Sugar maple is one of the most widespread and abundant deciduous forest trees in eastern North America. Populations of sugar maple continue to expand in forests in the corn belt states of the U.S., but at the same time appear to be declining in parts of the northeastern U.S. and southeastern Canada. A number of causes for this "sugar maple decline" have been suggested, including effects of both gaseous air pollutants (especially ozone), and acidic precipitation. Rather than acting as the proximate cause of death of individual trees, such diffuse environmental factors act by placing trees under chronic stress. Such stress-weakened trees then become more susceptible to attack by insect pests or diseases which actually kill the tree. In this study, the investigators will examine the effects of ambient ozone, and of slightly elevated ozone, on a number of important, basic physiological processes in sugar maple: photosynthesis, respiration, transport of sugars among plant parts, and accumulation of starch stores for overwintering. They hypothesize that ozone injury to leaves will reduce the photosynthetic rate and force the plant to use more of the energy it does obtain through photosynthesis to repair damage. In addition, they hypothesize that the system the plant uses to transport sugars from leaves to roots may be directly damaged by ozone. These changes will have the effect of reducing the amount of food (as starch and sugar) that the plant can accumulate for survival over the winter. Thus, they expect overwinter mortality to increase, and growth during the subsequent spring to be lower, in plants exposed to ozone. The investigators will also extend these experiments to screen individuals from populations in areas of sugar maple decline and unaffected areas to determine if sensitivity to ozone varies geographically. The results of these experiments may reveal much about the basic ecophysiology of sugar maple, and will also allow insight into the more applied problem of sugar maple decline.
