Abundant and rare spiders on tree trunks in German forests ( Arachnida , Araneae )

The spider fauna active on the bark of trees in forests on eight sites in different regions in Germany was investigated. Trunk eclectors at about 2-4 meters height on living trees were used in different regions of Germany (SW Bavaria, Hesse, Brandenburg) between 1990 and 2003. In Hesse eclectors were also used on dead beech trees (standing and lying). In this study data, mainly from beech (Fagus sylvatica) and spruce (Picea abies), from May to October are compared – whole year samples (including winter) are only available from Hesse. A total of 334 spider species were recorded with these bark traps, i.e. about one third of the spider species known from Germany. On average, each of the eight regions yielded 140.5 (± 26.2) species, each single tree 40.5 (± 12.2) species and 502 (± 452) adult spiders per season (i.e. May to Oct.). The 20 most abundant species are listed and characterised in detail. Six of the 20 species were not known to be abundant on bark, three prefer conifers and three beech/broadleaf. Even in winter (December-March) there was a remarkably high activity on the trunks. However, only a few species occur exclusively or mainly in winter. Finally, the rarity of some bark spider species is discussed and details (all known records in Germany, phenology) of four of them are presented (Clubiona leucaspis, Gongylidiellum edentatum, Kratochviliella bicapitata, Oreonetides quadridentatus). The diversity and importance of the spider fauna on bark in Central Europe is still underestimated.

The spider fauna of tree trunks in Germany and Central Europe is much less well known than the epigeal fauna active on the forest floor.Only the fauna of the tree crowns is more poorly known (SIMON 1995, GOSSNER 2004, RÖDER et al. 2010).Furthermore, the epigeal spider fauna of common forest types -at least in Central Europe -is significantly less intensively investigated than that of the epigeal fauna of special habitats, such as those that are extremely dry or wet (HÄNGGI et al. 1995).I estimate that this disproportion is much higher, if one compares the epigeal spider fauna in forests with the tree trunk spider fauna.In Central European forests knowledge about the tree trunk fauna reaches an estimated maximum level of 5% compared with the epigeal fauna.
Before the research in Strict Forest Reserves in Hesse started, estimates of the species richness of animals (biodiversity) in a beech forest were 1500-2000 species (all animals) (ELLENBERG et al. 1986).Now we know that there are closer to 5000-6000 (DOROW et al. 2004(DOROW et al. , 2010)).In each of the four Strict Forest Reserves investigated until now 162 to 202 spider species were recorded (BLICK 2009).
As the complete spider coenoses and species lists were already published (see below) or will be published elsewhere, the focus here is on the following topics: (a) totals of species and abundances, (b) the 20 most abundant spider species, (c) winter activity, and (d) notes on rarely or very rarely recorded species.

Methods
Trunk eclectors (Fig. 1, method after BEHRE 1989, see also BRAUN 1992, DOROW et al. 1992, ENGEL 1999) at heights from 2 to 4 meters on living trees were used in different regions of Germany (SW Bavaria, Hesse, Brandenburg) (Fig. 2).In Strict Forest Reserves in Hesse eclectors were also used on dead beech trees (on standing trees and with an adapted type of trap, also on lying trees) (DOROW et al. 1992).
The traps were operated during different time periods in the different regions and projects.The trapping periods from May to October were available for comparison from nearly all sites (except Brandenburg).Hesse (Strict Forest Reserves) was the only area where data were available for the whole year, i.e. two entire years including two winters.
Most data sets are from beech -Fagus sylvatica and spruce -Picea abies; see details below.• In four Strict Forest Reserves and their reference areas, where forestry is continued, in Hesse (nos.2-5 in Fig. 2) trunk eclectors were installed on two living and two dead standing beech trees during two whole years (MALTEN 1999, 2001, MALTEN & BLICK 2007, BLICK 2009).Additionally at least two eclectors on lying dead beech trees were installed in each reserve.In total 30 standing and 13 lying dead trees were investigated.

Totals
In total 334 spider species, i.e. one third of the spider fauna of Germany, were recorded with 98328 adult spiders.In the 8 different regions between 106 and 177 spider species were recorded, with an average of 140.5 species (± 26.2 standard deviation) (Tab.1).

Twenty most abundant spider species
In Tab. 2 the 20 most abundant spider species are sorted in descending abundance.Regional distribution (focus or exclusivity -if nothing is noted the species was recorded in every one of the 8 regions), the tendency to occur on broadleaves/conifers and other notes are added.
To summarise: • 8 of the 20 species (40%) were linyphiids, 10 families were present • only 2 species belong to the same genus (Xysticus) • 6 species are not known yet as abundant bark species (compare e.g.WUNDERLICH 1982) • 14 species are abundant in (nearly) all regions investigated, 6 species have a more regional distribution • 3 species prefer conifers, 3 species prefer beeches/ broadleaves [3 other species were only or mainly trapped in regions, where all/most of the traps were on beech/broadleaf ] • these 20 most abundant species make up 65281 adults, i.e. 66.4 % of the adult spiders from the trunks

