Lab and Field Warming Similarly Advance Germination Date and Limit Germination Rate for High and Low Elevation Provenances of Two Widespread Subalpine Conifers

Abstract

Accurately predicting upslope shifts in subalpine tree ranges with warming requires understanding how future forest populations will be affected by climate change, as these are the seed sources for new tree line and alpine populations. Early life history stages are particularly sensitive to climate and are also influenced by genetic variation among populations. We tested the climate sensitivity of germination and initial development for two widely distributed subalpine conifers, using controlled-environment growth chambers with one temperature regime from subalpine forest in the Colorado Rocky Mountains and one 5 • C warmer, and two soil moisture levels. We tracked germination rate and timing, rate of seedling development, and seedling morphology for two seed provenances separated by~300 m elevation. Warming advanced germination timing and initial seedling development by a total of~2 weeks, advances comparable to mean differences between provenances. Advances were similar for both provenances and species; however, warming reduced the overall germination rate, as did low soil moisture, only for Picea engelmannii. A three-year field warming and watering experiment planted with the same species and provenances yielded responses qualitatively consistent with the lab trials. Together these experiments indicate that in a warmer, drier climate, P. engelmannii germination, and thus regeneration, could decline, which could lead to declining subalpine forest populations, while Pinus flexilis forest populations could remain robust as a seed source for upslope range shifts.

Moisture availability limits subalpine tree establishment

“…Germination rates of Engelmann spruce are negatively affected by warmer and drier conditions (Kueppers et al. ), and seedling mortality of both species is typically very high (>85%) during the first year, particularly on dry south aspects (Noble and Alexander ). In the years following germination, seedling mortality remains high (typically >99% mortality after 4 yr) and in the southern Rocky Mountains is strongly influenced by soil moisture conditions (Noble and Ronco , Kueppers et al.…”

“…Yet previous research in the southern Rockies has primarily focused on the influence of climate on alpine treeline dynamics (Weisberg and Baker , Maher and Germino , Elliott and Cowell , but see Kueppers et al. , ), leaving uncertainty about how climate influences establishment across a majority of the area inhabited by subalpine tree species (i.e., “core” species range). Thus, research is needed to determine the frequency and trends in annual establishment events across the core range of subalpine tree species and to understand the sensitivity of establishment to interannual climate variability in the context of expected increases in temperature of 1.7°–2.8°C by mid‐century (Lukas et al.…”

Bacterial endophyte communities in Pinus flexilis are structured by host age, tissue type, and environmental factors

“…Seed was collected locally, processed as described in Kueppers et al (2017a) and sown in the common gardens in the autumn of each year. Emergent seedlings were surveyed weekly in spring and summer, and individually tracked from one season to the next to quantify germination (Kueppers et al 2017b) and recruitment up to 4 years (Kueppers et al 2017a). The work was carried out with permission from the U.S. Forest Service via the University of Colorado Mountain Research Station.…”

A δ18O and δ2H stable water isotope analysis of subalpine forest water sources under seasonal and hydrological stress in the Canadian Rocky Mountains

“…Despite these challenging growing conditions, subalpine forests manage to rely largely upon limited soil moisture stores to sustain ET (Day et al, 1990; Small & McConnell, 2008). Current research on dominant subalpine forest species (i.e., Abies lasiocarpa ( S. fir , hereafter fir) and Picea engelmannii (Engelmann spruce, hereafter spruce)) has focussed on physiological and biological differences by elevation and climate (Kueppers et al, 2017; Sala, 2006; Sowell & Spomer, 1986), and species management (Alexander, 1987). However, there are few studies of subalpine ET and water use (Brodersen et al, 2006), particularly in the hydroclimatically distinctive Canadian Rockies (Fagre et al, 2014) highlighting a gap in knowledge of water use by these species at high latitudes.…”

“…Picea engelmannii (Engelmann spruce, hereafter spruce)) has focussed on physiological and biological differences by elevation and climate (Kueppers et al, 2017;Sala, 2006;Sowell & Spomer, 1986), and species management (Alexander, 1987). However, there are few studies of subalpine ET and water use (Brodersen et al, 2006), particularly in the hydroclimatically distinctive Canadian Rockies (Fagre et al, 2014) highlighting a gap in knowledge of water use by these species at high latitudes.…”