Introduction
There has been a net increase in nitrogen deposition in the form of ammonia and ammonium across the European Union largely as a result of the introduction of agricultural subsidies through the Common Agricultural Policy (CAP), which has favoured intensification of livestock and crop farming (Asman et al. Reference Asman, Sutton and Schjørring1997; Fowler et al. Reference Fowler, Coyle, ApSimon, Ashmore, Bareham, Batterbee, Derwent, Erisman, Goodwin, Grennfelt, Hornung, Irwin, Jenkins, Metcalfe, Ormerod, Reynolds and Woodin2001). At the same time SO2 emissions have decreased since the late 1970s due to successful environmental amelioration (Heidam Reference Heidam2000; Fowler et al. Reference Fowler, Coyle, ApSimon, Ashmore, Bareham, Batterbee, Derwent, Erisman, Goodwin, Grennfelt, Hornung, Irwin, Jenkins, Metcalfe, Ormerod, Reynolds and Woodin2001). Although the effect of wet deposited ammonium on sensitive lichen taxa has been shown to occur many hundreds of kilometres from the source (van Herk et al. Reference van Herk, Mathijssen-Spiekman and de Zwart2003), the major part of nitrogen deposition occurs within 200m from the source (Asman Reference Asman1998; Fowler et al. Reference Fowler, Coyle, ApSimon, Ashmore, Bareham, Batterbee, Derwent, Erisman, Goodwin, Grennfelt, Hornung, Irwin, Jenkins, Metcalfe, Ormerod, Reynolds and Woodin2001), where it can cause considerable impact on natural vegetation (Bak et al. Reference Bak, Tybirk, Gundersen, Jensen, Conley and Hertel1999), particularly lichens.
Lichens form a major component of species that are sensitive to changes in atmospheric nutrient conditions (Barkman Reference Barkman1958; Sutton et al. Reference Sutton, Leith, Pitcairn, van Dijk, Tang, Sheppard, Dragosits, Fowler, Wolseley, James, Lambley and Wolseley2004a; van Dobben & ter Braak Reference van Dobben and ter Braak1998; van Herk Reference van Herk1999, Reference van Herk2001) and have long been used as bioindicators of pollution, such as sulphur dioxide (Hawksworth & Rose Reference Hawksworth and Rose1970) and more recently of ammonia (van Herk Reference van Herk1999, Reference van Herk2001; van Herk et al. Reference van Herk, Mathijssen-Spiekman and de Zwart2003; Seaward & Coppins Reference Seaward and Coppins2004; Sutton et al. Reference Sutton, Pitcairn, Leith, van Dijk, Tang, Skiba, Smart, Mitchell, Wolseley, James, Purvis, Fowler, Sutton, Pitcairn and Whitfield2004b; Wolseley et al. Reference Wolseley, James, Leith, van Dijk, Pitcairn, Sutton, Leith, van Dijk, Pitcairn, Wolseley, Whitfield and Sutton2005, Reference Wolseley, James, Theobald and Sutton2006; Frati et al. Reference Frati, Santoni, Nicolardi, Gaggi, Brunialti, Guttova, Gaudino, Pati, Pirintsos and Loppi2007; Sparrius Reference Sparrius2007; Berthelsen et al. Reference Berthelsen, Olsen and Søchting2008).
