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FIRES AND FOREST SUCCESSION IN THE BITTERROOT
MOUNTAINS OF NORTHERN IDAHO
J. A. LARSEN
Iowa State College
Foresters have recently begun to seek a more intimate knowledge of the
natural, successional stages by which forests regain terrain lost by extensive
fires or other pronounced denuding agencies. Studies in this field lead to a
closer understanding of the factors which control the distribution, composition and density of the present forest, the silvical requirements of the
various species which compose the existing forest and of the soil building
or soil deteriorating influences which are operative after large fires. It is
the author's privilege in this paper to record certain observations on forest
succession in the Bitterroot Mountains in northern Idaho, gathered in the
course of several years of forest research in that region. This is the territory lying north of the Salmon river between the crest of the Bitterroot
divide and the Columbia river plateau.
Perhaps no other region in the United States is visited by forest conflagrations of similar magnitude, frequency or degree of destruction. The
contributing causes are mainly as follows: the moist winter and spring
which give rise to a profuse and luxuriant forest vegetation of a highly inflammable nature; an invariably dry summer with afternoon temperatures
often ranging between 95 and ioo degrees F. (350 to 380 C.); the low
atmospheric humidity which frequently falls to I5 per cent and lower. These
critical conditions are combined with strong, desiccating winds sweeping in
from the arid region to the southwest. The fires, therefore, when once
under way, travel with great speed and rapidly assume uncontrollable proportions. Not infrequently an entire township (approximately 95 sq. km.)
of timber is wiped out in the course of one or two days.
These large forest fires kill all of the trees and the seedlings. In from
ten to twenty years most of this dead timber lies prone, and it then presents
a fire hazard of greater magnitude than existed in the green, virgin forest.
The second fire, on this account, is even less controllable and more destructive
than the first.
The return of the forest to the climax composition subsequent to these
devastating fires proceeds along certain well defined steps or stages which
may be considered parts of the entire cycle of succession, or the natural
process of regeneration. The first steps in this succession begin with the
appearance of species intolerant of shade, and capable of withstanding considerable drought and exposure, and concludes with the establishment of
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Ecology, Vol. X, No. i
A. LARSEN
shade-tolerant trees or species which are both tolerant and moisture loving.
The exact number of the steps in the succession, depends, however, in a
great measure upon the factors of topography, soil, degree of exposure, the
intensity of the fire and the amount of destruction wrought.
It is of considerable interest to note that the subordinate forest vegetation also develops after these fires in regular successional stages in its progression toward the climax subordinate vegetation, and that the final, climax
association, or ultimate type, is attained and developed underneath the climax
type or association of the forest trees. Those subordinate species of temporary occupancy become established immediately after the forest fire and
generally simultaneously with the appearance of the temporary arborescent
species, while those of permanent occupation follow directly upon the heels
of the climax arborescent species.
In composition the subordinate vegetation which follows the fires is also
governed by the condition of the habitat, the degree of destruction and the
requirements of the individual species, so that in the subordinate flora there
is also distinct grouping in accordance with these characters or qualities.
However, since the seedlings of the aborescent species develop very slowly
during the first three or four years, and much slower than the annuals of the
subordinate flora, the latter are in much greater evidence in the beginning.
A superficial examination would ordinarily give the impression that there
existed nothing but these rapidly growing and very dense annuals during the
first few years following the fire.
FOREST
ZONES
AND
SILVICAL
REQUIREMENTS
OF THE
TREES
In this region, in which the elevations of the mountains vary from 6,ooo
to 8,ooo feet (I,800 to 2,400 meters) above sea level, and the valleys from
2,000
to 3,ooo feet (6oo to goo meters),
the western yellow pine forests,
which are composed mainly of Pinus ponderosa, occupy the lowest zone of
tree growth; the western white pine (Pinus monticola), with its associated
species of western red cedar (Thuja plicata), western hemlock (Tsuga
heterophylla), grand fir (Abies grandis), Douglas fir (Pseudotsuga taxifolia), and western larch (Larix occidentalis), occupy the slopes of intermediate elevation ranging from 2,500 feet to 5,500 feet.
Above this middle
zone the forest is composed of subalpine species represented by Engelmann
spruce (Picea engelmannii), mountain hemlock (Tsuga mertensiana), white
bark pine (Pinus albicaulis), alpine fir (Abies lasiocarpa), and lodgepole
pine (Pinus murrayana var. contorta). These subalpine species are frequently individually present also within the intermediate forest zone. The
discussion in this article deals exclusively with the conditions surrounding
the succession within the intermediate forest zone.
