Many semi-natural fragments of the formerly extensive British forests are relatively small (Spencer & Kirby 1992) and widely separated from one another (Kirby et al. 1994). They are frequently isolated within an agricultural matrix (Peterken 1993) and usually have a complex and intensive management history (Rackham 1980). The field layer tends to be more uniform in character in English lowland forest, where there is less geo-climatic variability than in the open forests of Scotland and western Wales. The imbalance in the geographical distribution of forest specialists, with a slight bias towards southern lowland forest types, may also reflect this, although it may be in part an artefact of the regions for which species lists were available (Kirby et al. 2006). As elsewhere (e.g. Hermy et al. 1999), many plant species found in the field layer were not forest specialists.
While British forests can differ greatly from those in North America or continental Europe (Bunce 1981), many of the factors found to be significant in this study (e.g. climatic, biotic, management, disturbance) also influence the distribution of specialist and non-specialist forest species elsewhere. Therefore, while the approach used here focused on those factors known to influence species composition in Britain, the methodology employed is of far wider applicability.
Variables that control species composition may operate at a range of scales considerably broader than the simple division of site and plot scales presented here. However, forward selection indicates the significance of any variable, regardless of the set into which it was placed, while intersecting variance regions illustrate where variables may operate at multiple levels, as suggested in Fig. 5(a). Our approach allows useful separation and quantification of responses with respect to the scale at which they are likely to exert greatest effect; this method can therefore be used to facilitate comparable assessments of variation in analogous ecosystems.
The values of total variation explained (2.0–19.7%), derived from a data set containing a wide range of forest types sampled at a national level, are comparable with values reported elsewhere for smaller-scale analyses also based on unimodal response models, e.g. abundance of tree species in an area of c. 0.5 km2 (sample area 40 000 m2, TVE = 36.7%; Borcard et al. 1992) and seed bank data from a 2-ha forest stand (sample area 784 m2, TVE = 8.0–20.6%; Olano et al. 2002). In addition, much of the apparent ‘unexplained variation’ is likely to be due to lack-of-fit of data to response models (Økland 1999).
A large pool of environmental variables was used because environmentally conditioned variation is underestimated if important variables are missing (Økland & Eilertsen 1994). To balance this and guard against overestimation of TVE, forward selection was employed to retain only significant and non-collinear variables. It is possible that the threshold of multicollinearity used was too generous and may have allowed inclusion of collinear climate/spatially structured variables, such as winter cloud cover with ground frost, or site altitude with easting. This may explain the apparent discrepancy in selection position and TVE between climate/spatial and boundary/grazing variables. Another possibility is that as gradient-based models fit data representing clear gradients (e.g. climate), before anthropogenically dependent and less consistent data (e.g. boundary structure), the latter may not produce a good fit along an early ordination axis, yet still explain much variation.
Nevertheless, the strength of this approach lies in provision of realistic relative values (Økland 2003), and these results provide the first quantitative assessment of factors strongly correlated with the floristic component of a range of forest types sampled across an entire country.
Ordination of species within forest plots confirmed and elaborated the key ecological and geographical gradients described for Britain by Bunce (1981) and Rodwell et al. (1991). Forests with Fraxinus excelsior and an understorey of Corylus avellana and Crataegus monogyna displayed a strong association with deep, damp, base-rich soils, while those dominated by Alnus glutinosa were clearly associated with wet environments, such as streams and bogs.
There were also strong correlations between climate and vegetation composition. Precipitation and temperature gradients were found to be of particular importance, along with ground frost and cloud cover, again similar to the splits in the NVC between types typical of the south-east vs. the north-west.
