Plant-plant interactions are important for regulating plant species distributions, affecting population processes at range edges and influencing species responses to climate change. Singleleaf pinyon pine (Pinus monophylla), which is among the most widespread tree species in the western US, often regenerates in association with big sagebrush (Artemisia tridentata). Ecological theory predicts biotic interactions to vary with spatial or temporal variation in environmental stress, and sagebrush effects are also likely to vary depending on the life stage and associated physiology of pinyon juveniles. In this study, we used experimental manipulations of tree-shrub interactions to examine how big sagebrush affects pinyon juveniles of varying age classes and across an elevational gradient of water availability (1900-2400m). Over two years, we monitored pinyon juveniles and planted seeds in under-shrub, shrub-removal, and interspace treatments along the full elevational gradient across which pinyon pine occurs in central Nevada. Additionally, we used measurements of soil moisture, temperature, nutrient availability, and light environment in all treatment types to explore how big sagebrush modifies the environment of associated pinyon juveniles, generating hypotheses of underlying mechanisms for future work.
Results from the first year of the experiment showed that the removal of nurse shrubs increased mortality rates for young pinyons with juvenile foliage. These results were most pronounced at the lowest elevation, where shrub removal decreased the probability of survival by nearly 50% compared to the under-shrub treatment, suggesting that the interaction between sagebrush and pinyon juveniles shifts toward stronger facilitation with increasing aridity. Older juveniles with mature foliage showed no response to shrub removal in either mortality rates or annual terminal leader growth, suggesting that the facilitative effect of nurse shrubs is most important at early stages of regeneration, but is not limited to the initial germination and early establishment phases. We found that sagebrush stabilizes the under-shrub environment by moderating daily and seasonal fluctuations of soil moisture and temperature. Available N was higher under shrubs than in interspaces, suggesting that sagebrush creates islands of fertility through litter inputs, especially in more arid microsites where shrub interspaces are not occupied by herbaceous cover. Shrub removals initiated a pulse of available N at all elevations, likely the combined result of reduced uptake from shrubs and increased rates of microbial processes due to higher soil temperatures.