This middle forest zone is designated by foresters in the region as the
western white pine type, chiefly because of the abundance and economic importance of the white pine. This tree is not a climax species, however, for
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hemlock, western red cedar and grand fir are the only true climax representatives among the trees.
The western white pine and its associated species thrive best on well
drained and fertile soil such as is found on the protected easterly or northerly gradients and on lower slopes, bottoms and benches. In these locations
the soil remains moist throughout the entire summer, being sheltered by a
broken topography, a very dense crown canopy of the trees, and covered by
deep layers of " duff " and humus.
The forests composed chiefly of western white pine are usually evenaged and rather dense. Over large areas the pine is the predominant species. It maintains a rather rapid and remarkably uniform rate of height
growth by which, in most cases and situations, it overtops its competitors.
Although both the lodgepole pine and the western larch grow as fast as the
pine in most locations, these two are more intolerant of shade and therefore
not in a position to arrest its growth seriously or to crowd it out. Mixed
with the western white pine there is generally an understory of hemlock,
cedar or grand fir wherever the soil is rather moist or the site well protected, especially at the low elevations, while toward the upper margin of
the zone and more particularly on the exposed south of west aspects we find
more of the Douglas fir and lodgepole pine.
Another characteristic of the western white pine, and one which gives
it a considerable advantage over the associated species, is the ability of its
seed to lie over in the duff on the forest floor for a period of two or three
years and then germinate upon the removal of the overhead canopy. Some
of this seed is ordinarily deeply buried by rodents and may escape injury in
the course of a forest fire. On account of this prolonged or delayed germination and preservation of the seed there is usually a greater quantity of
seed of this species available for germination subsequent to a fire than of any
of the other trees in the forest.
All of the forest trees which are found in mixture with the white pine
are listed below. The arrangement is such as to show the relative requirements of the trees for light, moisture and soil fertility, and to bring out their
Tolerance
Moisture
Soil
Permanency
W. yellow pine
Western larch
Lodgepole pine
W. yellow pine
Lodgepole pine
Douglas fir
Lodgepole pine
W. yellow pine
Douglas fir
Lodgepole pine
West. larch
Douglas fir
W. white pine
Grand fir
West. larch
W. white pine
Grand fir
West. larch
Grand fir
W. white pine
Douglas fir
W. white pine
Grand fir
Alpine fir
Eng. spruce
W. red cedar
W. hemlock
West. hemlock
Alpine fir
W. red cedar
Eng. spruce
Alpine fir
Eng. spruce
W. red cedar
W. hemlock
Alpine fir
W. hemlock
Eng. spruce
W. red cedar
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A. LARSEN
Ecology, Vol. X, No. I
permanency or lack of permanency in the forest as this develops toward the
climax association for a particular site. The least shade-tolerant, the least
moisture demanding, and the least fastidious as to soil quality, are placed
first on the list.
The effect of these differences in requirements of light, moisture and soil
is observed in adjustments of the species in distribution and composition
according to local variations in the habitat. Whenever there occur a great
variety of the species of trees in the unburned virgin forest there is a distinct grouping in harmony with the above factors and requirements.
FIRST STAGE IN SUCCESSION
It would simplify our position greatly if we could consider first all fires
occurring in the climax forest, for from this viewpoint we could more
easily trace the influences of the fires and the various steps in the succession.
Unfortunately fires do not wait for the completion of the cycle. They are
of frequent occurrence, and relatively few stands are permitted to reach
the true climax complexion before being overtaken by conflagrations.
Nevertheless, whatever the condition of the forest at the time of its destruction, lodgepole pine and western larch become established first on the most
severely burned and denuded areas. After small fires the forest ordinarily
comes back directly to cedar and hemlock, but after the larger fires the other
species may come first, though sometimes with the climax trees in the mixture. One of the most noticeable effects of extensive and repeated burns is
the appearance of considerable bodies of lodgepole pine and western larch.
These two species usually appear groupwise according to local conditions.
The lodgepole pine grows nearly pure on the dry knolls and exposed ridges
or otherwise denuded and deteriorated sites, especially toward the upper
part of the western white pine zone. Pure stands of lodgepole pine may
also be seen on gravelly slopes and benches of relatively low elevation. On
north and east exposures, however, where the soil possesses greater moisture holding capacity than on south aspects, western larch generally appears
in extensive bodies of pure forest in the first stage of the succession. Although both of these trees are intolerant of shade, the lodgepole pine seedlings survive and the larch seedlings succumb on dry warm and exposed
sites. On the cool and moist aspects, however, the situation is reversed.
