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Universität Kiel, Zoologisches Institut, LS für Ökologie, Olshausenstr. 40, 24098 Kiel, Germany
Received for publication February 26, 1997. Accepted for publication July 13, 1998.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLiterature Cited |
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Key Words: ecomorphology • Evernia • growth form • lichenophagy • microclimate • morphogenesis; • Parmeliaceae • plant–animal mutualism • variability by disturbance
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLiterature Cited |
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). (The term "growth form" will be used in order to express the detailed shape—dense, loose, aerodynamic, etc. of a body, not just its basic crustose, foliose, or fruticose form.) Morphogenesis in such phanerogams depends on a highly complicated architecture of different tissues. Lichens completely lack such tissues. Correspondingly, their growth forms are often only two-dimensional, especially on wind-exposed surfaces. Such crustose or foliose growth forms do not exceed the laminar microscale boundary layer covering the substrate (Nobel, 1981
) and therefore do not suffer the detrimental effects of turbulent wind. How then can fruticose lichen species successfully develop their tall shrub-like thalli on wind-exposed surfaces in spite of the primitive morphology?
Evernia prunastri (L.) Ach. is the most common of such fruticose lichen species on exposed tree trunks in Europe (Poelt, 1969
). The individual branches of E. prunastri grow basically flat and isotomic-dichotomous (Beltman, 1978
), but with frequent anisotomies in their (annual) bifurcations (Stone and McCune, 1990
) as well as occasional adventive branches on the branch surface (also "lobuli"; Beltman, 1978
). Although the morphogenesis mainly consists of only the two-dimensional basic processes, one finds extremely variable, polymorphic growth forms (Beltman, 1978
; Wirth, 1980
), which seem to correlate to wind exposure, as in phanerogams (Zimmer, 1994
). Therefore I investigated three questions: (1) What morphogenetic patterns make the polymorphism of E. prunastri possible? (2) How do these morphogenetic patterns interact with the environment? Important environmental factors on the solitary tree trunks studied are (a) the microclimate of the exposed substrate and (b) grazing by arthropods, which occur in densities of up to 25 animals/thallus and mostly prefer E. prunastri compared to other epiphytic lichens (Prinzing, 1996
, 1997
; Prinzing and Wirtz, 1997
). (3) Do the (combined) effects of these mechanisms influence the ontogenetic development of wind-tolerant growth forms?
Unfortunately, a direct experimental test of the effect of grazing on the phenotypic acclimation of the whole-thallus growth form is hardly feasible: application of insecticides would probably not prevent grazing for a sufficiently long period to affect the very slow growth of branches. The effects of earlier morphogenetic disturbances due to grazing would not be suppressed anyway. Mechanical exclusion of grazers (e.g., by small fences) is problematic, because it would also influence the microclimatic wind exposure. Nor is it possible to exclude all grazers from the whole tree trunk for months or years because they are dispersed by wind (Farrow and Greenslade, 1992
) and have a very wide distribution (Prinzing and Wirtz, 1997
). Thus, the overall effect of grazing on the shape of the whole thallus can only be inferred from an investigation of the growth patterns behind this effect. The investigation, therefore, focused on the thallus description at all scales from the single feeding trace to the complete growth form of the thallus. The respective patterns had to be recognized by eye on photographs of the living thalli in the field (for comparable photographic observations see Stone and McCune [1990]).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLiterature Cited |
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). Evernia prunastri was found on trunks of solitary trees situated along avenues in villages (sites 6, 10, 11), or small roads through agricultural landscape (sites 4, 14–19, 21), in hedgerows (sites 7, 20, 24), in clearings (site 1), at forest edges (sites 2, 8, 13, 18), or within meadows or fields (sites 3, 5, 9, 12, 22, 23), mainly on oaks (Quercus robur; sites 1–8, 11, 12, 14, 16–24) , lindens (Tilia spp.; sites 4, 6, 10), ash (Fraxinus excelsior; sites 9, 15), and sometimes beeches (Fagus sylvatica; sites 1, 13). On these trunks E. prunastri mostly occurred at heights of 0.4–3.0 m above ground level, mainly on the west or northwest faces. All trees investigated had a circumference of >2 m at a height of 1.5 m.
