BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Pinus strobiformis | Southwestern White Pine 



GENERAL BOTANICAL CHARACTERISTICS : 
Southwestern white pine is a native, evergreen conifer with an open,
irregular crown.  It can grow to 90 feet (27 m) tall with diameters to
3.2 feet (1 m) [15,53,64].  The bark is thin, rough, and furrowed
[15,54].  Branches are long and horizontal to pendant [53].  Needles are
2.4 to 4 inches (6-10 cm) long in fascicles of five [53].  The cones are
2.8 to 9.8 inches (7-25 cm) long with reflexed, thick scales; cones are
dehiscent when mature [15,46].  Seeds are essentially wingless and 0.4
to 0.5 inch (10-12 mm) long [15,53].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Southwestern white pine reproduces sexually.  It begins to produce cones
when pole sized or about 15 years [39,44].  The interval between seed
crops for any one tree is 3 to 4 years [44].  Seed traps were placed in
clearcut mixed-conifer forest in Arizona to monitor regeneration over 3
years.  No southwestern white pine seeds were found in the traps [39].

Southwestern white pine seeds require 2 weeks to 4 months stratification
before germinating [43,44,78].  Germinability can vary between 52 and 95
percent [43,44].  Seed collection and germination methods are discussed
in detail [36,44].

Seeds of the southwestern white pine ripen synchronously throughout a
forest and overwhelm the harvesting efforts of predators [11,34].  Red
squirrels clip entire cones and cache them [34].  The wingless seeds are
dispersed by birds, primarily by the Steller's jay and Clark's
nutcracker [11,47,72].  Animal caches result in clustered stands [72].

Southwestern white pine seedlings root deeply (to about 8 inches [20.3
cm]) the first year, which increases their survival under drought
conditions [40,43].  All of the southwestern white pine seedlings died
in a greenhouse experiment that assessed the drought resistance of
conifers along an elevational gradient.  Southwestern white pine died
after significantly (p<0.05) fewer days than other conifer species from
lower elevations [10].

With the initial deep root growth, southwestern white pine seedlings had
the slowest top growth rate of four conifer species measured.  At about
6 years of age, the average height of southwestern white pine seedlings
was 13.3 inches (33.8 cm) [40].  In another study, 2-year-old seedlings
were between 4 and 8 inches (10.2-20.3 cm) tall [36].



SITE CHARACTERISTICS : 
Southwestern white pine is widespread in mesic sites on ridges, slopes,
and canyons of montane zones [15].  The best growth of this species
occurs on moist, cool sites with deep soil [14,53].  The climate is
semiarid, characterized by mild winters and warm summers.  There are two
wet seasons:  July to September and December to March [2,10].  Average
precipitation ranges from 19.2 to 45 inches (487-1,143 mm) per year
[25,27,57].

Southwestern white pine occurs in ravines or on mesic lower slopes at
5,000 feet (1,525 m) in southeastern Arizona and goes up to timberline
in southwestern Colorado [52,76].  It is typically found at elevations
from 6,000 to 10,000 feet (1,830-3,048 m) [17,52].  Southwestern white
pine often occurs on north- to east-facing slopes, but it has been
reported on all aspects [1,11,17,57,71].

Southwestern white pine is found on sites with loamy soil textures
ranging from shallow, gravelly loams to deep, sandy loams to stoney
silty clay loams [23,30,32,40].

Two to nine conifer species occur in the southwestern mixed-conifer
forest type.  Their proportions vary depending on site characteristics
[37,48].  Some associated species not mentioned in Distribution and
Occurrence are listed below.  Associated tree species are New Mexico
locust (Robinia neomexicana), Gambel oak (Quercus gambelii), and
silverleaf oak (Quercus hypoleucoides) [14].  Associated shrubs are
mountain snowberry (Symphoricarpos oreophilus), bush oceanspray
(Holodiscus dumosus), Arizona honeysuckle (Lonicera arizonica), and
Fendler ceanothus (Ceanothus fendleri) [8,14,23,57].  Other associated
plants are Arizona fescue (Festuca arizonica), screwleaf muhly
(Muhlenbergia virescens), pine dropseed (Blepharoneuron tricholepsis),
Arizona wheatgrass (Elymus arizonicus), and western yarrow (Achillea
lanulosa) [8,23,51].


SUCCESSIONAL STATUS : 
Facultative Seral Species

Southwestern white pine is frequently a persistent, long-lived seral or
climax species in mixed-conifer forests [4,26,51].  It has been
classified as an early successional major tree [60].  Southwestern white
pine infrequently is a climax dominant or codominant in open stands
[4,51,69].

Southwestern white pine is relatively shade intolerant compared to other
associated conifers in the southwestern mixed-conifer forests.  It is
relatively resitant to damage from full sunlight [41].  In 8 out of 12
sites in New Mexico, southwestern white pine was in the overstory, but
in only 2 of those 12 sites was it in the understory.  Ahlstrand [1]
suggested that canopy closure prevented southwestern white pine
replacement at these sites.


SEASONAL DEVELOPMENT : 
Southwestern white pine flowers in June.  Cones mature in September, and
seeds are dispersed from September to October [44].  Southwestern white
pine seeds germinate either in the spring or in the summer after the
rains begin [40].

Related categories for Species: Pinus strobiformis | Southwestern White Pine