Both of these species possess characteristics which enable them to reclaim burned ground speedily. The lodgepole pine produces seed at an astonishingly early age, and the seed is withheld and preserved within the
unopened cone until the fires have prepared a suitable germinating bed.
It is well known that some of the cones open and shed the seeds shortly
after the fire, while numerous others remain firmly closed for an indefinite
period. Unopened cones with enclosed seed have been found entirely overgrown with wood. The western larch, on the other hand, is favored by a
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remarkably long life and a very high degree of resistance to injury from fire,
in that it possesses an unusually thick bark and a foliage of less inflammable
nature than the lodgepole pine. On this account very good trees of larch
will survive and bridge several critical fires, thereby retaining, after the
fire, large seed-trees which become important factors in a speedy restocking
with larch-provided only that the soil is not too warm and dry for the
rather tender larch seedlings.
On sites suitable for the western white pine, and especially where the
pine existed in large numbers before the occurrence on the fire, the white
pine usually comes in strongly after the burn and becomes a part of the first
stage in the succession.
Much depends also upon the size of the area burned. If the fire which
takes place in the climax forest is small, the climax arborescent species will
succeed themselves directly without going through the temporary and intermediate stages. The same is true of any climax forest anywhere. It may
be observed to good advantage in the Beech-Birch-Hard maple forest of the
northeast United States.
SECOND STAGE OF SUCCESSION
In the absence of fires in the temporary stands of lodgepole pine and
western larch the western white pine and Douglas fir will establish themselves under and among the former and eventually crowd them out. These
intermediate species, when in possession of the ground, represent the second
stage in the forest succession. Stands properly belonging to this intermediate step in the succession are on the whole rather abundant. The reason is
that the forest of this composition is an excellent soil builder, and because
white pine and Douglas fir reproduce unusually well after single fires.
Stands of these two species are considered the second stage in forest succession whether they come in under lodgepole pine and larch, or directly
after a single fire which has destroyed a climax cedar-hemlock forest or a
white pine or Douglas fir forest. In fact the larger percentage of the western
white pine stands in Idaho at present have succeeded directly after burns
which have destroyed white pine and Douglas fir stands, that is, single fires.
It is mainly after double fires that lodgepole pine and larch seed is in abundance.
THIRD AND FINAL STAGE IN SUCCESSION
As soon as the western white pine and its associates of intermediate light
and moisture requirements reach maturity, begin to decay and thereby produce a broken forest canopy, the western hemlock, western red cedar and
grand fir begin underseeding. The forest is then in the early stages of the
climax association. These last species of greater shade-tolerance than the
former effectually prevent the natural reproduction of the white pine and
the Douglas fir. It should be made clear that the grand fir, which is less
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Ecology, Vol. X, No. i
A. LARSEN
tolerant than the hemlock and cedar, is often found growing with white pine
and Douglas fir in the intermediate stage. The fir is then present only as
scattered individuals on northerly aspects, but may be in greater abundance
on south slopes, particularly the lower parts of such slopes where the soil is
deep and moist.
These climax species respond readily to local differences in the habitat.
The western red cedar requires distinctly wet soils but not stagnant conditions of water; the hemlock needs a rather moist soil without free water,
and the grand fir thrives with much less moisture than either cedar or hemlock. The result of these different demands is a grouping of the species
according to variations in the soil moisture. The cedar occurs in greatest
abundance near the streams and on well watered soil. The hemlock is found
much oftener on moist flats and on lower north aspects, while the grand fir
prefers the rolling lowland and favorable south aspects.
Within the western white pine forest zone one may find considerable
Engelmann spruce and alpine fir, but these trees do not properly belong in
the intermediate zone. They are at home in the higher zone above the white
pine. When found in mixture with the white pine, etc., it is only along the
cool streams and in deep sheltered canyons where the soil is moist but cold
and where there is comparatively little sunlight.
Of the three climax species mentioned above, the cedar is king. It is by
far the largest and the longest lived tree in this region. It attains diameters
from six to ten feet and an age up to 6oo years. It is furthermore much
less afflicted with fungous and insect attacks than the other trees. The hemlock and grand fir are in the great majority of cases defective. For these
reasons many large, old cedars exist as the only remnants of a former
climax forest. These large trees remain chiefly along the stream flats and
in deep canyons, where they have escaped destruction by the fires which
ravaged the slopes.
