Facilitation of Pinus albicaulis seedling regeneration by Vaccinium scoparium

Abstract

Plants often facilitate each other in harsh environments, and facilitation appears to be important for conifer recruitment in subalpine forests and the formation of treelines. This study investigated the potential role of facilitation for the establishment of Pinus albicaulis (whitebark pine), a dominant Rocky Mountain tree species that is in decline due to an exotic blister rust disease (Cronartium ribicola), bark beetle outbreaks, and fire suppression. Surveys were conducted at five subalpine sites for spatial associations between naturally regenerating P. albicaulis seedlings and other plant species, and found that the cover of Vaccinium scoparium was higher in proximity to P. albicaulis seedlings. In transplant experiments, survival of P. albicaulis seedlings was higher when planted with V. scoparium and in shade cloth treatments, but neither survival nor growth rates were altered by clipping or rhizome severing of V. scoparium, suggesting a below-ground rather than above-ground facilitation mechanism. In contrast, Carex species reduced the survival and growth rates of P. albicaulis. In a fertilization experiment, no effect of nitrogen and/or phosphorous fertilization was found on Pinus albicaulis seedling growth, but seedlings growing with V. scoparium had higher growth rates and increased leaf phosphorus concentrations relative to seedlings growing in bare ground. In a microcosm experiment, P. albicaulis seedlings growing with V. scoparium had higher growth rates and leaf phosphorus concentrations than when grown alone, but only if P. albicaulis needles were added on the soil surface. My results suggest success of restoration plantings of greenhouse grown P. albicaulis seedlings may be improved by planting in areas where V. scoparium occurs, and avoiding areas dominated by Carex species. Removal of adjacent vegetation prior to planting seedlings may be ineffective in significantly altering either positive or negative effects of the neighboring plants. Selection of sites with adjacent shading elements, such as rocks, logs, stumps, or snags, may also improve seedling survival.