In this post Beth Atkinson discusses her recent paper ‘A comparison of clearfelling and gradual thinning of plantations for the restoration of insect herbivores and woodland plants’
Forests are valued across the globe for their history and heritage, as well as their importance for biodiversity. They display their historical use, for example through coppice stools, and are evocative like perhaps no other habitat, conjuring images of the wild woods and enchanted forests of folklore. For all these reasons the restoration of degraded woodland is happening all over the world. However, it is often focussed on trees with less attention given to ground flora and fauna.
In this paper we studied non-native conifer plantations on ancient woodland sites (sites in England and Wales which have had continuous tree cover since at least 1600) in South West England. Ancient woodlands are one of the most diverse habitats in Britain and are also historically and culturally important. Furthermore, due to the continuous tree cover, plantations on ancient woodland sites are a good starting point for the restoration of native woodland. We compared the effects of two methods of plantation restoration, clearfelling of planted trees and their gradual removal, on the ground flora and the plant–invertebrate herbivore relationship. We also compared sites undergoing restoration to native woodlands and unrestored plantations.
Whilst the species richness of the ground flora differed between plot types, with clearfelled plots having more species, the number of woodland species in the ground flora was the same on plots being restored through clearfelling, plots being restored through gradual thinning, native woods and unrestored plantations. This is important for restoration. Firstly it confirms that plantations on ancient woodland sites are a good starting point for restoration as they have maintained woodland species in their ground flora. Secondly, during restoration it is important to conserve any species native to the target community that are already present. This is especially important for woodland species due to their slow colonisation rates. We found that both clearfelling and gradual thinning are able to do this, at least for the ten-year post-felling window. Furthermore, this was true for two woodland types, acidic Quercus woodland and mesotrophic Fraxinus woodland.
Although both restoration methods maintained woodland species in the ground flora they had different effects on the plant species – herbivore species richness relationship. On plots undergoing gradual thinning for restoration, as well as native woodland plots and plantations not undergoing restoration, there was a positive relationship between plant species richness and leaf-miner species richness. However, on clearfelled plots there was a negative relationship. Further work is needed to determine what drives this negative relationship, but it may be due to the increased plant cover on clearfelled plots. As the plant species richness increases on these densely vegetated plots leaf-miners may have difficulty locating host plants if they don’t form dense stands or if they are obscured by other plant species. This could have consequences for restoration if the aim is to restore specific species on a particular host plant. The success of this might differ with restoration method and/or plant species richness.
During restoration it is important to conserve any target species already present, as well as restoring those that are absent. It is also important to test assumptions, such as an often assumed positive relationship between plant and invertebrate herbivore species richness. It is also important to compare different methods so that informed decisions can be made about which to use.