Associated Forest Cover
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Western larch is a long-lived seral species that always grows with other
tree species. Young stands sometimes appear to be pure, but other species
are in the understory, Douglas-fir (Pseudotsuga menziesii var.
glauca) is its most common tree associate. Other common tree
associates include: ponderosa pine (Pinus ponderosa) on the lower,
drier sites; grand fir (Abies grandis), western hemlock (Tsuga
heterophylla), western redcedar (Thuja plicata), and western
white pine (Pinus monticola) on moist sites; and Engelmann spruce
(Picea engelmannii), subalpine fir (Abies lasiocarpa), lodgepole
pine (Pinus contorta), and mountain hemlock (Tsuga
mertensiana) in the cool-moist subalpine forests (44).
Western larch makes up a majority or plurality in the forest cover type
Western Larch (Society of American Foresters Type 212) (43). It is
included in 11 other cover types:
205 Mountain Hemlock
206 Engelmann Spruce-Subalpine Fir
210 Interior Douglas-Fir
213 Grand Fir
215 Western White Pine
218 Lodgepole Pine
220 Rocky Mountain Juniper
224 Western Hemlock
227 Western Redcedar-Western Hemlock
228 Western Redcedar
237 Interior Ponderosa Pine
Classification systems based on potential natural vegetation have been
developed for much of the geographic area where western larch grows. Larch
is a seral species in 13 of the 21 habitat types described for eastern
Washington and northern Idaho (7). In Montana, larch is a significant
component in 20 of the 64 forest habitat types (21). Of these 20 habitat
types, larch is a major seral species in 12, and a minor seral species in
8. These habitat types are found within the following forest series: the
relatively dry-warm Douglas-fir; the moist grand fir, western redcedar,
and western hemlock; and the cold-moist subalpine fir.
Larch forests typically have a rich understory flora with dense
herbaceous and less dense shrub layers. It is not unusual to find as many
as 7 tree species and 40 undergrowth species in plots of 405 m²
(4,356 ft²) (21). On a 40-ha (100-acre) study area on the Coram
Experimental Forest in northwestern Montana, 10 conifer, 21 shrub, and 58
herbaceous species were recorded (31). Some of the common understory
species associated with larch are the following:
Shrubs
Rocky Mountain maple
Acer glabrum
Sitka alder
Alnus sinuata
Serviceberry
Amelanchier alnifolia
Oregongrape
Berberis repens
Menziesia
Menziesia ferruginea
Mountain lover
Pachistima myrsinites
Ninebark
Physocarpus malvaceus
Rose
Rosa spp.
Thimbleberry
Rubus parviflorus
Common snowberry
Symphoricarpos albus
Dwarf huckleberry
Vaccinium caespitosum
Blue huckleberry
Vaccinium globulare
Scouler willow
Salix scouleriana
Spiraea
Spiraea betulifolia
Herbs
Wild sarsaparilla
Aralia nudicaulis
Kinnikinnick
Arctostaphylos uva-ursi
Arnica
Arnica latifolia
Pinegrass
Calamagrostis rubescens
Queenscup
Clintonia uniflora
Fireweed
Epilobium angustifolium
Twinflower
Linnaea borealis
Beargrass
Xerophyllum tenax
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Climate
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Western larch grows in a relatively moist-cool climatic zone, with low
temperature limiting its upper elevational range and deficient moistures
its lower extremes (44). Mean annual temperature within the larch zone is
about 7° C (45° F), but annual maximums average 29° C (84°
F) and minimums average -9° C (15° F) (table 1) (35). Average
temperatures during the May through August growing season are about 16°
C (60° F) with July the warmest month. The frost-free season varies
from about 60 to 160 days, usually from early June through early
September. Frosts can occur any month of the year.
Table 1- Summary of weather data from within the range
of western larch¹
°C
°F
Average Temperature
Annual maximum
29
84
Annual minimum
-9
15
Annual mean
7
45
Annual absolute maximum
41
106
Annual absolute minimum
-37
-34
Growing season only
15
59
mm
in
Average precipitation
Total annual
710
28
Total during growing season²
160
6
Total snowfall
2620
103
¹Data compiled
from 12 weather stations in Idaho, 10 in Montana, 3 in Oregon, and 4 in
Washington using U.S. Department of Commerce summaries for 1951 through
1960 (35).
²May through August.
Annual precipitation in larch forests averages about 710 mm (28 in) in
the north part of its range to 810 mm (32 in) in the south. The extremes
where larch grows are about 460 mm (18 in) and 1270 mm (50 in). About
one-fifth of the annual precipitation occurs during the May through August
growing season, most of it in May and June. July and August are usually
dry and are characterized by clear, sunny days (60 to 80 percent of the
daylight hours), low humidity, and high evaporation rates (44). Elevation
and geographic location affect both the amount and the form of
precipitation. On midelevation sites, snow commonly blankets most larch
forests from November to late April and accounts for over half the total
precipitation. Snow accounts for an even higher proportion of the total
precipitation in the northerly higher elevation Portions of larch forests.
