Old field succession on a Minnesota sand plain: effects of deer and other factors on invasion by trees

Invasion of old fields by trees occurs much more slowly at the Cedar Creek Natural History Area, Minnesota, than is typical of most areas in the eastern and central United States. Many old fields abandoned from agriculture more than 50 years ago lack a tree canopy. Tree density, height and average distance from the forest margin were all positively correlated with time since field abandonment. Most tree saplings over 20 cm tall were browsed by white-tailed deer during winter. Tree growth was significantly greater inside deer exclosures; however, the difference in growth rate for trees inside and outside of exclosures was much smaller than anticipated. Pocket gophers killed 1-2% of tagged trees per year, thus representing a significant source of mortality over the extended time period during which Cedar Creek old fields are invaded by trees. Mortality during and immediately after a drought in 1988 was more than double the mortality in other years. Removal of herbaceous vegetation around oak saplings resulted in a significant increase in stem diameter growth. Although cover of woody plants is positively correlated with soil nitrogen, fertilization did not increase growth of oak saplings. Nutrient-poor soils, slow growth rates, herbivory, and climatic factors all probably contribute to the slow invasion of Cedar Creek old fields by trees, and the extended period during which herbaceous plants dominate old field succession at Cedar Creek.