Twentieth century forest–grassland ecotone shift in Montana under differing livestock grazing pressure

Abstract

We studied 20th century lower forest–grassland ecotone shift in the Centennial Valley in southwestern Montana and investigated the effects of livestock herbivory on ecotone shift at a local spatial scale and a decadal time scale. A total of 525 aspen ( Populus tremuloides) and 1703 Douglas-fir ( Pseudotsuga menziesii) trees were cored and 10,168 saplings were mapped along the ecotone at five sites that represented five different grazing regimes. 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 corroborate the dendrochronological data and to detect changes in the ecotone, historic and current aerial photographs were compared. To examine livestock herbivory effects on ecotone shift at a decadal time scale, the number of new trees was correlated with 10 different levels of cattle grazing pressure during the last 60 years. Two different types of ecotone shift were documented. Previously documented simple linear relationships between livestock herbivory and tree encroachment were not consistently observed at decadal time scales. Livestock herbivory had a complex relationship with tree encroachment. Our results suggested that no consistent conclusions could be made regarding grazing effects on tree establishment across broad ranges of grazing pressure at decadal time scales. We suggest that the way forward needs to use a process-based approach to devise strategies for observing and quantifying multiple interacting processes of inhibition and facilitation effects.