While some lichen species are tolerant of increasing atmospheric nitrogen, others are highly sensitive and these include many species of high conservation value that are also indicators of ecological continuity (Rose Reference Rose, Bates and Farmer1992; Coppins & Coppins Reference Coppins and Coppins2002). Recent work in the Netherlands (van Herk Reference van Herk1999, Reference van Herk2001) and the UK (Sutton et al. Reference Sutton, Leith, Pitcairn, van Dijk, Tang, Sheppard, Dragosits, Fowler, Wolseley, James, Lambley and Wolseley2004a; Wolseley et al. Reference Wolseley, James, Leith, van Dijk, Pitcairn, Sutton, Leith, van Dijk, Pitcairn, Wolseley, Whitfield and Sutton2005; Wolseley et al. Reference Wolseley, James, Theobald and Sutton2006) has shown a correlation between increasing ammonia concentration and loss of nitrogen sensitive species (nitrophobes) and increase in nitrogen tolerant species (nitrophiles) on trunks. These were combined to form a lichen index where negative values are correlated with increasing ammonia and increasing nitrophiles, and positive values with increasing nitrophobes (Sutton et al. Reference Sutton, Wolseley, Leith, van Dijk, Tang, James, Theobald, Whitfield, Sutton, Reis and Baker2008; Wolseley et al. Reference Wolseley, Leith, van Dijk, Sutton, Sutton, Reis and Baker2008). Investigations of total nitrogen in thalli of the nitrophile Xanthoria parietina and the nitrophobe Evernia prunastri have shown that X. parietina accumulates nitrogen in the thallus while E. prunastri does not, making it intolerant of high nitrogen (Gaio-Oliveira et al. Reference Gaio-Oliveira, Dahlman, Palmqvist, Martins-Loucao and Maguas2005).
Bark pH varies with tree species, age of tree and with local conditions. Trees with a low bark pH, such as oak and pine, support a predominantly nitrophobe, low nutrient lichen flora, while trees with a higher bark pH such as ash, elm and poplar support more nitrophilic species (Wolseley et al. Reference Wolseley, James, Leith, van Dijk, Pitcairn, Sutton, Leith, van Dijk, Pitcairn, Wolseley, Whitfield and Sutton2005). A loss of nitrophobes has been observed in situations, where an increase in bark pH is occurring due to an increase in atmospheric ammonia concentrations (van Herk Reference van Herk1999; Sutton et al. Reference Sutton, Leith, Pitcairn, van Dijk, Tang, Sheppard, Dragosits, Fowler, Wolseley, James, Lambley and Wolseley2004a; Wolseley et al. Reference Wolseley, Leith, van Dijk, Sutton, Sutton, Reis and Baker2008), this is accompanied by an increase in nitrophiles. The shift from nitrophobes to nitrophiles is more clearly seen on trees with a lower bark pH than on trees with a higher bark pH such as ash (van Herk Reference van Herk1999; Wolseley et al. Reference Wolseley, James, Leith, van Dijk, Pitcairn, Sutton, Leith, van Dijk, Pitcairn, Wolseley, Whitfield and Sutton2005).
Twigs offer an annually renewed bark substratum in a microclimate that is exposed to extreme atmospheric conditions. Therefore lichen colonization and succession on twigs are much influenced by local environmental conditions (Degelius Reference Degelius1978; Esseen & Renhorn Reference Esseen and Renhorn1998; Wolseley & Pryor Reference Wolseley and Pryor1999; Hilmo & Holien Reference Hilmo and Holien2002; Hilmo et al. Reference Hilmo, Holien and Hytteborn2005; Wolseley et al. Reference Wolseley, James, Leith, van Dijk, Pitcairn, Sutton, Leith, van Dijk, Pitcairn, Wolseley, Whitfield and Sutton2005, Reference Wolseley, James, Theobald and Sutton2006). With increasing age, trunk bark provides many microniches due to variation in exposure, inclination, bark structure, etc. (James et al. Reference James, Hawksworth, Rose and Seaward1977). Stone (Reference Stone1989) found that competition had no influence on macrolichen and bryophyte communities on oak twigs within the first 15 years of growth, whereas she considered that atmospheric conditions are more important. The UK survey of trunks and twigs showed that lichens on twigs responded at lower concentrations of atmospheric ammonia than those on trunks (Sutton et al. Reference Sutton, Leith, Pitcairn, van Dijk, Tang, Sheppard, Dragosits, Fowler, Wolseley, James, Lambley and Wolseley2004a, Reference Sutton, Wolseley, Leith, van Dijk, Tang, James, Theobald, Whitfield, Sutton, Reis and Baker2008; Wolseley et al. Reference Wolseley, James, Leith, van Dijk, Pitcairn, Sutton, Leith, van Dijk, Pitcairn, Wolseley, Whitfield and Sutton2005a, Reference Wolseley, Leith, van Dijk, Sutton, Sutton, Reis and Baker2008), suggesting that the lichen communities on the new substratum on twigs better reflect current environmental conditions, while the older trunks may support relict communities reflecting previous conditions of the tree and its environment.
