Report from the Sub-comittee on the environment and health

Effects on forest-floor flora
76
<strong>the</strong> drift zone than for plants directly under <strong>the</strong> sprayer. An American
study showed that sweet cherry is damaged by herbicide drift
(chlorsulfuron) at doses down to 1/100 <strong>the</strong> normal field dose (Al-Khatib
et al. 1992). At doses 1/3 to 1/10 <strong>the</strong> normal dose, several herbicides
(2,4-D, glyphosate) caused significant damage. A number of studies
(Marrs et al. 1989, 1993; Davis et al. 1993, 1994) of <strong>the</strong> effect of sprayproduct
drift on plants showed that plants were affected up to 50 m <strong>from</strong>
<strong>the</strong> sprayed area. However, <strong>the</strong> majority of <strong>the</strong> plants were only affected
over a distance of 0 to 5 m <strong>from</strong> <strong>the</strong> field.
Actual studies of <strong>the</strong> effect of pesticides on forest-floor flora are rare, so
<strong>the</strong> assessment has been based on knowledge concerning <strong>the</strong> mode of
action of herbicides and knowledge concerning <strong>the</strong> ecology of forestfloor
flora. It is only <strong>the</strong> effect of herbicides that has been considered
because, according to Elmegaard et al. (1996), <strong>the</strong>re are not stated to be
any known effects <strong>from</strong> o<strong>the</strong>r groups of pesticides on forest-floor flora.
Østergaard et al. (1998) state that glyphosate is applied once before clear
cutting of deciduous trees and conifers and once or twice in <strong>the</strong> first few
years <strong>the</strong>reafter. This application practice is thought to have a radical
effect on <strong>the</strong> forest-floor plants, so that all individuals of <strong>the</strong> flora
associated with <strong>the</strong> type of forest in question may be eradicated (F. Rune,
Danish Forest and Landscape Research Institute, personal
communication). There will still be a seed pool, but it will also be
seriously reduced, partly because of <strong>the</strong> repeated spraying and partly
because of changes in <strong>the</strong> forest climate for a time when renewal takes
place by means of clear cutting. Plants <strong>from</strong> seeds that germinate in <strong>the</strong>
first period after clear cutting, possibly provoked into germinating by <strong>the</strong>
increased amount of light, will often die, ei<strong>the</strong>r as a consequence of
spraying or because of drought, frost-nipping or scorching by <strong>the</strong> sun. In
addition, forest-floor plants have a short-lived seed pool, so <strong>the</strong>se species
have no possibility of surviving an unfavourable period during <strong>the</strong> seed
stage (Graae 1999). Even if <strong>the</strong> treatment is carried out over a limited
number of years, it will have a serious impact on <strong>the</strong> forest-floor flora.
Seen over a rotation period, <strong>the</strong> relatively small treatment frequency
index thus has a big impact. In Christmas tree and ornamental greenery
cultures, where herbicides are used during <strong>the</strong> entire lifetime of <strong>the</strong>
culture, <strong>the</strong>re is virtually no forest-floor flora. So massive are <strong>the</strong> effects
that <strong>the</strong>re is no flora at all – which is, of course, <strong>the</strong> whole idea of using
herbicides in <strong>the</strong>se cultures. When Christmas trees and ornamental
greenery are produced without herbicides, a flora develops that is
dependent on <strong>the</strong> way <strong>the</strong> soil is treated, and <strong>the</strong> method of renewal can
be more or less au<strong>the</strong>ntic (M. Strandberg, DMU, personal
communication).
The rate of recolonisation by forest-floor flora species is very slow (0 -
1m/year) (Brunet & von Oheimb 1998), so it is only in areas in <strong>the</strong>
immediate vicinity of <strong>the</strong> forest with elements of <strong>the</strong> original flora that
one can expect partial recolonisation by <strong>the</strong> species that have
disappeared. Indeed, Brunet and von Oheimb (1998) also recommend
that <strong>the</strong> slow recolonisation rate of forest-floor flora be taken into
account in forestry planning. Inghe and Tamm (1985) have shown in
Swedish forests that individuals of blue anemone are more than 40 years
old, and it is not unlikely that many species of forest-floor flora can reach