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1. Introduction

Tree trunks are often directly exposed to wind, sun and precipitation and their fast, irregular changes. Additionally, the trunk surfaces are separated from the soil, and thus from a large water reservoir. Temperatures change between opposite trunk faces for by as much as 37°C and exceed the temperature of the free atmosphere for by as much as 32°C (STOUTJESDIJK & BARKMAN 1992). These conditions are most pronounced on solitary trees (HAARLOV & PETERSEN 1952) or on the upper parts of forest trees (BRAUN 1992).

Nevertheless, such extremely exposed bark surfaces are largely colonised by species which are also found on completely sheltered bark (e. g. below moss cushions on the bark of trunks in forests) and mostly even in the litter layer of forests (for exposed trunks: ANDRÉ 1975, 1976, 1979, 1983, PRINZING & WIRTZ in press; for sheltered trunks: DEN BOER 1961, GJELSTRUP 1979, NICOLAI 1985, 1986, BÜCHS 1988, 1990, WUNDERLE 1992 a, BRAUN 1992; for both: PSCHORN-WALCHER & GUNHOLD 1957, TRAVÉ 1963; for litter layer and trunks GISIN 1943, 1960, TRAVÉ 1963, WEIGMANN & KRATZ 1981, WUNDERLE 1992 a, PONGE 1993). In fact, the taxa to which these species belong are mostly considered as soil/litter-dwelling animals (Collembola, Oribatei, Isopoda, linyphiid spiders, trombidiformous mites) or as partially litter-dwelling (Psocoptera).

The animals of this epicorticolous fauna also inhabit bark-crevices, but do not penetrate into the compact bark. In contrast, in the alternative habitats (litter, plant cushions), such species strongly depend on sheltered resources and only leave them after precipitation (HALE 1972, BAUER 1979, BAUER & CHRISTIAN 1993).

These corticolous microarthropod taxa lack several adaptations found in arthropods in other exposed, harsh habitats: a sclerotization such as in arachnids, scorpions or tenebrioned beetles (HADLEY 1970, CLOUDSLEY-THOMPSON 1988); power or social co-operation, which enables the digging of deep burrows such as in desert isopods (SHACHAK 1980); extremely long legs and hairs such as in some tropical arboreal Collembola (DELAMARE-DEBOUTTEVILLE 1951); a flexible dormancy during which animals are very drought tolerant such as in desert snails (TISCHLER 1990); a very rapid ontogenesis such as in desert orthoptera (CLOUDSLEY-THOMPSON 1991) or a resistance to waterfilms such as in most larger or more hydrophobic animal species (PRINZING & WIRTZ in press).

All this brings about questions on (1) the animals’ benefits of such a frequent colonisation of trunks, (2) the animals’ ability to use the extremely exposed crypto-gams on the bark surface and (3) the animals’ effects on these cryptogams. Therefore I investigated: the climatic living conditions on exposed trunks, the animals’ use of climates, the animals’ microhabitat use and the effect of grazing on the morphogenesis of a lichen species that serves as a specially preferred microhabitat.

I) The analysis of microclimatic living conditions on trunks should help us to understand the supply of thermal energy and of humidity at the trunk surface in its mechanisms, size, variability and predictability - with regard to both, the absolute level of supply as well as the relation to the surrounding macroclimate. The potential relevance for the microarthropods shall then be inferred. Such information would also give an insight into living conditions of epiphytic cryptogams and into the climatic stress a tree suffers at its exposed trunk.

Available climatic analyses mainly provide an insight into the diurnal or seasonal temperature fluctuations (KRENN 1933, HAARLOV & PETERSEN 1952, NICOLAI 1985, 1986, 1989) and into the temperatures at different heights of a trunk and under exposures to N, E, S, W (HAARLOV & PETERSEN 1952, BRAUN 1992, NICOLAI 1985, 1986). Only NICOLAI (1985, 1986) also measured gradients thoroughly on a smaller scale within the relief of the bark. However, no investigations were conducted on the zonation of humidity, the fluxes of water vapour, the significance of climates on different scales, the degree of heat-humidity-antagonism and the predictability of climates. Thus, the present study will describe and explain the interaction of temperatures, humidities and the respective gradients with each other and with external impacts such as weather, time of day, wind speed and exposure to wind, sun or precipitation.

