School of Natural Sciences, University of California Merced, Merced, CA 95343, USA.
Oecologia. 2013 Jan;171(1):271-82. doi: 10.1007/s00442-012-2410-0. Epub 2012 Aug 9.
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 m(3) 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 m(3) 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.
预计气候变化的持续变化将改变植物物种的地理分布，部分原因是影响个体从种子建立的位置。我们通过减少高海拔地区的低温限制和增强低海拔地区的干旱胁迫来检验变暖促进亚高山树种种群向上坡重新分布的假设。我们在低亚高山森林、林线生态交错带和高山冻原位置的实验点，用红外加热和加水处理的组合对白皮松（Pinus flexilis）种子进行播种。2010 年，对无雪生长季节的生理表现和成活率进行了评估。种子在仲夏出苗，大约在雪融后 5-8 周。在出苗到 10 月之间，低温并没有被观察到限制幼苗的光合作用或呼吸作用，因此实验性的变暖似乎并没有减轻高海拔地区的低温限制。相反，所有地点的气体交换和水势表明，夏季水分胁迫对光合作用和碳平衡有主要影响。红外加热器提高了土壤生长度日（基础 5°C，p < 0.001）和 8 月至 9 月的平均土壤温度（p < 0.001）。尽管各地点的植被覆盖和气象条件存在显著差异，但 5-10 厘米深的土壤体积含水量（θ）与中度和重度干旱胁迫下的中午茎水势、气孔导度、光合作用和呼吸作用一致。与加热处理相比，浇水处理的种子成活率更高（p = 0.01），与土壤生长度日和暴露于 θ < 0.08 m³ m⁻³的时间呈负相关，在逐步线性回归模型中（p < 0.0001）。我们的结论是，季节性水分胁迫和高土壤表面温度对包括林线在内的广泛海拔梯度上白皮松幼苗的建立构成了强烈限制，而这些限制可能会因进一步的气候变暖而加剧。
