Scattered hedgerow trees out across the landscape can be regarded as keystone habitats for biodiversity, where traditionally keystone habitats would have been located within woodlands (including wood pastures) and forests. This is crucial, as these scattered keystone trees act as ‘go-betweens’, enabling species to ‘hop’ between larger wooded sites. In essence, habitat connectivity increases.
The authors state that the reason this phenomena has emerged for various reasons, and they use three examples (within the UK):
1. The rove beetle Batrisodes adnexus (a Red Data Book species listed as ‘endangered’), within the UK, was historically found only within medieval forests (Windsor and Epping, most notably), though has recently been found within the heartwood of a single mature Fraxinus excelsior in a farmland hedgerow boundary in Leicestershire, alongside many other saproxylic invertebrates. Without this hedgerow, the species would remain isolated to woodlands only. The emergence of mature hedgerow ash, following a reduction in management of hedges, may be a reason for this.
2. Dorcatoma serra, a saproxylic beetle, which relies on Pseudoinonotus dryadeus upon Quercus spp. for its habitat, has usually been found only within old parkland areas but has now also been found within farmland hedgerow boundaries. It is suspected that this may be because these scattered old oak pollards are the remnants of what was once the Forest of Essex (located throughout large swathes of the county).
3. Upon Crataegus spp., the hawthorn jewel beetle Agrilus sinuatus can now be found throughout farmland hedgerow boundaries that contain mature hawthorns. Originally found only within woodland fringes and wood pasture sites where hawthorns were allowed to reach maturity un-managed, the abandonment of traditional hedge-laying techniques has lead to many field hedgerow boundaries growing into maturity and attaining decent sizes. This dramatically increases the viable habitat of the jewel beetle, and the scattered hedgerow hawthorns act as ‘connecting’ trees (because the beetle is unable to ‘jump’ great distances without ‘stopping off’ along the way) linking different woodland fringes and wood pastures.
Source: Butler, J., Green, T., & Alexander, K. (2012) Collections of ancient trees: hotspotting biodiversity, heritage and landscape value. In Rotherham, I., Handley, C., Agnoletti, M., & Samojlik, T. (eds.) Trees Beyond the Wood: an exploration of concepts of woods, forests and trees. UK: Wildtrack Publishing.