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  • Thumbnail for The Effects of Growing Season Soil Temperature on Engelmann Spruce Seedling Establishment, Survival, and Growth in an Abrupt Treeline on Pikes Peak, Colorado
    The Effects of Growing Season Soil Temperature on Engelmann Spruce Seedling Establishment, Survival, and Growth in an Abrupt Treeline on Pikes Peak, Colorado by Cutler, Victoria

    Previous studies have found treeline dynamics to be related to macroclimatic factors (Harsch et al., 2009) and microclimatic factors such as local snowpack, wind, and air and soil temperature (Germino, Smith, and Resor, 2002; Harsch et al., 2011; Smith et al., 2003). Seedling health has been thought to indicate the future of treeline position (Germino et al., 2002; Smith et al., 2003; Harsch et al., 2011) and has appeared to be highly related to soil temperature (Grace, Berninger, and Nagy, 2002; Harsch et al., 2011; Smith et al., 2003; Körner et al., 2004; Malanson et al., 2011). This study examines the effects of soil temperature on seedling establishment, survival, and growth, and the degree to which soil temperature may influence seedling dynamics relative to other factors. Daytime and nighttime soil temperature, as well as seedling establishment, survival, and growth data, were collected in an abrupt treeline consisting entirely of Engelmann Spruce (Picea engelmannii) on Pikes Peak, Colorado (~3,550 m). The study site was split up into three different microsites with unique microclimates – the Lower Sheltered Zone, Upper Sheltered Zone, and Tundra – where the relationships between seedling dynamics and soil temperatures could be analyzed separately, and post hoc analyses were conducted to compare the broader zonal relationships. Seedling establishment within the Lower Sheltered Zone was generally greater than expected in the warmer areas and less than expected in the colder areas. Establishment in the Tundra, however, followed the opposite pattern. Survival was largely statistically unrelated to soil temperature, unless a certain threshold was surpassed within the Lower Sheltered Zone and Upper Sheltered Zone. Growth was statistically significantly related to daytime soil temperatures within the Lower Sheltered Zone by a second-degree polynomial, with an optimum at 7.87°C (R2 = .39, n = 16, p = 0.04), but statistically unrelated to soil temperatures within the other two zones. Post hoc zonal comparisons of seedling dynamics revealed relatively large statistically significant differences, which suggested that other microclimatic factors were influencing seedling dynamics more than soil temperatures.