The natural tendency, therefore, of repeated forest fires in this forest
zone is to limit more and more the numbers and distribution of the tolerant
forest trees and relegate them to flats and canyons or lower slopes, while at
the same time extending the ranges and increasing the numbers of the intolerant species, particularly western larch and lodgepole pine, and, to some
extent also, the western white pine.
SUCCESSIONAL
STAGES
IN THE
SUBORDINATE
VEGETATION
In the first stage of the succession the subordinate species are mostly
those of herbaceous character and of relatively rapid growth, as well as those
whose seed is carried to great distances by the wind. The principal members of this association are:
Antennaria densiflora Nutt.
Antennaria racemosa Hook.
Pearly everlasting
Everlasting
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Chaemenarion angustifolium Scop.
Epilobium palmer Rydb.
Geranium carolinianum L.
Salix scouleriana Barr.
Spiraea lucida Dougl.
Solidago sp.
Monarda menthaf olia Graham
Sidalcea neomexecana A Gray
Xerophyllum tenax (Pursh) Nutt
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IDAHO
Fire weed
Willow herb
Wild geranium
Willow
Spiraea
Golden rod
Horsemint
Hollyhock
Pine grass
Among the above species the everlasting, hollyhock, pine grass, golden rod,
horsemint and spiraea occupy southerly aspects, and the geranium, fireweed
and others prefer the northerly slopes. Practically all of these temporary
and shallow-rooted species are beneficial to the site and to the establishment of the arborescent species which follow, for, although they do not prevent erosion or appreciably increase the quality of the soil and site they
throw considerable shade the result of which is a reduction of surface temperatures and decreased transpiration of the tender evergreen seedlings
underneath. This protection is, of course, most needed on the south and
west aspects. On moist and fertile soil the temporary herbaceous vegetation is sometimes altogether too dense for the survival of evergreen seedlings,
especially those of larch, lodgepole pine and Douglas fir.
SECOND
STAGE
IN THE
SUCCESSION
OF THE
SUBORDINATE
SPECIES
The subordinate species listed above give way after two or three years
to a second stage of plants composed mainly of perennials, the majority of
which possess berry fruits so that the seedling is effected by birds. This
seedling evidently begins during the first season after the burning of the
forest. In the case of the various species of Ribes information is far from
complete. The investigations on the seedling of Ribes carried on in connection with the blister rust control will undoubtedly yield some very interesting
and valuable information in this respect. The principal species composing
the second set of plants are:
Alnus tenuifolia Nutt.
Acer glabrum Torr.
Betula fontinalis Sarg.
Ceanothus velutinus Dougl.
Cornus canadensis L.
Cornus stolonifera Michx.
Fragaria sp.
Holodiscus dumosus Nutt.
Lonicera utahensis Wats.
Lonicera involucrata Banks
Alder
Rocky Mountain maple
Mountain birch
Mountain balm
Trailing dogwood
Willow dogwood
Strawberry
Ocean spray
Honeysuckle
Honeysuckle
74
J. A. LARSEN
Prunus demissa Nutt.
Ribes reniforme Rydb., Nutt.
Ribes viscossimum Dougl.
Rubus parviflorus Nutt.
Rubus strigosus Michx.
Sambucus glauca Nutt.
Sambucus melanocarpa Gray.
Symphoricarpos racemosa Michx.
Sorbus angustifolia Rydb.
Vaccinium nembranaceum Dougl.
Vaccinium occidentale A. Gray.
Ecology, Vol. X, No. i
Choke cherry
Currant
Currant
Thimble berry
Raspberry
Elderberry
Elderberry
Snowberry
Mountain maple
Huckleberry
Huckleberry
Among the above species the Acer, Holodiscus, Ceanothus and Betula
may be either wind disseminated or arise by sprouts from earlier specimens.
Lonicera, Ceanothus, Prunus, Holodiscus and Vaccinium are distinctly
dry site species, while Betula, Alnus, Sambucus, Rubus parviflorus, Rubus
strigosus and Cornus are moisture loving. The remaining species are somewhat intermediate in their moisture requirements. After a second burn dry
south slopes may support nothing but a scattered growth of Ceanothus velutinus or C. sanguineus. Along the water courses where the forest canopy
is broken and small patches of fertile soil occurs, one may find impenetrable
thickets of Cornus stolonifera, Spiraea tomentosa and Ribes reniforme.
