BOTANICAL AND ECOLOGICAL CHARACTERISTICS
SPECIES: Pinus attenuata | Knobcone Pine 



GENERAL BOTANICAL CHARACTERISTICS : 
Knobcone pine is a rapidly growing, native tree from 20 to 40 feet (6-12
m) tall and 13.5 to 23 inches (34-58 cm) in d.b.h. [13,41].  The crown
is dense and broad when young, becoming open when mature.  Trees
typically have multiple trunks with thin bark [36,42].  Excavtion of
knobcone pine roots in the Santa Ana Mountains showed that vertical
roots grew to bedrock in the shallow soil.  Average root depth was 10.4
inches (26.2 cm) [41].  Roots in less restrictive sites are reported as
"wide and deep" [36].

Trees produce female cones in groups of four or five, all firmly
attached to stout branches in a tight whorl.  The asymmetrical cones are
arched in configuration, as are the individual ovuliferous scales.
Cones remain closed and attached to the tree for life [40,48].  The
enclosed seeds are small and light, with thin seed coats and long seed
wings [15,41].  The lifespan of knobcone pine is relatively short.  Some
trees reach ages of 75 to 100 years [14], but in a typical 60-year-old
stand, over half the pines are dead [41].


RAUNKIAER LIFE FORM : 
   Phanerophyte


REGENERATION PROCESSES : 
Knobcone pine reproduction is controlled exclusively by fire; trees
occur in even-aged stands dating back to the last fire [42,43].  Unlike
other closed-cone species whose cones open with hot weather, upon
falling, or with age, unburned knobcone pine cones remain closed even
after trees have decayed and fallen.  Cones are sealed with a hard resin
that requires high temperatures (average: 397 degrees Fahrenheit [203
deg C]) to liquefy, boil, and vaporize.  Cone scales open gradually
following heating.  The first seeds fall within 1 to 12 hours after
fire, when the ground has cooled.  The arched scales continue to slowly
expand and drop seed for at least 4 postfire years.  Scales partially
contract during periods of rain or other high relative humidity, but
resume expansion when relative humidity drops [41].  The small, light
seeds are wind dispersed.  Knobcone pine has the greatest seed wing
length:seed size ratio of all the California closed-cone pines, allowing
for seed dispersal well beyond the edges of a fire [15].  Santa Ana
foehn winds, which blow during periods of low relative humidity, spread
seed for great distances.  Seed wings from charred or scorched cones
often have fire-seared tips, causing seeds to fall in a slower spin than
seeds with unburned wing tips.  Seeds with burned wings fall closer to
the parent tree.  Birds aid in disseminating some seed.  Steller and
scrub jays, attracted to partially opened cones, pound them heavily to
extract seeds.  This results in additional seed dropping to the ground.
Hairy and downy woodpeckers may also jar seed from cones as they work
over burned stems in search of insects [41].  Western grey squirrel are
sometimes able to chew through unopened cones and may disseminate small
amounts of seed [42].

Trees begin seed production between 10 and 12 years of age.  Average
production of trees over 20 years old is 176 cones per tree [41].
Limited tests show seed viability does not decline with age.  Seeds
enclosed in cones for 27 [45] and 60 [41] years have proved viable.
Following release, seeds require cold stratification for 60 days [3,19].
Germinative capacity of seeds from mechanically opened cones has varied
from 57 to 91 percent [19,41].  Hot fire probably kills some seed.
Laboratory tests show that germination rates of seed from mechanically
opened cones are greater than those of cones opened by oven heat
treatment.  Seeds may require a rise from normally low soil pH for
germination, and fire creates such a condition [41].  Knobcone pine
germinates earlier than other pines.  Tested against Coulter and sugar
pines, it was the first of the three species to germinate [47].
Seedlings require bare mineral soil for establishment.  They are drought
tolerant, with a strong tendency toward deep rooting [47].  Seedlings
establishing on fertile sites compete poorly with chaparral shrubs and
other tree species.  Knobcone pine seedlings, however, can tolerate
nutrient-deficient soils which restrict the growth of most competitors
[14,29,41].  Knobcone pine does not vegetatively reproduce [19].