Background to survey in Denmark and Tycanol
The sampling of lichen and moss composition on twigs at 4 sites under a range of environmental conditions in West Wales at Tycanol National Nature Reserve (NNR) in 1995 (Wolseley & Pryor Reference Wolseley and Pryor1999) provided the basis for a replicate survey in 2003 using the same methodology. Meanwhile, in 2001 Larsen had used a similar method to sample twigs at Kås Forest in Denmark (Larsen & Søchting Reference Larsen and Søchting2002a). In addition to measuring pH of trunks, pH of twigs and nitrogen content of Hypogymnia physodes were investigated in Tycanol Forest, 2003 and Kås Forest, 2001. This provided an opportunity to compare lichen communities on twigs in sites of high conservation value from two geographical regions, together with pH and nitrogen data, and to identify areas where changes in lichen communities were occurring. Tycanol Forest is a grade 1 Site of Special Scientific Interest listed for its rich epiphytic and saxicolous lichen communities. This area of Quercus petraea woodland supports 396 taxa of lichens of which 36 are nationally scarce species (Wolseley & Douglass Reference Wolseley and Douglass2008; Woods & Coppins Reference Woods and Coppins2003), 34 are Indicators of Ecological Continuity (NIEC) with 6 bonus species (Coppins & Coppins Reference Coppins and Coppins2002). Many of the nationally scarce species are dependent on low pH substrata supporting epiphytic species such as Hypotrachyna endochlora, Lecidea doliiformis, Usnea articulata and U. wasmuthii and young bark species Arthothelium ruanum, Buellia erubescens, Celothelium ischnobelum and Graphina ruiziana. Lobaria pulmonaria was noted in 1972 on Fraxinus (Rose Reference Rose1975), but has since disappeared.
Kås Forest is a protected EU habitat area of mixed Quercus petraea and Q. robur forest situated on a peninsula facing the brackish water of Limfjorden in Northern Jutland. In addition to having the highest density of Lobaria pulmonaria in Denmark, Kås Forest is a sanctuary for many red-listed species such as Cyphelium sessile, Lecanora confusa, Mycobilimbia pilularis, Opegrapha niveoatra, Pachyphiale carneola, Pertusaria hemisphaerica, P. pupillaris, Porina borreri (Pedersen Reference Pedersen1980), and more species are being added when lichenologists visit the area (Larsen & Søchting Reference Larsen and Søchting2002b; Larsen & Søchting Reference Larsen and Søchting2003; Alstrup et al. Reference Alstrup, Svane and Søchting2004; Søchting et al. Reference Søchting, Alstrup, Kocourková, Vondrak and Larsen2007).
Objectives
There were three objectives in the present study. First to compare lichen and bryophyte communities on twigs, nitrogen concentration in thalli of Hypogymnia physodes, pH of bark, and the frequency of nitrophobes and nitrophile lichen species at sites of high conservation value in the climatically different regions of West Britain (Tycanol) and West Denmark (Kås) (Fig 1, Table 1).
Fig 1. Location of the sites at Kås in Denmark and Tycanol, West Wales, United Kingdom.
Table 1. Site location and data for Tycanol, west Wales, and Kås, Denmark, including sulphur and nitrogen deposition
Second, to assess changes in lichen and bryophyte communities on twigs between 1995 and 2003, using frequency of nitrophile and nitrophobe lichen species and pH of trunks at four different sites within Tycanol.
Third, to trial a method using lichens on twigs as indicators of changes in atmospheric conditions that might be deleterious at sites of high conservation value.
