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 concent ration, and water potential were monitored on foliage of burned and no nburned regeneration of northern red oak (Quercus rubra L.), white ash (Fraxinus americana L.), and sugar maple (Acer saccharum Marsh.) foll owing spring fires in 1989 and 1990. Fire led to a stimulation of ligh t-saturated photosynthesis in foliage of resprouting plants, but the e ffect varied in magnitude among species. On average, photosynthesis of post-fire maple was 42% higher than that on nonburned plots, with cor responding increases occurring in stomatal conductance and leaf N conc entration, Ln general, fire had a marginal effect on leaf properties o f oak and ash. In 1990, the ratio of root area to leaf area and the wa ter potential of sunlit foliage were significantly higher on burned th an nonburned plots for maple, but not for oak or ash, paralleling tren ds in photosynthesis. There was little evidence of a treatment differe nce in soil moisture or nitrogen availability, and it appeared that fi re-induced stimulations in photosynthesis were mediated primarily by c hanges in intrinsic (e.g., root area/leaf area ratio) as opposed to ex trinsic (e.g., soil moisture) factors governing resource availability to the canopy of post-fire sprouts.