Colonization by oak seedlings into a heterogeneous successional habitat

Abstract

The pattern of oak (Quercus velutina L. and Quercus alba L.) seedling establishment in relation to ground-cover vegetation was examined over a 5-year period in a 30-year Michigan old field. Three types of vegetation cover were identified: primary patches were areas of bare ground or those covered by moss, lichen, or a moss–lichen matrix over mineral soil (sand); transitional patches were areas of herbaceous vegetation over a ground layer of primary vegetation; secondary patches were areas of herbaceous perennials and (or) grasses over organic soil. In 1976, 27% of the study field was covered with primary patches but by 1980 only 8% of the area was covered with primary patches. Age distribution data indicated that oaks began colonizing the field about 15 years after abandonment; lack of seed input to the field or inability of seedlings to survive beyond 15 years in this field could account for the apparent absence of colonization in the first 15 years. Oak seedlings colonized principally the primary patches: 84% of 1-year-old seedlings were found in primary patches in 1976 and 59% of 1-year-old seedlings were found in primary patches in 1980, despite the small proportion of the field covered by primary patch types. Older seedlings (>7 years) were found principally in transitional and secondary patch types as the primary patch types became transitional and secondary types. There are indications that the appearance of acorns on primary patch types is due to blue jays which preferentially disperse acorns to open ground cover. In contrast, growth rates of seedlings were greatest in secondary patches and lowest in primary patches. The study suggests that oak seedlings invading an abandoned field become first established in bare ground but then survive only if the surrounding vegetation invades the microsite.