COS 273-1 - Can growth-related traits and trait plasticity overcome the seedling bottleneck for a dryland tree species?

Abstract: Climate change is increasing the frequency and severity of droughts in arid and semi-arid ecosystems worldwide, pushing many tree species beyond their physiological limits. Forest resilience and recovery following drought-induced mortality will depend on successful seedling establishment, which can be a major bottleneck. Pinus monophylla, or singleleaf pinyon pine, is a foundational tree species in the western United States, which covers a large geographic area and is considered the most xeric pine in the nation. However, a regional drought in the Great Basin between 2013-2015 caused extensive mortality and canopy dieback, which is likely to occur again with future climate projections. Adequate recruitment is required for population persistence following overstory mortality, yet increasing drought conditions means that future recruitment will depend on the presence of drought-adapted seedling phenotypes. The goal of this study was to determine how seedling functional traits and trait plasticity varied among different seed source locations of P. monophylla in response to a gradient of water availability, using a common garden greenhouse experiment. We collected seeds from 23 sites across nine mountain ranges that encompassed rangewide gradients of aridity and seasonal moisture availability. We propagated 3,320 seedlings and subjected them to four watering treatments representing a range of water availability. After six months of growth in a climate-controlled greenhouse, we measured aboveground and belowground growth-related traits. Trait values and trait plasticity, here representing the degree of variation among watering treatments, were modeled as a function of watering treatment and environmental seed source conditions (climatic water deficit, precipitation seasonality, and available soil water storage capacity) using linear mixed effects models. We show that under all treatments, seedlings from more arid climates had larger aboveground and belowground biomass compared to seedlings from sites experiencing less growing-season water stress, even after accounting for differences in seed size. Additionally, seedlings from locations experiencing episodic monsoonal rainfall events had greater trait plasticity in response to watering treatments. Regional differences in seedling responses to water availability suggest that P. monophylla seedlings exhibit local adaptation in both traits and trait plasticity. Having high trait diversity will likely influence the potential for future seedling recruitment in pinyon-juniper woodlands that may experience extensive drought-related overstory tree mortality. Moreover, understanding these differences in seedling trait responses has implications for future ecosystem restoration.