One high elevation larch site at Roland, ID, receives an average of 620 cm
(244 in) of snow annually. Lower elevation sites commonly receive an
average of more than 150 cm (60 in) of snow.
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Damaging Agents
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Mature larches are the most fire-resistant
trees in the Northern Rockies because of their thick bark, their high and
open branching habit, and the low flammability of their foliage. Poles are
moderately resistant, but seedlings and saplings have very little
resistance to fire (44).
Larch is moderately to highly resistant to windthrow because of its
extensive root system. Isolated old-growth seed trees or those along
cutting boundaries, however, are susceptible to windthrow, particularly
those on upper slopes and ridgetops, or those in narrow canyons and
saddles where winds are channeled (35).
Because larch is deciduous, its branches seldom accumulate excessive
amounts of either snow or ice. Early fall or late spring snows
occasionally catch larch with a full complement of needles and cause
severe bending. After a heavy June snow on the Coram. Experimental Forest,
young larch were completely flattened, but they recovered surprisingly
well with little apparent long-term damage (34).
Young larch is extremely sensitive to noxious fumes, but because it is
deciduous, the tree accumulates fewer harmful deposits than other
conifers. Fluorine and sulfur dioxide are both harmful, but fluorine is
the more toxic. Fluorides at levels of 30 to 35 p/m produce toxic needle
effects (5).
Dwarf mistletoe (Arceuthobium laricis) is the most damaging
disease-causing parasite of larch. It can infect seedlings as young as 3
to 7 years old and continue throughout the life of the tree (49). In
addition to killing tree tops, reducing seed viability, creating
conditions suitable for entry of other diseases and insects, and causing
burls, brashness, and some mortality, it decreases height and diameter
growth. Basal area growth reductions can be expected as follows (22):
light infection, 14 percent; medium infection, 41 percent; and heavy
infection, 69 percent.
Infected residual-stand overstories left after logging or fires promptly
infect understory stands. Mistletoe seed can be ejected as far as 14 m (45
ft) (42). Thus 50 evenly-spaced, diseased trees per hectare (20/acre) may
infest understory trees with just one crop of mistletoe seeds. Proper
harvest-cutting systems, particularly clearcutting, can substantially
reduce the mistletoe problem.
Three other important diseases are found in larch: needlecast caused by
Hypodermella laricis, the quinine fungus Fomitopsis
officinalis, and red ring rot caused by Phellinus pini. Many
other less common but potentially dangerous fungi, such as Encoeliopsis
laricina, infect larch but have not caused significant problems in the
past (35).
Larch casebearer (Coleophora laricella) and western spruce
budworm (Choristoneura occidentalis) are currently the two most
serious insect pests of western larch (35). Casebearer was first detected
in the Northern Rockies in 1957 and since then has spread throughout
virtually the entire larch forest type (11). Introduced and native
parasites, plus adverse weather conditions on many larch sites, appear to
be reducing the casebearer problem, however. Severe defoliation by the
casebearer can substantially reduce tree growth, but mortality usually is
low.
Western spruce budworm has been a persistent problem wherever heavy
populations of budworm overlap the range of larch (12). The most serious
damage to larch is severance of the terminal leader, which results in an
average loss of about 25 to 30 percent of the height growth for that year
(32).
Other insect species affecting larch include the larch sawfly (Pristiphora
erichsonii) and the larch bud moth (Zeiraphera improbana) that
cause heavy, but sporadic, damage. The western larch sawfly (Anoplonyx
occidens), the two-lined larch sawfly (Anoplonyx laricivorus),
and the larch looper (Semiothisa sexmaculata incolorata) also
damage larch from time to time. Bark beetles are not generally a serious
problem for larch, but the Douglas-fir beetle (Dendroctonus
pseudotsugae) occasionally attacks weakened trees. At times, the
engraver beetle (Ips plastographus), the larch engraver (Scolytus
laricis), and the false hemlock looper (Nepytia canosaria) damage
larch.
Damage from larger animals is relatively minor. Rodents, because of
their seed- and seedling-eating habits, can greatly influence seedling
establishment. Larch is apparently unpalatable to most big game species.
In addition, most larch forests occur in areas of heavy snowpack not
suitable for winter game range (35). Bears, however, can be a local
problem. They strip the bark on the lower bole of the most vigorous trees
in young sapling and pole-sized stands during the spring of the year and
often kill the trees.
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Flowering and Fruiting
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Western larch is monoecious; both
staminate and ovulate flowers develop throughout the crown. Buds are found
at the end of short spurlike lateral branchlets. Vegetative buds are
smaller than flower buds-usually about 2.5 to 3.0 mm (0.10 to 0.12 in) in
diameter, whereas flower buds range from about 3.0 to 4.8 mm (0.12 to 0.
19 in) in diameter. Ovulate buds are one to one and one-half times longer
than they are wide and are rounded or conical on the end. Staminate buds
are usually globose and about one and one-half to two times longer than
wide. Vegetative and flower buds can be detected early in the fall, about
1 year before subsequent cone crops mature. Methods of sampling buds and
conelets have been devised for forecasting larch seed crops on individual
trees, as well as stands (24).
