Man's activities pose a number of threats to the functioning,
structure and diversity of natural and semi-natural ecosystems. One of
the main threats is the increase in concentrations in air pollutants in
this
century (Wellburn, 1988; Tamm, 1991). This paper is a commentary on the
effects of tropospheric
ozone (O3) and airborne nitrogen deposition (both oxidized (NOx)
and reduced (NHy) compounds) on
natural and semi-natural ecosystems, based upon the oral presentations
and the discussions during the
Symposium, extended with a personal overview and some suggestions about
future challenges for
research. The most important effects of these air pollutants on
natural and semi-natural vegetation are
summarized and evaluated in ecological terms, with respect to the functioning
and structure of
unaffected systems. Air pollutants are transported over both
short and long distances (as far as a few
thousand km) before being deposited on surface water, vegetation or
soil. In this way, vegetation over a
large area or in remote regions can be influenced by airborne pollutants
(see Fowler et al. (1998); Asman, Sutton & Schjørring
(1998)).