Most abundant spider species active in winter
A similar analysis was made for the winter active species (trapping months December to March, including long winter trapping periods in the regions which ended at the end of April or even at the beginning of May in some years).This means that mainly data of the regions 2-5 (see Fig. 2) and only some additional data of region 6 could be analysed.All these data came from the federal state Hesse.A total of 140 species, 7356 adults and 15714 juveniles (9833 determinable to species level) were recorded during the winter periods.Even in winter, numerous spiders were found in the trunk eclectors.Only two representatives of the commonly known winter-active spiders (e.g. the linyphiid genera Centromerus, Macrargus and Walckenaeria; the dictynid Cicurina) are on this list (Tab.3).The majority of the species are also included in Tab. 2. Compared to the forest floor, fewer species on bark were exclusively active in winter.Only Cicurina cicur and Thyreosthenius parasiticus can strictly be placed in the latter category, but at some sites also Monocephalus castaneipes.The 9 species with more than 100 adult individuals comprise 5930 (81%) of the adult spiders from the trunks in the winter -a much less balanced pattern than in the summer (see above).

Rare spider species
There are several 'types' of rare spider species: (a) widely distributed but rare, (b) at or near the border of their distribution area, (c) with restricted distribution areas (Central European endemics?).Examples of species for these types are listed as follows: species name (family), adults recorded (ind.), and distribution area.PLATNICK (2009), STAUDT (2009) and MIKHAILOV (1997) were used as main sources for the total distribution of each species.(b) at or near the border of their distribution Cinetata gradata (Simon, 1881) (Linyphiidae), 76 ind., central and southern Europe, mainly in mountainous areas, its northern border of distribution is situated in Germany.Clubiona leucaspis Simon, 1932 (Clubionidae) (Figs. 3a,4a), 583 ind., southern and central Europe, i.e. the northern border is located in Germany, in Germany only in the north-eastern and south-western    Miller, 1951 (Figs. 3b, 4b, 10 ind., central Europe (Germany, Czech Rep., Austria), N Italy, SE France, most numerous record (31 specimens) is from a rotting beech stump in Hesse (BLICK 2009), see also below.Kratochviliella bicapitata Miller, 1938, 878 ind., central Europe (Germany, Austria, Czech Rep., Slovakia, Poland), Bulgaria; besides the specimens presented here there is just one other record of a larger number of individuals in Poland (CZAJKA & BEDNARZ 1972: "only on the northern surface of the tree trunks.... We estimated that the whole population consisted of some 6000-7000 specimens at that time"), see also below.Oreonetides quadridentatus (Wunderlich, 1972)  record presented here probably refers to the greatest number of individuals found until now.Pseudocarorita thaleri (Saaristo, 1971), 64 ind., central Europe (Germany, Czech Rep., Austria, Switzerland, Belgium).
More details on four of the rare species mentioned above are presented in Figs. 3 and 4, because very little information has previously been published on them.The phenologies of these four species are presented in Fig. 4. The trapping periods are assigned to the month to which the majority of the days of each trapping period belonged; this may not be the month in which the trap was changed or emptied.All these data fit in the ranges given above for single trees.The eclector type without a bottle traps fewer specimens (and presumably less species) than the type with a bottle.Nevertheless, data from the bottle-less type fit within the range.A possible reason: the range of species and specimen numbers on older trees (with a larger diameter) compared to younger trees is larger than the range resulting from trap type.The data from different regions in this paper already show that it is difficult to compare the published data.The reasons are that different trapping periods, different tree ages and diameters and finally different types of eclectors were used.Nevertheless, important conclusions can be drawn (see also BLICK 2010): • spiders are a species rich group on forests floors, as well as on trees; • to estimate their biodiversity in forests data from both the forest floor and from trees are necessary; • the majority of the dominant species do not prefer conifers or broad-leafed tress; instead they are widely distributed; • in Germany forest spider species that are restricted in their distribution to Central Europe (s.l.) occur; • the diversity and importance of the spider fauna on bark in Central Europe was until now underestimated compared with the epigeal fauna (even though there is an overlap of both coenoses).
doi: 10.5431/aramit4002SitesThe data included in this study came from different sites in Germany (Fig.2): • Three pairs of pine forests, also part mixed with beech and oak, in Brandenburg (no. 1 in Fig. 2, two forests in the north, one in the south) (unpubl.data from 2000 and 2001, June to September, leg.T. Taeger & U. Schulz, det.TB).In each of the six forests six trunk eclectors were installed on Scots pine -Pinus sylvestris.A total of 36 trees were investigated.

Figure 1 :
Figure 1: Trunk eclectors on standing beech trees in northern Hesse, Germany, a -spring, b -winter, c -dead tree

Table 1 :
Totals of the spiders in the trunk eclectors from the 8 sites Between 21 and 88 spider species were recorded per tree and season (i.e.May-Oct.-based on 115 living trees/season and a total of 298 species), with an average (Ø) of 40.5 species (± 12.2).Altogether 122 to 2696 adult spiders per tree and season were trapped, Ø 502 (± 452).

Table 2 :
The twenty most abundant bark spider species (see text)