| Popis: |
In order to understand the response of vegetation to an air pollutant, definite cause and effect relationships must be established. Survey methods which document only the mutual occurrence of two events will not provide adequate information on which to base a cause and effect relationship. To administer experimental treatments, knowledge of the spatial, temporal, and other characteristics of ambient precipitation must be understood and technical equipment must be available to administer such treatments. For acidic deposition studies, exclusion of ambient acidic deposition must be accomplished so that the effects of administered treatments can be separated from the effects of ambient precipitation. Previous experience has demonstrated that the exclusion of ambient deposition and administration of experimental treatments should be accomplished under conditions which most closely approximate natural environments because vegetation responses have been shown to differ under controlled environmental conditions compared with field conditions. In experiments performed under natural field conditions, many plants per replicate and a large number of treatment replicates are necessary in order to separate treatment effects from random environmental variables. The following factors may make a linkage between acidic deposition and forest decline difficult to establish. The chemical, physiological, and episodic nature of acidic deposition exposures at either low or high elevation sites are not well documented. Visible symptoms exhibited by needles or trees in decline are not well characterized. The decline in forest growth does not appear to be of recent origin. Visible foliar symptoms which are directly attributable to ambient acidic deposition have rarely been observed. Environmental factors other than acidic deposition may be concurrently influencing these tree species. Consideration of these aspects is necessary to establish a relationship between acidic deposition and forest decline in the northern Appalachians. |
