Currently, in Germany there is a very controversial discussion between forest management and nature conservation concerning the cultivation of the neophytic tree species American Red Oak (Quercus rubra) and Douglas-fir (Pseudotsuga menziesii). No satisfactory determination can be made because of the deficient state of knowledge about the consequences of these tree species on faunistic-ecological relationships. The aim of this present work is to contribute to the establishment of a primary information base for future management policy by the study of the arthropod communities in different vertical strata for these tree species in comparison with the indigenous tree species Pedunculate Oak (Quercus robur) and Norway Spruce (Picea abies) in mature forest stands. In addition, data obtained by less intensive sampling of Beech (Fagus sylvatica) and White Fir (Abies alba) was also considered. The main focus of the study was the comparison of arboreal arthropod communities and how they vary by tree species and different stand compositions: Neophytic and indigenous Oak species in pure and mixed deciduous stands; and neophytic and indigenous conifer species of Douglas-fir, Spruce, and Beech dominated stands. Primary arthropod sampling was by activity traps: In the tree crowns flight-interception and branch traps were utilized, while in stem strata stem-eclectors were used. Special investigations of the crown dead wood, the acorn guild, and the winter fauna on Douglas fir (branch-beating) complemented the principal trapping program. The important indicator groups Coleoptera, Heteroptera, Neuropterida, and Araneae were examined intensively. Parasitoids were analyzed by Hymenoptera families. For a comparison of the diversity and structure of the communities between neophytic and indigenous tree species I took a current ecological statistical evaluation approach. In the tree crowns, Red Oak exhibited a poor arthropod community with regard to activity density, species number, and diversity in comparison to Pedunculate Oak, particularly in Coleoptera and Heteroptera that are rich in ecological guilds. Here, mainly species strongly specializing on the genus Quercus were affected, chiefly herbivores that have a narrow niche in regard to secondary plant compounds. The stronger the degrees of specialization by these species, the more obvious were the deleterious effects of Red Oak. An exemplary illustration from this study is the guild of insects developing in acorns. Thus, the monophagous Acorn Weevil (Coleoptera: Curculionidae: Curculio sp.) were more strongly affected than the polyphagous Acorn Tortricid Moths (Lepidoptera: Tortricidae). Also, the saproxylic guild was clearly reduced on Red Oak as compared with populations on native Pedunculate Oak and this was not explained by a lesser amount of crown dead wood; but rather, may be explained by differences in quality or decomposition degree of the dead wood. Among the predators, the differences were particularly distinct in the case of aphid antagonists (Neuropterida, Coleoptera, Heteroptera). They exhibited much higher densities of activity on Pedunculate Oak as compared to Red Oak. On the other hand, Red Oak appears to provide sufficient suitable resources for the structure-dependent Araneae. A negative influence of Red Oak on the parasitoid Hymenoptera was ascertained. The arthropod communities active in the lower stem area were less influenced by Red Oak. The causes probably lay in the lesser degree of specificity for a certain tree species of many of the arthropods that migrate between strata, but rather require a structural pathway into the crown. All together the species poverty of Red Oak was more clearly distinctive in the pure stand than in the mixed stand. The communities in the pure stand of red oak were determined primarily by generalistic species. Analogous results from Berlin, Tharandt, and Nürnberg speak for the transferability of the general results to other regions. Beech may be defined as a tree species poor in number of species and individuals as compared to Pedunculate Oak, thus Beech is rather comparable to Red Oak concerning these criteria. Nevertheless, due to the typical communities occurring on Beech, it is to be more highly valued in regard to nature conservation than Red Oak in that Red Oak offers habitats only to a partial set of insects indigenous to native Oaks. Because the bark structure of Red Oak is similar to that of Beech, its community is influenced to a limited extent by species also typical for Beech. Douglas fir did not exhibit poorer communities in the tree crowns than Spruce. Above all, in the Douglas fir-dominated stand type Coleoptera, Heteroptera, and Neuropterida were represented in even higher densities of activity and species number on Douglas-fir than on Spruce, which maybe primarily explained by the aphid-predators (aphidophagous species). In 2000, the aphid populations exhibited low abundance which coincided with especially higher activity densities and species numbers of aphid-predators on Douglas-fir. The high densities of the Douglas-fir wooly aphid (Gilletteella sp.) introduced from North America presumably led to concentration effects of aphidophagous species. However, the saproxylic Coleoptera exhibited clearly diminished communities on Douglas-fir when compared with Spruce. This effect appeared in particularly in the saproxylic beetle rich year 2001. Above all, species which directly feed on dead wood were negatively affected by Douglas-firs presumed inedibility. However, mould-eating species contributed a higher percentage on Douglas-fir. In this connection, the different composition of dead wood in tree crowns of Spruce and Douglas-fir seems to be of importance. The different needle structure and branch structure, respectively, of Spruce and Douglas-fir produced a spider community on Douglas-fir that was poorer in species and specimens, especially the net-building spiders. Specialized, phytophagous species exhibited only a slight difference between Spruce and Douglas-fir. In parasitoids there were indications of higher densities of activity on Douglas-fir. The stem fauna of Douglas-fir exhibited much lower densities of activity in comparison to Spruce. This could be connected to a bark structure of Douglas-fir which is more unfavorable for arthropod species changing between strata. With total species numbers the differences were not so distinct. Due to relatively high densities of arthropods, above all spiders, Spruce crowns offer an important food resource for bird species wintering in Northern and Central Europe. In Douglas-fir crowns the density of Araneae and Psyllidae are close to zero, possibly because of a freezing-out effect on the exposed crowns of Douglas-fir. Hence, tree crowns of pure stands of Douglas-fir are avoided almost completely by food-searching birds in winter. The White Fir fits rather well to the picture of both other conifers. Single species seem to show a special affinity to the White Fir. Nevertheless, all together the White Fir is estimated to be poorer in species and specimens as Spruce and Douglas-fir. This is possibly connected with the island-like occurrence of White Fir in the investigation area. Moreover, the low sample size requires further study of White Fir. The results show that the various effects depend on investigated tree species, stratum, arthropod taxa, and investigation year. This advises caution with a generalization of the results, especially when for forest management decisions. An isolated case consideration is always recommended. It is certain that, despite the consensus opinion, the neophytes Red Oak and Douglas-fir are not ecological dead spaces. Nevertheless, they represent, in any case, a changing influence on the structure of the arthropod communities. With American Red Oak the affect is much greater than with Douglas-fir. In view on the guilds relevant to nature conservation an expansion of neophyte cultivation is to be seen critical, in particular with Red oak, above all, for pure stands. Even if, with Douglas-fir the differences are subtle in the structure and density of activity of the arthropods communities at least in the crown area in comparison to Spruce and White Fir, pure stands should also be avoided. However, within the scope of a single tree admixture in a hardwood (Beech) stand, the potential high volume and financial yield of Douglas-fir maybe balanced with an adequate measure of ecological diversity. Besides, the Douglas-fir percentage of the whole spectrum of tree species should amount to only few percent of the region, and in any case should not rise above 10% in any one forest stand. The results obtained for Douglas-fir apply only to the region of middle Swabia. Hence, further national investigations on different locations are necessary to check the transferability of the results. In addition, different age stages should be taken into consideration. The inclusion of all forest strata (especially tree crowns) is necessary in order to develop a full understanding of the varied effects of Douglas-fir on native arthropod communities.
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