The dynamics of stands In old-growth ponderosa pine (Pinus
ponderosa) forests on the eastern flank of the central Oregon
Cascades are Investigated. Age structure and spatial pattern of
ponderosa pine within the Pringle Falls Experimental Forest
Research Natural Area coupled with the fire history of the area
aid In Interpreting stand development. Two 1.0-ha permanent reference stands were established in
the Pinus ponderosa/Purshia tridentata-Ceanothus velutinus/Stipa
occidentalis comunity type. Age data were taken from 294 Pinus
ponderosa and 223 Pinus contorta >10 cm dbh, and 98 Pinus
ponderosa and 57 Pinus contorta <10 cm dbh. Spatial dispersion of
trees within identifiable age cohorts was tested using Morisit&s
Index. The historical development of both stands since 1500 is
examined using a chronosequence of the location of stems of known
ages. Associations using chi-square tests are examined for the
identified cohorts of both species.
Both reference stands are composed of three identifiable age
cohorts. Comparison of the age data with a fire history of the
Pringle Falls Experimental Forest indicates that periods of
increased fire activity correspond to periods of decreased
survivorship in both stands. Regeneration of Pinus ponderosa and
Pinus contorta on both stands since the turn of the century, in
the absence of fire, indicates that successful regeneration within
these stands is not as dependent on fire-prepared mineral seedbed
as is regeneration in pine stands in the Southwest.
Trees on both stands in all three cohorts occur in clumps
ranging from 25-3500 m2. Distribution within the clumps is
aggregated as well, with small, usually even-aged clusters of
trees being a common component of the larger aggregations.
Two scales of pattern are identified in both stands. High
mortality during periods of increased fire activity produce
long-term temporal and coarse-grained spatial patterns of the broad age cohorts. Fine-grained spatial and shorter temporal
patterns result from contemporaneous reproduction in small areas.
Chi-square associations indicate that the spatial and temporal
patterns of the older cohorts have been maintained in the stands
for the last 250 years. Fire exclusion has caused a deterioration
in both types of patterns.
A fire regime that includes frequent (7-20 years)
low-Intensity burns must be introduced to these stands if the
desired goal is to maintain patterns characteristic of the
primeval forest. Furthermore, rare fires that are either larger
or hotter must be periodically included in the fire regime if
coarse-grained spatial and long-term temporal patterns are to be
maintained
