شاخص های گلسنگ تداوم زیست محیطی زمین جنگلی با آزمایش های انتقادی

ABSTRACT Critical tests for lichen indicators of woodland ecological continuity

Deforestation since the mid-Holocene has massively reduced the extent of primary, old-growth forest across the temperate zone, and especially in Western Europe (Hannah et al., 1995, and cf. Fig. 3.15 in the MEA 2005). Consequently, there is a strong emphasis in conservation on identifying forest remnants which retain two important ecological properties. The first of these properties is structural: the existence of key microhabitats analogous to those occurring in ‘old-growth’ forest stands. Examples include a unique soil quality (Verheyen et al., 1999; Wilson et al., 1997), and the greater accumulation of deadwood than might be associated with intensively managed woodland (Kirby et al., 1998). The second property is temporal: the time over which key microhabitats have persisted in the landscape, which determines the likelihood that dispersal-limited niche-specialists will successfully become established, as demonstrated for vascular plants (Brunet and Von Oheimb, 1998; Dzwonko and Loster, 1992; Matlack, 1994). These old-growth properties of (i) habitat quality and (ii) extended time-for-colonisation, form the basis of what has been referred to as ‘woodland ecological continuity’ (Coppins and Coppins, 2002).

Species from across a broad range of different taxonomic groups show a dependency on aspects of ecological continuity, including: invertebrates (Assmann 1999), mammals (Bright et al., 1994), lichens (Rose, 1976; Selva, 1994) and vascular plants (Peterken, 1974; Wulf, 2003). Focussing on vascular plants, it is established that sites with ecological continuity are distinctive in terms of their species composition and guild proportionality (Dzwonko and Loster, 1992; Hermy et al., 1999) with possibly greater species richness (Peterken and Game, 1984) than comparable woodland types which have a lower ecological continuity value. Given these implications for biodiversity, proxy indicators have been developed to readily identify sites with high ecological continuity and which are therefore conservation priority habitats. However, the interpretation of different indicators is dependent on the ecology of the guilds used. Vascular plant indicators may recover from extended periods of woodland degradation or even clear-felling, owing to the buffering capacity of the seed bank (Erenler et al., 2010). In contrast, epiphytes are primarily dependent on trees as their habitat substratum, and the absence of trees at a site, for example through clear-felling, represents a definitive break in species occurrence. Nevertheless, indicator groups have received general criticism as being ambiguous (cf. Nordén and Appelqvist, 2001; Rolstad et al., 2002) and in need of critical assessment. Here, we quantify the dependency of epiphytes on the continuity of tree cover to statistically test lichen indicators of ecological continuity, by drawing on the concept of ‘ancient woodland’ as it is adopted in the United Kingdom.

The UK’s Ancient Woodland Inventory provided cartographic evidence for the time over which woodland sites have had continuous tree cover (Roberts et al., 1992; Walker and Kirby, 1987). Different categories in the temporal continuity of tree cover do not map exactly onto trends in ecological continuity because certain ‘ancient woodlands’, which are defined as having continuous tree cover since c. 1750, have undergone periods of historic intensive management. Despite being classed as ancient woodland, such sites may not retain high values of ecological continuity, i.e. the long-term persistence of niche-specialist microhabitats, required for the occurrence of indicator species. This may be the case for coppice Oakwood in Britain, in which the rotation of coppicing in plots on a cycle of 20–30 yr (Smout, 2005; Smout et al., 2007) will have reduced structural complexity (e.g. over-mature trees, deadwood), instigating a break in associated microhabitat persistence over long time-scales, despite continuous tree cover. Thus, not all ancient woodland sites have high ecological continuity values, conversely however, it is only from among sites with a continuity of tree cover (i.e. ancient woodlands) that the properties of high ecological continuity can be drawn, and it is these sites with which epiphytic indicator species should be significantly associated. On this basis we compared occurrence records for previously established lichen epiphyte indicators, with sites that have had continuous tree cover for different periods of time. We tested the null hypothesis of no statistical association with ancient woodland, i.e. semi-natural habitats with persistent tree cover >250 yr, relative to occurrence rates among the same indicator species for sites which are known to have undergone a period of deforestation and subsequent regeneration in the more recent past. Species for which the null hypothesis is rejected provide provisionally strong indicators of ecological continuity, as they are consistent with an expected skewness to ancient woodlands, presumably those which retain ecological continuity. Species for which the null hypothesis cannot be rejected must be treated with greater circumspection; they may have been erroneously selected as indicators, or they may not be responsive to the full breadth of ecological continuity, perhaps signalling the existence of certain niche-specialist microhabitats (which may occur outside ancient woodlands) without the additional constraint of microhabitat persistence.

Hypothesis testing using British woodlands represents a casestudy region in which lichen indicators of ecological continuity were first developed (Rose, 1974, 1976), and where they have since been widely adopted in conservation assessment. However, we capture a broad problem, because among different regions the identification of ecological continuity indicator species remains largely subjective, and founded on qualitative (expert opinion) or semi-quantitative analyses (Rose, 1976; Selva, 1994; Tibell, 1992). Statistical tests in the validity of ecological continuity indicator species should therefore be considered a conservation priority.