SITE CHARACTERISTICS : 
The climate in which knobcone pine grows is mediterranean, characterized
by wet, mild winters and hot, dry summers.  Fog drip often precipitates
heavily beneath pines during summer months in coastal regions,
ameloirating the effects of hot weather [41].  The pines grow at
elevations between sea level and 5,500 feet (1,676 m) [44].

Soil parent materials are usually of volcanic origin [43]; serpentine is
the most common substrate [18,29].  Soils are typically shallow, rocky,
infertile, ultramafic, acid, and/or dry.  They may contain levels of
magnesium, chromium, nickel, and/or cobalt that are toxic to most plants
[44].  Calcium, nitrogen, and phosphorus are usually deficient [43].
Soil pH at a knobcone pine site in the Santa Ana Mountains was 5.0 [43].
Water-retaining capacity of knobcone pine soils are often favorable to
its growth.  The average saturation percentage of serpentine soils is
nearly double that of adjacent chaparral [41].  Slope angles range
between 0 and 38 degrees but are most commonly steep and subject to
continual erosion.  Knobcone pine communities often occur along fault
blocks where earthquake activity has produced fresh serpentine
escarpments [41].

Plant associates:  Overstory associates not listed in Distribution and
Occurrence include Monterey pine, Coutler pine (Pinus coulteri), Digger
pine (P. sabiniana), sugar pine (P. lambertiana), shore pine (P.
contorta spp. contorta), bigcone Douglas-fir (Pseudotsuga macrocarpa),
Pacific madrone (Arbutus mensiesii), tanoak (Lithocarpus densiflora),
giant chinkapin (Chrysopelis chrysophylla), incense-cedar (Libocedrus
decurrens), Pacific yew (Taxus brevifolia), tecate cypress (Cupressus
forbesii), Santa Cruz cypress (C.  abramsiana), and MacNab cypress (C.
macnabiana) [26,27,28,44].

Some shrub associates are Eastwood manzanita (Arctostaphylos
glandulosa), pinemat manzanita (A. nevadensis), chamise, chaparral
whitethorn (Ceanothus leucodermis), wartleaf ceanothus (C. papillosus
var.  rowaenus), wedgeleaf ceanothus (C. cuneatus), leather oak (Quercus
durata), chaparral currant (Ribes malvaceum var. viridifolium), Sargent
cypress (Cupressus sargentii), chaparral pea (Pickeringia montana), and
huckleberry (Vaccinium spp.)  [6,14,42,44,46].

Ground cover associates include Carey balsamroot (Balsamorhiza
deltoidea), Hooker balsamroot (B.  hookeri), fire reedgrass
(Calamagrostis koeleroides), houndstongue hawkweed (Hieracium
cynoglossoides var. nudicaule), big deervetch (Lotus crassifolius),
showy phlox (Phlox speciosa), and brome grasses (Bromus spp.) [14,41,46].


SUCCESSIONAL STATUS : 
Obligate Initial Community Species

Knobcone pine is a shade-intolerant pioneer species [9,36,40].  Fire
creates the conditions necessary for its continued survival.  Old
knobcone pine stands, undisturbed for 60 or more years, will show signs
of invasion and competition from surrounding communities because the
resultant soil genesis and organic matter deposition have begun to
reduce or cover the restrictive barriers produced by serpentine [41].
In the absence of fire, knobcone pine is replaced by chaparral shrub
species at lower elevations and other conifers at higher elevations
[1,41].


SEASONAL DEVELOPMENT : 
Seeds germinate from early February through late March, depending on
elevation [47].  Pollination occurs from March until May [33].

Related categories for Species: Pinus attenuata | Knobcone Pine