II) The analysis of the use of climatic gradients by corticolous arthropods should help to understand whether the animals are able to take advantage of certain microclimatic patterns (revealed in the first part of this study) by an adaptive distribution. The use of microclimatic benefits would permit a largely autarkic colonisation of the food sources on exposed trunks by arthropos. One microclimatic benefit of tree trunks had already been suggested by NICOLAI (1985, 1986): The upheating of trunks as an energetic benefit for arthropods (solar panel effect) resulting in a faster maturation of animals and in an extension of activity into the cold season (NICOLAI 1985, 1986). The majority of authors, however, considers the trunk as an only temporally suitable habitat for arthropods and mentions very stricted benefits of trunk colonisation, such as: migration route between soil and crown (i. e. the respective habitats of juveniles and of adults; for microarthropods: CHRISTENSEN 1980, ALLMEN & ZETTEL 1983), food sources (algae and lichens) which are only accessible during moist macroclimate (rainy weather, MAYER 1957, BAUER 1979, or night BOWDEN et. al. 1976), refuge from soaking of soils (BOWDEN et. al. 1976, FUNKE 1979, VEGTER 1983), or "jumping-off point", giving soil animals access to aerial dispersal (WASHBURN & WASHBURN 1984, BLACKITH & DISNEY 1988, FARROW & GREENSLADE 1992). The significance of these different benefits of exposed trunk bark shall be verified by comparing the corresponding climates to the arthropods' abundances, distributions and also age class compositions on trunks. Such comparisons have to be corrected for the differences in microhabitat composition during some of these climates.

A further aim is to discuss which species face either most unpredictable or most unfavourable climates (the former species track the redistribution of microclimates closely, the latter are more stationary). A literature review will then test whether such "r-" and "A-strategies" of distribution do indeed depend on the presence of the other traits expected for such strategies according to e. g. GREENSLADE 1983 and SOUTHWOOD 1988.

III) The analysis of the use of discrete microhabitats (cryptogam species, crevice types) by corticolous arthropods should help to understand how the animals are able to locate and combine food supplies and climatic microshelters. An animal needs to find both before it can take advantage of a climatic gradient (as investigated in the latter part).

Such microhabitat use is an especially difficult task for animals on the exposed trunks covered with algae and lichens, because these cryptogams hardly retain water (or accumulate dust and detritus that might store the water) and can thus be climatically more harsh than a bare trunk or rock surface (TRAVE 1963). Moreover, dry cryptogams are mostly stiff and thus less palatable for microarthropods like Collembola (GERSON & SEAWARD 1977, PRINZING & WIRTZ in press), and the cryptogams provide only little food due to their very low productivity per unit bark surface (for corticolous algae: TURNER 1975). Also, those cryptogam species that provide suitable food can be "hidden" from grazers among other species of similar shape and smell but different chemistry and consistency. Eventually, suitable cryptogam species can sometimes be restricted to small, isolated patches of some square millimetres. But elsewhere, they can also continuously cover a square metre of bark despite an apparently identical exposure and substrate. All this is different in most phanerogams, in litter layer detritus and in fungus mycelia - thus in the resources of the grazers usually studied (herbivores or microarthropods in soils). Directly or indirectly, these peculiar living conditions on corticolous cryptogams will also effect the predators of cryptogam grazers.

The aim of this part of the study is to test the importance of microhabitat use for the animals' access to food and microshelter. Microhabitat use can only have a substantial and adaptive effect, (a) if animals accurately colonise only certain types of discrete structures, even when these structures are isolated and rare, (b) if this microhabitat use corresponds to the rapid climatic fluctuations, (c) if the microhabitat use corresponds to the specific physiological needs of the respective species and hardly to non-physiological explanations.