Pollen and seed conelets appear several days before vegetative buds
open-usually from about April 15 to May 15 (44). Conelets are generally
very conspicuous, varying from bright red to green. Pollination occurs in
late May and early June (33). Cones complete their development in one
season and mature by mid- to late-August, reaching 2.5 to 4.5 cm (1.0 to
1.8 in) in length.
Cones usually begin to open by early September, but in cool-moist
summers cone opening may be delayed a month or longer. More than 80
percent of the seeds usually are dispersed by mid-October (44). Cones open
when they have dried to a moisture content of 35 to 40 percent, opening at
the same time on individual trees, but varying substantially among trees
in the same stand (39). Cones usually fall from the tree during the
following winter, but many may stay attached through the next summer.
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Genetics
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Population Differences
No differences in cold hardiness of 1-year-old larch seedlings were
detected from 78 populations before frost in early September (23).
Regardless of geographic origin, 2-year-old seedling populations separated
by 1000 m (3,300 ft) tended to differ by 1.4 days in bud burst, 1.1 weeks
in bud set, and 8 cm (3.1 in) in height (21 percent of the height
variance) when growing in the average test environment.
Races and Hybrids
Races of western larch are not known. Putative natural hybridization of
western larch and subalpine larch (Larix lyallii) occasionally
occurs in areas where their distributions overlap (4). Even where the
geographic ranges of the, two species overlap, usually elevations of 300 m
(1,000 ft) or more separate them. Interspecific hybrids of western larch
and Japanese larch (Larix leptolepis) were taller and more
vigorous than open-pollinated western larch progenies at the end of the
first and second growing seasons (48).
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Growth and Yield
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Western larch is long-lived and is the largest
of the world larches (20). Trees exceeding 230 cm (90 in) d.b.h. and 900
years of age have been found (44). Larch normally reaches 30 to 55 m (100
to 180 ft) in height at maturity and occasionally exceeds 61 m (200 ft).
Larch grows faster in height than any other conifer in the Northern
Rockies for the first century, giving this highly shade-intolerant species
the height advantage it needs to survive. For the first 50 years, larch
and lodgepole pine height growth are similar, but thereafter lodgepole
height growth declines in comparison with larch.
Differences in height growth of larch and its associated species are
readily apparent at early ages. Both larch and lodgepole pine start off
faster than their associates. Studies on good quality sites on Coram
Experimental Forest in Montana show larch and lodgepole pine growing at
about twice the rate of Douglas-fir and three to four times faster than
subalpine fir and Engelmann spruce for the first 20 years. On wetter sites
in northern Idaho, larch and lodgepole pine typically grow much faster
than western white pine, western hemlock, and western redcedar in
unthinned natural stands for the first half century. In thinned stands,
however, differences in height growth of western white pine and larch are
nominal. By age 100, the height growth advantage larch holds over its
associates typically becomes less pronounced (35,10).
Site productivity accounts for the largest share of the variation in
height growth of larch throughout its range. Site index curves for larch
(base age of 50) show heights at age 100 ranging from 20 m (65 ft) on low
sites to 40 m (130 ft) on high sites (table 2). Average site indices for
larch on different ecological habitat types are given in table 3.
Table 2- Height of average dominant and co-dominant
western larch by age and site index
______Site index at
base age 50 years______
Age
12.2 m or 40 ft
18.3 m or 60 ft
24.4 m or 80 ft
yr
m
m
m
20
3
4
6
40
9
14
19
60
14
21
29
80
17
26
35
100
20
30
40
yr
ft
ft
ft
20
9
14
19
40
31
47
63
60
47
70
94
80
57
86
115
100
65
97
130
Table 3- Average site incdices for larch (21,35)
Ecological habitat type
Average site index at
base age 50 years
m
ft
Northern Idaho and Washington:¹
Abies lasiocarpa-Xerophyllum tenax
14.9
49
Abies lasiocarpa-Pachistima myrsinites
17.7
58
Tsuga heterophylla-Pachistima
myrsinites;
Thuja plicata-Pachistima
myrsinites; Abies
grandis-Pachistima
myrsinites
20.1
66
Pseudotsuga menziesii-Physocarpus
malvaceus
18.9
62
Pseudotsuga menziesii-Calamagrostis
rubescens
16.8
55
Montana:
Pseudotsuga menziesii-Vaccinium
caespitosum
18.0
59
Pseudotsuga menziesii-Physocarpus
malvaceus
17.4
57
Pseudotsuga menziesii-Linnaea borealis
16.8
55
Picea-Vaccinium caespitosum
22.6
74
Thuja plicata-Clintonia uniflora
19.2
63
Tsuga heterophylla-Clintonia uniflora
24.4
80
Abies lasiocarpa-Clintonia uniflora
19.2
63
Abies lasiocarpa-Linnaea borealis
17.1
56
Abies lasiocarpa-Menziesia ferruginea
20.4
67
Abies lasiocarpa-Xerophyllum tenax
15.5
51
¹Based on
Daubenmire's classification (6).
