Wildfire responses and tree longevity in old-growth ponderosa pine/Douglas-fir forests

Author

Eric G. Keeling

Year of Award

2009

Document Type

Dissertation - Campus Access Only

Degree Type

Doctor of Philosophy (PhD)

Degree Name

Organismal Biology and Ecology

Department or School/College

Division of Biological Sciences

Committee Chair

Anna Sala

Commitee Members

Ragan Callaway, Elizabeth Crone, Tom DeLuca, Emily Heyerdahl

Abstract

Mixed ponderosa pine forests are widespread in the western United States and are experiencing less frequent wildfire today than they did historically. Higher stand densities associated with lack of fire raise concerns that trees in these forests are under physiological stress due to increased competition for resources, and that current wildfires are more likely to harm trees than fires of prior centuries because of higher fuel loads today. However, little research has addressed effects of wildfire and lack of fire by comparing contemporary unburned stands to frequently burned stands in forests that have not been logged.

The main focus of this research is to understand how wildfire and lack of fire affect ponderosa pine communities and individual ponderosa pine trees in forests that were never logged or directly managed. I utilized sites in old-growth ponderosa pine/Douglas-fir forests in Montana and Idaho, each with at least two stands, one not burned for at least 70 years and one burned at approximately the historical fire frequency. I found higher overall tree densities and densities of shade-tolerant tree species (Douglas- fir and grand fir) in unburned stands, although overstory and understory community composition and structure varied from site to site. Surprisingly, there were no significant biological differences between mature ponderosa pine trees in burned versus unburned stands in a suite of physiological variables. I found a trend of increasing negative growth responses to wildfire (reduced stemwood (basal area) growth in the burned stand compared to the unburned stand) in recent fire years that had drier winters.

I was able to study very old trees that are otherwise extremely rare across landscapes. Recent investigations on “age-related’' growth in trees have led to the common assumption that old trees grow slowly due to negative effects of age or height on growth. In contrast, I found that tree age and height have little effect on basal area growth, and I present evidence that slow growing trees may be more likely to reach old age. These findings highlight the potential importance of old-growth ponderosa pine/Douglas-fir forests as reservoirs and sinks for carbon in a future with novel fire regime and climate.

Recommended Citation

Keeling, Eric G., "Wildfire responses and tree longevity in old-growth ponderosa pine/Douglas-fir forests" (2009). Graduate Student Theses, Dissertations, & Professional Papers. 10835.
https://scholarworks.umt.edu/etd/10835