ABSTRACT RESULTS SUMMARY AND CONCLUSIONS REFERENCES MATERIALS & METHODS OBJECTIVES Background/Questions/Methods 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. Results/Conclusions Timing of seedling emergence was described by species-specific logistic functions of soil growing 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 growing 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. 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 Andrew B. Moyes (abmoyes@lbl.gov), Cristina Castanha, Lara M. Kueppers Earth Sciences Division, Lawrence Berkeley National Laboratory ACKNOWLEDGMENTS The Alpine Treeline Warming Experiment (https://alpine.ucmerced.edu) Overview The Alpine-Treeline Warming Experiment was installed in 2008-2009 on Niwot Ridge above Boulder, Colorado. It uses infrared heaters initially intended to warm soil and plant surfaces by an amount comparable to average projections of climate warming in the year 2100. We have three basic questions we hope to answer with the project: Will subalpine trees, currently restricted from cooler, higher elevations, move into alpine habitat and replace alpine plant species as a result of climate warming? Will subalpine trees be stressed by warmer temperatures and be less successful in their existing elevational ranges as a result of climate warming? Will ecosystem properties modify the effects of climate warming on subalpine or alpine species within and beyond their current elevational ranges? The Alpine Treeline Warming Experiment Warming lead to earlier snowmelt and emergence, with emergence corresponding with soil growing degree days. Mortality among new emergent was initially high, with about half of seedlings dying within 20-30 days. Following this period, mortality increased with time plants were exposed to soil moisture below 0.16 m3 m-3 and with accumulated growing degree days. Mortality of lodgepole pine and Engelmann spruce steeply approached 100% near the middle of the observed range of growing degree days accumulated after emergence. While warming facilitated early emergence and growth when temperature was limiting: As conditions continue to warm, sensitivity to prolonged low summer moisture and/or heat exposure may restrict seedling establishment across the elevation gradient, and especially at warmer, drier, low elevations, and exposed alpine sites.. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research, under Award Number DE- FG02-07ER64457. We thank the Mountain Research Station and Niwot Ridge LTER at the University of Colorado, Boulder for logistical support. Castanha C, Torn MS, Germino MJ, Weibel B,Kueppers LM. 2012. Conifer seedling recruitment across a gradient from forest to alpine tundra: Effects of species, provenance, and site. Plant Ecology & Diversity 1-12. Moyes A, Castanha C, Germino M,Kueppers L. 2013. Warming and the dependence of limber pine (pinus flexilis) establishment on summer soil moisture within and above its current elevation range. Oecologia 171: 271-282. Moyes, AB, MJ Germino, and LM Kueppers, Moisture cost of warming rivals temperature benefits for trees establishing beyond their cold-edge range limit (In submission). AGU Poster Number Alpine Treeline Forest Overall experimental design. At each of 3 sites along an elevation gradient there are 20 circular plots. Half of plots are warmed by 6 infrared heaters and half are watered weekly (3 mm only to replace water lost to surface heating). Plots were divided into 1m 2 subplots, with a soil moisture and temperature sensor in the center of each subplot inserted vertically to measure over 5-10 cm depth. Panoramic photographs taken at each of the three sites Engelmann spruce Limber pine Lodgepole pine Seeds of three species are sown into the subplots each October Survey crews visited plots every 1-2 weeks and counted emergent, surviving, and dead seedlings Heaters warmed plots by 1-4 degrees at 5-10 cm depth, with much variability in heating associated with differences in wind speed. Seedlings emerged in large numbers a few weeks to months after snowmelt each year (positive y-axis values). There were large numbers of seedlings that died (negative values) soon after emergence, with fewer dying in subsequent years. Most 1 st -year deaths occurred within 20-30 days after emergence Seedlings emerged fairly consistently in relation to growing degree days, but with some site, species, and inter- annual differences Mortality increased with time subplots spent with soil moisture below 0.16 m 3 m -3 (upper panels), and with growing degree days (lower panels) during the growing season period following the date of 75% emergence. Each data point is a subplot-year. Early greening of alpine vegetation is apparent inside a heated plot in early summer. Alpine vegetation senesces in late summer, when soil surfaces become hot and dry in the afternoon sun. These conditions were intensified by the heating treatments. View publication stats View publication stats