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Why are some hedgerows better than others for wildlife?

Species rich, mixed hedges

The more plant species found in a hedge, the greater the number of other species the hedge can support. Many plant eating insects, for example, depend on a particular plant species, so the more different plants there are, the more potential for insect diversity.

Since different shrub species flower and fruit at different times, this diversity and a good spread of plant species extends the flowering and fruiting period, beneficial to nectar and pollen feeding invertebrates, and thus to their predator species too!

Some species require the presence of just one shrub species to survive in a hedge but others need several for different roles throughout their life. A good example is the thrush which nests in the shrubby structure of the hedge, sings from hedgerow trees, hunts snails in the base of the hedge before swapping to berries later on in the season.

 

Food plants

Whilst all plants will be part of a food chain and be useful for some form of wildlife, the plants on the food list stand out as providing abundant food for a huge range of other species.

How a hedge is being managed affects the abundance of food that a hedge can supply, regardless of the species within. Most of our hedge species flower and fruit of second year wood, so any hedge that is trimmed to the same point each year will not be able to produce anywhere near so many flowers or fruits.

Hedgerow trees

Over half of the priority species associated with hedgerows are dependent or partially dependent, on hedgerow trees. Hedgerow trees, particularly old trees offer nesting sites for birds bats and bees, they contain the rare deadwood habitat that supports thousands of our invertebrate species, many of which are rare or threatened, and they provide leaf, flower and fruit forage for a huge number of species. Oak and willow trees can support over 400 plant eating insect species each, which then go on to support a large part of the local food chain.

Whilst undoubtedly mature and ancient hedgerow trees offer more for wildlife than young trees, it is essential that we ensure that a new generation of trees is encouraged. It is all too easy for a mechanical flail to strim the top of a potential new sapling tree, so these need protection and care if they are to become the mature hedgerow trees of the future.

Hedgerow plants

We don’t record individual species of the herbaceous vegetation at the base of the hedge as this information is season dependent, location dependent and would be really difficult to ascertain a hedge-condition assessment from. What we do instead is measure how available the hedge base is to accommodate a diverse flora.

Any area that has had high levels of nutrient enrichment will be less likely to support a diverse wildflower layer for a number of reasons. Some wildflowers cannot tolerate higher nutrient levels, some will lose their fungal root associations, and others may just be shaded out by the presence of plants that thrive in a high nutrient soil, such as nettles, cleavers and docks.

Whilst there is nothing wrong with nettles, docks or cleavers, their abundance at the base of our hedgerows can be a warning to us we use them as a proxy measurement for nutrient enrichment which we know can be detrimental to our wildflowers.

Our wildflowers also need space to thrive, and a margin of un ploughed and unsprayed land at the base of the hedge will not only provide this space for nature, but also act as a buffer which can protect the hedge from damage. Herbicides, insecticides and fertilisers often drift into hedgerows and close cultivation will damage the roots of hedge shrubs and trees; a good margin will help both of these issues as well as providing space for wildflowers, nesting birds and other wildlife.

How do we measure the wildlife value of a hedge?

Trying to record all the species that live, graze on, shelter or hunt in a hedge is a daunting task. Robert Wolton of the Devon Hedge Group undertook such a survey on an 85m stretch of hedge and over two years recorded 2070 species living in and using the hedge, even this he thought is an underestimate.

Although fascinating, it is not practical to take this approach at a larger scale and so we measure specific hedge features that evidence has shown to increase the value of that hedge for wildlife.

  • Measuring the number of woody species compromising the living body of the hedge, and how well distributed these are over a 30 m sample of the hedge) gives us an indication of the species richness.
  • The presence of a selection of these species, those on the ‘food list’, tells us more about whether the hedge provides a good year round food source for pollinating species and those that rely on fruits and berries.
  • Knowing the hedge structure, and so its position in the hedge life cycle helps understand the capacity of that hedge to provide a good food source.
  • We survey the number of hedgerow trees above and below 20cm dbh. In order to replenish our stock of hedgerow trees we need to ensure young trees are encouraged to grow or are actively planted in our hedges.
  • The percentage of nettles, docks and cleavers in the base vegetation of a hedge is used as a proxy for the level of artificial nutrient enrichment.
  • Recording the presence of invasive species in the hedge.

How do we improve hedges for wildlife?

  • If and when we need to gap up a hedge, we can use it as an opportunity to increase the number of plant species in the hedge.
  • If your hedge is poor in shrubs that produce an abundance of flower and fruit,
  • Alternatively, small gaps in a hedge can provide the perfect opportunity to plant a hedgerow tree.
  • Manage your hedges to maximise the flower and fruit your hedge can provide. Cutting every 3 years means a hedge can provide 3.4 times the mass of berries than those cut annually.
  • When cutting is required, but the hedges at the end of winter enabling the hedge to be a larder for wildlife over the long winter months.
  • A hedge margin will protect the hedge structure from root damage, and the hedge wildlife from drifting sprays, as well as provide great possible habitat for wildflowers.

PTES Hedgerow Survey

Current Hedgerow Statistics

Length of
hedgerow surveyed

545 km

Average number of woody species
reported per 30m

3.6

% hedges in each
main structure category

  • 52.5 Overtrimmed
  • 27.1 Dense and managed
  • 13.6 Tall and overgrown
  • 6.8 Recently rejuvenated

Average trees/100m

12.8