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• 1. [Article] Persistence of western juniper resource islands following canopy removal

  Western juniper (Juniperus occidentalis) cover has more than doubled within the last century and currently occupies over 9 million acres in the Intermountain West. Encroachment has altered the spatial ...

  Citation × Citation Citation Abstract Copy Title: Persistence of western juniper resource islands following canopy removal Author: Miwa, Christopher Western juniper (Juniperus occidentalis) cover has more than doubled within the last century and currently occupies over 9 million acres in the Intermountain West. Encroachment has altered the spatial distribution of soil nutrients and plants in these systems, forming nutrient enriched 'resource islands,' under tree canopies. The purpose of this study was to determine the persistence of resource island characteristics after restoration treatment (tree removal). The study site was a BLM grazing allotment in Eastern Oregon where trees had been cut 1, 8 and 15 years ago. In each age class and in uncut western juniper woodlands juniper stumps or trees were randomly selected for sampling. At each bole three radial transects, set at 120º from each other, were marked and soil cores were collected to 5-cm depth at distances of 50, 100, 150 and 300-cm from the bole then combined to a single composite sample per distance class. Samples were analyzed for total C and N, soluble P, K, Ca, Fe, Si, Al and Na, inorganic NH₄ and NO₃, pH and gravimetric water content. Fifteen years after canopy removal there was still strong evidence of western juniper resource islands. Canopy soils were generally significantly higher in Ctot, Ntot, NO₃‾, P, K and Ca compared to intercanopy soils in all treatments; however, accumulations for several elements appeared to degrade with time since canopy removal, and at different rates. We attribute resource island persistence to deep litter mats beneath relic canopies. Due to the strong zonal distribution of understory vegetation in juniper duff and bare zones these nutrient enriched areas will continue to influence long-term successional pathways and should be taken into account for restoration purposes, such as seeding of forbs and grasses. Title: Persistence of western juniper resource islands following canopy removal Author: Miwa, Christopher Western juniper (Juniperus occidentalis) cover has more than doubled within the last century and currently occupies over 9 million acres in the Intermountain West. Encroachment has altered the spatial distribution of soil nutrients and plants in these systems, forming nutrient enriched 'resource islands,' under tree canopies. The purpose of this study was to determine the persistence of resource island characteristics after restoration treatment (tree removal). The study site was a BLM grazing allotment in Eastern Oregon where trees had been cut 1, 8 and 15 years ago. In each age class and in uncut western juniper woodlands juniper stumps or trees were randomly selected for sampling. At each bole three radial transects, set at 120º from each other, were marked and soil cores were collected to 5-cm depth at distances of 50, 100, 150 and 300-cm from the bole then combined to a single composite sample per distance class. Samples were analyzed for total C and N, soluble P, K, Ca, Fe, Si, Al and Na, inorganic NH₄ and NO₃, pH and gravimetric water content. Fifteen years after canopy removal there was still strong evidence of western juniper resource islands. Canopy soils were generally significantly higher in Ctot, Ntot, NO₃‾, P, K and Ca compared to intercanopy soils in all treatments; however, accumulations for several elements appeared to degrade with time since canopy removal, and at different rates. We attribute resource island persistence to deep litter mats beneath relic canopies. Due to the strong zonal distribution of understory vegetation in juniper duff and bare zones these nutrient enriched areas will continue to influence long-term successional pathways and should be taken into account for restoration purposes, such as seeding of forbs and grasses. Close

• 2. [Article] Seed bank response to juniper expansion in the semi-arid lands of Oregon, USA

  Expansion 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 ...

  Citation × Citation Citation Abstract Copy Title: Seed bank response to juniper expansion in the semi-arid lands of Oregon, USA Author: Duncan, Corinne M. Expansion 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. Title: Seed bank response to juniper expansion in the semi-arid lands of Oregon, USA Author: Duncan, Corinne M. Expansion 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. Close