Soil texture associated with wide variation in forb communities in established non-native perennial grass seedings

Authors: Copeland, Stella; WILLOUGHBY, MOLLY - Non ARS Employee

Submitted to: Western North American Naturalist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Crested wheatgrass (Agropyron cristatum) and other non-native perennial grasses are widely seeded across sagebrush steppe rangelands. These non-native grasses can compete with invasive annual grasses, associated with fire risk, limit erosion, and increase forage in degraded sites. However, crested wheatgrass may also strongly compete with desirable native species, limiting native plant diversity, and reducing wildlife habitat value. More information is needed on the causes of variation in non-native perennial grass seeding outcomes to make informed management decisions given the extent and variability in these seedings. In this study we investigated the effects of crested seeding and other vegetation and environmental variables in lower elevation sagebrush steppe on perennial and annual forb abundance, important for wildlife habitat and biodiversity, and associated vegetation characteristics, across Burns and Vale BLM districts in southeastern Oregon. We employed two methods, a paired plot design with seeded and unseeded plots, and a regional, monitoring analysis based on matching monitoring plots based on management and environmental variables. In the paired plot analysis, we found seeding effects on forb communities were affected by sand content. Crested seeding led to lower invasive annual grass cover. Invasive annual grass cover was in turn associated with lower annual forb abundance and diversity. Perennial forbs selected microsites with different characteristics in seeded and unseeded sites, suggesting some flexibility. The regional analysis showed that forb abundance was generally lower in crested seeded areas. Neither analysis was able to separate the effects of competition from crested wheatgrass from other conditions related to seeding, such as higher degradation (pre-seeding) and the impacts of co-occurring treatments, such as vegetation removal and soil disturbance with plowing and drill-seeding. More research is needed to identify management approaches for enhancing diversity in crested wheatgrass seeded areas and to understand the effects of soil disturbance and vegetation removal on seeding outcomes.

Technical Abstract: Non-native perennial grass seedings for rehabilitating degraded lands and enhancing livestock forage are common across large areas in the western US and other dryland regions. While these seedings can provide benefits in terms of perennial grass abundance and competition with invasive plant species, they also may reduce native plant abundance and diversity, with negative impacts on habitat for some wildlife species. We evaluated the effects of non-native perennial grass seedings, with crested wheatgrass (Agropyron cristatum), on native plant biodiversity in sagebrush steppe in eastern Oregon with two sampling methods focusing on differences in native forb (non-grass) understory species composition. We monitored composition in 17 paired seeded and unseeded sites across the region in multiple years with a variety of methods to identify factors associated with native forb community composition and competition with surrounding plants in seeded areas. We also used a large monitoring dataset across federal lands in the same region to test for differences in forb communities with crested wheatgrass seeding after controlling for environmental and management factors. Paired plot comparisons suggested negative relationships between seeding and forb density, but this was affected largely by seeding density. We also found that increasing sand content and seeding were associated with reductions in the diversity and abundance of perennial forb species. In contrast, native annual forb abundance was limited by increasing invasive annual grass cover, while richness decreased with higher sand content in seeded, but not unseeded sites. Species traits were generally not strongly related to their response to seeding. Crested seeding was also associated with lower invasive annual grass cover and lower total and perennial forb cover and richness in the regional level analysis which balanced the effects of management and environmental factors. These results confirm the capacity of seeding non-native perennial grasses to control invasive annual grasses, related to fire frequency, and the wide variation in forb communities in crested wheatgrass seedings. Soil texture appears to exert strong effects on forb community outcomes with crested wheatgrass seedings. Neither analysis is able to separate the effects of seeding from those associated with degraded state pre-seeding or soil disturbance related to seeding method (e.g. drill seeding) and vegetation removal treatments (e.g. herbicide or plowing). Additional research is needed to identify appropriate management strategies across environmental gradients, such as soil texture, to meet multiple goals such as controlling invasive annual grasses and enhancing diversity across the large areas historically seeded by crested wheatgrass and other non-native perennial grasses.