Thalli were mostly shrub-like, standing out from the trunk up to 20 mm. Branches were 0.5–4.0 mm wide and 0.2–0.5 mm thick. Thalli covered basal surfaces of 35–500 mm2 each. On 
25% of the investigated trees all or many of the thalli were densely covered with green algae (Pleurococcus sp.; mainly sites 1, 2, 4, 6, 16). Specimens of E. prunastri v. herinii (Duv.) Maas G., which are characterized by a complete lack of the lichen substance usnic acid (Culberson, 1969
; Poelt, 1969
), were found at sites 6 and 7.
Observations and experiment
Large-scale photographs were taken (a) in the first year from August to April at intervals of 2–3 mo to document the development of thalli in seven trunk areas (trunk area = area covered by a photograph) on five trees (oaks and two ash trees; sites 15, 17, 23, 24) (scale 1:1.25) and (b) in the second year in January and April (three trunk areas on two oak trees [sites 23, 24], scale 1.2:1). Small-scale photographs (1:2) were taken simultaneously in year 1 from thalli in an additional seven trunk areas.
Five thalli, all strongly overgrown by Pleurococcus algae and located on solitary trees (sites 1, 6), were artificially injured in January. Cortex and phycobiont layers were cut away with a scalpel along two to four rounded edges of branches in each thallus. The thalli were photographed immediately and again after a period of 3 mo. For surgery and photographing (Leitz photo-macroscope M 400, Wetzlar, Germany) three of the thalli were taken into the laboratory for 1 d. They were re-affixed to the trunk with strong, short, needles exactly onto their former position (which had been marked in the meantime with colored needles). The initial orientation of the thalli was restored by fitting the piece of bark that had been removed with the thallus exactly into the respective gap in the trunk surface.
Thalli were collected from different microenvironments on one oak trunk (site 5). Eleven trunk microsites were differentiated according to the combination of the following parameters: (a) height on trunk (0.5 or 1.7 m), (b) orientation of trunk faces to main wind direction (directly exposed and both lateral faces [the wind-sheltered face was free of thalli] with main wind direction indicated by the shape of surrounding shrubs and tree crowns), (c) association of thallus (in solitary position or within groups of other E. prunastri thalli). On each of these sites three or four thalli of recognizably different growth forms were collected for a total of 43 specimens. In the laboratory, thalli were examined under a photo-macroscope (Leitz M 400). Thalli were cut longitudinally with a razor blade, the proportion of thallus surface with feeding traces was estimated (0–100%, in intervals of 10 %), and the inner and outer structures of the complete growth form were photographed, sketched, and noted. General elements of "growth form architecture" were deduced later from a comprehensive overview of the documentation.
Thalli or branches that had been detached from bark and fallen off the trunk were collected at seven sites near Kiel in October (15–25 specimens/site; sites 6, 7, 9, 10, 16, 17, 18).
The climatic conditions for thalli on tree trunks were determined according to the following rules: prevailing wind directions at a solitary trunk can be recognized from the shape of surrounding shrubs and tree crowns ("flag trees," Holroyd, 1970
; Noguchi, 1979
). Main wind direction is affected by shelter from surrounding hills, hedgerows, and forest edges (Barner, 1983
), which are very common in the areas of investigation. The wind exposure below the crown increases with increasing height on the trunk (Kershaw and Larson, 1971
). Wind exerts the highest traction with the most constant direction where the air current can move along unhindered in a tangential direction (White, Mottershead, and Harrison, 1992
; Häckel; 1993
). This is the case on the two lateral trunk faces adjacent to the frontally wind-exposed face. At a microscale, there is additional wind shelter due to "valleys" of the bark relief or to neighboring thalli (Kershaw and Larson, 1974
; Nicolai, 1985
). Thus, the thalli that were situated on such lateral trunk faces without being wind sheltered on a smaller scale could each be divided into an exposed and a sheltered thallus face: wind-exposed branches grew in the direction of main wind exposure, and sheltered branches grew away from that direction; intermediate areas were not considered (see example in Fig. 3).