Variation partitioning indicated that, at the site level, management factors, including herbivory by deer, boundary type and spatial variation, accounted for a large proportion of TVE. Wild ungulates are important in determining forest field layer structure across a wide range of ecosystems (e.g. Mcinnes et al. 1992; Augustine & McNaughton 1998). They can facilitate the long-distance dispersal of a large number of plant species (Vellend et al. 2003; Eycott et al. 2004), but are often associated with a decline of palatable species (Augustine & DeCalesta 2003; Kirby 2001). Forest plants are often poorly adapted to high grazing levels (Rackham 1980) compared with non-forest, particularly grassland, species and this was reflected in ordination analyses. However, while heavy grazing is often detrimental to growth of vascular plants, light grazing can be beneficial in controlling the spread of taller competitive or ruderal pioneer vegetation, thus reducing competition for regenerating seedling and floristic components (Kirby et al. 1994; Truscott et al. 2004). Oak (Q. robur, Q. petraea) seedlings in particular, are noted for their ability to establish in open grazed conditions (Vera 2000). In contrast, ash (Fraxinus excelsior) produces abundant seedlings and young saplings under canopy, but these are easily lost to grazing (e.g. Crampton et al. 1998).
Boundary type, which was also found to be a significant factor, can influence vegetation in a variety of ways, acting as a physical barrier to ungulate movement or, in the case of hedges, acting as refugia (McCollin et al. 2000; Smart et al. 2001) or dispersal pathways for forest herb species, as demonstrated in central New York, USA (Corbit et al. 1999). Adjacent water bodies such as streams and rivers aid dispersal of both terrestrial and riparian plant species (e.g. Johansson et al. 1996; Merritt & Wohl 2002), and can considerably extend the range of those with a limited terrestrial dispersal capacity (Boedeltje et al. 2003). However, in this analysis, the road variable was selected before all other boundary types. In addition to facilitating the spread of exotic species into forests (Watkins et al. 2003), roads are known to have associated effects that alter interior-forest conditions; petrol combustion and the application of salt can lead to an increase of N, Na, Mg and Ca, which significantly alters vegetation and soil composition (Bernhardt et al. 2004). Clearly, more research is required into the effects of this ‘sleeping giant’ (Forman & Alexander 1998).
Forest spatial variation (e.g. shape and size) affects both structure and dynamics of species assemblages in a number of ways, including invasion of forest edges by weeds (Honnay et al. 2002b), pesticide and fertiliser impacts (Gove et al. 2004) and area-dependent extinctions (Jacquemyn et al. 2002). The spatial component of surveyed forests was found to be important, despite the fact that the same area was sampled per site, regardless of site size, which may have resulted in an underestimation of the effect of spatial dynamics; perhaps as a consequence, total variation explained by this set was smaller than that of other site-scale sets.
Gradient analysis using ordination demonstrated that, at the time of the survey, fallow deer were predominantly lowland species (Arnold 1993), whereas sheep were more typical of upland landscapes (Fuller & Gough 1999). Following roughly the same geographical pattern, thick hedges, banks and ditches were more frequent lowland boundaries, and water a more typical upland, forest boundary. However, there was little interaction between the effects of spatial and boundary/grazing sets, suggesting that boundary type and grazing are not acting primarily as a surrogate for larger scale spatial variation, such as surrounding land use or geo-climatic variability, but are significant in their own right. Thus, while an understanding of forest spatial dynamics is important, buffer zones and provision of appropriate exclosures in areas of significant species loss (e.g. Mcinnes et al. 1992; Cooke 2002), or desired expansion (Romagosa & Robison 2003; Palmer et al. 2004; Truscott et al. 2004) may prove as, or more, significant to the delivery of conservation objectives.
Local factors, such as soil pH, slope and the presence of wet areas or large glades, accounted for a large fraction of plot level variation. Canopy gaps can play a major role in maintaining or enhancing field layer species diversity (Peterken & Francis 1999; Ott & Juday 2002; Rantis & Johnson 2002). The variable representing current large open areas within forests (glades > 12 m), was selected relatively early for the plot model, whereas those suggestive of recent gaps (fallen uprooted tree, fallen broken tree) entered very late. Small gaps may therefore have only a minor and transient effect (Reader 1987; Peterken & Francis 1999), promoting whatever species diversity is already locally present. They may therefore uncouple control of species richness and abundance from resource-based niches, as observed in the neo-tropical forests of Central America (Hubbell et al. 1999).
