Influence of Soil Moisture, Herbaceous Competition, and Small Mammals on Lodgepole Pine (Pinus contorta var. murrayana) Seedling Establishment in Sierra Nevada Meadows of California

DOCTORATE DISSERTATION: To test the hypotheses that vegetative competition and high water tables prevent successful establishment of lodgepole pine (Pinus contorta var. murrayana) in montane California Sierra Nevada meadows, three month old lodgepole seedlings were transplanted into dry, mesic and wet meadows in 1982 and 1983. Soil moisture was increased in dry and mesic meadows and decreased in wet meadows in combination with reduction of vegetative competition and protection from small mammal grazers. Seedling response to abiotic factors was assessed with a stress/mortality index based on non-green needles (NGN) which was linearly related to mortality. Shade reduced mortality. In 1982 with soil water potential between -0.2 and -0.3 MPa, NGN exceeded 0.6 but averaged 0.15 in plots without and with meadow vegetation. NGN and light exposure were correlated. Unusually high summer-long soil moisture in 1983 depressed NGN, which ranged from 0.15 to 0.40 compared to 0.15 to 0.80 in 1982. Survivorship after 3 months in protected control plots was 99 percent and 69 percent for 1983 and 1982 cohorts. I wet meadows after 3 years 0 percent seedlings at ambient water tables survived compared to 17 percent in plots elevated 15cm. First season unprotected control plot mortality from small mammals in 1982 was 8, 37, and 42 percent in dry, mesic and wet meadows and in 1983 was 37, 25, and 18 percent. After three years 50 percent of the 1982 cohort was killed by small mammals. Given the apparent lack of competition and the tolerance of seedlings to moderately high water tables, small mammals are critical in excluding seedlings from meadows. It was hypothesized that small mammals consume lodgepole seeds, but that cleared patches within meadows discourage seed predation thereby increasing seed longevity. Percent lodgepole seeds remaining in Petri dishes in cleared 0.00, 0.35, 1.00, 2.00 m diameter plots averaged 10, 26, 21, 60 percent after 24 hours. Presence of small mammal feces was linked with seed predation. Seeds remaining on the ground in 0.00, 0.35, 0.77, 1.00 m patches averaged 33, 40, 88, 79, percent after 5 days. Small mammal seed predation in meadows can be substantial but was reduced in cleared areas larger than 0.5m.