Deadwood or saproxylic species depend on dead and decaying wood for at least part of their lifecycle (Speight, 1989). Deadwood invertebrates, mostly beetles, represent about 6% of all British invertebrates (Alexander, 1999) and 20% of the entire British fauna (Elton, 1966). There are 242 designated sites in Scotland (SSSIs, RAMSAR and SACs) featuring invertebrate interests, of which deadwood species, individually or as part of assemblages, are listed in 49 (~20%) sites.
Together with fungi, deadwood invertebrates contribute to the break-down and decomposition of plant matter, promoting the cycling of nutrients and soil enrichment, thus increasing overall forest productivity and biodiversity (Speight, 1989; Alexander, 2008; Stokland et al., 2012). Decaying wood and its saproxylic fauna play other vital roles in terrestrial ecosystems, including capturing carbon, shaping watercourses, influencing geomorphology, and contributing to the formation of humus (Hodge & Peterken, 1998; Dajoz, 2000; Cheesman & Wilde, 2003). Therefore the quality and quantity of deadwood and its fauna are considered indicative of the quality of forest habitats (Alexander, 2004; Lassauce et al., 2011).
Threats to deadwood invertebrates
Deadwood insects are the most threatened community of invertebrates in Europe (Alexander, 2004). There are around 1,700 species in Britain, of which about 40% are RDB or Nationally Scarce (Alexander, 2005). Of the 771 scarce woodland invertebrate species listed for the UK, 264 (34%) require deadwood habitats (Hodge & Peterken, 1998).
The main reason for these levels of threat and rarity are the removal and reduction in quality of dead and decaying wood, which historically have been considered as signs of poor forest management (McGee et al., 1999; Larsson & Danell, 2001; Siitonen, 2001). In commercial forestry, dead and decaying wood have been cleared to make way for new tree planting, or felled and burned to protect crops from pests (Winter, 1993); in urban environments, old trees are removed due to safety concerns. Unmanaged forests accumulate annually 50-200 m3 of deadwood per hectare, whereas in a conventionally managed forest this volume could be as low as 1-5 m3 (Albrecht, 1991).
Deadwood habitats are dynamic and ephemeral; as their components decompose and mature, microhabitats continually disappear and new ones are created. For example, decaying sap under bark and decaying sapwood, which are required by most deadwood flies, last about four years; afterwards they dry out (Rotheray et al., 2001). Saproxylic species are unlikely to survive in the long run when the supply of deadwood is interrupted, and its natural levels cannot be re-created in forests from which timber is extracted (Hodge & Peterken, 1998).
The main threats to deadwood invertebrates in Britain are (Harding & Alexander, 1993; Key & Ball, 1993):
- Removal of diseased or dead trees and branches.
- Improvement of pasture woodlands, including ploughing and fertilizing.
- Harmful arboriculture practices such as filling of cavities.
- Planting exotic tree species unsuitable as deadwood habitats.
- Collecting of firewood.
- Invasive shrubs and trees.
- Introduced pests and pathogens.
The number of trees and their age distribution, woodland structural diversity (from open-grown trees to closed-canopy stands) and continuity of dead wood components are the main determinants of saproxylic habitat quality (Alexander, 2008; Sverdrup-Thygeson et al., 2010; Bergman et al., 2012; Stokland et al., 2012).
There are methods for calculating the volume and number of deadwood components (e.g., Humphrey et al., 2002; Humphrey & Bailey, 2012), but they are not practical or affordable for most purposes. Moreover, deadwood volume alone is not sufficient for assessing site quality (Alexander, 2018).
Deadwood habitat assessment comprises two main objectives:
- Identify existing or potential threats.
- Note features most relevant to deadwood invertebrates.
Spot photographs are helpful for recording habitat condition and structure (Figure 5).
These images should be identified by date, precise location (coordinates), and compass bearing.
Most deadwood invertebrates require specific micro-habitats. For example, among the 44 species of beetle restricted to pine in Scotland, 38 are saproxylic, of which 25 are associated with the subcortical layer, 6 with bracket fungi, 4 with decaying heartwood and 3 with roots and stumps (Hunter, 1977).
These are the main deadwood components (Cathrine & Amphlett, 2011; Stokland et al., 2012):
- White-rotten or red-rotten heartwood wood (the dead core of a live tree or large branch).
- Loose bark and moss on living and dead trees and branches.
- Holes resulting from insect boring.
Rides, tracks and way-leaves with sunny, flower-rich margins can function as clearings if they are wide enough and properly maintained.
The following aspects of clearings are to be noted:
- Their size.
- The relative change in area in relation to previous monitoring.
- Encroachment by scrub and tall vegetation.
- Tree planting or regeneration schemes around clearings.
- Grazing regime, particularly if there were changes.
Nectar sources in clearings are important. Umbellifers, holly (Ilex aquifolium), wild privet (Ligustrum vulgare), crab apple (Mallus sylvestris), wild pear (Pyrus pyraster), rowan (Sorbus aucuparia) and bramble (Rubus spp.) are especially useful.
The publication Managing deadwood in forests and woodlands is recommended for further guidance for saproxylic invertebrates site assessment.
The first principle of management is to keep in mind that habitat quality is more important than deadwood volume (Kirby, 1992; Davies et al., 2008; Bremer & Farley, 2010; Bellamy & Charman, 2012).
Deadwood should decay naturally. Fallen wood should be left where it falls, especially large diameter (> 20 cm) trees, which are the most valuable deadwood components for invertebrate 11 richness. Partially submerged wood is particularly important; the majority of deadwood species require moist conditions, and waterlogged timber is a specialist habitat for some of the rarest deadwood species (Alexander et al., 1996). Moreover, woody debris is important for the stability and biology of riverine habitats in forest catchments (Gurnell et al., 1995).
Where cutting of dead or dying wood is unavoidable, trimming or removal of individual branches should be considered first, leaving other branches and the trunk standing. Stumps should be left as high above the ground as possible.
If it is not possible to leave deadwood on site, the material should be moved to a shaded area as close as possible to the site of origin. Dead wood in full sunshine quickly dries out. Stacks of wood should be as big as possible to retain moisture.
The best invertebrate habitats comprise plants of a wide range of age and height, which maximise the diversity of niches and microhabitats. Some of these plants (e.g., hawthorn and blackthorn) are sources of pollen. Gradual transitions at woodland edges and ride margins are also desirable.
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