|Abstract or Summary
- This study describes the soils, vegetation and age structure
of the Pinus contorta forests of Crater Lake National Park. Growth
rates of P. contorta, and levels of infection and impact of dwarf
mistletoe (Arceuthobium americanu1r) on growth of P. contorta
individuals are compared among communities.
Vegetation description is based on 81 circular 500 m² sample
plots in forest with at least 50% P. contorta. Communities were
defined using association tables and a computerized similarity
ordination. Tree ages were determined at 30 cm, and age classes
were defined. Severity of dwarf mistletoe infection was estimated
using a seven class rating system.
Eleven communities are defined, of which three are climax,
or at least persistent P. contorta, and eight are seral. All
but one are found on Steiger soil, or this soil mixed with non-pumiceous
rock. An important factor controlling P. contorta
climax community distribution may be soil temperature, as determined
by length of the snow-free period and topography. Presence
and distribution of seral communities may reflect climate and
result from higher soil nutrient levels, as determined by the
distance to non-pumiceous material.
There is considerable variability in size of P. contorta at a
given age. Both diameter and height seem to reflect site quality
and the two are highly correlated; maximum tree sizes differ
among some communities. Growth is greatly reduced after 100 years.
Age analyses and historical records suggest that, though fires
occurred in Crater Lake National Park before the arrival of white
man, they were unusually frequent from 1850 to 1900 because of his
activities. Most fires in P. contorta probably were intense
enough to destroy, or nearly destroy, a stand, and could occur
at intervals less than ten years. A minimum estimate of mean
fire frequency is between 25 and 50 years. The area of P. contorta
forest probably increased when white-man caused fires burned
Invasion patterns of shade tolerant species suggest that
repeated low intensity fires were common in two communities. In
one, fire-scarred trees support this hypothesis. Widespread fires
were probably rate in the P. contorta/Carex-Stipa community.
I suggest that true climax forest of Abies and Tsuga does
not exist here on pumice soils. A model is presented in which P.
contorta develops to Abies-Tsuga forest, which eventually
degenerates, burns, and recycles to P. contorta.
Dwarf mistletoe infection slightly reduces height and diameter
growth in heavily infected trees. Sapwood thickness is reduced
only slightly, if at all. Phloem thickness may be reduced in
the P. contorta/Carex-Lupinus community by heavy mistletoe infection;
trees large enough to be susceptible to mountain pine beetle
(Dendroctonus ponderosae) attack have slightly greater dwarf mistletoe infection than smaller trees in this and the Abies lasiocarpa/
Haplopappus/Aster-Elymus communities. Stand age accounts for almost
30% of the variability in mean stand infection level which is
highest in old stands and lowest in isolated or repeatedly burned
areas. Evidence of bark beetle activity is found in all communities.
There probably have been no irreversible changes in vegetation
because of fire suppression over the past 75 years. Permitting
lightning fires to burn and suppressing man-caused fires should
reestablish a more varied age structure and permit some P. contorta
to develop to Abies-Tsuga, while some Abies-Tsuga will probably
burn and recycle to P. contorta. Meadow area should increase as
will the cover of some shrubs, such as Ribes cereum.