Warming and the dependence of limber pine (Pinus flexilis) establishment on summer soil moisture within and above its current elevation range
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Continued changes in climate are projected to alter the geographic distributions of plant species, in part by affecting where individuals can establish from seed. We tested the hypothesis that warming promotes uphill redistribution of subalpine tree populations by reducing cold limitation at high elevation and enhancing drought stress at low elevation. We seeded limber pine (Pinus flexilis) into plots with combinations of infrared heating and water addition treatments, at sites positioned in lower subalpine forest, the treeline ecotone, and alpine tundra. In 2010, first-year seedlings were assessed for physiological performance and survival over the snow-free growing season. Seedlings emerged in midsummer, about 5–8 weeks after snowmelt. Low temperature was not observed to limit seedling photosynthesis or respiration between emergence and October, and thus experimental warming did not appear to reduce cold limitation at high elevation. Instead, gas exchange and water potential from all sites indicated a prevailing effect of summer moisture stress on photosynthesis and carbon balance. Infrared heaters raised soil growing degree days (base 5 °C, p < 0.001) and August–September mean soil temperature (p < 0.001). Despite marked differences in vegetation cover and meteorological conditions across sites, volumetric soil moisture content (θ) at 5–10 cm below 0.16 and 0.08 m3 m−3 consistently corresponded with moderate and severe indications of drought stress in midday stem water potential, stomatal conductance, photosynthesis, and respiration. Seedling survival was greater in watered plots than in heated plots (p = 0.01), and negatively related to soil growing degree days and duration of exposure to θ < 0.08 m3 m−3 in a stepwise linear regression model (p < 0.0001). We concluded that seasonal moisture stress and high soil surface temperature imposed a strong limitation to limber pine seedling establishment across a broad elevation gradient, including at treeline, and that these limitations are likely to be enhanced by further climate warming.
KeywordsLimber pine Treeline Experimental warming Moisture stress
This research was supported by the Office of Science (BER), U.S. Department of Energy. We thank the Mountain Research Station and Niwot Ridge LTER at the University of Colorado, Boulder for logistical support. M. Barlerin, S. Barlerin, J. Beauregard, J. Darrow, S. Ferrenberg, S. Love-Stowell, A. Peterson, and K. Riddell set up and/or maintained the treatment and microclimate sensing infrastructure, and D. Billesbach, D. Christensen, and J. Norris assisted with microclimate data retrieval, quality assurance, and archiving. J.B. Curtis, M. Daly, A. Faist, A.P. Farnham, H. Finkel, E. Flemming, C. Foster, R. Gaffney, A. Howell, A. Qubain, M. Redmond, A. Slominski, S. Sawyer, S. Taylor Smith, and F. Zust collected and sowed seeds and/or conducted germination and survival surveys. Thanks to K. Reinhardt for valuable discussions and helpful feedback on early drafts of the manuscript. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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