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Elemental Composition of Epiphytic Lichens from Okefenokee Swamp

1981; American Bryological and Lichenological Society; Volume: 84; Issue: 1 Linguagem: Inglês

10.2307/3242977

1938-4378

Robert W. Bosserman, Jane E. Hagner,

Botany and Plant Ecology Studies

Elemental compositions of two epiphytic Okefenokee Swamp lichens, Usnea and Parmelia, were examined. Differences between the two genera are ascribed to morphological and metabolic characteristics. These lichens occupy the same habitat in Okefenokee; therefore, elemental composition should reflect niche separation that allows the two genera to coexist. The fruticose lichen, Usnea, appears to depend more on atmospheric wetfall than dryfall, having higher concentrations of dominant precipitation elements (Na, Ca, Mg). The foliose lichen, Parmelia, appears to depend more on atmospheric dryfall than wetfall, having higher concentrations of dustfall elements (P, Si, Al, Fe, Zn, K). Substrate composition affects both lichens through either stemflow or direct substrate uptake. Elemental concentrations in Okefenokee lichens are similar to those of epiphytic lichens found elsewhere. Differences between genera are greater than differences between locations in Okefenokee. Lichen elemental content has become only recently a topic of interest in lichen research. The discovery that lichens concentrate pollutants has inspired researchers to look more closely at the role of lichens in ecosystem mineral cycling (Pike 1978; Lang et al. 1976; Touminen & Jaakkola 1973). Epiphytic lichens are inconspicuous components of most temperate forest ecosystems; however, in freshwater swamps, which can provide ideal habitats for epiphytic growth, lichens are often abundant and may play more important ecosystem roles. Much attention has been given to the abilities of lichens to absorb elements from their nutrient-poor environments; many lichens maintain greater concentrations of elements than do sympatric, terrestrial plants because of more efficient uptake mechanisms and longevity (Lounamaa 1965). Lichens are known to accumulate various trace elements and have been studied as effective pollution indicators and collectors of atmospheric radioisotopes (Jenkins & Davies 1966). Although lichen elemental composition has often been studied in terms of atmospheric pollution, little is known about the elemental content of epiphytic lichens in undisturbed ecosystems (Touminen & Jakkola 1973). This study examines distributions of 18 elements in two epiphytic lichen genera, Usnea and Parmelia, from Okefenokee Swamp. Differences in the morphological characteristics of the two genera, in substrate type, and in location within Okefenokee Swamp are hypothesized to account for much of the variation in elemental composition. Different species of epiphytic lichens often occupy the same habitat and partition their common nutrient environment by uptake characteristies, substrate preferences, and morphological adapta007-2745/81/48-58$1.35/0 This content downloaded from 157.55.39.249 on Wed, 03 Aug 2016 05:05:52 UTC All use subject to http://about.jstor.org/terms 1981] BOSSERMAN & HAGNER: ELEMENTAL COMPOSITION 49 tions. Among fruticose, crustose and foliose lichens, morphological differences that affect exposure to their environments should be reflected in elemental composition. In Okefenokee Swamp, Parmelia and Usnea coexist on dead and living branches of numerous Okefenokee trees and shrubs. Usnea is a fruticose or beardlike lichen with a stringy body that hangs from a single point of attachment. Usnea greatly depends on atmospheric factors because of its large surface area exposed to air. Atmospheric moisture dominates Usnea's ecology, hence the common name fog lichen. Usnea is particularly sensitive to air pollution, making it an effective indicator of environmental conditions (Ferry et al. 1973). Although Usnea is attached to bark with fungal strands that penetrate cracks and lenticels, it apparently receives no nutrition through its basal holdfast (Brodo 1973). The most common Usnea species present in Okefenokee are U. florida (L.) Web., U. longissima (L.) Ach., and U. strigosa (Ach.) Eaton. Parmelia, on the other hand, is a foliose lichen, having a flat, leaflike thallus, which extends along branches and trunks of trees. Parmelia has more contact with its substrate than does Usnea, which has only a small basal holdfast. Usnea's thallus consists of numerous, long, thin