Responses of Above- and Belowground Forb and Plant Species Diversity to Grazing Exclusion and Fire in the Northern Great Basin Sagebrush Steppe
AuthorBoldis, Mariel Tereza
Environmental and Natural Resource Sciences
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The legacy effects of improper grazing regimes in the pre-1936 Taylor Grazing Act era and historical fire suppression have contributed to an overall decrease in native deep-rooted perennial bunch grasses and forbs, an increase in annual invasive grasses, and greater sagebrush dominance. Although not as widely used as perennial bunchgrasses, forbs of the Intermountain west have also been tested for use in rehabilitation purposes in the Intermountain West. Forbs provide the majority of plant species richness in stable-state sagebrush systems of the Northern Great Basin, are important seasonal food sources for wildlife like the Greater sage-grouse, provide erosion control through rapid establishment, and help prevent soil-nutrient loss. We assessed differences in above- and belowground diversity between burned and adjacent unburned areas, and between grazed and long-term grazing excluded areas, using soil seed bank and aboveground cover attributes in order to provide insight into ecological potentials of sagebrush sites in the northern Great Basin.Based on soil texture, elevation, species richness and composition of the seed bank in burned areas at each site, aboveground diversity (Effective S) increased as the seed bank became more diverse and was likely dominated by annual herbaceous species (except Claypan 14-16 #1). Below 1660-m in elevation (Loamy Slope 10-14), non-native annual grasses and forbs generally dominated the seed bank, suggesting that in the event of disturbance, aboveground cover may recover into a non-native annual herbaceous community. At sites between 1660—1970-m in elevation (Loamy 14-16, Gravelly North Slope 14-18, Claypan 14-16 #2, Claypan 10-14), annual herbaceous plants dominated the seed bank, and the composition of native forbs and native perennial grasses increased while the composition of non-native forbs and grasses decreased with increasing elevation (except at Claypan 14-16 #2), suggesting predicted aboveground diversity (Effective S) would generally have a native, mixed annual-perennial herbaceous plant community at higher elevations. Claypan 14-16 #2 did not follow this trend with 91% of total seed density in the burned area characterized as annual, composed mostly by cheatgrass and non-native forbs. The irregularity exhibited by the seed bank composition at Claypan 14-16 #2 can be related to the availability of soil-moisture around the time of the fire. The years following the Holloway fire marked the end of a 9-year negative-phase Pacific Decadal Oscillation (PDO) weather cycle, reoccurring with the El Niño/Southern Oscillation which coincided with normal- to below-normal 30-year annual precipitation averages in 2011 and 2012, and below-normal averages following the fire at all sites (2013). Weather cycles that promote a seed bank that is dominated by annual non-native species and short-lived native perennial grasses, while underrepresenting sagebrush, and long-lived native deep-rooted perennial bunchgrasses and forbs can pose a risk for site potential, influencing structural shifts in plant communities at low and mid-elevations. At elevations above 1970-m (Claypan 14-16 #1), native perennial grasses and native annual sage-grouse forbs dominated the seed bank, suggesting that aboveground plant communities at this elevation or higher are more likely to recover as a native perennial grass and forb communities post-fire. As elevation increased, more native annual forb species, including sage-grouse forbs (M. gracillis, C. parvifolia, G. decipiens), and fewer non-native annual forbs (C. testiculata, S. altissimum) contributed to the seed bank, suggesting there may be selective influences that can provide certain native annual sage-grouse forbs a competitive edge with cheatgrass and other non-native invasive grasses and forbs, which may contribute to the long-term persistence of native forb populations. In grazing and grazing-excluded sites, there were generally few to no differences in richness (S) and evenness (J’) above- and belowground communities, indicating that diversity (H’) since pre-Taylor Grazing Act conditions is similar to 82-years since exclosures have been in place. Lack of differentiation in belowground diversity (H’) in fire affected areas and grazing/grazing excluded areas, suggests annual and perennial forb and grass communities are more or less similar across the landscape, making it difficult to assess how sites would cluster each year when forb germination depends on availability of soil-moisture. However, one year of data is not enough to suggest grazing pressure and fire effects do or do not drive biologically significant differences in above- and belowground diversity. Regardless of elevation, when only a few annual species are likely to contribute to total cover, sites can exhibit lower diversity (H’), more similar above- and belowground communities, and truncated post-fire and herbivory recovery of long-lived native perennial forbs, grasses, and sagebrush, negatively impacting sage-grouse survival long-term. In such a case, reduction in fine fuels through targeted grazing followed by mechanical reduction of decadent sagebrush can help reduce high-intensity wildfire risk and subsequently maintain more even plant communities.