The influence of this group of shrubs is distinctly beneficial, for the
rapidly accumulating leaf mold expedites the rehabilitation of the impoverished soil, the foliage shades and shelters seedlings of the mesophytic forest
trees of fir, spruce and white pine, and the roots provide an effective check
on soil erosion. This stage of the subordinate vegetation, therefore, becomes
an important nurse crop for the white pine and its associated species. After
eight or ten years the young white pines emerge from underneath the shrubs,
overtop them, and crowd them out entirely. The maple, only, may persist
within and underneath the new evergreen forest for a period of fifty to
sixty years.
With the elimination of this second set of subordinate species the forest
builds its own layers of litter and humus, paving the way for a third set of
subordinate vegetation of somewhat uninfluential and unassuming proportions composed of the following species:
Adenocaulon bicolor Hook.
Actea rubra Ait
Adiantum pedatum L.
Boykinea heucheriforme Rydb.
Aralia nudicaulis L.
Circea pacifica Asch. and Magn.
Claytonia chrysantha Greene
Silver green
Baneberry
Maiden hair fern
Boykinea
Wild sarsaparilla
Circea
Spring beauty
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FIRES
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Celernatis occidentalis Homen.
Clintonia uniftora (Schult) Kunth
Felix mas
Galium triflorum Michx
Galium boreale L.
Limnia spathulata (Dougl.) Heller
Mertensia spathulata (Dougl.)
Mentha canadensis L.
Mertensia ciliata (Torr) Don.
Osmorrhiza sp.
Seraphis gigantea Dougl.
Streptopus amplexifolius L.
Trilium petulatum Purch.
Valeriana septemtrionalis Rydb.
Vagneria liliacea (Greene) Rydb.
Valeriana sitchensis Bong.
Veronica scutellata L.
Veratrurn Eschscholtzianum (R. & S.) Rydb.
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IDAHO
75
Virgins bower
Clintonia
Wood fern
Three-flowered bed straw
Northern bed straw
Miner's lettuce
Howell
Wild mint
Bluebell
Sweet cicily
Hellebore
Trillium
Valerian
False Solomon seal
Veronica
Skunk cabbage
Of the above, Boykinea, Seraphis, Streptopus, Montia, Mentha and
Veratrum grow only along the streams or on the very wet ground under the
broken forest canopy. The remaining species may be found anywhere on
moist soil wherever the forest canopy is not dense. The influence of the
above species on soil building or in preventing erosion is almost negligible.
It is much less important than in any of the preceding associations.
Almost simultaneously with the invasion of this climax formation will
appear the climax species in the subordinate forest vegetation. The outstanding species in the subordinate climax associations are the following:
Arctostaphyllos uva-ursi L.
Asarum caudatum Lindl.
Berberis sp.
Chimalphila umbellata Nutt.
Coptis occidentalis Nutt. T. and G.
Fatsia horrida Benth. and Hook.
Heuchera ovalifolia Nutt.
Linnea borealis L.
Lycopodium selago L.
Lycopodium sitchensis Rydb.
Pachystima myrsinites Raf.
Pyrola uliginosa Torr.
Claytonia chrysantha Greene
Tiarella unifoliata Hook.
Viola glabella Nutt.
Kinnikinnick
Wild ginger
Oregon grape
Pipsissiwa or Princess pine
Gold thread
Devils club
Alumroot
Twin flower
Clubmoss
Clubmoss
Goat brush
Round shinleaf
Spring beauty
False miterwort
Violet
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Ecology,Vol. X, No. i
In this association, Fatsia and numerous ferns line the water courses;
Coptis, Asarum, Pyrola, and Lycopodium are of the most tolerant, for they
persist underneath the densest forest. In the intermediate class as to moisture and light requirements are Linnaea, Pachystima, Berberis, and among
these one finds many species of the foregoing group, especially Tiarella,
Claytonia, Galium, Montia and Circea.
The influence of this series upon erosion, soil building or protection to
the seedlings is nil.
SUMMARY
When the cedar-hemlock-grand fir forest in the Bitterroot Mountains has
been destroyed by fire the forest eventually returns to the climax composition through a series of associations which follow each other in regular succession, beginning in most cases with two intolerant or drought resistant
species, western larch and lodgepole pine. This first stage, together with the
distinct first stage of the subordinate vegetation improve the site and pave
the way for a second forest association composed chiefly of Douglas fir and
western white pine with its own peculiar subordinate type of vegetation.
This second stage eventually gives way to the climax forest association of
cedar, hemlock and grand fir and its own distinct and permanent subordinate
vegetation of perennial evergreen species.