Fire disturbance and forest structure in an old-growth Pinus ponderosa forest, southern Cascades, USA

Questions Did fire regimes in old-growth Pinus ponderosa forest change with Euro-American settlement compared to the pre-settlement period? Do tree age structures exhibit a pattern of continuous regeneration or is regeneration episodic and related to fire disturbance or fire-free periods? Are the forests compositionally stable? Do trees have a clumped spatial pattern and are clumps even- or mixed-age? How might information from this old-growth forest inform current restoration and management practices? Location A 235-ha old-growth forest in the Ishi Wilderness, southern Cascade Mountains, California. Methods Age, size, and spatial pattern of trees were quantified in seven stands. Fire history was reconstructed using fire scar dendrochronology. The influence of fire on stand structure was assessed by comparing fire history with age, size, and spatial structure of trees and identifying and measuring trees killed by two recent fires. Results Species composition in plots was similar but density and basal area of tree populations varied. Age structure for P. ponderosa and Quercus kelloggii showed periods of episodic recruitment that varied among plots. Fire disturbance was frequent before 1905, with a median period between fires of 12 years. Fire frequency declined after 1905 but two recent fires (1990, 1994) killed 36% and 41% of mostly smaller diameter P. ponderosa and Q. kelloggii. Clusters of similar age trees occurred at scales of 28-1018 m2 but patches were not even-aged. Interactions between tree regeneration and fire promoted development of uneven age groups of trees. Conclusions Fire disturbance strongly influenced density, basal area, and spatial structure of tree populations. Fire exclusion over the last 100 years has caused compositional and structural changes. Two recent fires, however, thinned stands and created gaps favorable for Q. kelloggii and P. ponderosa regeneration. The effects of infrequent 20th century fire indicate that a low fire frequency can restore and sustain structural characteristics resembling those of the pre-fire suppression period forest.