Ed Lewis of the University of Edinburgh presents a detailed study to ask if introducing coppice in Scotland’s SACs would support threatened invertebrates?
Scotland’s Special Areas of Conservation
Approximately two decades after their inception, Special Areas of Conservation (SACs) form the largest network of protected areas in Europe (Habitats Directive, 1992; Pryor & Peterken, 2001). SACs are considered the pinnacle of habitat conservation in Scotland, as such, they require a habitat or species of European importance to qualify for designation (JNCC, 2004) (See Figure 1). Despite being of such high ecological importance, as of 2013, only 65% of SAC woodlands were considered in ‘favourable’ condition (JNCC, 2004; SNH, 2013). If Scotland’s prime areas of conservation are of limited quality, it is unsurprising that woodland biodiversity is also falling (Burns, et al. 2013). After failing to reach the biodiversity targets of 2010, the Scottish Government has set the aim of curtailing biodiversity loss by 2020 (Hochkirch, et al. 2013). This corresponds to wider legislation at a European and international level (European Commission, 2011; Woodley, et al. 2012).
The UK Biodiversity Action Plan (UK BAP) is used as a framework to measure the number of threatened species within the UK (BAP, 2010). Invertebrates act as an indicator for ecosystem health, as they provide a myriad of ecosystem services, from pollination to nutrient cycling, as well as a food source for birds and mammals (Fuller & Rotherey, 2013; Oxbrough, et al. 2012). Furthermore, they have the second highest number of UK BAP species of any clade in Scotland (BAP, 2010). As such, reversing the decline in UK BAP invertebrates has beneﬁts to other clades and UK BAP species.
This article will ﬁrst introduce coppicing in Scotland. Part 2-to come soon- will review the UK BAP communities of a coppice in a spatial and temporal context, using a case study, a brief overview of the applicability and suitability of coppice in SACs before the conclusions.
Figure 1: Scotland’s 243 SACs cover 940,000 ha (JNCC, 2004).They span 78 habitats and 43 species of European importance. Ten of these habitats are woodlands, many of which are located on the West coast of Scotland.
Coppicing in Scotland
Coppicing has been practiced in the UK for at least two millennia (Rackham, 1990; Peterken, 1996), reaching its zenith in Scotland in the 19th Century (Anderson 1967; Sansum, 2005); however there is little recorded data (Smout, 2006). Coppicing is the cutting of arboreal regrowth, close to the ground, producing ∼ 3 tonnes of air dried wood/ha/ yr (Peterken, 1992; Quine, et al. 2011) (See Figure 2). The two species used for coppice were: Quercus petraea, cut on a 20 year rotation, and used for charcoal (Anderson, 1967; Evans, 1992) and Corylus avellana, on a ten year rotation, harvested for hurdles and thatching (Harmer, 2004). Ash can also be coppiced, but the threat of Chalara fraxinea, makes it unsuitable (Pautasso, 2013). The increasing use of coal in the 20th Century led to a reduction in Scotland’s coppice from 230,000 ha in 1905, to under 1000 ha by 2005 (Harper & Howe, 2003; Quine, et al. 2011).
The ubiquity of coppice in certain woodlands means many sessile Oak SACs have histories of coppice until at least 1850 (SNH, 2005; SNH, 2008; SNH, 2008b; SNH, 2010; SNH, 2011). The potential of coppice as a means for biodiversity conservation has been studied for over 100 years (Adamson, 1912). Recently, the use of conservation coppice is gaining increasing attention (Van Calster, et al. 2008). It is being expanded in sessile oak wood SACs in Wales (CCGC, 2011), whilst having been done for decades in some National Nature Reserves in England (Hodgson, et al. 2009). At a European scale it is common to have active coppice in SACs, with 700,000 ha in Bulgarian SAC woodlands alone (Pryor, 2001).
Spatial communities of a coppice
SAC woodlands will be mature high forest stands, by opening up the canopy for coppice the micro-climate is altered (See Figure 2). To gauge the beneﬁts of coppicing, the communities of a coppice will be compared to that of a high forest stand using three vertically stratiﬁed zones: ground level, understory and canopy. These strata will be reviewed for the degree to which they are used by invertebrates for activities such as feeding, mating etc. Thereby giving some indication whether the inherent structure of a coppice could support more UK BAP invertebrates.