Physiographic position, directly interrelated with habitat type, also
influences height growth. Larch grows most rapidly in height on the deep,
moist soils of valley bottoms and lower north and east slopes, but poorly
on the upper south and upper west slopes (35):
Physiographic class
Average site index
m
ft
Valley bottoms
18.9
62
Midnorth and mideast facing slopes, lower south and
lower west facing slopes and benches
18.0
59
Upper north and upper east facing slopes
17.4
57
Midsouth and midwest facing slopes
16.2
53
Upper south and upper west facing slopes
13.4
44
Seedbed conditions at the time of seedling establishment influence
height growth in the formative years (27). Studies on Priest River
Experimental Forest in northern Idaho showed that on the average
2-year-old larch seedlings were twice as tall on burned seedbeds as they
were on bare mineral or duff-covered soil (14). Subsequent studies on
Coram Experimental Forest showed that these height growth differences
persisted into the teenage years, with larch growing about one-third
faster on burned seedbeds than on scarified or undisturbed seedbeds (35).
These differences may be due to changes in nutrient availability, water
infiltration into the soil, or competing vegetation. Microchemical tests
showed increased levels of manganese, magnesium, nitrogen, phosphorus, and
calcium in the upper soil layers of burned seedbeds (14).
Stand density also affects height growth very early in the life of the
stand (27). Heavy overstocking is common in young stands with densities
sometimes exceeding 86,500 trees per hectare (35,000/acre). In a
9-year-old stand at Coram Experimental Forest for example, dominant larch
were growing a third faster in height in stands with 12,400 trees per
hectare (5,000/acre) than they were in stands with 86,500/ha
(35,000/acre). Thinning these overstocked stands relieved this height
growth suppression, but even the dominant trees in unthinned stands
continued to grow well below their potential in height (30). By age 24,
dominant trees in the thinned stands averaged more than 9 m (30 ft) tall,
but their counterparts in the unthinned stands averaged 15 to 20 percent
less (29).
Diameter growth measured at breast height (1.37 m or 4.5 ft) for larch
largely parallels height growth and is affected by many of the same
factors. Larch has the potential for rapid diameter growth, but
overstocking, insects, and dwarf mistletoe often prevent full realization
of this potential.
Potential diameter growth curves have been developed for western larch
on different combinations of habitat type and site index to provide a
basis for evaluating tree and stand conditions (table 4) (35).
Table 4- Potential d.b.h. of western larch trees at age
50 and at age 100 years by ecological habitat type and site index (35)
______________Site
index at base age 50 years_____________
Ecological habitat type
Age
12.2 m or 40 ft
18.3 m or
60 ft
24.4 m or
80 ft
yr
cm
cm
cm
1. Abies lasiocarpa-Xerophyllum tenax
50
13.7
19.3
-
100
25.1
33.8
-
2. Pseudotsuga menziesii-Physocarpus malvaceus
50
14.5
19.8
-
and Calamagrostis
rubescens
100
26.7
35.0
-
3. Abies lasiocarpa-Pachistima myrsinites
50
-¹
20.3
25.9
100
-
35.8
44.7
4. Abies grandis-Pachistima myrsinites
50
-
20.6
26.2
100
-
36.6
45.2
5. Tsuga heterophylla-Pachistima myrsinites
50
-
20.8
26.2
and Thuja
plicata-Pachistima myrsinites
100
-
36.8
45.2
yr
in
in
in
1. Abies lasiocarpa-Xerophyllum tenax
50
5.4
7.6
-
100
9.9
13.3
-
2. Pseudotsuga menziesii-Physocarpus malvaceus
50
5.7
7.8
-
and Calamagrostis
rubescens
100
10.5
13.8
-
3. Abies lasiocarpa-Pachistima myrsinites
50
-
8.0
10.2
100
-
14.1
17.6
4. Abies grandis-Pachistima myrsinites
50
-
8.1
10.3
100
-
14.4
17.8
5. Tsuga heterophylla-Pachistima myrsinites
50
-
8.2
10.3
and Thuja
plicata-Pachistima myrsinites
100
-
14.5
17.8
¹Dashes indicate
that values are outside the data base.
These projections, based on relatively open trees, show larch at, age
50, reaching diameters ranging from a high of 26 cm (10.3 in) on high to
14 cm (5.4 in) on low quality sites; at age 100, 45 cm (17.8 in) to 25 cm
(9.9 in).
Larch diameter growth is very sensitive to stand density. For example,
in 9-year-old stands on Coram Experimental Forest, overstocking of 86,500
trees per hectare (35,000/acre) had already restricted diameter growth of
the dominant trees to half that of their counterparts in stands with
12,400/ha (5,000/acre) (27). At age 19 and 24, dominant trees in these
unthinned stands (with about 37,100/ha or 15,000/acre) continued growing
at about half the rate of their counterparts in thinned stands (with about
1,000 trees per hectare or 400/acre). For example, at age 24, dominant
trees in thinned stands averaged nearly 13 cm (5 in) compared to about 8
cm (3 in) for dominants in unthinned stands (29). Elsewhere, 30- to
50-year-old stands in Montana showed about the same diameter
relationships, with crop-trees in unthinned stands growing at about half
their potential (25).