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). On the other hand, there is a decrease of solar radiation from the south and southwest-exposed trunk faces towards east and west faces and eventually towards the north faces (Geiger, 1961
; Nicolai, 1985
). The bark's microrelief does not have a strong effect on incoming solar radiation because of the sun's changing position throughout the day and the year (Nicolai, 1985
).
Supply of rain and snow is at its highest below those points where the crown drains precipitation onto the trunk (stem flow). The drainage zones are 10–60 cm wide, range from the crown to the trunk base, and can be easily recognized from the strong cover of algae or even mosses on the bark (Matthey et al., 1989
).
Morphogenetic patterns of E. prunastri
Branches
The tip of a branch develops one branch pattern per year. Branches mostly grew along their young parts within the last two branch points, which was observed 155 times compared to only 46 cases of increase in older parts of branches (distributed over 15 sufficiently focused thalli on five large-scale photographs from year 1). Some parts of branches were uniform, flat, even, and branched isotomic-dichotomously, whereas others showed numerous irregularities in cross section, growth plane, and branching pattern (Fig. 1).
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). Photographs from year 1 could only be considered in one comparison due to the lower magnification. In that case the respective three successive observations from August to April were averaged to avoid pseudoreplication. The growth type was also compared between branches with and without preceding artificial injury (chi-squared test, suitable for frequency distributions).
The amount of grazing (expressed in percentages, i. e., not as a frequency) was compared between inner and outer parts of the 43 thalli from an oak (site 5, see above). The two values for each separate thallus were considered pairwise, in order to eliminate interthallus variability (Wilcoxon rank test; Lamprecht, 1992
). Upper and lower positions on the trunk were also compared (Mann-Whitney U test) with respect to (a) the extent of grazing and (b) the number of certain "architectural elements" (described in Fig. 2). Pairwise comparisons (Wilcoxon test) were avoided here due to difficulties in separating pairs of obviously corresponding thalli from the two heights. In (b) a chi-squared goodness-of-fit test would have yielded suitable and similar results too, but a consistent analysis of (a) and (b) was preferred.
Branch density and the relative number of observations of branch increase within 2 to 2–3 mo periods (parameters defined in the Results section) were compared between wind-exposed and wind-sheltered thallus faces on the photographed trunk areas (large-scale photos from year 1; criteria for exposure given above). I applied (1) a pairwise comparison (Wilcoxon rank test) between the averages of the measurements from wind-sheltered and wind-exposed branches in each separately photographed trunk area. This eliminated the interarea variability. (2) A repeated-measures ANOVA was performed on the single measurements at each date [after ln(x + 1) transformation of the data to nearly normal distribution]. This test considers the dependence between the values collected repeatedly from one trunk area. Chi-squared or Fisher tests (on distributions of branches with/without increase, or of dense/loose branches) were less suitable: the growth speed and especially the growth density of directly neighboring single branches might not be mutually independent.
Statistics were calculated with a SYSTAT 5.0 computer package (Wilkinson, 1992
). All Wilcoxon tests, Mann-Whitney U tests and ANOVAs were two tailed.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLiterature Cited |
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Artificial injury (Fig. 5), "simulated grazing," induced changes in growth plane in 13 out of 14 cases, whereas this was only observed in two out of the remaining 20 nonmanipulated parts of photographed branches with rounded edges (
2 = 19.7, P < 0.01).
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2 = 4.03, P < 0.05, N = 91 branches on three trunk areas (= five thalli) in year 2; Fig. 3]. Thalli from 1.7 m height above ground level did not differ significantly from thalli at 0.5 m height in the proportion of surface area grazed (factor 1.25, N = 43 thalli, P = 0.27 according to Mann-Whitney U test), nor was the number of changes of growth planes from trunk parallel to perpendicular or vice versa significantly different at the two heights (factor 1.13, P = 0.77, Mann-Whitney U test, N = 43 thalli). Growth plane changes were recorded as presence of parallel/perpendicular, and perpendicular/parallel changes in the four sections of each thallus (presented in Fig. 2); differentiation of single branches, growth points, or cross sections was impractical.
