Diversity and sensitivity of epiphytes to oxides of nitrogen in London

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304 L. Davies et al. / Environmental Pollution 146 (2007) 299e310Fig. 3. CCA results. Lichen and bryophyte distribution in relation to environmental variables. Nitrophytes are largely associated with transport pollutants and acidophyteswith bark pH (designation after van Herk, 2002).species (lichens and two mosses, 76 species) in each class andthe number of first records (the first time a species was recorded)in each class (Fig. 4). There was a significant inverserelationship between NO x and diversity (r ¼ 0.80, p < 0.001,n ¼ 15) and first records within each class (r ¼ 0.85,p < 0.05, n ¼ 6), (Fig. 4). Diversity was highest (60 species)in the first two classes (50e69 mg m ÿ3 ) and then declinedwith just fourteen species remaining above 200 mg m ÿ3 . Firstrecords were also highest at lowest concentrations, decliningto 3e5 species in each class up to concentrations of110 mg m ÿ3 , after which there were no further first records.There was a significant positive relationship (r ¼ 0.85,p < 0.001, n ¼ 76) between abundance (number of treescolonised cover) and the highest concentration at which specieswere recorded (Fig. 4), showing that the most NO x tolerantspecies are widely distributed and abundant. New recordsand rare species were generally confined to areas of low NO xwhere the ratio of NO to NO x is generally at its lowest (Fig. 1).However, there were a few notable exceptions. Species tolerantof NO x , but with low abundance and not recorded in areasof low NO x were: Phaeophyscia nigricans, Rinodina gennarii,R. subexigua, Lecanora barkmaniana and L. compallens. Thelatter two were recently described from the Netherlands andassociated with roadside trees (Aptroot and van Herk, 1999;van Herk and Aptroot, 1999). R. gennarii is designateda ‘‘nitrophyte’’ in the Dutch scale for ammonia (van Herk,2002). Rinodina exigua was also recorded with a high meanNO x value. These results suggest these species may be stimulatedby high NO x concentrations. However, such environmentsare generally also characterised by high levels ofammonia and particulate matter (dusts).3.2.2. NO 2Species were grouped into classes of equal size (5 mg m ÿ3intervals) and plotted against the number of species (lichensand two mosses) in each class and the number of first recordsin each class (Fig. 4). Biodiversity was highest (59 species)in the second class (35e39 mg m ÿ3 ) and then declined with seventeenspecies remaining in the highest class (60e64 mg m ÿ3 ).There was a significant inverse relationship between NO 2and first records (r ¼ 0.96, p < 0.01, n ¼ 5), declining to justone (Phaeophyscia nigricans) between 50 and 54 mg m ÿ3 .Species associated with high NO 2 were generally thoseassociated with high NO x concentrations with the exceptionof Flavoparmelia caperata.3.2.3. PM 10Species found in areas of high PM 10 are generally the samepollution tolerant species associated with NO x and SO 2 , withthe exception of Phlyctis argena and Micarea prasina. The
L. Davies et al. / Environmental Pollution 146 (2007) 299e310305Fig. 4. (a) Diversity and first records as a function of NO x (mgm ÿ3 ) (Diversity: r ¼ 0.8, p < 0.001, n ¼ 15; First records: r ¼ 0.85, p < 0.05, n ¼ 6); (b) Lichenabundance (trees colonised cover) and maximum concentrations of NO x (mgm ÿ3 ). Species recorded at concentrations above 200 mg m ÿ3 from left to right are:Physcia nigricans, Lepraria incana, Scoliciosporum chlorococcum, Lecanora dispersa, Melanelia subaurifera, Xanthoria candelaria, Parmelia sulcata, Lecanoraexpallens, Amandinea punctata, Physica tenella, Xanthoria polycarpa, X. parietina, Physcia orbicularis, P. adscendens (r ¼ 0.85, p < 0.001, n ¼ 76). (c) Biodiversityand first records as a function of increasing NO 2 (mgm ÿ3 ) (First records: r ¼ 0.96, p < 0.01, n ¼ 5).
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Diversity and sensitivity of epiphytes to oxides of nitrogen in London

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