BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Pinus cembroides | Mexican Pinyon 



GENERAL BOTANICAL CHARACTERISTICS : 
Mexican pinyon is a native, monoecious small tree.  It averages 23 feet
(7 m) tall with a trunk diameter of 7 to 12 inches (17.8-30.5 cm), but
in protected areas it can reach up to 50 feet (15 m) high and have a
trunk diameter of 14 inches (35 cm) [16,33,41,69,75].  Stout, spreading
branches make a compact to spreading, rounded crown [16,75].  The bark
is thin, 0.5 inch (1.3 cm) or less [33,57,70].  Evergreen needles are in
bundles of three or infrequently in bundles of two or four.  The needles
are 0.8 to 2 inches (2-5 cm) long [16,33,75,90].  Cones are 0.8 to 2
inches (2-5 cm) long [25,51].  The seeds are thick walled, wingless, and
0.5 to 0.75 inch (1.3-1.9 cm) long [16,33,69].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Mexican pinyon regenerates by seed; reports of vegetative reproduction
were not found in the literature.  Mexican pinyon begins bearing seeds
at about 25 years, although trees on dry sites have delayed maturity
[57].  Seed production increases with age [28,33].  Cones require 3
years to mature.  In Arroyo Ancho, Chihuahua, Mexico, Mexican pinyon
produced about 110 to 125 cones per tree during 1978 [29].  Large crops
are produced at intervals of 3 to 8 years [28,33,44].  Synchrony of cone
crops has not been found [29].  Rainfall is positively correlated with
the production of the cone buds.  Summer moisture stress at the time of
cone bud production occasionally may promote an above-average crop of
strobili the following spring, indicating that reproductive growth is
favored over vegetative growth when trees are stressed.  Temperatures in
late August and early September are negatively correlated with cone
production; maximum production occurred when the mean maximum weekly
temperature was below 75 degrees Fahrenheit (24 deg C) [28].

Cones can experience heavy predation by invertebrates; up to 90 percent
of the seed crop may be lost to insect predation [29,33].  Insects
destroy a higher percentage of the cones from small than large cone
crops [53].

Mexican pinyon and other pinyons produce only a few seeds per cone.
Seed viability is initially high (85-95%) but decreases within 1 year
[25,33].  The heavy, wingless seeds fall and germinate beneath the
relatively open seed tree canopies [80].  The seeds are adapted to
dispersal and burial by rodents, other mammals, and birds such as
Clark's nutcrackers and jays [48,53,57,85].  Clumped seedlings may
reflect animal facilitation and/or microsite differences.

Conditions for cone and seed collection and seed germination are
discussed in the literature [29,44,89].  Seeds are thick walled, but
stratification is not necessary for germination [25,29,44].

Establishment of Mexican pinyon seedlings depends on the availability of
adequate water during the first dry seasons.  Mexican pinyon
establishment is facilitated by nurse plants such as low shrubs and
trees [5,29,53].  In a transplant study, Mexican pinyon seedlings
without cover died within 8 weeks, and seedlings planted beneath
adjacent oak canopies survived [29].  Four-month-old seedlings of
Mexican pinyon and other pines were subjected to drought tolerance
experiments in a greenhouse study.  Mexican pinyon was the most drought
resistant species; its seedlings survived 60 days without water [6].

Tree growth rates vary widely [17,33,57].  Mexican pinyon stand
structure and regeneration patterns are strongly influenced by
disturbances such as fire [80].


SITE CHARACTERISTICS : 
Mexican pinyon occurs in semiarid climates with relatively high
temperatures and evaporation rates.  Precipitation is bimodal with wet
winters and summers [53,66,82].

Mexican pinyon is found on nearly level to steep slopes, foothills and
ridgetops, and in mid- to upper slope draws, ravines, and washes
[23,36,61,68,80].  It belongs to woodlands that are very dense on
north-facing slopes but open on south- and east-facing slopes [14].

Mexican pinyon typically occurs from 4,000 to 7,000 feet (1,219-2,188 m)
in elevation [14,19,33,84].  The maximum elevation of Mexican pinyon
changes with latitude, extending from as low as 2,000 feet (610 m) in
the northeastern part of its range to as high as 8,203 feet (2,500 m) in
southern Mexico [13,80,93].

Mexican pinyon occurs on soils that vary in texture and depth [29,70].
Soils may be shallow and eroded with textures ranging from sandy loam to
loamy sand [29,80].  However, soils also can be more than 60 inches (152
cm) deep with moderately fine to very fine textures or with alluvial
layers of stratified sands, gravels and cobbles [19,23,61,86].  Mexican
pinyon occupies soils derived from a broad range of parent materials
including granite, basalt, limestone, sandstone, and mixed alluvium
[17,59].

Along a moisture gradient from mesic to xeric sites in Arizona, Mexican
pinyon was not present on the mesic end of the gradient.  It had 124
stems per acre (306 stems/ha) at the midmesic point and increased to 618
stems per acre (1526 stems/ha) at the most xeric end of the gradient.
The increase in Mexican pinyon stem number may have been due to a
release from competition with other less drought-tolerant conifers such
as Chihuahua pine [96].


SUCCESSIONAL STATUS : 
Facultative Seral Species

Mexican pinyon does well as an understory tree when canopies are
relatively open.  Most Mexican pinyon seedlings establish beneath open
canopies in partial shade.  Mexican pinyon becomes less shade tolerant
as a sapling [20,25,33,80].

Mexican pinyon woodlands vary in community structure; succession is not
well defined for these systems.  The pine-oak woodlands and forests in
which Mexican pinyon occurs are considered both as ecotones that are not
well developed and as climax vegetation that is extensive and well
developed [11,30].

Since pinyons will invade dry rocky sites, Little [53] suggested that
Mexican pinyon could be considered a pioneer species.  However, most
authors identify it as a late successional or climax species, especially
since nurse plants facilitate Mexican pinyon establishment [20,24,25,
100].  Mexican pinyon is climax in pinyon, evergreen oak, and Chihuahuan
pine woodlands [49].  Mexican pinyon is an infrequent and minor seral
species in white fir (Abies concolor) and Douglas-fir forests [21,49].



SEASONAL DEVELOPMENT : 
New leaves are formed annually in the spring and persist for 3 to 4
years [33].  Seed production requires 3 years.  Female cone buds are
formed from August to September [28].  Pollination occurs the following
spring during March and April [98].  Cones mature from August through
October of the third year [28,90].  Seeds begin to drop by mid- to late
October [33].

Related categories for Species: Pinus cembroides | Mexican Pinyon