Spatial and Temporal Tree Responses to Water Stress in an Old-growth Deciduous Forest

Xylem water potential of flowering dogwood (Cornus florida) and pawpaw (Asimina triloba) and stem girth of white oak (Quercus alba) and tulip poplar (Liriodendron tulipifera) were measured over 3 yr in an old-growth forest in SW Indiana. The 3 yr encompassed periods ranging from moderate drought to over average rainfall. Both precipitation events and the distance of a stem to a nonforested edge had an important influence on predawn xylem water potential. Individuals closer than 30 m to the edge of the forest had consistently lower water potentials than those farther into the forest. This correlation was greater for pawpaw than for dogwood, and differences were generally significant on drier dates. Stem girth was not linked as closely to spatial differences within the woods as to fluctuations in the precipitation pattern. Neither white oak nor tulip poplar showed a net increase in girth during the severe drought of 1988. During 1989, a year with greater than average rainfall, white oak had a greater stem girth increase than tulip poplar, a reversal of the predrought growth pattern.