Warming and the dependence of limber pine (Pinus flexilis) establishment on summer soil moisture within and above its current elevation range

Abstract

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.

Soil Preferences in Germination and Survival of Limber Pine in the Great Basin White Mountains

“…These results show that despite the high pH, high magnesium, and low phosphorus properties of dolomite soils (Table 1), there must be at least some positive attributes of dolomite for limber pine establishment, namely water holding capacity and temperature, which may trump the negative aspects of dolomite. In recent studies of western North American sub-alpine forests establishing at and above treeline, drought stress was found to be a very strong limiter of establishment, especially for limber pine [15][16][17][18]. At least in the first year of life, the positive attributes of dolomite translate into positive effects on limber pine germination and establishment.…”

“…Properties of soil such as water holding capacity and albedo are likely major factors in successful recruitment of sub-alpine forests. Despite treeline being generally limited by cold temperatures [7], seedling establishment both at and above the treeline ecotone can be more immediately limited by low water availability and high temperatures [15][16][17][18].…”

Vegetation change at high elevation: scale dependence and interactive effects on Niwot Ridge

“…Warming without the addition of water negatively affected P. flexilis because it increased seasonal moisture stress (Moyes et al 2013). In 2010, seedling survival was greater in watered plus heated than heated only plots; warming was associated with reductions of first-year survival to as low as zero (Moyes et al 2013). Survival also was negatively related to both the length and warmth of the growing season and to the number of days below a low moisture threshold (Moyes et al 2013).…”

“…In these same sites, plots were experimentally warmed with aboveground infrared heaters, with or without the addition of water to counter increased evaporation. Warming without the addition of water negatively affected P. flexilis because it increased seasonal moisture stress (Moyes et al 2013). In 2010, seedling survival was greater in watered plus heated than heated only plots; warming was associated with reductions of first-year survival to as low as zero (Moyes et al 2013).…”

“…Perhaps more than other sites in the region, seedling establishment in the alpine tundra on Niwot Ridge appears to be strongly controlled by soil moisture and wind (Shea 1985;Humphries et al 2008;Moyes et al 2013). Age distributions indicate a relatively stable rate of establishment and persistence over time (Shea 1985), with fewer recruitment pulses than found regionally.…”