Where the botanical composition of a field margin has become severely degraded, rejuvenation may be possible through the alleviation of detrimental farming practices such as those outlined above. This natural regeneration of the habitat preserves the local flora (Baines et al, 1996). However, it can be a slow and unreliable process. The reasons for this are twofold; firstly there may be a lack of desirable species and a corresponding abundance of long-lived seeds of many 'undesirable' species within the seedbank (Mortimer et al, 1988; Radosevich and Holt, 1984). Seeds of dicotyledonous weed species generally remain viable in the seed bank for a more prolonged period than do those of grass species (Lewis, 1973). Therefore, the success of this method is partially dependent on the availability of 'desirable' seed sources and a corresponding lack of 'undesirable' seed in the vicinity of the margin habitats (Baines et al, 1996). Secondly, even where a desirable seed bank is in place, the lack of seed dispersal pathways may inhibit the rejuvenation of botanical diversity (Muller et al., 1998; Jefferson and Usher, 1989).
Where lack and limited dispersal of desirable species propagules is a difficulty, increased botanical diversity in the field margin may be achieved through the introduction of grass and wild flower seed mixtures (Sheridan et al, 2008). This method can have the additional benefit of reducing the abundance of undesirable weed species through their rapid exclusion by the development of the sown perennial species (Sheridan et al, 2008). However, this may be influenced by the diversity of the mixture used, with highly diverse mixtures more likely to suppress undesirable species than low diversity mixtures (van der Putten et al, 2000). The principle difficulty associated with this method of botanically diverse field margin establishment is the poor establishment rate of some herb species (Sheridan et al, 2008; Bokenstrand et al, 2004; Hopkins et al, 1999). In addition, due to the high labour intensity involved in their collection, seed mixtures tend to be relatively expensive. However, these difficulties can at least to some extent, be alleviated through the selection of reputable seed suppliers, careful tailoring of the mixture to soil type and other environmental conditions prevalent in the area, and fine seed bed preparation.
Careful consideration should be given to the provenance of any seed or other plant propagules being introduced, as the widespread use of non-native seed can compromise the genetic diversity of any particular species or population (Price, 2003, Smith et al, 2005). In addition, some species may demonstrate the home site advantage hypothesis i.e. propagules of local origin showing enhanced fitness when compared with non-local propagules (Montalvo and Ellstrand, 2000). Hay strewing and brush harvesting of seed from species rich donor sites can be effective methods of introducing desirable propagules of native provenance to a recipient site. Although results from individual species are known to differ depending on their structural location within the sward e.g. colonisation results by lower growing species such as Lotus corniculatus (birdsfoot trefoil) and Prunella vulgaris (self-heal), tend to be greater with hay strewing compared with brush harvesting techniques. Hay strewing has the additional benefit of not requiring the use of specialised machinery for its adoption by farmers (Edwards et al, 2007).
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