Alpine Seedling Establishment: The Influence of Disturbance Type

Abstract

The effects of disturbance type on seedling environment and establishment of alpine species with different physiological and life history traits were examined during a 2—yr study on the Beartooth Plateau in southwestern Montana, USA. We compared soil temperatures, water potentials, and nutrients on mineral soils of a gravel borrow area with those on highly organic soils of a Geum turf area. Seedling emergence, growth, and survival of six seeded species (Geum rossii, Artemisia scopulorum, Potentilla diversifolia, Sibbaldia procumbens, Deschampsia cespitosa, and Festuca idahoensis) and emergence and survival of five unseeded species (Draba crassifolia, Draba incerta, Cerastium arvense, Arenaria rubella, and Androsace septentrionalis) were evaluated on both areas. The effects of N and P nutrient addition and surface organic mulch on the soil environment and seedling establishment were evaluated on the borrow area, while differences between uncleared turf and turf cleared of vegetation were compared on the Geum turf area. Plots cleared of vegetation on the Geum turf area had higher levels of soil N (NO3—) and P than uncleared turf and both higher levels of N (NO3— + NH4+) and P and higher soil temperatures (surface, 5, and 15 cm depths) than fertilized or not—fertilized borrow area treatments. Fertilization increased N and P on borrow area soils, but after 2 yr N had decreased significantly. Soil water potentials (5 and 15 cm depths) did not differ between cleared plots on the Geum turf area or any of the borrow area treatments and were never low enough to cause plant stress. Vegetated Geum turf had significantly lower water potentials than cleared plots, especially late in the growing season. Mulch had no effect on soil water potential or nutrients on the borrow area and increased soil temperatures only on clear days during the first growing season. Wind removed or redistributed the mulch over time, thus decreasing potential effects. Seedling emergence was highly dependent on soil surface stabilization and reflected species life history traits. Growth of seedlings was slow, and varied among species and treatments: 0.005—0.04 and 0.02—0.20 g total mass after the first and second growing seasons, respectively. Significantly higher total seedling mass was observed on cleared Geum turf plots than on any of the borrow area treatments, and on fertilized than on not—fertilized plots on the borrow area. Seedling mortality of most species was much lower than previously found for alpine ecosystems, rarely exceeding 50% even after 2 yr. On the borrow area mulch increased survival, probably through microenvironmental amelioration. The nutrient pulse from fertilization mortality of several species, presumably by creating plant nutrient demands in excess of availability during year 2. Both disturbance characteristics and species life history and physiological traits affected seddling establishment. Pretreatment soil properties of the two disturbance types had the greatest effects on soil temperatures and nutrients and, consequently, on seedling growth and survival. Soil surface characteristics had the largest effects on seedling emergence; surface stabilization was essential for holding both soil and seed in place. Single species responses varied in magnitude but were similar on both disturbance types. In general, there were larger differences among species in emergence and growth than in survival. Thus, successful seedling establishment on different alpine disturbance types may depend more on obtaining the necessary conditions for seedling emergence and on species interactions than on the ability of seedlings to survive different environmental conditions.