old. Examination of older burns on
similar sites or pre-burn examination of a site to look for old or dead remnants
of previous shrub communities may help “predict” the possibility of their
presence after a fire. Careful
identification of seedlings may reveal their presence within 2 years after a
fire.
After
a wildfire, however, it is often difficult to predict the extent to which
conifer or non-sprouting hardwoods will become established.
As discussed above, seed supply is important and is dependent not only on
species, but on the proximity and number of seed trees and the frequency or
periodicity of seed production. Prescribed fire is done with the goal of saving
overstory trees on the site, but a wildfire may destroy seed bearing trees and
thereby eliminating certain species from a site. Timing of seed availability and
fire is very important. Plentiful seedling establishment is most likely when
seed production occurs within one or two years following fire.
When seed availability is delayed, cover of forests from wind dispersed
seed and shrubs from seedbanks or sprouts will substantially inhibit
establishment by competing for water and light, as well as providing cover for
seed consuming rodents. There are
many examples in western forests where establishment of a conifer forest was
delayed for many decades because forests shrubs and hardwoods inhibit early
establishment of species like Douglas-fir, Ponderosa pine, and Sugar pine.
In these cases, sites remain in shrubs or shade tolerant species like
true firs gradually become established in the shrubs and over top them after
about 30 to 50+ years.
Establishing
a New Forest:
Because
of the uncertainties of establishment of conifers due to lack of seed or timing
of seed availability and competition from other plants, conifers are often
planted following intense stand replacing fires. By planting and controlling competition, one can facilitate
prompt establishment of conifers on sites where there is a lack of tree seed
and/or severe competition from shrubs and forests. Reforestation by planting is routine on sites where timber
production is the primary forest management objective.
However, it can be used on any site to insure establishment of species
like ponderosa and sugar pine and Douglas-fir.
In Lodgepole pine forests that are well adapted to fire, natural
regeneration is often plentiful after a fire because there is ample seed and
little competition from other plants.