The Pattern of Tree Seedling Establishment Relative to Staghorn Sumac Cover in Michigan Old Fields

Abstract

Trees invading 10-year and 16-year-old abandoned fields in Michigan were aged to determine year of establishment. Vegetation cover was mapped to develop correlates between tree establishment patterns and vegetation type. Few trees established in the 1st 5 years after the fields were abandoned from cropping. Most (74%) of the 947 individual trees were between 2 and 5 years of age. The density of tree seedlings in all fields was significantly higher under a canopy of the shrub, staghorn sumac (Rhus typhina), compared to areas without a sumac canopy. It is argued that while sumac cover promotes tree seedling success, the effect is indirect, mainly by reducing the amount and type of ground-level vegetation which inhibits the establishment of tree seedlings, and secondly by increasing tree seed arrival via animal activity. INTRODUCTION The mechanisms that determine how late successional species appear in a successional sere have been under discussion for some time (see reviews by McCormick, 1968; Drury and Nisbet, 1973; Horn, 1974; Connell and Slatyer, 1977; McIntosh, 1980). There are three general views: The first is that the earlier colonists prepare the way (called reaction by Clements, 1916, or by Connell and Slatyer, 1977) for later colonists by modifying the environment so that it is more suited for the later successional species. This view is perhaps the most common presented in textbooks and in popular literature, but has been questioned repeatedly by ecologists, notably by Egler (1954) and further by McCormick (1968), Niering (Niering and Egler, 1955; Niering and Goodwin, 1974), Drury and Nisbet (1973) and others. Most likely it describes a mechanism that operates in primary succession, but is not very important in secondary succession. The second view is that the first colonists inhibit invasion of later arrivals and are replaced only when they die naturally; there is much evidence to support this claim presented in the reviews by McCormick (1968) and Drury and Nisbet (1973). The third view is that the species sequence is determined solely by the life history characteristics of the organisms, that late-appearing species are those which arrived later and grew more slowly to larger sizes, eventually dominating the community. To what degree do early colonists inhibit, tolerate or facilitate (terminology of Connell and Slatyer, 1977) later arriving species? Experimental studies would be required to test the various models proposed. Some experiments have been conducted in communities which show rapid successional change (e.g., estuarine fouling communities of invertebrates, Dean and Hurd, 1980), with the results showing inhibitory interactions more prevalent than facilitation interactions. Experiments in terrestrial communities are rare, perhaps because they require such a long time that they are impractical; an analysis of pattern in existing communities may still be among the best evidence in regard to this problem. In the field study reported here, invading trees were aged in fields abandoned from agricultural practice for 16 and 10 years to determine whether they had been a part of the initial floristic composition (Egler, 1954) or whether they 1Present address: B. P. 817; Yaounde, Cameroon, Africa.