Stand structure in eastside old-growth ponderosa pine forests of Oregon and northern California

Abstract

Quantitative metrics of horizontal and vertical structural attributes in eastside old-growth ponderosa pine ( Pinus ponderosa P. and C. Lawson var. ponderosa) forests were measured to guide the design of restoration prescriptions. The age, size structure, and the spatial patterns were investigated in old-growth ponderosa pine forests at three protected study areas east of the crest of the Cascade Range: Metolius Research Natural Area and Pringle Butte Research Natural Area in central Oregon and Blacks Mountain Experimental Forest in northern California. The three study areas represented sites characterized by deep accumulations of pumice from Cascade volcanism. All stems ≥15 cm in height (minimum height of an established seedling) were mapped and measured on a total of 27 1-ha plots. The distribution of trees within each individual plot was investigated by second-order spatial analysis with Ripley’s K( d) function, and then evaluated across each study area with functional data analysis. Coarse woody debris was sampled by using the strip-plot method to determine log density, mean log size, volume, and cover. The oldest trees were 618 years at Metolius, 613 years at Pringle Butte, and 330 years at Blacks Mountain. Stands were multi-aged, with as many as 16 cohorts at Metolius and 22 cohorts at Pringle Butte. Density of live old-growth ponderosa pine in the upper canopy ranged from 34 to 94 trees ha −1 at Metolius, 35 to 79 trees ha −1 at Pringle Butte, and 15 to 73 trees ha −1 at Blacks Mountain; the differences between study areas were not significant, resulting in an overall mean density of 50 ± 3.5 live old-growth trees ha −1. Mean diameters of these old-growth trees did not differ among the three study areas; the overall mean was 60.0 ± 1.55 cm dbh. Large dead ponderosa pines (overall mean diameter 61.7 ± 4.33 cm) were a common feature at all three study areas; the overall mean density was 9.0 ± 0.97 trees ha −1. Ripley’s K( d) analysis of spatial point patterns using upper canopy trees revealed significant departure from randomness in 24 of the 27 plots. Functional data analysis of the spatial relationship of all sample plots by study area revealed two strong patterns. At scales of 1.2 ≤ d ≤ 2.6 m at Metolius and 1.6 ≤ d ≤ 8.4 m at Blacks Mountain, the deviation from random was not significant, suggesting the distribution of old-growth trees was random. More important, significant positive deviation from complete spatial randomness at larger scales at Metolius and Blacks Mountain suggested a clumped distribution. Maximum radii of the clumps were about 22.5 m in diameter at Metolius and about 24 m in diameter at Blacks Mountain. In contrast, old-growth trees at Pringle Butte were randomly distributed. Density of logs at Metolius and Pringle Butte was 47.0 ± 5.28 logs ha −1, their mean large-end diameter was 37.6 ± 2.41 cm, the mean length of each log was 4.2 ± 0.09 m, the cumulative length of all logs averaged 512.9 ± 78.12 m, the total volume averaged 62.3 ± 6.30 m 3 ha −1, and the cover averaged 1.7 ± 0.08%. A majority of the logs were in an advanced stage of decomposition, suggesting that they were in place for considerable time. These results are discussed in the context of reference conditions for restoration of ecosystem health and ecological integrity in eastside ponderosa pine forests.