cylinders, a form exposing to the atmosphere more surface area per unit biomass than does Parmelia's flattened thallus. Parmelia is attached to its substrate by numerous penetrating rhizines, but these apparently do not absorb nutrients from the substrate (Brodo 1973). Parmelia's gray thallus is comprised of numerous cup-like apothecia and depressions, suggesting an ability to accumulate atmospheric dryfall and wetfall. Rocks and trees in habitats are substrates for the cosmopolitan Parmelia. The most common Parmelia species in Okefenokee is P. perforata (Jacq.) Ach. METHODS AND SITE DESCRIPTION Okefenokee Swamp is a large wetlands area in southeastern Georgia and northern Florida, comprised of shrub swamp (34% of the total area), cypress forests (23%), aquatic macrophyte and grass sedge marshes (21%), and bay forests (6%) (Hamilton 1977). Study sites were chosen in aquatic-marsh shrub-swamp ecotone areas abutting Grand, Chase, Chesser, and Durdin Prairies and Billy's Lake. Lichens of the genera Parmelia and Usnea were collected from shrubs (angiosperms) and cypress (gymnosperms) from three randomly selected sites in these areas. Dominant shrubs included Cyrilla racemiflora, Ilex cassine, Lyonia lucida, Leucothoe racemosa, and Clethra alnifolia, with Cyrilla and Ilex being most abundant. Immature pond cypress (Taxodium distichum var. nutans) are intermixed with shrubs. In the laboratory, Usnea and Parmelia were separated from their respective cypress and shrub substrates; care was taken to remove as much bark as possible from lichens. Sorted lichens and their substrates were dried at 60'C in a forced air drying oven and ground with a Wiley mill. Half-gram samples were ashed at 500'C in a muffle furnace, dissolved in a 20% nitric acid solution, and analyzed with a Jarrell Ash plasma emission spectrograph for 18 elements: P, K, Ca, Mg, Fe, Mn, B, Cu, Zn, Na, Al, Si, Co, Cr, Ni, Pb, Cd and Sr. Orchard leaf standards from the National Bureau of Standards were used to calibrate the spectrograph. Interferences from the ionic matrix were examined by making standard additions of the measured elements to a single plant sample (Bosserman 1979). All elements in the sample were above the detection limit of the instrument. More information about detection limits, matrix interferences, machine variability, and percent recovery for this method and this particular instrument are being published elsewhere (Blood et al. 1980; Blood, pers. comm.). Combined wet and dry precipitation samples in various Okefenokee locations were collected in polyethylene containers over two week periods during the year preceding the sampling period. The precipitation samples were concentrated on a hotplate, placed in a 20% nitric acid solution, and analyzed for the above 18 elements on the plasma emission spectrograph. Water samples with known concentration were used to calibrate the instrument (Blood et al. 1980). Statistical analyses were done using SAS 76 (Barr et al. 1976). Analysis of variance and correlation analyses were done using GLM and CORR procedures, respectively. Duncan multiple range tests were used to describe significant differences among means (0.05 level). Genus, substrate type, and location were factors included in a statistical model, in which a percentage of element content variThis content downloaded from 157.55.39.249 on Wed, 03 Aug 2016 05:05:52 UTC All use subject to http://about.jstor.org/terms 50 THE BRYOLOGIST [Volume 84 TABLE 1. Elemental concentrations (ppm) of Usnea and Parmelia from Okefenokee Swamp. Values are means from all locations and substrates. * P(Ho:equal means) < 0.01, ** < 0.001, *** < 0.0001. Usnea (ppm) Parmelia (ppm) ? (SD) i (SD) Element n = 40 n = 43 Usnea concentrations greater than Parmelia Ca 3600 (1300) 2000 (1300) *** Mg 810 (170) 410 (210) *** Na 240 (50) 160 (60) Sr 15 (4.1) 10 (4.1) *** Parmelia concentrations greater than Usnea K 1500 (230) 2000 (470) *** Fe 290 (70) 370 (90) *** B 13 (6) 32 (33) ** Zn 23 (4) 31 (7) Al 440 (110) 760 (240) *** Si 310 (39) 360 (100) * P 190 (30) 320 (90) Usnea concentrations same as Parmelia Ash 19,000 (6000) 26,000 (2700) Mn 64 (22) 71 (59) Cu 4.7 (0.9) 5.6 (3.3) Co 0.3 (0.12) 0.2 (0.20) Cr 1.0 (0.3) 0.8 (0.4) Ni 1.2 (0.3) 1.3 (1.5) Pb 29 (7.0) 31 (10.3) Cd 0.3 (0.1) 0.3 (0.2) ability is attributed to each factor. From this model, the relative importance of elemental sources and the influences of generic differences, substrate type and location are established.