The old wood pastures of the UK that were once (or still are) grazed by livestock are home to many of our ancient trees. These ancient trees may be particularly crucial for saproxylic (deadwood) insects, which rely upon their host trees having adequate space to reach their maximum sizes. Because pastures were (or are) routinely grazed, regeneration from seed was (or is) very limited – this has lead to many ancient trees within such pastures being exposed, and having very full crowns (very much the opposite to what we see in closed canopy situations). As saproxylic species require expanses of heartwood and large lateral limbs that have died yet remain attached to the trunk, the conditions generated by grazing are ideal – trees become very broad in girth (to support the heavy crowns) and form wide crowns with large lateral limbs (to maximise photosynthetic capability).
Many saproxylic insects also require significant amounts of light, with particular species of longhorn beetle needing large fallen branches to be exposed to plenty of light in order for there to be a viable habitat. Of course, the exact light conditions required will vary between species of insect (some moths will very much pursue shaded hosts), though the incubation effect of the wood from sun-exposed trees (the wood will be warmer) will usually mean that breeding is more frequent and mortality rates of larvae are lower. However, the exact required balance of sun-exposure and shade is not currently known.
The benefits of grazing to saproxylic insects do not end just with the tree, however. Because many saproxylic insects require nectar as a food source, there is a necessary access to tall and white flowering herbaceous species such as hogweed. Whilst such plant species are found in pastures, they would not be found under closed canopy conditions. Other flowering plant species, which are usually to be considered trees (namely hawthorn and elder), also act as a critical nectar source. Again, these species are infrequently found within woodlands, and if they are then it would commonly be at a woodland edge or clearing (hawthorn will not flower under dense shade).
Sources:
Alexander, K. (2013) Ancient trees, grazing landscapes and the conservation of deadwood and wood decay invertebrates. In Rotherham, I. (ed.) Trees, Forested Landscapes and Grazing Animals – A European Perspective on Woodlands and Grazed Treescapes. UK: Earthscan.
Siitonen, J. & Ranius, T. (2015) The Importance of Veteran Trees for Saproxylic Insects. In Kirby, K. & Watkins, C. (eds.) Europe’s Changing Woods and Forests – From Wildwood to Managed Landscapes. UK: CABI.
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