BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Larix occidentalis | Western Larch 



GENERAL BOTANICAL CHARACTERISTICS : 
Western larch is a rapidly growing, deciduous, coniferous tree which may
live for more than 700 years [34].  Evidence provided by stumps in
Montana indicates that early cutting included western larch older than
900 years [30,66].  Trees may reach 260 feet (80 m) in height [58].  In
the Pacific Northwest a typical mature western larch is 164 feet (50 m)
tall and 55 inches (140 cm) d.b.h. [34].

Western larch has a deep and extensive root system that provides
moderate to high resistance to windthrow [118].  Short roots are
ectomycorrhizal [55].  The ectomycorrhizal root tips are found in
mineral soil more often than those of other conifers, although some are
also found in decayed wood and humus [52,53,54].  

The thick bark of western larch is furrowed into large plates from which
cinnamon-colored scales may flake off [58].  The crown is relatively
short, narrow, and less dense than most conifers [82].  Young twigs are
glabrous or pubescent rather than tomentose like alpine larch [4,58].
The thin, light green, deciduous needles occur at the tips of short
lateral spur shoots in a whorl of 15 to 30 needles on a spur.  Needles
on seedlings and leaders of older plants are decurrent and arranged in
spirals along the stem [4,58].  The deciduous habit allows larches to
avoid winter desiccation.  Western larch can withstand defoliation by
insects or disease better than evergreen conifers and competes well with
them because its needles require less carbon to construct; it
efficiently relocates nitrogen before needlefall.  Its photosynthetic
capacity is high, and its needles receive ample light through the open
crown [4,40].


RAUNKIAER LIFE FORM : 
   Undisturbed State:  Megaphanerophyte
   Disturbed State:  Megaphanerophyte
   Disturbed State:  Therophyte


REGENERATION PROCESSES : 
Cones:  Although trees may produce some cones when 8 or 10 years old,
cone production is infrequent before the age of 25, and abundant
production does not begin until trees are 40 to 50 years old [116].
Good cone crops are produced irregularly but average 1 every 5 years
[116,118,131].  Cones are born on short spur branches; more female cones
occur the upper crown and more male cones in the central and lower crown
[25,58,106,116].  Since cones are produced throughout the crown, larger
trees produce most of the cone crop [116,131].  Details of pollination
and early seed development within western larch cones have been
described [92,93].  When mature, the reddish-brown to brown cones are
between 1 and 1.5 inches (2.5-4 cm) long and have thin, papery scales
and small, pointed bracts extending beyond each scale [4].

Seed:  Western larch seeds are small, averaging 137,000 per pound
(302,000/kg), and light with a large wing [106].  It requires more
filled seed to establish a seedling when the seeds are light than when
they are heavy, as are the seeds of some other conifers [127].  In one
study it took an average of 53 filled seeds to produce a live western
larch seedling [127].  Western larch may form normal appearing but
inviable seeds if pollination does not occur or the embryo aborts [93].
Seeds are stratified naturally over winter and should be stratified for
prompt germination in nursery planting [71,106,133].  Seed treatments
are discussed by various authorities [132,133].

Seed dispersal:  Cones open when they have dried to a moisture content
of 35 to 40 percent [91,116,125].  Generally over 80 percent of the
cones open by mid-October, although seed dispersal may extend into
winter [48,116].  The light seeds are dispersed by wind.  Frequently,
they are carried 800 feet (250 m) and sometimes over 984 feet (300 m)
[33,76].  Patterns of dispersal in clearcuts show the amount of seed
decreases rapidly for the first 400 feet (122 m), then remains at a low
level.  Thermal upslope winds aid dispersal at mid to low elevations,
but on upper slopes, where seed fall is later, storm fronts aid
dispersal [76].

Seedling establishment:  Western larch seed germinates about the time of
snowmelt [116,133].  Normal first year growth is about 2 inches (5 cm),
but seedlings may grow to 5 inches (13 cm) in the Blue Mountains of
Oregon [85,116].  Germination is good on a variety of materials,
including duff, rotten wood, and mineral soil [31,73].  However,
survival is poor on undisturbed litter, humus, sod, and in areas with
heavy root competition.  Survival is usually best in mineral soil.
Seedling roots are not able to penetrate layers of duff rapidly enough
to obtain sufficient water [126,127].  In central Idaho, the best growth
of 3- to 5-year-old western larch seedlings occurred on moss mats and
next best on bare mineral soil [38].  During early development, seedling
mortality is often caused by fungi, rodents, birds, and insects [116].
In the summer, most seedling mortality is caused by the physical factors
of excess heat (from insolation) or drought and is highest on south- and
west-facing slopes [122,132].

Vegetative reproduction:  Western larch does not sprout.  Researchers
have rooted cuttings and grafted western larch [116].


