Inhibition of Conifer Regeneration by an Herbaceous Perennial, Wyethia mollis, in the eastern Sierra Nevada, California

DOCTORATE DISSERTATION: Wyethia mollis is a long-lived, herbaceous perennial. It dominates extensive herbaceous patches of the Sierra Nevada on the east slope yellow pine forests resulting from overgrazing by domestic ungulates after loss of forest canopy. Little regeneration of pines occurs in these patches which are believed to persist for greater than 60 yrs. Previous studies using tissue extracts in controlled settings and in situ over-wintering bioassays indicated that Wyethia exhibits a potential for allelopathic interference on germinating pine seeds. Also, rodent predation of pine seed was found to be effective in reducing the availability of seed within these patches. A second study found that the presence of W. mollis was correlated significantly with decreased survival and growth of Pinus jeffreyi saplings. The current study was designed to further examine the role of allelopathy in pine suppression; to examine seedbed microclimate and water availability; to study pine growth in association with Wyethia (field and controlled environments); and to characterize pine seedlings grown with montane chaparral, Great Basin steppe, and yellow pine forest. Results indicate that pine seedbed microclimate is greatly influenced by snowpack size and duration as well as the presence of vegetation and litter. Soil water, while not limiting in the early summer, becomes limiting to pines later in association with W. mollis, restricting establishment and survival through summer drought. Nutrient availability is not limiting in W. mollis patches. Seed predation by rodents, seedling herbivory and bioturbation damage both by rodents and domestic sheep reduced pine establishment. Allelopathy in the seedbed appears to vary with precipitation and snowpack. Mycorrhizae, both ectomycorrhizae of pines and vesicular-arbuscular mycorrhizae of Wyethia and bioassay species, are not reduced in association with Wyethia suggesting that patches are not hostile sites for these fungi. Patch persistence is discussed in light of continued conifer suppression. Pine establishment is inhibited in Wyethia patches by a synergism of factors rather than a single causal element. These include rodent herbivory and bioturbation, variability in snowpack and precipitation, and seedbed allelopathy.