MECHANISMS OF ARRESTED SUCCESSION IN SHRUBLANDS - ROOT AND SHOOT COMPETITION BETWEEN SHRUBS AND TREE SEEDLINGS

Relative effects of above-ground and below-ground competition from shrubs (Cornus racemosa and Rhus glabra) on the growth of tree seedlings (Acer rubrum and Fraxinus americana) were assessed with a field experiment in the Hudson Valley, New York. Shade on tree seedlings planted in clumps of shrubs was reduced by guy-wiring back overtopping shrub crowns. Root competition was reduced by cutting a 0.5 m deep slit around the seedlings and inserting root restriction cloth. We also applied a combined treatment to some seedlings and reserved others as controls. In the field with the most severely drained and nutrient-poor soil, reduction of root competition increased relative growth rates of the seedlings while opening the canopy had no effect. Trenching and shade reduction around tree seedlings planted in shrubs clumps in a more mesic old field with a stony silt loam soil increased relative growth rates only if the treatments were combined. In a wet field with alluvial soil, guy-wiring back overtopping shrubs greatly increased seedling growth but trenching alone had no effect. Tree encroachment into shrub-dominated stands can be reduced by both root and shoot competition, but the severity of competition in these arenas varies with site conditions; below-ground competition is intense where soil resources are limited whereas the effects of shade are relatively more severe on sites with good soil.