Understory Species Increase Project: Investigating the Revegetation of Native Herbaceous Species from Seed in Urban Forest Fragments

Abstract

Management of urban forest fragments often aims to reduce invasive species and promote native species abundance and diversity. Often, these environments lack natural establishment of native forest species, including herbaceous species that are especially sensitive to site conditions. While herbaceous understory species may represent a small proportion of forest biomass, they perform important functions within forest environments, including nutrient cycling, erosion and runoff control, and providing habitat for wildlife, as well as hosting the greatest biodiversity among other forest strata. However, many restoration projects focus primarily on the revegetation of dominant woody forest species, such as hardwood trees and shrubs. Herbaceous understory species may be overlooked due to limited understanding of their importance, technical information, budget, and plant material availability. The Understory Species Increase Project (USIP) is a collaborative effort started by Clean Water Services, the City of Portland’s Revegetation Program, and Metro that aims to fill this gap in knowledge and resources by researching, developing, and amplifying diverse herbaceous species in the region. The current stage involves two distinct but complementary investigations: Examining seeding effectiveness, species performance, and environmental conditions related to germination and cover through the installation of in-situ trial plots in multiple forest sites, and Examining the current commercial market, and challenges and opportunities in the production of native forest herbs through a survey of local plant material producers. Results from trial plots show benefits of a seeding treatment, including increased cover of target species and native species richness. However, these benefits were not realized at all sites. Modeling analysis revealed that soil properties are the strongest predictors of whether a seeding treatment will be effective, as sites with degraded soil conditions failed to establish strong cover of target species and generally had a greater presence of invasive species. Additionally, seeding success varied across species. Unfortunately, many of the most successful species to establish from seed were also found to have limited commercial availability in the current market. While challenges to producing herbaceous species are numerous, producers identified the lack of a stable demand as the largest challenge to production. Recommendations to address this challenge include continued research and creation of guidance documents, education and outreach to share information and foster interest in these species, and ongoing support of diverse partnerships to help expand and stabilize demand.