20th Century forest-grassland ecotone shift and effects of livestock herbivory
Sankey, Temuulen Tsagaan
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I studied 20th Century lower forest-grassland ecotone shift in the Centennial Valley in southwestern Montana, USA and the Darhad Valley in northern Mongolia and investigated the effects of livestock herbivory on ecotone dynamics. A total of 525 aspen (Populus tremuloides) and 1,703 Douglas-fir (Pseudotsuga menziesii) trees were cored and 10,168 seedlings were counted at five sites along the ecotone in the Centennial Valley. A total of 2,968 Siberian larch (Larix sibirica) were cored and 4,709 seedlings were counted at five ecotones in the Darhad Valley. Tree-age distribution was constructed to determine 20th Century tree establishment. Tree age and location within the ecotone were correlated to describe the process of ecotone shift into the adjacent grassland. To examine livestock herbivory effects on ecotone shift, the number of new trees was correlated with ten different levels of cattle grazing intensity during the last 60 years in the Centennial Valley and with five different grazing regimes during the last 80 years in the Darhad Valley. Three different types of ecotone shift into the adjacent grassland were documented: forest boundary shift, densification, and fairy ring establishment. No evidence of ecotone shift upslope towards the forest was found. Grazing intensity had a complex relationship with tree encroachment. Aspen and Douglas-fir tree encroachment was low at medium levels of grazing intensity, but aspen establishment was higher at low and high grazing levels and Douglas-fir establishment was higher at low grazing levels. Siberian larch tree encroachment was higher at low and high grazing intensities by sheep and cattle, but it was lower at low and medium grazing levels by goat-sheep and goatsheep- cattle mixes. My results implied that grazing can both facilitate and inhibit tree encroachment. I propose a model of grazing effects on tree encroachment that integrates both inhibition and facilitation effects of grazing disturbance. I also propose a conceptual model of lower forest-grassland ecotone shift and a conceptual model of ecotone shift and livestock herbivory effects. My models suggest that grazing can be used as a tool to maintain the equilibrium between forest and grassland vegetation and to increase or decrease forest expansion.