Basal area increases rapidly to about age 40 years, decelerates, and
nearly levels off after age 100. At age 100, basal area of larch forests
approaches 69 m²/ha (300 ft²/acre) on high quality sites and
about 46 m²/ha (200 ft²/acre) on low quality sites. On high
sites, the average annual increase in basal area is about 0.7 m²/ha
(3 ft²/acre) for the first century. Average increase during the 100-
to 200-year period is only about one-tenth the rate noted in the first 100
years. As basal area stocking approaches site potential, increment drops
off rapidly-the site is fully occupied.
Larch forests can produce heavy timber volumes. The increase in volume
follows a similar pattern as basal area but peaks later. Because of their
influence on diameter and height growth, site quality, age, and stocking
level play the major roles in volume yield. Projected cubic yields for
larch forests at age 100 range from 308 m³/ha (4,407 ft³/acre)
on low quality to 813 m³/ha (11,608 ft³/acre) on high quality
sites (table 5). With full stocking (but not overstocked), 544 m³/ha
(7,765 ft³/acre) is a reasonable objective by age 100 on medium
quality sites for larch forests.
Table 5- Total volume of western larch trees 1.5 cm (0.6
in) and larger in d.b.h. (35)
_____Site index at
base age 50 years_____
Age
12.2 m or 40 ft
18.3 m or 60 ft
24.4 m or 80 ft
yr
m³/ha
20
¹17
30
45
40
105
184
275
60
191
336
502
80
258
454
¹678
100
308
544
¹813
yr
ft²/acre
20
¹246
434
648
40
1,494
2,632
3,934
60
2,724
4,801
7,176
80
3,680
6,484
¹9,692
100
4,407
7,765
¹11,608
¹Values in
italics are extrapolated beyond the range of the basic data.
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Reaction to Competition
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Larch is the most shade-intolerant
conifer in the Northern Rockies. Only during the seedling stage can it
tolerate partial shading. If larch is overtopped its crown rapidly
deteriorates, and its vigor declines severely.
Because of its intolerance to shade, larch grows in even-aged stands or
age-classes. Its primary associates are usually the same age as larch but
often give the appearance of being younger because they grow slower than
larch and form the lower strata in the stand. As larch stands mature,
however, shade-tolerant associates continue to establish and form younger
understories.
Fire is essential to the maintenance of western larch in natural forest
stands. Most fires that occur on mountain slopes are usually small and of
low or moderate intensity (8). Fire intensity, however, increases on steep
slopes with heavy fuels, or on dry ridgetops. These fires thin stands,
reduce fuels, rejuvenate undergrowth, and prepare seedbeds that promote
mixed conifer stands with small pockets of regeneration dominated by seral
species, particularly western larch. Intense fires often create definite
even-aged stands. At Coram, multiple burns occurring at less than 50-year
intervals favor lodgepole pine or shrub fields. Historically, within the
mixed conifer/pinegrass communities of the Blue Mountains of Oregon,
underburns occur at 10-year intervals and maintain western larch and other
seral species in the stands (15). Here, all species, including western
larch, often overstock and can stagnate unless periodic fires release some
trees. Without fire, grand fir and Douglas-fir replace the seral species.
Although larch normally remains in the dominant position, understory
trees and other vegetation vigorously compete with larch for available
water and nutrients. In one harvest-cutting study, diameter growth of
residual mature seed trees after logging increased 67 percent over
pre-logging growth (44). When all understory trees were also cut, the seed
trees increased an additional 36 percent in diameter increment.
Even-aged silviculture systems of shelterwoods, seed-tree cuttings, and
clearcuts best fit the ecological requirements of larch forests. They
provide an adequate seed source and the microsite conditions needed for
establishing the new seedlings. They are also compatible with the site
preparations of prescribed burning 'or scarification needed to reduce the
duff layers and vegetative competition for the new seedlings. Prescribed
burning most closely approximates the natural wildfires that historically
have perpetuated larch forests. No detrimental impact on site quality has
been attributed to harvesting or prescribed fire on the soil microflora
(16).
Conversely, uneven-aged silviculture systems have limited utility in
most larch forests. Not only does the residual stand show little overall
growth response after partial cuttings, the growth increases that do occur
are mainly on the more tolerant and generally less desirable species, such
as subalpine fir. In addition, partial cuttings discriminate strongly
against larch and its shade-intolerant associates in the regeneration
process, and larch becomes a minor stand component in stands it formerly
dominated. Prescribed burning or scarification needed to regenerate larch
are very difficult in partial cuttings. For management considerations
other than timber production, such as esthetics or wildlife, there may be
rationale for uneven-aged silviculture systems in some larch forests. Even
here, however, it should be recognized that these practices violate the
normal regeneration sequence in most of these forests, accelerate the
succession to tolerant species, and increase insect and disease problems.
