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© 2015 . Biological assessments of forest systems often involve a single ground-invertebrate sampling method that may ignore the biological component of the non-sampled canopy. Pitfall trapping for ground-active arthropods is a widely implemented technique for biological assessment in forested and open habitats. Although much evidence highlights the biases of pitfall trapping, this evidence typically comes from open-habitat crop and grassland systems. In forest systems where much of the biodiversity is found within the above-ground structure, management recommendations based solely on ground sampling may not represent the diversity within the three dimensional forest habitat. We provide evidence from combined ground and canopy sampling of three major forest types within the study region. We use canopy insecticide fogging to compare with more traditional ground-based pitfall trapping, and use spiders as a comparative species-rich biota that is able to colonise most terrestrial habitats and is strongly affected by changes in environmental condition.We identified 3933 spiders from 109 species from the 18 forest patches sampled. Both types of sampling defined differences in community composition between forest types in a similar manner; hence, either method could be used to evaluate differences or test management regimes in well-replicated experiments of forest type. However, the association in community composition between ground and canopy assemblages at the individual site-based level was weak; we found low correlation between the two data sets indicating that surrogacy between methods was not supported at this level. Furthermore, disparities in spider habitat association, body size, hunting guild and vertical stratification of spider families indicates that where detailed species and family-based information is required, or if inventorying is necessary, then multiple targeted surveys are essential.

Globally, the total area of plantation forest is increasing as deforestation and fragmentation of native forest continues. In some countries commercial plantations make up more than half of the total forested land. Internationally, there is growing emphasis on forestry policy for plantations to deliver biodiversity and ecosystem services. In Ireland, native forest now comprises just 1% of total land cover while non-native spruce forest makes up 60% of the plantation estate and approximately 6% of the total land cover. The majority of plantation invertebrate biodiversity assessments focus on ground-dwelling species and consequently a good understanding exists for these guilds, especially ground-active spiders and beetles. Using a technique of insecticide fogging, we examine the less well understood component of forest systems, the canopy fauna (Coleoptera, Araneae, Diptera and Hemiptera), in Irish spruce plantations (Sitka and Norway) and compare the assemblage composition, richness and abundance to that of remnant native forest (ash and oak). In addition, we examine the potential for accumulation of forest species in second rotation spruce plantations and identify indicator species for each forest type. From 30 sampled canopies, we recorded 1155 beetles and 1340 spiders from 144 species and over 142 000 Diptera and Hemiptera from 71 families. For all taxa, canopy assemblages of native forests were significantly different from closed-canopy plantation forests. No indicators for plantation forest were identified; those identified for native forest included species from multiple feeding guilds. Plantations supported approximately half the number of beetle species and half the number of Diptera and Hemiptera families recorded in native forests. Although assemblages in Norway spruce plantations were very different to those of native forest, they had consistently higher richness than Sitka spruce plantations. No differences in richness or abundance were found between first rotation and second rotation Sitka spruce plantations. Compared to other forest types, Sitka spruce plantations contained far greater total abundance of invertebrates, due to vast numbers of aphids and midges. Under current management, Sitka spruce plantations provide limited benefit to the canopy fauna typical of native forests in either first or second rotations. The large aphid populations may provide abundant food for insectivores but may also lead to reduced crop production through defoliation. Progressive forestry management should attempt to diversify the plantation canopy fauna, which may also increase productivity and resilience to pest species.

As the area of plantation forest expands worldwide and natural, unmanaged forests decline there is much interest in the potential for planted forests to provide habitat for biodiversity. In regions where little semi-natural woodland remains, the biodiversity supported by forest plantations, typically non-native conifers, may be particularly important. Few studies provide detailed comparisons between the species diversity of native woodlands which are being depleted and non-native plantation forests, which are now expanding, based on data collected from multiple taxa in the same study sites. Here we compare the species diversity and community composition of plants, invertebrates and birds in Sitka spruce- (Picea sitchensis-) dominated and Norway spruce- (Picea abies-) dominated plantations, which have expanded significantly in recent decades in the study area in Ireland, with that of oak- and ash-dominated semi-natural woodlands in the same area. The results show that species richness in spruce plantations can be as high as semi-natural woodlands, but that the two forest types support different assemblages of species. In areas where non-native conifer plantations are the principle forest type, their role in the provision of habitat for biodiversity conservation should not be overlooked. Appropriate management should target the introduction of semi-natural woodland characteristics, and on the extension of existing semi-natural woodlands to maintain and enhance forest species diversity. Our data show that although some relatively easily surveyed groups, such as vascular plants and birds, were congruent with many of the other taxa when looking across all study sites, the similarities in response were not strong enough to warrant use of these taxa as surrogates of the others. In order to capture a wide range of biotic variation, assessments of forest biodiversity should either encompass several taxonomic groups, or rely on the use of indicators of diversity that are not species based.

The importance of plantation forests for biodiversity conservation is greatest in landscapes that have experienced significant loss of natural forest ecosystems and the plantation forest estate continues to expand, as is the case in Ireland. We investigated the role of plantation forests in supporting forest plants, invertebrates and birds of conservation concern in comparison to semi-natural woodlands in Ireland. Of the 169 vascular plant species, 97 bryophyte species, 162 spider species, 159 beetle species and 36 bird species recorded 5, 3, 9, 1 and 5 species of conservation concern, respectively, were recorded. Many of these were found in semi-natural woodlands, highlighting the importance of the retention or restoration of these habitats for forest biodiversity. A number of species of conservation importance were also recorded in plantation forests demonstrating that the role of these forests in the provision of habitat for biodiversity conservation should not be overlooked.