Persistence of western juniper resource islands following canopy removal

Abstract

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.