Studies on Coram Experimental Forest have demonstrated many of the
problems with single-tree selection cuttings. Even with special care, it
is extremely difficult to use group-selection cuttings in old-growth larch
forests.
Exceptions to the above are possible in some of the drier phases of
Douglas-fir and grand fir habitat types, particularly in the Blue
Mountains of Oregon and some lower elevation areas of western Montana.
Here, natural underburns at 20- to 30-year intervals perpetuated more
open-grown stands and allowed the establishment of western larch and
ponderosa pine regeneration under the main forest canopy (1,15).
Uneven-aged silviculture systems that mimic these natural conditions are
plausible in these types of larch forests.
Thinning in young western larch stands, preferably before age 20,
enhances the growth of diameter and height during the juvenile years when
response potential is greatest. Drastic reduction in the densities found
in most unthinned stands is advisable. Studies in young larch show that
larch responds well in diameter, height, and crown retention under a
fairly broad range of densities after thinning, usually exceeding what
were thought to be maximum growth rates (30). Even at ages 30 to 50, larch
responds well to release (25,36). By this age, however, overstocking has
reduced the crown and response is usually delayed. Timing and extent of
response is a function of length and severity of overstocking. Individual
tree growth once lost can never be regained.
Branch turnups following thinning can be a problem in young larch
stands. If a tree is cut off above a live branch, it may turn up, reform
the tree, and reduce the effectiveness of the thinning (35). Older larch
sometimes produce sprouts from adventitious buds on the upper bole of the
tree after thinning of older stands, but this effect may not have
practical significance. The amount of sprouting increases with the
severity of the thinning (25).
Preliminary studies of fertilization in Montana (2) show a positive
diameter growth response to fertilization with nitrogen, but the effects
last only about 3 years. Similar studies in Idaho showed a short-term
diameter growth response to nitrogen (13), but neither study showed any
height increase.
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Rooting Habit
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Larch develops a deep and extensive root system,
but little information is available about its root growth. Root lengths on
first-year natural seedlings usually reach 5 cm (2 in). Under good nursery
conditions, well-developed fibrous roots 20 cm (8 in) or longer develop on
1-0 growing stock. Observations in soils under young larch stands indicate
extensive fibrous rooting in the top 50 cm (20 in), substantially less in
the 50-100 cm (20-40 in) depths, and practically none at greater depths.
Soil water depletion studies verify these observations in young larch
stands (29). Heavy rooting at depths greater than the above has been
observed along roadcuts through old-growth stands. Evaluations of roots of
windfallen overmature larch show that nearly all of them were infected
with root rots (35). Apparently, these rots play an important role in wind
stability of overmature trees, but their importance in young trees is not
known.
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Seed Production and Dissemination
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Larch is a good seed
producer, but cone crops vary substantially by year and location.
Long-term records of larch seed production in Montana show that good seed
crops are produced at about 5-year intervals with fair to poor crops in
the intervening years (44). Two good crops or several poor crops, however,
may occur in close succession. Overall, the ratio of good or fair to poor
seed crops is about 1 to 1.
Cone production is infrequent on larch trees less than 25 years old,
although trees as young as 8 years occasionally produce cones. Larch
starts bearing abundant cone crops from 40 to 50 years and continues
bearing heavily for 300 to 500 years (35). Only dominants and codominants
produce significant numbers of cones (44).
Cone production usually is a function of crown size because larch bears
cones throughout the crown. Trees with the largest crowns produce the most
cones. During a good cone year, production ranged from a low of 56 cones
in one tree with 45 major branches to a high of 2,090 cones in another
tree with 95 major branches. Also, vigorous, full-crowned, mature trees
averaging 56 cm (22 in) in diameter produced about five times as many
seeds as 36-cm (14-in) trees in the same stand and age class (44).
A mature cone may have as many as 80 filled seeds per cone, but the
average is about half that number (39). Seed viability is related to
cone-crop size, ranging from a low of 5 to 10 percent viability in poor
crops to 70 to 80 percent in good crops. Young trees usually produce seeds
of higher viability than overmature trees.
Larch seeds are small and lightweight, averaging 302,000/kg (137,000/lb)
(45). Because of their relatively large wing, they are dispersed to
greater distances than the heavier seeds of Douglas-fir and subalpine fir,
but to about the same distance as the light seed of Engelmann spruce (37).
Larch seed may be dispersed 240 m (787 ft) from clearcut boundaries under
normal wind conditions (fig 1). Although the seeds traveling that distance
are only about 5 percent of that falling within the timber, they may
amount to 100,000/ha (40,000/acre) in a heavy seed year-more than is
adequate to restock favorable seedbeds. Overstocking often occurs near the
seed source when bare soil is exposed. Seeds are disseminated more
uniformly in seed tree and shelterwood cuttings than in clearcuts.
Figure 1- Dispersal characteristics of sound western
larch seed from a
seed source along a clearcut boundary.
