Complex restoration challenges: Weeds, seeds, and roads in a forested Wildland Urban Interface

Background/Question/Methods

Dry coniferous forests in the western US are experiencing severe wildfires, insect and disease outbreaks, and a growing presence of invasive plant species. Policies strongly emphasize reducing hazardous fuels at the Wildland Urban Interface (WUI) where human communities and forests intersect. However, these areas present restoration challenges as they tend to have existing populations of exotic invasive species, frequent human disturbance, and the presence of roads. Understanding plant species seed banks is important in planning for desirable post-disturbance community conditions, developing integrated weed management programs, and for complying with State and Federal regulations. We characterized the aboveground vegetation and seed bank in mixed age dry-mixed conifer forest stands in eastern Washington State, and examined the patterns and relationships between aboveground vegetation and seed germinant abundance and richness in relation to seed bank layer source, distance to road, and herbicide treatments.  Herbicide was applied to noxious weeds in treatment plots, and aboveground vegetation, ground cover and site characteristics were measured. Litter and mineral soil samples were collected, cold-moist stratified and grown in a greenhouse.

Results/Conclusions

Noxious weed frequency, as well as exotic weed and annual native forb cover, significantly decreased with distance from road edge, while graminoid cover increased, declining at some point beyond 20 m.  The herbicide treatment did not affect any above ground plant functional group. Six hundred and thirty seeds germinated from litter and mineral soil samples and forty three species were identified. Most germinants (77%) and species (36 species) emerged from the litter layer compared to mineral soil and the majority was annual forbs, followed by perennial forbs, graminoids and exotic species. Little similarity was found between the largely mid- and late-seral vegetation and early-seral seed bank floras.  Fourteen percent of germinants were exotic and invasive species, and were found in similar abundances regardless of proximity to road or herbicide treatment. These findings suggest that (1) some herbicide treatments may be ineffective and do not impact seed banks in the short term; (2) seed bank contributions to desirable post-disturbance understory development may be relatively low, especially if activities remove the litter layer; and (3) weed populations are largely confined to near road environments, but the weed seed bank, which is a substantial part of the total seed bank, is not.