Responses of hardwood regeneration to fire in mesic forest openings. II. Leaf gas exchange, nitrogen concentration, and water status

The physiological responses of tree regeneration to fire were studied in openings in a mesic hardwood forest. Gas exchange, nitrogen concentration, and water potential were monitored on foliage of burned and nonburned regeneration of northern red oak (Quercus rubra L.), white ash (Fraxinus americana L.), and sugar maple (Acer saccharum Marsh.) following spring fires in 1989 and 1990. Fire led to a stimulation of light-saturated photosynthesis in foliage of resprouting plants, but the effect varied in magnitude among species. On average, photosynthesis of post-fire maple was 42% higher than that on nonburned plots, with corresponding increases occurring in stomatal conductance and leaf N concentration. In general, fire had a marginal effect on leaf properties of oak and ash. In 1990, the ratio of root area to leaf area and the water potential of sunlit foliage were significantly higher on burned than nonburned plots for maple, but not for oak or ash, paralleling trends in photosynthesis. There was little evidence of a treatment difference in soil moisture or nitrogen availability, and it appeared that fire-induced stimulations in photosynthesis were mediated primarily by changes in intrinsic (e.g., root area/leaf area ratio) as opposed to extrinsic (e.g., soil moisture) factors governing resource availability to the canopy of post-fire sprouts.