Seed production in mature natural stands of larch may exceed 1.2 million
seeds per hectare (0.5 million seeds/acre) in a heavy seed crop. Records
at Coram. Experimental Forest indicate that small rodents eat only about 1
to 3 percent of the seeds during the overwintering period (41). In
contrast, rodents usually feed heavily on the larger seeds of Douglas-fir
and ponderosa pine during this same period.
Larch seed germinates about the time of snowmelt from late April to
early June, usually 1 to 2 weeks before associated tree species (38).
Germination is epigeal (45). Natural stratification of larch seeds during
the winter prompts rapid and complete germination. Without stratification,
spring-sown larch seeds germinate slowly and erratically, with some seeds
holding over until the next season. Artificial stratification methods
using cold-moist conditions work well for preparing seed for field
germination. These same seed treatments, as well as those using
stimulants, such as hydrogen peroxide, are particularly useful for testing
germinative energy and capacity (26). Air temperatures of about 27° C
(80° F) are ideal for larch seed germination, but seeds germinate at
temperatures 10° to 15° C (17° to 27° F) cooler than
that.
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Seedling Development
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Western larch is a seral species well
adapted to seedbeds exposed by burning (9) or mechanical scarification
(35,40). Seedbeds of undisturbed litter, humus, sod, and areas with heavy
root competition are poor for larch seedling survival. Most seedling
losses occur the first growing season- after 3 years seedling losses are
minor (35). For example, studies on areas favorable for larch show that 54
percent of the seedlings survived the first season; 85 percent of the
remaining seedlings survived the second season; and by the fifth season
the remaining seedlings survival was 94 percent. In other studies, an
average of 39 percent of the larch seedlings survived the first 3 years
(44).
Seedling survival is affected mostly by biotic factors early in the
growing season and by physical factors late in the season. Until about
mid-July mortality is caused primarily by fungi, rodents, birds, and
insects. Most losses of first-year seedlings, particularly those growing
on duff, are caused by fungi, usually immediately after germination.
Seedlings growing on mineral soil seedbeds are far less susceptible to
fungi than their counterparts growing on duff under both full sun and
partial shade. Under full shade, however, susceptibility on the two types
of seedbed is reversed (44). Seedling losses to animals, insects, and
birds are relatively minor overall but may be heavy in specific locations
and years.
Insolation is the most important physical factor affecting larch
seedling survival (38). High soil surface temperatures exceeding 57°
C (135° F) are not uncommon starting in late June, resulting in heat
girdling of seedlings at the soil-air interface. Again, duff is the least
desirable seedbed, with lethal temperatures occurring earlier in the
season and on more days. Lethal soil temperatures are reached most
frequently on duff, less on burned mineral soil, and least on scarified
mineral soil. On south and west slopes, soil surface temperatures exceed
79° C (175° F), and few larch seedlings survive regardless of
the type of seedbed (38).
Drought is the major physical factor affecting mid-to late-season
seedling survival. Unlike insolation, drought losses are heaviest in full
shade because of the heavy competition for moisture by all the associated
tree and understory vegetation.
Although aspect affects germination very little, it has a pronounced
effect on seedling survival. North, northwest, and northeast exposures and
gentle to flat topography provide the most favorable conditions for larch
seedling survival. High surface temperatures and droughty conditions on
the south and west exposures preclude survival of any significant number
of larch seedlings. As a result, larch is either absent or but a minor
stand component on hot, dry slopes.
Larch seedlings grow about 5 cm (2 in) the first growing season. In
shade, root penetration may average only 2.5 cm (1 in) the first year,
while its counterparts growing in the sun or partial shade may have 23 cm
(9 in) roots. Seedlings growing in partial shade usually grow faster in
height than seedlings in full sunlight for the first few years, but faster
in full sunlight after that.
Larch seedlings break dormancy very easily. Buds usually burst by late
April, well before those of any other native conifers. Shoot growth starts
from late May to mid-June.
Larch seedlings grow rapidly in spite of the relatively short growing
season of the Northern Rockies. Average annual height growth of about 30
cm (12 in) for the first 4 years is common (44). Of its major associates
only lodgepole pine matches the rapid juvenile height growth of western
larch. Douglas-fir seedlings grow at about one-half the rate of larch, and
Engelmann spruce and subalpine fir seedlings grow at about one-fourth the
rate of larch (28).
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Soils and Topography
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Silvics of North America tarafından sağlandı
Western larch grows on a wide variety of soils. The most extensive soils
have developed in glacial till or colluvium composed of materials derived
from limestone, argillite, and quartzite bedrocks of the Precambrian belt
geologic series. Larch also grows on soils developed in Recent and
Tertiary alluvium and Pleistocene lake sediments. Most soils suitable for
the growth of western larch are deep and well drained. Soils developed in
glacial till, colluvium, and recent alluvium have nongravelly to gravelly
loamy surfaces and gravelly to extremely gravelly loamy subsoils. Volcanic
ash is often incorporated into the surface horizon. Soils developed in
Tertiary sediments or Pleistocene lake sediments have silt loam surfaces
and silt loam, silty clay loam, silty clay, or clay subsoils.
