Biology ETDs

Publication Date

Summer 5-16-2022

Abstract

Fire-exclusion has acted as a major perturbation on dry conifer forests, increasing tree density and, in mixed-conifer forests, the dominance of shade-tolerant species. Restoration efforts aim to reverse these effects by reducing stand density, restoring relative proportions of tree species, and reintroducing recurrent fire, but the long-term effects of repeated burning on tree regeneration have not been quantified. We analyzed two decades of seedling and overstory data from the Teakettle Experimental Forest in the southern Sierra Nevada to determine how thinning and repeated burning affect seedling establishment and overstory recruitment. Across treatments, pine seedling densities remained much lower than shade-tolerant seedling densities. We found repeated burns led to modest increases in sugar pine (Pinus lambertiana) seedling densities and substantial increases in incense-cedar (Calocedrus decurrens) seedling densities four years post-burn. No significant differences in seedling densities among treatments were detected for Jeffrey pine (P. jeffreyi) or white fir (Abies concolor). Estimates of natural midstory recruitment were much higher among white fir and incense-cedar than pines, even following restoration treatments. However, rates of pine midstory recruitment increased substantially in overstory thinned treatments, which included post-harvest planting. Our results suggest that fire-exclusion may have shifted the ecosystem out of its initial domain of attraction, creating a forest dominated by shade-tolerant species that exhibits hysteresis by resisting a return to a historic range of variability even after restoring structure and process through thinning and burning. Planting pine species may be effective at overcoming this resistance to restore the forest to a pine-dominated state.

Language

English

Keywords

Sierra Nevada, regeneration, recruitment, hysteresis, domain of attraction

Document Type

Thesis

Degree Name

Biology

Level of Degree

Masters

Department Name

UNM Biology Department

First Committee Member (Chair)

Matthew D. Hurteau

Second Committee Member

Harold S.J. Zald

Third Committee Member

William T. Pockman

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