Surfaces of exposed tree trunks have extreme climatic conditions for most small arthropods. Nevertheless, the food sources on exposed bark are largely used by arthropod species (most of which belong to Collembola, Oribatei, Psocoptera, linyphiid spiders and Isopoda) which equally dwell in the climatically favourable litter layer or in sheltered moss cushions. The prerequisites of this arthropod colonisation are largely unknown, just like the benefits achived by the animals and the animals' distributional strategies or their effect on the epiphytes. Therefore, I investigated: (i) the microclimatic processes at the bark with respect to their potential effects on arthropods, (ii) the significance of the revealed potential microclimatic benefits for the arthropods as compared to other possible purposes of trunk colonisation for arthropods, (iii) the animals' ability to use exactly the climatic microshelters and food sources they need within the microhabitat mosaic, and (iv) the effects of arthropod grazing on the morphogenesis of a climatically strongly exposed lichen.
I) Analysis of microclimatic living conditions on exposed tree trunks
Which variables and processes determine the animals' heat and humidity supply at the trunk surface? Which climatic disadvantages and advantages for arthropods can be expected to result from this compared to the alternative habitats?
I measured the climate of the atmospheric boundary layer on tree trunks from August to April 1993/1994 in northern Germany on several deciduous tree species at a height of 80 to 180 cm above the ground. The surrounding climates on four separate spatio-temporal scales (from the bark microrelief up to the macroclimate at 2 m distance from the trunk). The climatic parameters were: momentary exposures to wind, sun and precipitation, temperature, saturation deficit, water vapour pressure, wind speed, daytime and weather condition. The analysis by multiple regression and subsequent path analysis covered several hundred complete climatological data-sets. Thus, there was no restriction to a few, selected day-cycles. Like this, the effect of a factor could even be recognised, when it was indirect or simultaneous to that of another factor
The microclimate was primarily determined by the conditions at the trunk, especially by the accumulation of heat and humidity on certain trunk faces.
The strong upheating of trunks could be explained by the low evapotranspirative cooling and by the thick atmospheric boundary layer covering rough barked trees.
Water vapour fluxes were primarily due to evaporation from or sorption at the trunk surface. This strongly resembles surfaces of soil/litter but not of living leaves, and might contribute to the fauna similarity between the trunk and soil/litter. Thermally, the trunk was more similar to the living leaf.
More than at the surface of soil litter or of branches in tree crowns, the arthropods on exposed tree trunks faced drought, climatic fluctuations and seasonal harshness. Moreover, the climatically important trunk face zonations were variable, differentiated on large scales (metres), but superimposed by a differentiation on a cm-scale. This made it probably difficult for the animals to assess their position in relation to the tunk face zonation. On the other hand, there were also regular climatic zonations and water vapour fluxes. They would have always permitted the animals to improve their heat and humidity gain by redistribution. The animals even seemed to have the opportunity to improve heat and humidity gains simultaneously - the one by a redistribution on a larger scale, the other by a, climatically equally efficient, small-scale differentiation of this distribution.
II) Use of climatic gradients by corticolous arthropods
Can the arthropods adapt their distribution to the microclimatic patterns on exposed tree trunks and thereby take advantage of them? Or is the trunk only temporally suitable/useful under certain climates?
Arthropods were sought and collected with a 10xlens simultaneously to the above-mentioned climate measurements. The eight predominant microhabitat types were sought separately. This permitted to take into account the changing frequencies of microhabitat types when comparing arthropod densities and climates. Twelve species were found to occur with sufficient frequency for such an analysis. Five climatic scales were investigated: macroclimate, trunk / macroclimate differences, trunk faces, microrelief zones and climatic profiles at trunk surface
Most species' distributions could be explained best by the use of microclimatic advantages of the habitat tree trunk. Such species had to redistribute actively to track the rapidly changing microclimatic patterns. Such animals could use the food sources on the exposed trunks permanently and live largely independently from the soil. Often even the juvenile stages and eggs of these species could be found on these trunks. Mostly, the spatiotemporal pattern of a specific heat- or humidity-gain (as detected in the first part of this study) strongly resembled the distribution of a species on several scales at the same time. For example, the isopod Porcellio scaber equally experienced high temperatures with respect to the used macroclimates, the heating of the trunk over the macroclimate and the colonisation of warmer zones within the microrelief - especially on the cooler trunk faces. Equivalent coincidences were found between the distribution of (a) the spider Entelecara penicillata and high temperatures, (b) the Collembola Entomobrya nivalis and also of the juvenile Carabodes-oribatids and an upheating of the trunk above ambient temperatures, (c) the two Collembola Orchesella cincta and Entomobrya albocincta and high air humidity, (d) Entomobrya nivalis and an influx of water vapour. Porcellio scaber, on the other hand, used water vapour effluxes at the trunk, which P. scaber indeed needs physiologically according to the literature. Many species corresponded additionally to the distributions of further kinds of heat- or humidity gains - but only with respect to a single scale (Porcellio scaber, the Collembola and the predatory bug Temnostethus gracilis). The flexibility between the climatic zonations was reflected by all above mentioned species, a trombidiformous mite and the Psocoptera. This flexibility was utilised especially by species that can easily perceive climatic gradients and effectively use heat metabolically.