Most earlier evidence for microhabitat use on bark is indirect (but provides an important frame): the arthropod fauna correlated to the distribution of certain groups of cryptogams (TRAVÉ 1963, DUFFEY 1969, NIEDBALA 1969, WOLTEMA-DE 1982, NICOLAI 1985, 1986, BÜCHS 1988, STUBBS 1989, BRAUN 1992, MAN-HART 1994). In particular, the moss-/lichen-zonations (PSCHORN-WALCHER & GUNHOLD 1957, GJELSTRUP 1979), the distribution of algae- and lichen-patches, the basic lichen growth forms or the distribution of certain species of foliose lichens (ANDRÉ 1979, 1983, 1976) appeared relevant to the fauna. In contrast, the overall biomass of cryptogams was not significantly correlated to the fauna accordin g to NICOLAI (1985). All of the mentioned patterns of cryptogam cover, however, simultaneously corresponded to different heights and exposures on the trunks (N, E, S, W, main wind direction; above mentioned authors). The effect of bark crevices on the fauna was only inferred by comparing the frequencies of animal species either between barks of different thickness (ANDRÉ 1976), or between tree species of differently fissured bark (NICOLAI 1985, 1986, 1989, BÜCHS 1988). Only in very few studies had the effect of single discrete microhabitats been differentiated from the effect of the climatic gradients on the trunk: two crustose lichen species differed strongly in their oribatid faunas (ANDRÉ 1975), and even within single thalli of a fruticose (shrub-like) lichen species there was a differentiation of the living conditions and of the animals' distribution and grazing (PRINZING & WIRTZ in press).

IV) Corticolous epiphyte grazers often colonise the shrub-like, and thus wind-exposed, thalli of the lichen Evernia prunastri in high densities (PRINZING & WIRTZ in press). The arthropods´ grazing was especially likely to influence the thalli in their morphogenesis because the animals feed very selectively on distinct structures of the thalli (LAUNDON 1971, PRINZING & WIRTZ in press). Therefore, an analysis of the morphogenesis of Evernia prunastri under grazing and wind exposure was conducted. This morphogenesis could be very significant for the survival of such shrub-like thalli because windfall and desiccation on these extremely exposed substrates probably depend on the growth form1) of the thallus. In fact, in phanerogams, such an adaptive value of growth forms under wind exposure is already well known, e. g. from "flag trees" (HOLROYD 1970).

The morphogenesis of such phanerogams, however, depends on a highly complicated architecture of different tissues. Lichens completely lack such tissues. Correspondingly, the growth forms of most other lichen species on wind-exposed surfaces are only flat. The growth form of Evernia prunastri, however, is three-dimensional and, additionally, extremely variable (Wirth 1980, Beltman 1978) and seems to correspond to the wind exposure of the trunk (ZIMMER 1994). The individual branches of Evernia prunastri grow basically flat and isotomous-dichotomous (BELTMAN 1978), but with frequent irregularities in branching patterns (STONE & McCUNE 1990) as well as sometimes adventive branches on the branch surface ("lobuli", BELTMAN 1978). Due to this simple, but easily disturbed growth pattern it becomes even more likely that external impacts, such as grazing, are significant for the variable morphogenesis of the thalli and its adaptability to e. g. wind.

Therefore, I investigated: (1) the variation of morphogenetic patterns of branch growth, since they permit the polymorphic growth forms of whole Evernia prunastri thalli; (2) the influence of grazing and microclimate on these growth patterns; (3) the (combined) effect of these mechanisms on the development of wind-tolerant growth forms.

The study re-examines qualitative, occasional observations by PRINZING (1992). According to this study, corners along edges of branches and destroyed tips of branches result in a diversification of the branch growth and thus of the overall growth forms of thalli. Growth was found to be faster in some wind-sheltered as compared to some wind-exposed branches. It was speculated that only thalli with branches of highly variable growth can develop an overall shape which reflects the exposure to wind. It was also speculated that grazing might induce this variability of the thallus growth because of the observed similarity between the feeding-traces of grazers and the branches' corners or destroyed tips, and also because the variability of growth was generally higher on strongly grazed thalli - namely such without alternative food (epibiotic algae) or with especially low lichen acid concentrations.

