Restoration impacts on understory plant species in a Colorado Front Range ponderosa pine and Douglas-fir forest
Ertl, Elizabeth, author
Smith, Frederick, advisor
Fornwalt, Paula, advisor
Norton, Andrew, committee member
Land managers working in Colorado’s ponderosa pine – dominated forests are faced with many challenges concerning forest health and resiliency, such as higher tree densities, greater risk of high severity wildfire, and depauperate understory plant communities. Restoration treatments designed to move forests toward less degraded conditions that are more in line with those found prior to the settlement era are being increasingly implemented, and have been shown to decrease stand density, minimize ladder fuels, and decrease severe fire risk. However the impacts of such treatments on understory plants are less clear, especially over the long-term. To address this knowledge gap, I quantified changes in the richness, cover, and composition of understory grass, forb, and shrub species, and in the density and composition of trees regenerating in the understory, following restoration treatments at a 383-ha site located near Trumbull, Colorado. The site, which was treated in 2002, was chosen by the Upper South Platte Watershed Protection and Restoration Project (USPWPRP) as a priority area for restoration and was the first area on the Pike National Forest to receive such treatments. In 2004, 15 randomly located 1000-m² plots were established in treated stands, with five north-, five south-, and five east- or west-facing plots. Twenty randomly located plots were also established in nearby untreated stands, with slopes, aspects, and elevations comparable to treated plots. Data were collected in 2004, 2005, 2006 and 2014 (two, three, four, and 12 years post-treatment). Overstory stand structure post-treatment indicated that the goals of the restoration treatment were met, with basal areas reduced from 20.3 to 8.1 m² ha⁻¹ on north aspects, from 14.8 to 10.9 m² ha⁻¹ on east and west aspects, and from 12.7 to 5.1 m² ha⁻¹ on south aspects. Site factors such as treatment and aspect influenced tree seedling (<1.37 m tall) recruitment twelve years post-treatment. Tree regeneration in treated plots consisted of mostly ponderosa pine less than 45 cm tall, suggesting that they largely established post-treatment, while regeneration in untreated plots consisted of mostly Douglas-fir of varying sizes. Understory richness, cover, and composition also changed significantly during the 10-year period of observation. The most dramatic changes occurred on north-facing aspects, perhaps because these relatively mesic aspects are generally considered to be the most favorable for plant growth, and because the considerable reduction in basal area that they experienced ensured that ample resources were available for understory vegetation. Treated plots on north aspects saw increases in total, forb, graminoid, and native species richness in one or more years. Exotic species richness also increased on north aspects following treatment, although values were low. Plant communities further revealed changes in composition between treated and untreated plots on north and east/west aspects post-treatment. This study shows that restoration treatments can stimulate understory vegetation in the long-term, having few negative effects, which would be the increase in exotics and overly abundant tree regeneration on north aspects. Site factors can further influence understory response, with the most dramatic responses occurring on more mesic sites and/or where treatments are most aggressive.
Includes bibliographical references.