Microclimate factors affecting seedling emergence and survival of three conifer species within and above their current elevation ranges: results of a multi-year climate manipulation study in the Colorado Rocky Mountains
Emergence and survival of seedlings are requisite, limiting steps of tree species range persistence and expansion with climate change. In these early life stages seedlings are uniquely sensitive to microclimate conditions near the soil surface, yet the abiotic conditions governing their emergence and mortality have yet to be precisely quantified for use in distribution models. To investigate microclimate sensitivity of seedling emergence and initial mortality of Rocky Mountain conifers, we sowed seeds of three species (Picea Engelmannii, Pinus contorta, and Pinus flexillis) in common gardens in lower subalpine forest, the treeline ecotone, and in the alpine (above treeline). Over three years, we conducted replicated climate manipulation treatments (infrared heating and water additions), and recorded seeding emergence and mortality every 7-10 days. We use soil temperature and moisture data, logged every 15 minutes to determine which abiotic variables best explained the timing of seedling emergence and mortality across treatments, sites, and years.
Timing of seedling emergence was described by species-specific logistic functions of soil degree days, with all species reaching 50% emergence between 250-300 °C d despite site and treatment differences in snow accumulation, snowmelt, soil type, and sun exposure. Because heated plots accumulated soil degree days faster than unheated plots, seedlings emerged 5-40 d earlier with heating. Seedling mortality decreased with seedling age, with >50% mortality occurring within the first 30 days following emergence. Additional mortality events coincided with exposure to combinations of low soil moisture and high soil temperature. Limber pine was most resilient to these conditions. Our results indicate a contrast between a purely temperature-dependent predictor of seedling emergence (soil degree days) and episodic mortality events dependent on both moisture and temperature. Snowmelt and soil water depletion occurred earlier at maximum and minimum elevation sites (within windswept alpine sites and lower elevation forest) than within the treeline ecotone. This indicates that, in the absence of additional precipitation, warming may forestall uphill advance at cold edge limits of subalpine conifers in addition to lowering recruitment rates near warm edge limits.