Effects of Mechanized Thinning and Prescription Fire on Stand Structure, Live Crown, and Mortality in Jeffrey Pine

ABSTRACT

Forest thinning, using cut-to-length and whole-tree harvesting systems with subsequent underburning were assessed for their influence on stand structure, health, and fire resilience in uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf). Stand attributes, derived from measuring trees ≥ 10.2 cm diameter at breast height (DBH), were collected from permanent plots. Trees were divided into three size classes that generally corresponded to dominant/codominant, intermediate, and suppressed crown classes. Comparisons of post- to pre-burning mortality revealed significant thinning and fire main treatment effects as well as significant interaction between these two treatments in the two larger size classes. Mortality increased by 250% in the intermediate crown class within the burned stand portion of the whole-tree treatment, whereas among dominant/codominant trees mortality rose by 160% in the burned cut-to-length treatment combination. Pre- to post-burning shifts in live crown, expressed as a percentage of total tree height, were significantly influenced by both thinning and fire main treatments in the two larger size classes, while the interaction of these treatments was also significant among the largest trees. Within both of these size classes, decreases in live crown percentage were greatest in the burned portion of the unthinned treatment, where intermediate crown class trees lost over 20% of their crowns, while reductions in dominant/codominant trees averaged nearly 25%. The second highest losses for both size classes occurred within the burned cut-to-length treatment. In the smallest trees, mortality rose sharply and live crown decreased substantially after burning in both thinning treatments and in the unthinned control. Within the two larger size classes, preburn live crown size was negatively correlated with changes in crown size subsequent to underburning while DBH was negatively correlated with postburning changes in mortality, but only in intermediate crown class trees. These results present land managers with outcomes of differing management practices presently being evaluated for their potential to enhance forest health and reduce wildfire risk in the Sierra Nevada and similar dry forest regions.

KEYWORDS:

• Density and stocking
• forest thinning
• prescribed fire
• forest health
• crown scorch
• mortality
• Jeffrey pine
• Pinus jeffreyi

The authors are indebted to D. Brunson, J. Chacon, G. Fernandez, T. Gunder, J. Holcomb, K. Jones, C. McCarthy, B. Melville, K. Murphy, J. Roubique, R. Salas, G. Schall, R. Shane, J. Spurlock, and A. Wallace for their assistance.

Support for this research was provided by the Nevada Agricultural Experiment Station Projects 523M, 523P, and 5262 funded by the McIntire-Stennis Cooperative Forestry Research Program. Publication No. 52077014 of the Nevada Agricultural Experiment Station.