The present investigation should re-describe these growth patterns and test the earlier, largely occasional, qualitative observations by: (a) observation of growth patterns of all focused branches in the series of photographs in PRINZING (1992) and, in addition, a new series of photos, and subsequent statistical testing; (b) field-observation and documentation of the regeneration along feeding traces on a scale of millimetres; (c) experimental testing of the effect of grazing on the development of single branches.

Unfortunately, a direct experimental test of the effect of grazing on the adaptability of the whole thallus’ growth form is hardly feasible: application of insecticides would probably not prevent grazing for a sufficiently long and continuous period in order to effect the very slow growth of branches. The effects of earlier morphogenetic disturbances due to grazing would not be suppressed by insecticides anyway. Mechanical exclusion of grazers (e. g. by little 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 & Greenslade 1992) and have a very wide distribution (summarised in Prinzing & Wirtz, in press). Thus, the overall effect of grazing on the shape of the whole thallus can only be inferred from a detailed knowledge of the growth mechanisms behind this effect. The investigation therefore had to include all scales from the single feeding trace towards the complete growth form of the thallus. The respective patterns had to be recognised by eye on photographs of the living thalli in the field (for comparable photographic observations see STONE & McCUNE 1990).

The four studies just introduced required specific new methodologies:

Part I (microclimatology): the sampling positions had to be arranged individually on each trunk according to the momentary exposures to wind, sun and precipitation on two spatial scales. The diversity of climatic variables and situations had to be analysed by multiple regression analysis.

Part II (climate use): the microclimate and the distribution of animals had to be investigated almost simultaneously in the same zones of the same trunks. Changing frequencies of microhabitat types under different climates had to be standardised for. This was done by sampling each microhabitat type separately under each climate and by standardisation for sampling efforts in the statistical analysis. Such a separation between microhabitat types was only possible by optical searching. In the data analysis, the distribution of animals was compared to those climatic gradients and scales that were most relevant for heat and humidity conditions at the trunk according to the prior analysis of the microclimate. The consideration of several scales also permitted to recognise, for example, the thermal effect of a use of warmer trunk faces even in a species that is restricted to winter and thus to cold average temperatures. This procedure was equivalent to the system advocated by ANDREWATHA & BIRCH (1984): the consideration of several "ecological webs" effecting a species' population.

Part III (microhabitat use): the employed stratified optical sampling technique permitted sufficient differentiation between all common microhabitat types even on a mm-scale. Then, relative densities could be calculated for each microhabitat type during different climates. And the statistical tests could be standardised for the different frequencies of certain microhabitat types during different climates. The optical sampling of defined plot sizes of microhabitat surface provided biologically meaningful density values: the surface describes the size of a habitat from the point of view of the considered animals, which feed on crusts on this surface or thin layers below it or prey upon such grazers (STEVENSON & DINDAL 1982, PRINZING & WIRTZ in press). The developed density parameter was therefore largely immune against the general uncertainties in applying density data (WIENS 1989) and fulfilled SCHWERDTFEGER's (1968) very strict criteria for a reasonable density parameter.

Part IV (lichen morphogenesis): different types of growth patterns of the lichen were separated. These types had to be counted under different environments such as different neighbouring or previous growth of branches, different climatic exposures, different general grazing intensities or different simulated grazing.

To my knowledge almost each single of the above mentioned steps is new, at least for the study of microarthropods or lichens.

The studies on microclimates and arthropod colonisation were conducted from August 1993 to April 1994. This was useful because during that time, the climatically most sensitive arthropod taxa are most common (Collembola and Isopods, LEPOINTE 1957, BÜCHS 1988 and BRERETON 1957), and in winter thermal effects are most likely to influence microarthropods (AGRELL 1941, NICOLAI 1985). This period also included a large amplitude of microclimatic conditions. As far as we know, the basic mechanisms of trunk microclimate during the investigated period are not substantually different from the remaining months: under equivalent weather conditions and sun exposures the diurnal transpiration patterns and shifts in temperature zonations are the same in summer and in winter (GEURTEN 1950 and KRENN 1933, HAARLOV & PETERSEN 1952, NICOLAI 1985). Multiple regression analysis of data permitted considering only such truly comparable conditions.

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