Plants in stressful environments experience adverse abiotic conditions and elevated herbivory. These factors often affect mortality in seedlings more than in other life stages. For long-lived species in stressful environments, seedling mortality may be even higher proportionally. Thus sheltered micro-sites, whether provided via positive interactions with other plants (facilitation) or via non-living structures, can be important for recruitment in many species in these environments. The Great Basin bristlecone pine (Pinus longaeva D. K. Bailey) is an extremely long-lived species found at high altitude in the harsh desert ranges of the western U.S. Despite relatively high seed production, annual recruitment in this species is very low. The goal of this study is to illuminate some of the factors affecting bristlecone survival at early life-stages. We followed bristlecone seedling transplants over two years in two field experiments in the White Mountains of California to examine the impact of herbivores and shelter on seedling survival. To assess the comprehensive and spatial effects of herbivores, we planted seedlings in cages and unprotected (an exclosure experiment) inside and outside of a bristlecone forest. To test the effects of shelter provided by sagebrush (Artemisia tridentata Nutt.) and dead wood, we planted seedlings underneath sagebrush, in the open spaces between sagebrush plants, and underneath shade structures made from blocks of wood.
Results/Conclusions
We found that herbivore pressure was stronger inside the forest than outside the forest, suggesting that herbivores are not an important factor influencing the spatial extent of the forest. Survival of seedlings shaded by wood was highest of the three treatments. Seedlings under sagebrush fared little better than seedlings in the open, indicating the facilitative effects of sagebrush are weak. Due to herbivore pressure, successful recruitment episodes may only occur when herbivore populations are low. In addition, recruitment outside of typical microsites may occur in years with favorable weather conditions. In conclusion, rare recruitment may be driven by the low likelihood that both of these conditions are met in a given year. Thus, in most years, sheltered microhabitats may be a critical resource for successful recruitment in bristlecone pines.