Most soils supporting the growth of western larch are classified in two
orders of the soil taxonomy: Inceptisols and Alfisols. Occasionally
western larch is found on soils of the order Spodosols, but Spodosols are
not extensive within the range of western larch and generally occur above
the upper elevational limits of the species. A majority of the soils
supporting the growth of western larch are the Cryoboralf, Cryochrept, and
Cryandept great groups. Mean annual soil temperature of the soils within
the great groups is about 5° C (41° F) at 51 cm (20 in). At low
elevations on southern or western exposures within the range of western
larch, soil temperatures are warmer and soils supporting the growth of
western larch are in the Eutroboralf and Eutrochrept great soil groups.
Western larch grows best on the more moist Eutrochrepts or Eutroboralfs
and the lower elevation (warmer) Cryochrepts and Cryoboralfs. It is
commonly found growing on valley bottoms, benches, and north- and
east-facing mountain slopes. South and west exposures are often too severe
for larch seedling establishment, particularly on the drier sites found at
larch's lower elevational limits and the southern portion of its range. On
moist sites found in the mid-to northern-portion of its range and on mid-
to high-elevation sites, larch grows on all exposures.
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Special Uses
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Silvics of North America tarafından sağlandı
Western larch forests are valued for their multiple resource values. The
seasonal change in hue of larch's delicate foliage from light green in the
spring and summer, to gold in the fall, enhances the beauty of these
mountain forests.
Because larch is an aggressive pioneer species, it quickly reforests
areas denuded by natural or man-caused disturbances, providing protection
for those important watersheds in the Columbia River Basin. Western larch
is an important component of high water-yielding forests-areas where
management can influence water yield through harvest cuttings (19) and
young stand culture (29).
Larch forests provide the ecological niches needed for a wide variety of
birds and animals. Hole-nesting birds comprise about one-fourth of the
bird species in these forests, and studies on Coram Experimental Forest
show that broken-topped larch is a preferred site for the hole-nesters
(18). Deer, elk, moose, and the black and grizzly bear are widespread and
numerous throughout the range of larch.
Larch timber is used extensively for lumber, fine veneer, long-straight
utility poles, railroad ties, mine timbers, and pulpwood (35). Larch wood
is strong and hard and contains about 4 to 23 percent arabinogalactan. It
is the best domestic source of this water soluble gum used for offset
lithography and in food, pharmaceutical, paint, ink, and other industries.
Arabinogalactan has the consistency of honey and contains 16 percent
volatile pinene and limonene (44).
Timber harvesting practices in larch forests are now utilizing more of
the woody biomass formerly left in the woods after logging. Studies in the
last decade have aimed at characterizing this biomass and the
environmental consequences of removing biomass from larch forests (46).
Typically, large volumes of standing live and dead tree biomass are found
in old-growth larch forests (3). For example, of the 512 m³/ha (7,318
ft³/acre) found on a larch study area on Coram Experimental Forest in
western Montana, 55 percent was in standing green trees, 20 percent in
standing dead, and 25 percent in down material. In addition to tree
biomass, shrubs and herbs account for additional biomass (31). In terms of
weight, the average total biomass was 325 t/ha (145 tons/acre) with the
following distribution:
Pct.
Standing green and dead 7.6 cm (3 in) diameter and
larger
49
Crown material less than 7.6 cm (3 in) diameter
12
Down wood 7.6 cm (3 in) diameter and larger
11
Down wood less than 7.6 cm (3 in) diameter
3
Shrubs and herbs
2
Litter
1
Duff
22
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Vegetative Reproduction
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Larch does not reproduce by sprouts.
Cuttings have been successfully rooted by researchers at the Intermountain
Forest and Range Experiment Station, but methods have not been fully
tested at this time. One technique requires cutting 8 to 10 cm (3 to 4 in)
scions from young larch trees, dipping the lower portion of the cutting in
a powder mixture of 0.8 percent indolebutyric acid and 10.0 percent Captan
50 wettable powder (mixed with talc), and placing them in a rooting
chamber at about 24° C (75° F). Researchers at the Intermountain
Station have successfully grafted western larch.
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Distribution
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Western larch grows in the Upper Columbia River Basin of northwestern
Montana, northern and west central Idaho, northeastern Washington, and
southeastern British Columbia; along the east slopes of the Cascade
Mountains in Washington and north-central Oregon; and in the Blue and
Wallowa Mountains of southeastern Washington and northeastern Oregon.
- The native range of western larch.
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Brief Summary
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Silvics of North America tarafından sağlandı
Pinaceae -- Pine Family
Wyman C. Schmidt and Raymond C. Shearer
Western larch (Larix occidentalis), a deciduous conifer, is also
called tamarack and western tamarack; less commonly used names are
hackmatack, mountain larch, and Montana larch (17). It is largest of the
larches and is the most important timber species of the genus. Western
larch is used for lumber, fine veneer, poles, ties, mine timbers, and
pulpwood.
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