Thalli of E. prunastri var. herinii were recognizably more grazed upon than the neighboring specimens of E. p. var. prunastri with the latter's higher lichen acid contents (N = 20 thalli were found). Also the number of branches with corners along edges or with destroyed tips was much higher in any E. p. var. herinii. The growth forms of E. p. var. herinii were denser and more hemispherical than that of any neighboring E. p. var. prunastri of the same tree (Fig. 6a).
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). These thalli were much more regular in growth than neighboring "normal" specimens (based on 65 algae-covered thalli at all three sites where both types were growing next to each other [Fig. 6b]).
Correlation between climatic exposure and the growth of E. prunastri
Increase and shape of differently exposed branches
An increase in size at the tips of branches that were exposed to wind (for determination of exposure, see Material and Methods section) was observed only 49 times in comparison with 106 times under wind-sheltered conditions (2–3 mo periods from August to April of year 1, with 15 sufficiently focused thalli distributed over five photographed trunk areas; Fig. 7). A greater increase in branch size in sheltered than in exposed parts of branches was also found when considering the relative numbers of observations of branch size increase (number defined as [number of observations of size increase within 2–3 mo]:[total number of visible branches]; results: repeated-measures ANOVA: P = 0.05, Wilcoxon test: P = 0.043, N = all five sufficiently focused photographed trunk areas). Moreover, branches were more densely packed at wind-exposed thallus faces (branch density = [the number of branches that were close to their neighboring branches, i. e., separated by less than about twice the branch's thickness] ÷ [overall number of visible branches]; results: repeated-measures ANOVA: P < 0.001, Wilcoxon test: P < 0.02, N = 7 photographed trunk areas with 18 thalli sufficiently focused). As mentioned above, the frequency of branches with irregular cross section (Fig. 1) was proportionally smaller on the exposed than on the more sheltered thallus faces.
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In thalli with little grazing (conditions opposite to the above mentioned) such a differentiation of growth forms was not observed (Fig. 6). In a few cases the branches were found to be bent slightly but consistently in the same direction. Sometimes the wind-exposed parts of the less grazed thalli were smaller (e.g., in the upper zone of the E. p. var. prunastri thallus in Fig. 6a).
Sunlight is distributed differently on the trunk surface than wind (see Materials and Methods). Thus sunlight exposure did not match any of the above-mentioned patterns of thallus growth, nor any other recognizable growth pattern, nor were any of the above-mentioned patterns in growth form correlated with crown-drainage zones on the trunk where precipitation is most intense. Sometimes a luxuriant, hanging growth of thalli was striking there.
Mechanical effects of wind
Falling off the bark was the most common cause of death observed for E. prunastri in the area of investigation (Fig. 7: X). This happened mainly during strong wind with simultaneous precipitation. And, in fact, detached parts of thalli (i.e., photographed parts of thalli that were later found missing on subsequent photos) were demonstrated to have had a more wind-susceptible (i.e., loose and unaerodynamic) growth form compared to undetached neighboring parts of thalli (confirmed in 47 out of 52 observations in year 1 in all small- and large-scale photographs of 11 trunk areas with missing thalli: example in Fig. 7). Also, the corresponding single branches that became detached by wind were more regular (see Fig. 1) than neighboring, intact ones (65 out of 91 observations of single branches). All of the 88 collected thalli that had been torn off the trunk were rather regular in growth pattern. The algae cover of these thalli (as a possible indicator of the attractiveness of the lichen phycobionts for grazers) was not assessed because it might well have been affected by the microclimate, grazers, and fungi at the ground where the thalli had been collected. Detachment of thalli by vertebrates was rarely observed: squirrels were very rare, and birds collected thalli only exceptionally and only during spring (as nesting material).
Wind also bent branches when they were soaked, flexible, and mostly spread away from the trunk. When such branches dried out under windy conditions, they became stiff and temporarily fixed in their flexed position.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLiterature Cited |
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that anisotomic-polytomous branching patterns of branches are caused by injuries at the branch tips.