Coppicing opens up the canopy thereby increasing the amount of solar radiation that reaches the ground (See Figure 2). The primary beneﬁt of this radiation is the diversiﬁcation of the ground layer and understory (Ulyshen, 2011), this has been shown to strongly correlate to invertebrate diversity (Barsoum, 2013). This counters Greatorix-Davies & Marrs, 1992 who found that the ground level supported fewer species, but larger populations than the levels above. Nonetheless, the increased abundance and diversity of plants directly beneﬁts herbivorous UK BAP species, especially Moths, who constitute 67% of UK BAP woodland invertebrates. These conditions would beneﬁt many threatened invertebrates, as temperature or light have signiﬁcant aﬀects on the distribution on 5 of the 7 clades shown in Table 1. The secondary beneﬁt of the solar radiation is the warmth it provides, essential for thermophilious UK BAP species such as Wood Ants, who only nest in high light areas, or Golden Lantern Spiders who mate on exposed patches of leaf litter (Dolek, 2012). Other thermophilious species require solar radiation for thermoregulation of adult activity (Bouget & Duelli, 2004). Conversely, coppice will reduce the amount of leaf litter, detrimentally aﬀecting saprophagous invertebrates, such as annelids and Coleoptera (Grove, 2002). Invertebrates (e.g. Lepidoptera), being ectothermic, often spend the winter in the egg stage using the ground for insulation, showing ground habitats to be essential throughout the year (Fuller & Warren, 1993; Hodgson, et al. 2009; Ulyshen, 2011). It is important to note that coppices not only support a more diverse community than high forest, but due to the rarity of intra-woodland glades, rarer species (Spitzer, et al. 2008, Van Calster, et al. 2008b).
Table 1: Scotland’s UK BAP species. Of the species with known distributions 64% associate with woodland, the vast majority being Moths.More species have an unknown distribution in Scotland than are known, meaning that coppice could in reality beneﬁt many more UK BAP species. Data compiled by author from: (JNCC, 2004; BAP, 2010; Barnes, 1993).
Figure 2: Coppice with standards provides more structural heterogeneity than high forest, and therefore more niches for UK BAP invertebrates. (A) High forest: High canopy cover produces an extensive habitat for UK BAP canopy species such as Lichen Runner spiders (I). This simultaneously suppresses understory and ground ﬂora, and therefore the species that use them. Saprophagous ground invertebrates, such as the Lesser Searcher beetle (II) beneﬁt from high leaf litter and biomass (B) Simple coppice: Opening up the canopy increases light exposure, temperature and wind speeds (Morecroft, et al. 1998). The complete lack of trees provides few resources for UK BAP canopy species, but plenty for species requiring heliophytic ground ﬂora, such as the Checkered Skipper butterﬂy (III). (C) Coppice with Standards: Standards are mature trees interspersed within the coppice, supporting arboricolous species such as the Groove Head Beetle (IV). The abundant ground ﬂora can support UK BAP species such as Oak Lutestring moth (V) and Four Spotted Ground beetle (VI). Eﬀectively combining the habitats of A & B. Source: Author.
A developed understory is scarce in high forest, therefore the species that utilise it would beneﬁt from coppicing (See Figure 2) (Ulyshen, 2011). It is important as a food source for many UK BAP invertebrates, such as the Aspen hoverﬂy. The stems of the coppice are inhabited by web spinning spiders, who feed on the high abundance of aerial species. These aerial species such as the Oak Hook Tip moth, also use it for mate ﬁnding (BAP, 2010). Despite this, there is not the abundance or diversity found at the ground or canopy.
The complex structures of woodland canopies may support an order of magnitude more species than herbaceous plants (Floren & Schmidl, 2008). Coppices are entirely dependent on the standards to support the bark dwellers, catkin and seed feeding invertebrates (Waring & Townsend, 2009). Bark and lichen dwellers are only found on mature trees (Nicolai, 1993), as are Osmia uncita bees, which nest in arboreal cavities (Floren & Schmidl, 2008). Not all canopy species are limited to mature trees, phytophagous invertebrates can migrate to the canopy to feed, or be fed upon, such as the UK BAP Guest Ant and leaf aphids (Seifer, 2008). The canopy provides sources of crypsis from predators such as birds (Fuller, 2013).