Materials and Methods
Lichens were sampled on twigs following Wolseley and Pryor (Reference Wolseley and Pryor1999). Four sites were chosen in Tycanol NNR including a sheltered glade (Gl), a woodland edge facing moorland (Mo), ancient trees in old pasture (P2) and a woodland edge facing improved pasture (P1). All sites were investigated in 1995 and again in 2003. At each site at Tycanol a 20m edge of woodland was selected where twigs were accessible and a 20m tape laid down parallel with the edge. Random numbers were used to select 10 points and at each point the nearest well-illuminated twig above grazing level was selected.
Kås Forest was investigated in 2001. At the edge, 9 well-illuminated twigs above grazing level were selected from trees standing 15 m apart. Nine twigs from different trees in the interior canopy of Kås Forest were destructively collected from 13–18m above ground. Lichens and bryophytes growing on the youngest 15 annual increments were recorded at all sites, using girdle scars as separators. In each increment, species of lichen and bryophyte present were recorded, giving a maximum frequency of 15 for any species on each twig. Most specimens were identified in the field, but critical specimens were collected and identified using microscopes and HPTLC following Arup et al. (Reference Arup, Ekman and Mattsson1993) for Danish material and TLC following Orange et al. (Reference Orange, James and White2001) for Welsh material. Nomenclature mainly follows Blockeel & Long (Reference Blockeel and Long1998) for bryophytes and Coppins (Reference Coppins2002) for lichens, except Melanelixia which follows Søchting & Alstrup (Reference Søchting and Alstrup2007). Lichens described as ‘nitrophyte’ and ‘acidophyte’ are defined in van Herk (Reference van Herk1999). In this paper we are using the terms nitrophile and nitrophobe.
Trunk pH in Tycanol was measured with a Jencon flathead electrode on site several minutes after moistening the trunk with 10% KCl solution. At least 3 measurements were taken on each trunk (Wolseley & Pryor Reference Wolseley and Pryor1999). Twig pH was measured according to Kermit & Gauslaa (Reference Kermit and Gauslaa2001) where straight twigs, c. 10 cm long and c. 6 mm in diameter were collected and dried in paper bags. In the laboratory a 6 cm long piece was cut and sealed at both ends with paraffin wax. The sample was placed in a test tube with 10 ml of 10% KCl and shaken regularly for an hour prior to measuring the pH of the supernatant liquid using the flathead electrode.
Total nitrogen was measured in thalli of Hypogymnia physodes. Samples with developed soralia and no sign of necrosis were collected from horizontal twigs and dried in paper bags. Bark and fauna were removed, and samples of 0·045–0·145 g of thallus were weighed and analysed in a Leco FP-428 nitrogen determinator system, which incinerates the lichen thalli, and measures the concentration of nitrogen in the combustion air (Leco 2007). Nitrogen is expressed as percentage of total dry weight.
Multivariate analysis of lichen and bryophyte species data was carried out using PRIMER_6. Within this, Principal Component Analysis (PCA), Non-Metric Multi Dimensional Scaling (MDS), and Cluster Analysis (CA) were used to indicate similarities and differences between sites and species components. For MDS and Cluster Analysis, site similarity was estimated using the Bray Curtis coefficient, calculated from the frequency of species at each site. Individual species contributions to the groupings indicated were elucidated using the SIMPER (SIMilarity PERcentage contribution) routine in PRIMER 6 (Primer 6 2007).
Results
Lichen and bryophyte flora, nitrophile and nitrophobe lichen frequency, nitrogen concentration in Hypogymnia physodes and bark pH
The total number of lichen species recorded on twigs at both sites was 67 of which 60 were recorded at Tycanol and 22 at Kås. The nitrophobe lichens observed in this study were Evernia prunastri, Hypogymnia physodes, H. tubulosa, Lepraria incana, Parmelia saxatilis, Platismatia glauca, Usnea cornuta, U. flammea, U. florida and U. subfloridana, and the nitrophile lichens found were Candelariella reflexa, Physcia tenella, Xanthoria parietina and X. polycarpa. (grouped respectively as acidophyte and nitrophyte lichens in van Herk Reference van Herk1999, Reference van Herk2001). Out of 40 species occurring with >1% frequency only 15 species occurred in both sites, and only one species occurred at Kås that was not found at Tycanol. Tycanol also supported a higher number of nitrogen sensitive species as well as a higher diversity on twigs. Furthermore, 7 species of bryophytes were found at Tycanol, and none on twigs at Kås. The dendrogram of site similarity (Fig. 2) shows that there was only c. 28% similarity between the twig floras at Kås and Tycanol. The major differences between the twig lichen communities at the two sites were that Tycanol has a high frequency of nitrophobes and a low frequency of nitrophiles, while at Kås there is a low frequency of nitrophobes and a high frequency of nitrophiles (Table 2 & Fig. 6).
