Plants in stressful environments experience adverse abiotic conditions and elevated herbivory. These factors cause higher mortality in seedlings than in other life stages. For long-lived species in stressful environments, seedling mortality may be even higher proportionally. Positive interactions between seedlings and other plants (e.g., nurse effects) may thus play an important role in these communities. The goal of this study is to illuminate some of the factors affecting survival of early life-stages of a long-lived, high-altitude desert conifer, the Great Basin bristlecone pine (Pinus longaeva). We performed two field experiments with transplanted bristlecone seedlings in the White Mountains of California to examine the impact of herbivory and habitat amelioration on seedling survival. To assess the overall and spatial effects of herbivory, we performed an exclosure experiment inside and outside of a bristlecone forest. To test the effects of habitat amelioration by sagebrush (Artemisia tridentata) and dead wood, we planted seedlings underneath shade structures made from blocks of wood, underneath sagebrush, and in the open space between sagebrush plants.
Herbivory was stronger inside the forest than outside the forest, suggesting that herbivory is not an important factor influencing the boundary of the forest. Mortality was most reduced in seedlings shaded by wood (mean mortality ± 1 SE = 5.56% ± 2.3). Seedlings under sagebrush (33.3% ± 6.8) had reduced mortality compared to seedlings in the open (62.2% ± 7.1). Sheltered microhabitats may thus be a critical resource for successful recruitment in bristlecone pines.