SITE CHARACTERISTICS : 
Western larch is not as tolerant of summer drought as many other
conifers and is generally found on north- or east-facing slopes and
other relatively moist sites [2,33,82].  On drier sites at western
larch's lower elevation or southern range limits, it is frequently
unable to establish seedlings on south- or west-facing slopes, but in
moist areas in the middle and northern portion of its range, it grows on
all exposures [116].  Height growth is most rapid in valley bottoms and
on lower north and east slopes and poor on upper south- and west-facing
slopes [116].

Western larch in southwestern British Columbia is common in the interior
Douglas-fir zone, frequent in the interior cedar-hemlock and montane
spruce zones, and less frequent in the ponderosa pine zone [65,67].  In
mountain hemlock stands in northern Idaho and western Montana, it does
best below 5,600 feet (1,707 m) [43].  In the same region, it grows in
western redcedar, western hemlock, grand fir, moist subalpine fir, and
Douglas-fir habitat types [30,44,95].  In the Cascade Mountains of
northern Oregon, western larch is very frost tolerant and grows with
Pacific silver fir, mountain hemlock, fool's huckleberry (Menziesia
ferruginea), beargrass (Xerophyllum tenax), blue huckleberry (Vaccinium
membranaceum), grouse whortleberry (V. scoparium), bunchberry dogwood
(Cornus canadensis), and dwarf Oregon grape (Mahonia nervosa) [57].

Soil and soil nutrients:  Usually western larch grows on soils
classified as Inceptisols and Alfisols and occasionally on Spodosols
[116].  Most of the soils supporting western larch developed in glacial
till or colluvium containing argillite, quartzite, and limestone
bedrock, and are deep and well drained [116].  Results of one study
indicate that trees growing on sites with compacted subsoil and thus
slow drainage grow well as juveniles, but growth slows when root growth
becomes restricted and nutrients in the rooting zone are depleted.  In
contrast, trees on rapidly draining sites grow slower initially, but
subsequent productivity is higher [134].

In British Columbia, western larch grows on moderately dry to moist,
medium-nutrient-rich to very-nutrient-rich soils [65].  It needs high
levels of calcium and magnesium in the soil [67].  In many areas,
including western Montana, western larch is associated with moist soils
containing volcanic ash [87,116].  Western larch retranslocates 87
percent of foliage nitrogen before needle fall, possibly due to poor
soil availability and uptake of nitrogen [40,39].

Elevation:  Elevational ranges in some western regions are [4,23]:  

                     Minimum                   Maximum
                   feet      meters         feet      meters

British Columbia   2,000      610           5,500     1,676
Montana            3,200      975           8,100     2,469
Oregon             3,000      930           7,000     2,134
Washington         2,000      610           5,500     1,676


SUCCESSIONAL STATUS : 
Western larch is a long-lived and highly shade-intolerant seral species
[116].  It can tolerate partial shading only as a seedling.  If it is
overtopped later, its crown deteriorates and it loses vigor and dies
[116,117].  For the first 100 years of life, larch grows faster in
height than any other conifer in the northern Rocky Mountains [116,118].
Since it is shade intolerant, it grows in even-aged stands, although
other tree species may appear younger because they are smaller.  As
these stands mature, shade-tolerant conifers continue to establish and
form younger understories [116].  In the western redcedar-western
hemlock zone of northern Idaho and western Montana, western larch is
frequently a close associate of lodgepole pine (Pinus contorta) in
pioneer communities.  It may be a minor element in these communities
initially, but fire recurrence during early succession can greatly
increase the proportion of western larch [44].  In the Garnet Mountains
of western Montana, western larch seems less able than lodgepole pine to
establish under an open canopy, and thus less shade tolerant [142].


SEASONAL DEVELOPMENT : 
Leaves of western larch begin to develop early in spring before the
cambium becomes active.  Spring growth appears to be controlled by
photoperiod rather than temperature [16].  The buds of western larch
flowers (conelets) are initiated in early spring, develop during the
summer, and remain dormant during fall and winter [91].  The following
spring, pollen and seed cones open before leaf buds, usually between
mid-April and mid-May.  Pollination occurs in late May and early June
[116].  Cones ripen in August and seeds are dispersed in September and
October.  At high elevations ripening and dispersal may occur 2 to 4
weeks later [76].  In October the needles turn from green to a golden
yellow before falling [2].  Results of an Idaho study indicate that the
time of leaf fall may be correlated with soil moisture [24].

Phenological observations of western larch made over a 10-year period in
northern Idaho and western Montana are summarized below [114]:

                        Earliest       Average        Latest
                          Date          Date           Date

Buds burst              March 20       April 30       June 11
Terminal shoot 
 growth begins          April 10         May 14       July  2
Pollen begins           April 26         May 20       June 29
Pollen ends               May 10        June  3       July 14
Shoot growth ends        June 18        Aug.  1       Sept. 6
Winter buds formed       July 11        Aug. 22       Oct. 11
Cones full size          July  2        Aug.  6       Sep. 11
Cones open               July 31        Sep.  4       Sep. 30

Related categories for Species: Larix occidentalis | Western Larch