Fig. 2. Dendrogram of site similarity based on epiphytic lichen and bryophyte species found on oak twigs at four sites at Tycanol National Nature Reserve, Wales, in 1995 and in 2003 and at 2 sites in Kås Forest, Denmark, in 2001. Tycanol sites include Gl – glade, Mo – moorland, P1 – pasture 1, P2 – pasture; Kås sites include twigs in the forest edge (ed) and interior (int.).
Table 2. Frequency of lichen species recorded on twigs at sites at Tycanol NNR, Wales, and Kås Forest, Denmark. Values are the number of branch increments on which the species was recorded (out of 15 expressed as a %). Species with frequency value <% excluded. Nitrophiles in bold and nitrophobes underlined (after van Herk Reference van Herk1999)
* Tycanol: Gl = glade, Mo = moorland, Pa 1 = pasture 1, Pa 2 = pasture 2; Kås: ed = forest edge, int. = interior.
† Hypogymnia initials were only registered when specimens were too small to identify.
Presence of species on each annual increment (between girdle scars) is recorded for 15 years growth on each twig and 10 twigs at each site.
Within Kås Forest, the dissimilarities between the two sites expressed in Fig. 2 are mainly caused by fewer and less abundant lichen species on the twigs from the interior site compared to the edge (Table 2).
The percentage nitrogen content in Hypogymnia physodes from all sites is positively correlated with nitrophile frequency and negatively correlated with nitrophobe frequency (Fig. 3A). The correlation is stronger when using a combined score of nitrophobes (positive) and nitrophiles (negative) (Fig. 3B and 3D), hereafter named ‘lichen index of nitrogen’. The loss of nitrophobes is already taking place at pH 4·8 on twigs prior to the increase in nitrophiles on the twigs above pH 5 (Fig. 3C). The percentage nitrogen content in H. physodes from Kås and Tycanol is highly correlated with bark pH of twigs (Fig. 4), but not with pH of trunks at Tycanol (data not shown).
Fig 3. The relationships between nitrophobes (no. ▴), nitrophiles (ni, ○) and the ‘Lichen Nitrogen Index’ (no–ni, ) and nitrogen content in Hypogymnia physodes and twig bark pH at Tycanol, Wales, and Kås Forest, Demark. A & B, lichen frequency and % nitrogen; C & D, lichen frequency and twig bark pH. R 2 values and linear trendlines are indicated. The significant correlations [Pearson Product Moment P<0·05, (based on H+-ion concentration for pH) indicated with *]. Data are based on measurements at the 4 sites at Tycanol in 2003 and the edge site from Denmark in 2001. Danish site is marked with arrows in B&D.
Changes occurring between 1995 and 2003 at Tycanol
The dendrogram (Fig. 2.) shows that there have been major changes in the lichen flora between 1995 and 2003 at Tycanol. At the same time there has been an increase in bark pH on the trunks (Fig. 5). The 4 sites at Tycanol show a greater similarity to the year of sampling than to each other (Fig. 2), except the woodland glade site which is dissimilar to all sites in 2003, due to an increase in bryophyte cover associated with increased shade from canopy increase and bracken cover (Table 2). There was little change in species diversity at Tycanol between 1995 and 2003 (Table 2), but there was a conspicuous loss in the frequency of nitrophobes at all sites, for example, Evernia prunastri, Hypogymnia spp., Usnea spp. (except U. cornuta) and Parmelia saxatilis, and an increase in nitrophiles at the pasture sites (Fig. 6, Table 3). The most remarkable changes in lichen frequency were in Physcia tenella which were increased from 2% to 45% at Pasture 1 and from 1% to 11% at pasture 2, and Xanthoria polycarpa is observed for the first time at Tycanol in Pasture 1 (Table 2).
