Age structure and spatial pattern of old-growth ponderosa pine in Pringle Falls Experimental Forest, central Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9p290d777

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  • 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.
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