Dose-response relationships have been generated for various agricultural crops and some
temperate forest tree species, but data on native herbaceous vegetation are sparse. The
information used in deriving such relationships have been based on controlled fumigations,
under quasi-field or defined environmental conditions, from filtration experiments and from
field studies such as transects along sulfur dioxide gradients. Field and filtration studies provide
data on responses under realistic conditions but are confounded by the presence of other
pollutants and variable environmental conditions. Nevertheless, reasonably accurate values for
no-response thresholds for adverse effects have been derived for broad categories of plants, as
used in the documentation on critical levels of the United Nations Economic Commission for
Europe