Report from the Sub-comittee on the environment and health

Impact on <strong>the</strong> flora of
nature areas through
atmospheric transport of
pesticides
The effect of pesticides in
rainwater
a higher age than trees. Danish studies have shown that forests with long
continuity (>200 years) have a better developed forest-floor flora than
younger forests (Graae 1999), so for Danish forests, too, it is reasonable
to expect impacts on <strong>the</strong> forest-floor flora if herbicides are used in
connection with clear cutting and reestablishment of cultures. According
to Graae (1999), species propagated mainly by clonal growth will
recolonise very slowly, while those dispersed by animals and <strong>the</strong> wind
recolonise more quickly. Mechanical soil treatment instead of <strong>the</strong> use of
herbicides presumably has similar effects on <strong>the</strong> forest-floor flora, but
this has not yet been studied.
Herbicides are not normally used on nature areas, but many herbicides
are transported over long distances and small amounts of <strong>the</strong>m are
<strong>the</strong>reby deposited on nature areas. No information has yet been found on
measurements of direct impacts <strong>from</strong> pesticide drift on <strong>the</strong> flora in nature
areas apart <strong>from</strong> boundary areas such as hedgerows. It is <strong>the</strong>refore
necessary to use model calculations. In <strong>the</strong> Ne<strong>the</strong>rlands, <strong>the</strong> average
combined herbicide consumption is 1.35 dose equivalents per year, 5.5%
of which evaporates. On this basis and import/export considerations
concerning atmospheric transport, it was calculated in a Dutch model
study (Klepper et al. 1998) that <strong>the</strong> Dutch nature areas received an
average of 0.02 dose equivalents per year. By means of a dose-response
model for <strong>the</strong> potentially affected natural vegetation, this deposition was
used to predict how large a percentage of <strong>the</strong> species were affected
beyond NOEC (No Observed Effect Concentration). The result was that
2% (median value) of <strong>the</strong> species were affected beyond <strong>the</strong>ir NOEC. The
percentage of affected species was highest in agricultural areas and in
areas with fruit and berry growing. There are no similar calculations for
Danish conditions, but <strong>the</strong> use of herbicides with a treatment frequency
index of 1.65 in 1997 (DEPA 1998a) is comparable to <strong>the</strong> Dutch
consumption, since <strong>the</strong> frequency index corresponds to <strong>the</strong> Dutch dose
equivalent. It cannot be concluded directly <strong>from</strong> <strong>the</strong> Dutch calculations
that around 2% of <strong>the</strong> species in Danish nature areas are unacceptably
affected by herbicides. The level of impact depends, in part, on how <strong>the</strong>
nature areas are situated in relation to areas in which herbicides are used
and on <strong>the</strong> magnitude of <strong>the</strong> emission, <strong>the</strong> wind conditions and <strong>the</strong>
sensitivity of <strong>the</strong> local plant community. In <strong>the</strong> Dutch calculations, <strong>the</strong>
biggest uncertainties were reportedly due to <strong>the</strong> emission calculation and
<strong>the</strong> effect models (Klepper et al. 1998). A diffuse dispersal of pesticides,
e.g. via <strong>the</strong> atmosphere with precipitation, must be regarded as having
less effect on <strong>the</strong> composition of <strong>the</strong> flora in nature areas than increased
supply of nutrition and changed nature management.
The occurrence of pesticides in rainwater in Denmark is discussed in
section 4.5. Herbicides have also been detected in rainwater in countries
in Scandinavia and Nor<strong>the</strong>rn Europe (Kirknel, Felding 1995ab), but no
direct effects on flora as a consequence of this have yet been found
(Felding 1998b). It is known <strong>from</strong> American studies that deposition of
atmospheric herbicide residues (sulphonylurea) can produce symptoms
of damage in some crops, such as peas and beans (Felsot et al. 1996). It
has similarly been shown that even small doses of chlorsulfuron (1/100th
to 1/1000th part of normal dose) greatly reduce <strong>the</strong> plant biomass and
seed production in persecaria (Fletcher et al. 1996). In a Danish project
on <strong>the</strong> occurrence of pesticides in precipitation and effects on plants and
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