Grazing, natural or simulated, apparently caused most of these injuries and irregularities of branch growth (Fig. 8: 3) in my study. Further causes might have been wind or any windborne particles, e.g., sand or ice crystals (Häckel, 1993
). But grazing seemed to be the prevailing impact, because of (a) the coincidence in the distributions of irregular growth and of grazing but not of wind (Table 2: comments on branches), and (b) the structural similarity between feeding traces and corners along edges/destroyed tips of branches.
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In basal branches, which were hidden from photographic observation in the present study, a third type of asymmetries in branch cross sections was sometimes observed: strong infoldings of the lower side of branches into the medulla correlated to extreme bending. Such asymmetries were often found to be correlated with mining by juvenile oribatids (Prinzing and Wirtz, 1997
). These animals then enlarged their tubes into the medulla, increased the risk of a rupture toward the branch's upper side, and enhanced the morphogenetic effect of cross sections with such infoldings on further branch growth. Similar cases of mining by oribatids in the medulla of lichen thalli including ruptures to the branch's upper side are described by Travé (1963)
and for fruticose thalli similar to E. prunastri by Bachmann (1929)
and Bellido (1979)
. Bachmann also shows how such tubes lead to strong infoldings and deep groves in branch cross sections. Zopf (1907)
found that branches of fruticose lichens also become hollow and wider when mined. It is therefore probable that in E. prunastri also the juvenile oribatids could not only have enhanced the effect of tube formation and infoldings in branches, but even induced them. I did not observe any other organisms that create/enlarge such tubes in E. prunastri, nor did I observe tubes that were too narrow or too wide to be attributable to the effect of a juvenile oribatid.
Complete thalli
The growth form of complete thalli, especially the branch density, matched the wind exposure only when the branches were also grazed upon (Table 2). Without grazing wind exposure did not have a clear effect on growth form besides occasional bending of branches. Without wind exposure (on wind-sheltered faces of thalli or trunks or in bark valleys) all growth forms were rather loose and susceptible to air currents.
The described variation in growth form even on differently exposed faces within the same thallus would have hardly been conceivable if all the single branches had been growing evenly and isotomic-dichotomously. Instead, such differentiated growth forms were able to develop, if (a) branches, including the young ones, were variable in their growth (Fig. 8: 5) and if (b) among such branches those increased fastest that grew into a wind-sheltered direction (Fig. 8: 6). Both of these conditions can be demonstrated. From Fig. 8 the mechanism can now be recognized that is probably most relevant for the observed interaction between grazing and the ontogenetic ability to vary complete growth forms: grazing disturbance mainly generated the variability of the branch growth, i.e., one of the prerequisites for growth form differentiation.
The observed selective effect of wind on growth speed could have been caused by (a) convective desiccation of the poikilohydric thalli, (b) minute cracks in the cortex or its superficial erosion as a result of microscopic abrasion, (c) ruptures of the cortex when branches are bent by wind, or (d) the stress effects of shaking. All such mechanisms have already been demonstrated for phanerogams (Nobel, 1981
). Convective cooling by wind might have additionally reduced metabolism while thalli were still wet, whereas it could not have led to serious chill damage since this occurs in lichens only at extremely low temperatures (Henssen and Jahns, 1974
).
Ultimately, these processes of induced variation and selective growth lead to thallus growth forms that are phenotypically acclimated to wind (Fig. 8: 7). These ontogenetic, modifying processes mimic to some degree processes in the phylogenetic evolution of adaptations (Darwin, 1859
). The ontogenetically acclimated thalli were much less endangered by wind dislodgment than those of non-wind-acclimated growth forms showing growth patterns that are very regular and simple (Fig. 8: 2). The aerodynamic effect of such wind-acclimated, dense growth forms is demonstrated by a desiccation experiment (Prinzing and Wirtz, 1997
): wet thalli are dried with a hair drier from a fixed position and distance in front of the thallus for 1 min. The percentage of water loss of different thalli indicates the wind accessibility and turbulence at the surface of the respective thalli. This water loss correlates well to a decreasing branch density of the frontal faces of thalli (rs = -0.96, P < 0.001).
