Biodiversity indicators of ground-dwelling spiders in Irish plantation forests and native woodlands

Irish forest cover was reduced to less than one percent by the early 1900s, and since then the amount of forested land has risen steadily, with approximately 10% of the total land area forested today. Just 10% of this forested land is accounted for by native woodlands, while the majority is comprised of plantation forests predominately made up of non-native conifers. The Irish government ultimately aims to achieve a national forest cover of 17% and with such large-scale afforestation planned it is necessary to assess both the impact of such large areas of plantations on the native flora and fauna and also the potential of plantations to provide a refuge for forest-associated biota. Biodiversity research in Ireland’s plantation forests is a relatively new field and native woodland research has focused on floristic differences between stands while largely ignoring forest fauna. This study was undertaken at 25 sites with a wide geographical spread across Ireland in the following forest types: 5 first and 5 second rotation commercially mature Sitka spruce (Picea sitchensis) plantations; 5 commercially mature Ash plantations (Fraxinus excelsior); 5 Oak (Quercus petraea and Q. robur) and 5 Ash (F. excelsior) dominated native woodlands. Pitfall traps were used to sample the ground-dwelling spider fauna using three plots of 5 traps per site. At each plot plant species diversity, plant structural diversity, canopy cover, soil pH and organic content and litter and deadwood cover were recorded. Spiders were identified to species level and assigned habitat associations as follows: woodland specialists, open-habitat specialists, or generalists. Spider assemblages were assessed among the different forest types and Generalised Linear Modelling was used to identify potential biodiversity indicators and make management recommendations. Existing sustainable forest management plans frequently overlook smaller, less charismatic organisms such as spiders, which are difficult to capture and identify. Identification of links between their biodiversity and the structural and compositional characteristics of the forest ecosystem, which can be readily identified by non-specialists, will facilitate their incorporation into forest management plans.