Fig 4. The correlation between percentage total nitrogen in Hypogymnia physodes and bark pH of oak twigs (♦) at 4 sites at Tycanol, Wales, and the edge site from Kås Forest, Denmark. The correlation (R 2=0·89, Pearson Product Moment P<0·05) is based on H+-ion concentration for pH.
Fig. 5. Bark pH on oak trunks at sites in Tycanol, Wales, in 1995 (white columns) and 2003 (black columns). Gl = glade, Mo = moorland, P1 = pasture 1, P2 = pasture 2.
Fig. 6. The average frequency per twig of nitrophiles (black) and nitrophobes (white) on twigs at four sites at Tycanol NNR, Wales, in 1995 and 2003 and Kås Forest, Denmark, in 2001. Sites at Tycanol: Gl = glade, Mo = moorland, P1 = pasture 1, P2 = pasture 2; sites at Kås: ed = forest edge and int = interior.
Table 3. Frequency of lichens and bryophytes at all sites at Tycanol, Wales, and Kås Forest, Denmark
n.m.= not measured
* note that average number of bryophytes, nitrophobes and nitrophiles per twig can exceed the number of increments (15), as there are several species included in each group.
† see Table 2 for explanation of abbreviations.
Discussion
Lichen and bryophyte flora, nitrophile and nitrophobe lichen frequency, nitrogen concentration of Hypogymnia physodes and bark pH
The forests of Tycanol in Wales and Kås in Denmark both support elements of the Lobarion community, including species that are indicators of long ecological continuity, many of which are also highly sensitive to increases in atmospheric nitrogen (Wolseley et al. Reference Wolseley, James, Theobald and Sutton2006). Although the composition of the lichen and moss floras in West Wales and Denmark reflect different climatic conditions, particularly of rainfall, both sites show a trend towards an increase in nitrophilous species on twigs, providing an opportunity to test the application of a Lichen Index of Nitrogen in different areas of northern Europe. The strong statistical correlations of nitrogen content in H. physodes from this study to acidophytes (nitrophobes) and nitrophytes (nitrophiles) as defined in the Netherlands by van Herk (Reference van Herk1999)(Fig. 3B) indicate that the method is applicable in a range of different regions at least including the Netherlands, Great Britain and Denmark. However, regional differences in distribution patterns may affect the definitions of nitrophile and nitrophobe. For example Physcia aipolia is not included in the list of nitrophytes in van Herk (Reference van Herk1999), although Wirth (Reference Wirth, Ellenberg, Weber, Dull, Wirth, Werner and Paulissen1992), Frati et al. (Reference Frati, Santoni, Nicolardi, Gaggi, Brunialti, Guttova, Gaudino, Pati, Pirintsos and Loppi2007) and Nimis and Martellos (Reference Nimis and Martellos2008) considered it to be a nitrophyte. In this study, P. aipolia has shown an increase in the pasture sites concomitant with P. tenella (Table 2). Physcia aipolia is rare in the Netherlands, which is probably the reason that it is not included in the list of nitrophytes in van Herk (Reference van Herk1999). The variation in sensitivity between species of the same genus has as yet been little investigated, but the results at Tycanol suggest that there are differences in sensitivity between species of Usnea in the glade site, where U. cornuta increased whereas all other species of Usnea decreased.