Only few species depended clearly on macroclimatic humidity. This was most pronounced in the middle-aged stage of the Collembola. These animals could apparently only use the trunk as a food source, not for the microclimatic benefits.
For some species the trunks could have been also - but not exclusively - a migration route between soil and crown (Porcellio scaber, Temnostethus gracilis and the oribatid Xenillus discrepans). During the period of investigation the trunks were not very significant as a shelter from soil soaking or as jumping-off point for wind dispersal.
The same kind of heat or humidity supply could be exploited by species of very different morphology and physiology (as taken from literature). Species that reacted opportunistically to their respective favourite climate (by redistribution or by accumulation of water reservoirs in the body), however, also resembled to each other in many other ways: They showed largely characters of "r-strategists". This seems to aid the exploitation of microclimatic benefits on exposed tree trunks. The other species were largely "A-selected".
III) Use of discrete microhabitats (cryptogam species, crevice types) by corticolous arthropods
An animal needs to have access to discrete patches of food and climatic microshelter in order to use the climatic gradients at the trunk surface as investigated in the preceding chapters. Both is difficult and rarely combined, since cryptogams on the surface of exposed tree trunks are microclimatically especially harsh habitats, often extremely patchy and change quickly in their palatability. How much can arthropods actively improve their access to food sources and microshelters on the trunk by the spatio-temporal pattern by which such cryptogam species and the bark crevices are used?
Arthropods were optically sampled on each cryptogam species and crevice type separately during the above-mentioned investigations. The sampling unit was determined according to the substrate´s surface area, since this is where the arthropods walk on and graze upon. Densities on a microhabitat were considered relative to the overall densities on all types of microhabitats and were then compared to the climates. Statistical tests were adapted to the different frequencies of certain microhabitat types under different climates. Also the combined effect of several microclimatic variables on microhabitat use was investigated (by discriminant function analysis).
The animals colonised the microhabitats in very different densities - even such lichen species which primarily only differ in their chemistry. The animals changed between microhabitat types - corresponding to the quick climatic fluctuations and the physiological requirements of the respective species. Even small, isolated patches of microhabitats could be utilised. These redistributions did probably hardly correspond to other impacts such as reproduction, competitors or predators. Thus, the microhabitat use is probably a highly efficient behavioural adaptation to ensure access to food and microshelter even in this seemingly unsheltered, unproductive environment.
The shrub-like thalli of Evernia prunastri provided both food and shelter, especially from heat. Crusts of Lepraria incana (for a bug species: crusts of Pertusaria albescens) provided soft palatable food for small Collembola, soft substrate for poking predators (bugs) and for mining oribatids and also a shelter against waterfilms. Algae were rarely colonised, mostly when they were completely sheltered against waterfilms and from convection or radiation. Horizontal bark crevices mostly offered shelter during chill, vertical crevices mostly during heat.
A species' need for shelter in microhabitats depended on its drought tolerance (age-dependent), its seasonal and diurnal distribution on the trunk, its capability to dig mines into the substrate or to migrate into other, sheltered macrohabitats such as crown or litter layer.
The predominant species are eurytopic and only one or few millimetres in size - thus they’re climatically sensitive and little specialised. But this also permits them to exploit even the smallest patches of a microhabitat type and to utilise and combine several types of microhabitats.
IV) Morphogenesis of the heavily grazed and wind-exposed lichen Evernia prunastri
The preceding analyses revealed by far the highest densities of grazers on the lichen Evernia prunastri. How does the grazing influence the morphogenesis of these very wind-exposed shrubby thalli?
Series of photographs showed that the branches were basically even and flat with isotomous-dichotomous ramifications. Numerous deviations from this basic regular pattern were found and traced back to their origins in asymmetries of the branches' cross-section.
The structure and distribution of these important asymmetries in branch cross-sections could only be explained by the effect of grazing. Very small-scale growth observations and experimental simulation of grazing confirmed this.
Branches growing windwardly grew more slowly. This selection resulted in a wind-adapted shape of the whole thallus growth form - provided that the thallus developed many small-scale variations in branch growth due to grazing. Such thalli were well protected from windfall and desiccate slowly.