Date of Award

Spring 1-1-2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geography

First Advisor

Thomas T. Veblen

Second Advisor

Carol Wessman

Third Advisor

Tim Seastedt

Fourth Advisor

Holly Barnard

Fifth Advisor

William Travis

Abstract

In recent years, concern has grown among researchers, land managers, and the public regarding potential shifts in forest resiliency to disturbances such as wildfire under warming climate conditions. We examined conifer regeneration after fire in low-elevation, ponderosa pine (Pinus ponderosa) forests of the Colorado Front Range (CFR). Given preliminary observations of limited post-fire conifer establishment, we developed the general hypothesis that warming temperatures and associated drought are less suitable for post-fire conifer regeneration. We surveyed juvenile conifer densities in six recently burned areas of the CFR and found that juvenile ponderosa pine and Douglas-fir (Pseudotsuga menziesii) densities were typically lower than needed for sufficient stocking levels. We also identified several site characteristics that were associated with conifer presence including higher elevation, more northerly aspect, and shorter distance to seed source. In addition to surveying post-fire conifer densities, we implemented a field experiment to examine the effects of microclimate manipulations on the growth and survival of ponderosa pine and Douglas-fir seedlings planted in a low-elevation, recently-disturbed setting. We found that average growth and survival was highest in the watered only plots, followed by the control, warmed + watered, and warmed plots, respectively. Lastly, we assessed past relationships between climate variability and post-fire conifer establishment. We dated 413 seedlings collected from five recently burned areas, using a dendrochronological method that yields annually-resolved estimates of tree age. We found that conifer establishment was concentrated in years of above-average precipitation and positive Palmer Drought Severity Index (PDSI) for the growing season (April-September).

Collectively, our findings suggest that warming temperatures and associated drought are likely to inhibit post-fire regeneration of ponderosa pine and Douglas-fir in low-elevation forests of the CFR, especially in xeric settings (i.e. at low elevations and on south-facing aspects). Future vegetation composition and structure may differ notably from historic patterns. In the absence of abundant conifer regeneration, some previously forested areas may be replaced by persistent grasslands or shrublands. We expect that similar changes are imminent or underway in other low-elevation forests where warmer climates may limit post-fire tree regeneration.

Share

COinS