Plot level management accounted for a significant proportion of TVE, although other variables included in the biotic set (e.g. glades, dead wood, etc.), may also have been the result of human activity. Ordination and bivariate SD ellipse analysis suggested that forest specialist species were strongly associated with coppice stools and cut stumps, including vernal species (e.g. Hyacinthoides non-scripta), shade tolerators (e.g. Lonicera periclymenum), and species that can survive shade phases in the seed bank or on rides and clearings, such as Veronica montana and Lysimachia nemorum (Mason & MacDonald 2002). This may represent an historic association with the long tradition of coppicing in Britain, which persists despite the widespread abandonment of the practice that occurred during the 20th century (Peterken 1993). In contrast, old conifer stumps within surveyed forest patches were found to be associated with species of open, rather than forest interior habitats, reflecting the history of forestry practice in the UK. Since the mid-1900s many woodlands have been planted or extensively under-planted with non-native conifers such as Picea sitchensis, Larix spp. and Pinus sylvestris beyond its natural range (Peterken 1993; Truscott et al. 2004). Recovery of former native forest, through removal of introduced plantation species (as encouraged by a recent policy shift; The UK Steering Group 1995) may not therefore, at least in the short term, necessarily result in the expansion of desired native forest species (e.g. Meier et al. 1995). Outcomes will depend on factors such as productivity of the forest soil and composition of the site species pool (Romagosa & Robison 2003).
CCA illustrated that the presence of cattle and sheep was strongly correlated with non-forest field layer species of grass- or heath-land. Our findings are thus consistent with other research, which suggests that over-grazing by livestock can reduce survival and growth of forest field layer species (Gustafsson 2004) and heavily influence woodland regenerative success (Pigott 1983).
Spent cartridges, Prunus lauroceracus and, by implication, management for game such as the common pheasant (Phasianus colchicus), were not closely associated with any forest specialists, or with many forest species. Pheasant release pens have more bare ground, a reduced vegetation structure, and lower species diversity and cover of herbs, compared with control areas (Sage et al. 2005).
woody species and forest structure
Forest succession models (Shugart 1984) have developed considerably in recent years, particularly in the USA (e.g. Robinson & Ek 2003; Busing & Mailly 2004). However, successful linking of gap models with field layer dynamics will need to place the complex relationships between the light environment and herbaceous species within the wider context of other factors contributing to field layer composition (Weisberg et al. 2003). On any particular site, clear differences in the ground flora between tree species are apparent (e.g. Mitchell & Kirby 1989), but this study demonstrated that the association between field-layer vegetation composition and particular tree and shrub species may not be that strong, relative to other plot and site conditions. Stand-based variables (basal area and stem density) also explained only a relatively small amount of variation in the data set, compared with other plot scale variables. This finding has important implications for the development of models based on stem data.
However, the wide geographical spread of this study may have prejudiced the analysis against variables important at finer scales, e.g. light penetration and tree species composition (Graae et al. 2004). Thus the effects of the local structural environment may have been swamped by the stronger pull of countrywide gradients such as pH or climate. Moreover, NWS sampling, in common with other survey work, was optimized to capture floristic variation, rather than gradients of potential explanatory variables. Therefore some gradients may, by chance, be longer than others and thus able to explain more variation. In addition, direct factors such as heavy grazing by sheep can create ‘bowling green’ swards (Fuller & Gough 1999) under the tree canopy, composed of species more characteristic of open habitats. This may cause the response of field layer species to become statistically uncoupled from passive drivers such as relative light regime. Recent work on changes in the vegetation of these plots from 1971 to 2001 indicates that changes in nitrogen regime may also interact with responses to changed light conditions (Kirby et al. 2005). Further research is required to test these hypotheses.