Seed bank response to juniper expansion in the semi-arid lands of Oregon, USA

Graduation date: 2009Expansion of Juniperus occidentalis into the sagebrush steppe has resulted in significant changes in understory composition. A consequence of increased J. occidentalis dominance may be a depletion of the seed bank. The potential for depletion is problematic because a reduction in the amount of species available from the seed bank may compromise site resiliency. During disturbances that disrupt current vegetation, a reduction in recruitment from the seed bank could cause the site to be more vulnerable to colonization by weedy species or could lead to a reduction in the amount of cover protecting the soil surface and thus increase site susceptibility to erosion. This research evaluated the effects of the relative abundance of J. occidentalis on the soil seed bank. Questions addressed were: 1) is species richness negatively correlated with J. occidentalis cover; 2) does the diversity of weedy species increase along a gradient of J. occidentalis cover; and 3) does seed density decrease as J. occidentalis cover increases? We also attempted to identify seed bank depletion thresholds based on seed density. Two eastern Oregon sagebrush steppe sites where chosen to represent the juniper woodland-sagebrush steppe region. These sites displayed a range of J. occidentalis canopy cover. Soil and litter samples were collected in the fall of 2006 and 2007 and subjected to both cold-wet and warm-dry stratification. Germination occurred over a period of eight months under greenhouse conditions. Linear and Poisson regression techniques were used to evaluate relationships. A non-parametric deviance reduction approach was used for threshold detection and a threshold strength index was used to evaluate the strength of the identified thresholds. No statistically significant relationships were detected between J. occidentalis cover and species richness or weedy species diversity at either site in either year, with the exception of Bridge Creek in 2007 which showed a positive correlation between J. occidentalis cover and seed bank species diversity. The results for seed density were more complex. In 2006, J. occidentalis cover was strongly related to seed density at Devine Ridge but in 2007 there was no evidence of a correlation. At Bridge Creek in 2006, there was a weak relationship between increasing J. occidentalis cover and decreasing seed density but in the second year the pattern differed suggesting a positive correlation between J. occidentalis cover and seedling density. This variability in results suggests a complex relationship between J. occidentalis cover and seed bank composition. This relationship may partially depend on weather conditions, and a more lengthy study period would be valuable in discerning whether such a correlation exists. Concerning threshold detection, seed bank depletion thresholds were identified across both sites and years but their strength was very weak. These findings indicate that a reliable seed bank depletion threshold, as defined in this study, may not exist for these sites at least over the course of this study. Though these results were negative, the approach was useful and pursuit of its further development is suggested for evaluation of other suspected thresholds