- Prior to European settlement, native prairie dominated the landscape of the
Willamette Valley. Today, due to urbanization, agriculture, and the cessation of
burning, only isolated remnants of these grasslands still exist. In response to
conservation concerns, there has been a move to restore the remaining prairies in the
Willamette Valley, and prescribed burning and sowing native seed are often top
candidates for grassland restoration. However, the effects of burning on native seedling
establishment and the spread of weedy exotics are largely unknown.
In this study, I investigated how prescribed burning affects native and exotic
species seedling establishment on three upland prairie vegetation types in the Willamette
Valley. The general approach was to sow a known number of seeds from several native
and exotic grassland species into experimentally manipulated plots, designed to separate
the effects of litter removal from the other effects of burning, and to monitor seedling
densities. Germinability and dormancy characteristics of the sowed seeds were also
addressed. An additional study focused on the fire temperatures at soil surface in three
grassland vegetation types and two burn sizes.
Burning increased the establishment of most or all of the sown native species in
the two low quality, exotic grass sites. Exotic seedling establishment also tended to
increase, but did not differ significantly from unburned plots for most species tested. On
the higher quality, native bunchgrass site, burning did not significantly improve native species establishment, but did significantly increase the establishment of short-lived exotic species over those in unburned plots.
The germination and dormancy characteristics of the native and exotic species tested indicate that grasses, both native and exotic, are more likely than forbs to be non-dormant in the autumn following dispersal. Forbs, especially native forbs tend to require cold-stratification for maximum germinability.
During grassland fire, temperatures at soil surface were relatively cool. Fire temperature intensity was highest in the higher quality native bunchgrass vegetation. These burns reached higher temperatures significantly closer to the soil surface than the plot burns in lower quality sites dominated by annual or perennial exotic grasses. Average temperatures in a two hectare broadcast burn and in replicated 2m x 2.5m plot bums in an annual exotic grassland were very similar.
Prescribed burning can be an excellent tool for the restoration of low-quality upland prairies when combined with sowing native seeds in the fall. However, on high-quality prairie, prescribed burning may be a poor restoration choice for promoting native seedlings, as burning promoted weedy species without enhancing native seedling establishment.