The basic adaptive value of shrubby growth forms is apparently the same as in crowns of phanerogams (Nobel, 1981
): an aerodynamic shape with a mutual sheltering of branches. Morphogenetic processes analogous to those in Fig. 8 (simple growth patterns, environmental disturbances [e.g., grazing] and selective influences on growth speed) are found in the phanerogam root system and they result in an analogous polymorphism with advantages to different forms in different contexts (Simberloff, Brown, and Lowrie, 1978
; F. Hallé, URA, Montpellier, personel communication).
How do other species of fruticose cryptogams exposed to turbulent currents develop stable thalli? In many taxa, thalli can generate high variability in branch growth on their own, without additional disturbance by grazers. This could be due to rounded cross sections of branches (e.g., the lichen genera Usnea, Cladonia, Stereocaulon, mosses, and some calcareous red algae). Such cross sections do not lead to a branching symmetry, i.e., there is no pattern analogous to 1 in Fig. 8 and thus processes 3 and 4 (Fig. 8) are superfluous. Fruticose cryptogam species with more flattened branches often have apical, spirally dividing cells (Phaeophyceae) or actually consist of bunches of cell threads each dividing dichotomously but oriented into separate directions (multiaxial Rhodophytina). Here, the variable growth of branches (analogous to 5 in Fig. 8) depends on asymmetries on a cellular level, instead of asymmetries in the branch's cross section (4 in Fig. 8).
In many species of benthic fruticose algae branches grow symmetrically and nonvariably. Thallus growth forms are correspondingly even, often fan-like, and very susceptible to water current (analogous to 2 in Fig. 8). And in fact these thalli do not have rigid growth forms that remain unmodified by water movement, but instead their soft, water-saturated bodies can flexibly "match" these currents. The risk of detachment is reduced by a cartilaginous or leather-like consistency. Moreover, populations of many marine benthic fruticose algae can easily compensate for occasional detachment because they are able to grow much faster than terrestrial epiphytic lichens.
In all these examples grazing is not necessary for a morphogenesis of current-tolerant thalli. But there are still many fruticose, terrestrial cryptogams with flat branches, lacking spiral growth patterns. It would be very interesting to find out their strategies to cope with wind exposure and to test the importance of grazing. The existing qualitative descriptions of grazer-induced variation in such fruticose lichens (Zopf, 1907
; Bachmann, 1929
) suggest that the mechanisms might be similar to those in E. prunastri.
Compared with the indirect beneficial effects of grazing, the detrimental direct effect might be of subordinate importance for those thalli that grow on strongly wind-exposed positions. It is at exactly such exposed trunks and positions within the bark relief that the grazers do prefer E. prunastri most (compared to other lichens; Prinzing, 1996
, 1997
). A completely detrimental effect of grazing has until now only been found under wind shelter on trunks in a forest (extreme overgrazing reported by Laundon [1971]
). Mostly, however, the thalli are protected because grazing is always restricted to certain specific regions within a thallus shrub depending on the surrounding climate (Prinzing and Wirtz, 1997
).
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FOOTNOTES |
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2 Current address: Center for Environmental Research Leipzig—Halle Ltd., Department of Community Ecology, Theodor Lieser Str. 4, D-06120 Halle/Saale, Germany (tel. +49 / 345 5585 315; fax +49 / 345 5585 329; e-mail: prin@oesa.ufz.de).
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———. 1997 Spatial and temporal use of microhabitats as a key strategy for the colonization of tree bark by Entomobrya nivalis L. (Collembola Entomobryidae). In N. Stork, J. Adis, and R. Didham [eds.], Canopy arthropods, 453–476. Chapman and Hall, London.
———, and H.-P. Wirtz. 1997 Epiphytic lichens (Evernia prunastri L. 1753) as a habitat for arthropods: shelter from desiccation, food-limitation and indirect mutualism. In N. Stork, J. Adis, and R. Didham [eds.], Canopy arthropods, 477–494. Chapman and Hall, London.
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Simberloff, D., B. J. Brown, and S. Lowrie. 1978 Isopod and insect root borers may benefit Florida mangroves. Science 201: 630–632.
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0.01; Fisher test for branching patterns, chi-squared-test for growth planes.)