The nitrogen content in Hypogymnia physodes has been widely used as a measure of nitrogen deposition (e.g. Bruteig Reference Bruteig1993; Søchting Reference Søchting1995). The present results show a strong positive correlation between the Lichen Nitrogen Index (nitrophobe minus nitrophile frequency) and both percentage nitrogen in the thallus of H. physodes (Fig. 3B) and bark pH (Fig. 3D). The nitrogen content in H. physodes is highest at Kås and results in the lowest Lichen Nitrogen Index (arrows to Danish site in Fig. 3B & D) suggesting that at this site atmospheric nitrogen has had an effect on the lichen community over a considerable period of time, whereas at Tycanol the effect can be shown over a period of eight years. The results of lichen surveys in Holland (van Herk Reference van Herk1999) and of macrolichens across the UK in sites adjacent to ammonia monitoring stations showed that the loss of nitrophobes and increase in nitrophiles is occurring over a wide geographical range (Wolseley et al. Reference Wolseley, James, Leith, van Dijk, Pitcairn, Sutton, Leith, van Dijk, Pitcairn, Wolseley, Whitfield and Sutton2005, 2007; Sutton et al. 2007) and is correlated with increased ammonia concentration and with an increase in bark pH. Percent nitrogen in H. physodes also correlates with twig bark pH in this study (Fig. 4). However, bark pH varies with the species and age of the tree whereas the shift in lichen communities is occurring on all tree species subjected to atmospheric ammonia (Wolseley et al. Reference Wolseley, Leith, van Dijk, Sutton, Sutton, Reis and Baker2008). At rural sites livestock farming is the major source of reduced nitrogen, but in urban areas oxidized nitrogen from heating and traffic plays a major role affecting the lichen flora. Gombert et al. (Reference Gombert, Asta and Seaward2003) showed that percentage nitrogen in Physcia adscendens was better correlated to traffic density in areas around Grenoble than to percentage nitrogen in Hypogymnia physodes. Likewise, the number of nitrophile and nitrophobe lichens living on oak trunks in the parks of London correlated with traffic related NOx concentration (Larsen et al. Reference Larsen, Bell, James, Chimonides, Rumsey, Tremper and Purvis2007; Davies et al. Reference Davies, Bates, Bell, James and Purvis2007), suggesting that lichens are affected by a variety of sources of nitrogen.
Changes occurring between 1995 and 2003 at Tycanol
The increase in trunk bark pH at all sites at Tycanol from 1995 to 2003 indicates that there is an ongoing change in the atmospheric environment. This may reflect the general trend of increased nitrogen deposition across Europe. Tycanol is on the Atlantic coast of Wales where long distance pollution is assumed to be very rare, but where local changes in adjacent land management may have an effect on lichens. The frequency of nitrophobes and nitrophiles on twigs in all sites in 1995 and 2003 shows a loss of nitrophobes at all sites at Tycanol, whereas the increase in nitrophiles is occurring mainly in the most exposed sites; Pastures 1 and 2 (Fig. 6). Our results suggest that loss of nitrophobes occurred prior to the increase in nitrophiles (Fig. 3A & C), which is in accordance with other observations (Sutton et al. Reference Sutton, Leith, Pitcairn, van Dijk, Tang, Sheppard, Dragosits, Fowler, Wolseley, James, Lambley and Wolseley2004a, Reference Sutton, Wolseley, Leith, van Dijk, Tang, James, Theobald, Whitfield, Sutton, Reis and Baker2008; Wolseley et al. Reference Wolseley, James, Leith, van Dijk, Pitcairn, Sutton, Leith, van Dijk, Pitcairn, Wolseley, Whitfield and Sutton2005, Reference Wolseley, James, Theobald and Sutton2006).