Overall, the ground and canopy strata provide both the highest abundance of food sources, and niches, for habitation for invertebrates in general as well as UK BAP species. Some of the canopy species may be supported within the already present high forest of SACs, but the light dependent species of the ground will only be found in clearings such as coppices.
Temporal communities of a coppice
A coppice stand with standards may support more niches than high forest, however, the coppice will undergo successional changes towards a sere more akin to high forest within the space of the rotation. Essentially, will coppicing beneﬁt threatened invertebrates throughout the rotation, or would continual glade conditions be more beneﬁcial?
The early rotation is immediately after cutting to approximately three years (Harmer, 2004). It provides a now rare set of woodland conditions, therefore supporting rare woodland species. Thermophilious species diversity peaks around three years, mirroring that of herbaceous plant species (Fuller & Warren, 1993; Mason & MacDonald, 2002). The early rotation sees the highest abundance of ground beetles, wolf spiders and butterﬂies (Ziesche & Roth, 2008).
Between four and six years the increasing coppice canopy shades out heliophytic plants (See Figure 3). Invertebrate diversity doesn’t decline with the plants due to species being facultative insectivores (Hill, Roberts & Stork 1990; Greatorix-Davies & Marrs, 1992). The change in vegetation composition results in a transition in herbivorous species to shrub pollinators, such as the Aspen hoverﬂy which feeds on Brambles (Fartmann, Muller & Poniatowski, 2013). The coppice crop itself now supports species, especially moths (Broome, et al. 2011). Lepidoptera have distinct communities at every stage of the rotation (Broome, et al. 2011) with early stage species gradually declining throughout the ten year rotation (Benes, et al. 2006).
From seven years onwards species are primarily arboricolous phytophagous herbivores such as the hazel pot beetle, moths, or species that inhabit the bark and branches of the coppice and standards (Nicolai, 1993). This transience creates a dynamic community of interactions such as predation, parasitism and intra/inter species competition, continually driving population dynamics (Waring & Townsend, 2009).
The early rotation could be considered the most useful, as it supports species not found in the surrounding high forest. As the rotation develops, that disparity is reduced. It is diﬃcult to disentangle the role of the coppice crop, or the rotational disturbance on invertebrate biodiversity. The diversity of the early rotation is largely due to the inherent impacts of coppicing, whereas towards the end of the rotation, it is the coppice itself that provides the diversity.
Figure 3: Despite conditions changing to be less favourable for some species the structure of the coppice remains beneﬁcial for many UK BAP species, such as the Small Blue butterﬂy (IX), as well as woodland edge species such as Autumnal Rustic moth (X) (BAP, 2010). (A) Early Rotation: Open habitats support many UK BAP species including the pearl bordered fritillary (I), the small pheonix moth (II), the violet oil beetle (III) and Red shanked carder bee (IV). These species all require the light, warm conditions, as well as the heliphytic woodland plants found only in woodland glades. (B) Mid Rotation: Despite the rapidly changing conditions, many UK BAP species, e.g. the Dark bordered beauty moth (V), which feeds on hazel, and the Six Spotted beetle (VI) can still thrive. (C) Late Rotation: More shade tolerant UK BAP species, such as the Aspen hoverﬂy (VII) and Scottish Yellow Splinter ﬂy (VIII) will become more abundant. Species may migrate between stands within a rotation, making a gradient of diﬀerent levels of rotation essential for any trial.
This essay has assumed the coppice stand is established. The island biogeography and metapopulation theories would suggest that if the coppice was newly made it could take considerable time to be inhabited by UK BAP species (Whittaker, et al. 2008; Hanski, 1991). Balancing the appropriate level of disturbance is key to conservation coppice, generally a 10 year rotation has been shown to maintain and support the vast majority of the invertebrate community (Welch, 1969 ; Fowler, 1985).
Read Part 2 for a detailed case study of coppice in a Scottish SAC and its potential to support the invertebrate community, and conclusions.