FIRE EFFECTS
SPECIES: Pinus ponderosa var. ponderosa | Pacific Ponderosa Pine 



IMMEDIATE FIRE EFFECT ON PLANT : 
Fire has a wide variety of potential effects on Pacific ponderosa pine.
These effects vary according to size, configuration, and density of the
stand, in addition to fire severity.  Generally, well-spaced seedlings
and saplings are able to withstand low-severity fires, as are pole-sized
and mature trees.  Moderate- to high-severity fires, however, will kill
trees pole-sized and smaller.  Mature Pacific ponderosa pines have a
higher survival rate than younger trees due to their enhanced
adaptations to fire [16,39,77].

The principal cause of mortality following fire is crown scorch rather
than damage to the cambium or roots.  The size of tree determines its
ability to withstand scorch.  A model has been developed to predict
mortality using tree d.b.h. and scorch heights as independent variables.
Fire effects are also dependent upon other factors such as season, site
condition, tree age and vigor, available moisture, and occurrences of
insect and disease attack [39,56].


DISCUSSION AND QUALIFICATION OF FIRE EFFECT : 
If fire consumes any part of tree canopy, the total leaf area is
reduced, thus decreasing photosynthesis.  If burning results in damage
to the bole or roots, nutrient and water transport is impaired.  Heat
from fire may kill living tissue and result in a certain amount of
stress [26,39,55].

Crown damage:  Crown scorch appears to be the leading factor in the
majority of damage to Pacific ponderosa pine.  Estimation of percent
crown volume scorch has been proven to be the best predictor of tree
mortality following fire.  Crown damage is most severe in spring and
early summer due to low foliar moisture content and the succulent nature
of the buds and twigs [39,55].  Survival of buds is also crucial to
postburn survival of Pacific ponderosa pine.  Buds can tolerate
temperatures 68 degrees F (20 deg C) higher than the needles can due to
their protective outer scales.  Therefore, large trees can sometimes
survive a 100 percent crown scorch provided not all the buds are heat
killed [26,39,44,55].

Root damage:  Following prescribed burning of old-growth Pacific
ponderosa stands in Crater Lake National Park, Oregon, mortality was
higher in burned areas (19.5 percent) than in unburned areas (6.6
percent).  A major factor contributing to postfire mortality was the
reduction of fine roots.  Burning reduced fine-root dry weight 50 to 75
percent from 1 to 5 months after burning [64].

Bole damage:  This pine is fire tolerant because it has a fire-resistant
bark containing a 1/8- to 1/4-inch (0.3-0.6 cm) thick layer at 2 inches
(5 cm) diameter [77].  It also has a very moist core of high density
wood that dissipates the heat energy it receives, thus protecting the
bole from lethal heat levels [55].  Ryan and Frandsen [53], however,
found that mature Pacific ponderosa pine trees suffered more basal
injuries from smoldering fires than did immature trees because of the
greater quantities of accumulated duff surrounding their boles.  Cambium
damage most often occurs after the passing of high-severity fires.
Young trees are most susceptible to cambium damage as a result of
thinner bark and a higher occurrence of girdling [26,78].  Partially
girdled trees may survive up to 25 percent basal loss if root and crown
damage is minimal [71].

Season of burn:  Pacific ponderosa pine can withstand low-severity fires
which generally occur during the wet months of early spring or late
fall.  A dry spring fire may occur when trees are in stress during leaf
and bud burst, resulting in higher mortality rates.  Trees become
dormant toward fall and thus are more fire resistant.  In fall, Pacific
ponderosa pine can withstand up to 50 percent crown scorch, while in
spring only 30 percent [26,45].


PLANT RESPONSE TO FIRE : 
Pacific ponderosa pine's response to fire will vary according to fire
severity, tree age, and season.  High-severity fires that occur during
periods of high stress will generally result in death.  Low- to
medium-severity fires will generally restrict the growth and
regeneration of the tree, but recovery is usually evident the following
year [39].  Immediately following fire, Pacific ponderosa pine may
experience a large needle drop as a reaction to hot convectional air
movement through the canopy [55].  

Postfire seedling establishment:  Fire creates favorable seedbeds for
seedling establishment.  The soil is often rich in available inorganic
nitrogen that benefits tree growth [52].  Postfire stocking rates depend
upon site characterisitcs, fire severity, and weather.  The potential
for regeneration after fire is generally considered good [39].  On the
Eldorado Natinal Forest, California,a low-severity burn resulted in
20,000 seedlings per acre (49,400/ha) on burned sites and no seedling
establishment on unburned sites [39].  In a western Montana study,
Pacific ponderosa pine produced 12 percent of the total number of sound
seeds found on a burned clear-cut site over a 5-year period [64].  A
postburn study in the Plumas National Forest, California, found that
Pacific ponderosa pine had the highest postburn percent increase of all
other associated species [39].  Postfire stocking rates depend upon site
characterisitcs, fire severity, and weather.  The potential for
regeneration after fire is generally considered good [39].  In a western
Montana study, Pacific ponderosa pine produced 12 percent of the total
number of sound seeds found on a burned clear-cut site over a 5-year
period [64].  A postburn study in the Plumas National Forest,
California, found that Pacific ponderosa pine had the highest postburn
percent increase of all other associated species [39].

Postfire growth and recovery:  Information concerning Pacific ponderosa
pine's response after fire is variable.  This may be attributable to the
beneficial effects of reduced competition and increased nutrient
availability, along with the detrimental effects of damage to the crown,
cambium, and roots.  Some studies found reductions in diameter and height 
growth [46,76], while others reported increases in postfire growth 
[8,46,69,73].



DISCUSSION AND QUALIFICATION OF PLANT RESPONSE : 
NO-ENTRY


FIRE MANAGEMENT CONSIDERATIONS : 
The last 100 years has produced unplanned, radical changes in stand
structure, fuel loadings, and role of fire in Pacific ponderosa pine
ecosystems [2].  Postsettlement fire suppression has resulted in dense
stockings of shade-tolerant species and the increase of insects and
disease.  These results have led to other concerns such as loss of
timber productivity, loss of natural diversity and aesthetic values, and
the increased risk of severe fire damage to homes and harvestable timber
[26].

Prescribed fire:  Reduction of fuel loads beneath existing stands of
Pacific ponderosa pine by the use of prescribed fire has proven useful
in reducing the potential threat of wildfires, while also favoring
natural regeneration of seral species through site preparation.  In
western Montana, prescribed burning on an interval of 20 to 25 years is
suggested to maintain seral species and open stocking.  This would also
prune lower branches thus increasing timber values, while also lowering
the risk of wildfire [2,22,26,39].

Nutrient depletion:  Prescribed fire often leads to the loss of volatile
nutrients from the site, especially nitrogen (N).  Following a
prescribed fire on a Pacific ponderosa pine site in Oregon, all periodic
annual growth increments were reduced in surviving trees four growing
seasons later.  Foliar N concentration was not affected by the fire;
however, total foliar N content was reduced immediately after burning.
Foliar N content was significantly correlated with the observed
reductions in periodic annual increments [39,40].

Related categories for Species: Pinus ponderosa var. ponderosa | Pacific Ponderosa Pine