Persistence of Resource Islands Following Juniperus occidentalis (Western Juniper) Canopy Removal.

Western juniper (Juniperus occidentalis Hook.) encroachment has altered the spatial distribution of soil nutrients and plants in semi-arid systems of the northern Great Basin, 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 cutting). The study site was a BLM grazing allotment in Eastern Oregon, USA, where trees had been cut eight and fifteen 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 120º apart, 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, Al, Mg and Na, inorganic NH4 and NO3, and pH. Fifteen years after canopy removal there is still strong evidence of western juniper resource islands; canopy soils were significantly higher in C_tot, N_tot, NO₃^-, P, K, and Ca compared to intercanopy soils. However, accumulations for several elements degraded with time since canopy removal, and at different rates. The persistence of resource islands was likely due to the presence of litter mats beneath relic canopies. These resource islands may play a role in the spatial distribution of re-establishing vegetation, including invasive species.