|Abstract or Summary
- Quaking aspen (Populus tremuloides) is an important tree species in the western United States and there has been much concern about its persistence. In this thesis, I report on aspen in a portion of its range that has not been previously evaluated: the eastern and western slopes of the Central Oregon Cascades and the Willamette Valley. My primary research objectives for each of the three landscapes were to: (1) assess aspen overstory recruitment across time; (2) assess the condition of aspen sprouts; (3) identify environmental and biological stressors to the aspen overstory and understory; and (4) test the aspen overstory, recent recruitment, and sprouts for correlations to ungulate herbivory and conifer encroachment. In addition, on the eastern slope of the Cascades, I tested the efficacy of piles of coarse woody debris (CWD) or jackstraw as an herbivore deterrent.
On the east side of the Cascade Mountains, aspen stands were small (≤ 0.6 ha) and found in a dense conifer matrix. Small stands are expected to recruit overstory across time and expand with disturbance events. Aspen stands on the east side showed a steady decline in overstory recruitment that coincided with increase in conifer
encroachment and ungulate herbivory. Aspen stands with conifer mortality attributable to mountain pine beetle (Dendroctonus ponderosae), and subsequent release of overstory competition and formation of jackstraw, recruited overstory trees at expected levels. More than 75% of all recently recruited trees (< 20 years) were behind jackstraw. Diseased sprouts protected from herbivores recruited into the overstory. My results suggest that aspen sprouts increase in height when released from herbivory and increase in density when released from conifer shading and competition. Accordingly, I suggest that the mountain pine beetle has a powerful ecological effect that can help aspen recruit at the stand level and persist at the landscape level via: (1) removal of conifer shading and resource competition; (2) creation of snags, and subsequent increase in jackstraw; and (3) the ensuing increase in sprout density and heights, which leads to overstory recruitment.
Results in the two western Oregon landscapes show aspen stands are small (< 0.1 ha), rare, and in decline. Stand origins are unknown and could be genetically linked to stands on the east Cascades or to the north. While most aspen stems dated back to the early to mid 1900s, one stem originated in the mid 1800s. I classified 16 aspen stands from these two landscapes into 6 stand types: Snowfield, Upland Aspen/Conifer, Meadow Fringe, Lithic/Boulder, Pasture Valley, and Riparian Valley. One stand type was found increasing, 3 types were decreasing, and 2 types were at risk of loss. I found aspen sprouts were suppressed by herbivory, disease, and conifer encroachment. Some stand types showed aspen overstory at risk from conifer encroachment.
Aspen is on the fringe of its range on the east and west slopes of the Oregon Cascades and in the Willamette Valley. A scientific understanding of aspen in these three landscapes may help the persistence of this species in the face of climate change.