The present study shows that these changes are correlated with an increase in bark pH at all sites (Fig. 5), the lowest increase being in the Glade site and the highest increase in Pasture 2, adjacent to improved pasture and more intensive farming. The most exposed site adjacent to Moorland also shows an increase in bark pH, suggesting that the affect is not restricted to trees adjacent to pasture. At the sheltered Glade site with the lowest recorded percentage nitrogen in Hypogymnia physodes (Table 3) the loss of nitrophobes (mainly Hypogymnia species and Usnea subfloridana) together with the appearance of the nitrophile Physcia tenella shows that this area is also affected. There is also a marked increase in Hypotrachyna revoluta and Punctelia subrudecta s. lat. from 1995 to 2003 (Table 2), indicating that these species are tolerant of increasing nitrogen deposition, although not designated as nitrophiles. This is supported by Sparrius (Reference Sparrius2007) who has categorized H. revoluta and P. subrudecta s. lat. as neutrophytes together with Melanelixia subaurifera. Furthermore, the tremendous increase of Phaeographis smithii at Pasture 2 (Table 2), suggests that this species is tolerant of increasing atmospheric nitrogen. In sites in the west of Britain this species has been increasing rapidly (P. A. Wolseley, unpublished). Other factors that may have caused changes in conditions at Tycanol include increasing shade from canopy encroachment into glades. The woodland glade site is situated on the edge of prehistoric fortifications within the woodland. Since the first survey this long-established open glade has become increasingly shaded due to canopy and bracken encroachment resulting in a shift from photophilous lichens of the Parmelion to shade-tolerant bryophytes. Differences in light exposure can introduce bias, so that we recommend studying only well-exposed twig environments. However, the loss of nitrophobic species of Usnea and of Cetrelia and the appearance of Physcia tenella suggests that there is an ongoing change in atmospheric conditions in the Glade site. This suggests that small conservation sites containing nitrophobic species may be affected to a greater extent than larger sites.
Lichen flora of twigs as a biomonitoring method
Wolseley et al. (Reference Wolseley, James, Leith, van Dijk, Pitcairn, Sutton, Leith, van Dijk, Pitcairn, Wolseley, Whitfield and Sutton2005) investigated the lichen flora of trunks and twigs, bark pH and ammonia concentrations at two sites in Devon and Norfolk, UK, with different pollution histories. They demonstrated a good correlation between bark pH and ammonia concentrations with twig flora, and a poor correlation with trunk flora. The trunk flora continued to support nitrophobes whereas the twig flora was dominated by nitrophiles. Lichens are often relatively long-lived and nitrophobes may continue to survive on an older substratum despite moderately increasing ammonia concentrations. Furthermore, there is a hysteresis effect (resilience) in the bark properties following changes in the environment.
Canopy twigs are not useful for this biomonitoring method as conditions on the twigs of the interior are very different to those on the accessible twigs of the lower branches. The difference between the twig floras from the two Danish sites (edge and interior) is mainly due to the low frequency of lichens on the twigs from the forest interior canopy (Table 3). The low lichen diversity of the twigs from the interior forest may be caused by drought and exposure due to increased air turbulence at canopy height, rather than to differences in nitrogen deposition. Furthermore, canopy twigs are not only difficult to access, but also difficult to monitor without destructive collection of twigs.
The method used at Tycanol is relevant at sites where branches are accessible. In the Netherlands the surveys were made on wayside trees, where branches have been removed up to 3–4 meters from the ground, providing a well-lit substratum for lichen colonization. At sites where it has been possible to compare the trunk and twig floras, the twig flora has responded earlier to changes in atmospheric conditions, than that of the trunk (Wolseley et al. Reference Wolseley, James, Theobald and Sutton2006, Reference Wolseley, Leith, van Dijk, Sutton, Sutton, Reis and Baker2008). This may be especially important at sites of high conservation value where the rarer lichen species are associated with veteran trees. In this situation a change in frequency on the twigs of nitrophobe and nitrophile lichen species can act as an early warning system for long term changes associated with nitrogen or acidifying deposition.
Conclusions
The frequency of nitrophobe and nitrophile lichen species on twigs is combined in a Lichen Nitrogen Index which correlates well at both sites with twig bark pH and with percentage nitrogen in the thallus of Hypogymnia physodes, suggesting that the index is associated primarily with nitrogen deposition and possibly secondarily with decreasing SO2 deposition. This applies to both sites, despite their different climatic conditions.
The lichen flora on twigs can change according to current air conditions within a period of 8 years.
The method described is suitable for use in areas where branches are accessible from the ground, for example, along forest and glade edges or in parkland sites. It is reproducible, reflects current atmospheric conditions, and can be used to monitor changes over time.
Dansk Botanisk Forening and Foreningen til Svampekundskabens Fremme are thanked for providing funding for René L. Vilsholm for the survey; the Countryside Council for Wales and Kås Hovedgaard are thanked for permission to visit the sites.
