Ptatscheck et al. BMC Ecol (2018) 18:43 https://doi.org/10.1186/s12898-018-0198-4 BMC Ecology RESEARCH ARTICLE Open Access Is stemfow a vector for the transport of small metazoans from tree surfaces down to soil? Christoph Ptatscheck* , Patrick Connor Milne and Walter Traunspurger Abstract Background: Stemfow is an essential hydrologic process shaping the soil of forests by providing a concentrated input of rainwater and solutions. However, the transport of metazoans by stemfow has yet to be investigated. This 8-week study documented the organisms (< 2 mm) present in the stemfow of diferent tree species. Because the texture of the tree bark is a crucial determination of stemfow, trees with smooth bark (Carpinus betulus and Fagus sylvatica) and rough bark (Quercus robur) were examined. Results: Up to 1170 individuals per liter of stemfow were collected. For rotifers and nematodes, a highly positive cor- relation between abundance and stemfow yield was determined. Both taxa were predominant (rotifers: up to 70%, nematodes: up to 13.5%) in the stemfow of smooth-barked trees whereas in that of the oak trees collembolans were the most abundant organisms (77.3%). The mean number of organisms collected per liter of stemfow from the two species of smooth-barked trees was very similar. A higher number of nematode species was found in the stemfow of these trees than in the stemfow of rough-barked oak and all were typical colonizers of soil- and bark-associated habitats. Conclusion: This pilot study showed for the frst time that stemfow is a transport vector for numerous small metazo- ans. By connecting tree habitats (e.g., bark, moss, lichens or water-flled tree holes) with soil, stemfow may infuence the composition of soil fauna by mediating intensive organismal dispersal. Keywords: Forest ecosystems, Forest soil, Canopy, Nematodes, Rotifers, Collembolans Background dormancy than during the growing season. Staelens et al. Stemfow is the proportion of rain water that is held [3] reported that the minimal amount of precipitation back by leaves or branches and drained by the stem. that is required for the generation of stemfow is lower By connecting the vegetative canopy with the soil, this during the growing season than during the dormant hydrologic process has an important impact on the bio- season. For example, in Fagus sylvatica, 6–16% of the geochemical cycles of forest ecosystems [1]. According to incoming precipitation is funneled during winter and Levia and Frost [2], stemfow accounts for approximately early spring, while only 1–2% runs down the tree dur- 3.5%, 11.3%, and 19.0% of the precipitation in tropical, ing the rest of the year [4]. Tis diference is explained by temperate, and (semi)arid ecoregions, respectively. Te the presence during the growing season of leaves, which quantity and quality of stemfow refect numerous fac- shield the rain and thereby reduce the stemfow yield, and tors. Tus, in temperate forests, seasonality has a super- by the higher amount of precipitation during the dor- ordinate impact, with more stemfow generated during mant season. In general, the stemfow yield increases with increas- ing precipitation although during intensive rain events *Correspondence: christoph.ptatscheck@uni‑bielefeld.de it decreases because water reaches the ground as Animal Ecology, Bielefeld University, Konsequenz 45, 33615 Bielefeld, throughfall [1]. Conversely, when precipitation is low, Germany © The Author(s) 2018. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat​iveco​mmons​.org/licen​ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creat​iveco​mmons​.org/ publi​cdoma​in/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Ptatscheck et al. BMC Ecol (2018) 18:43 Page 2 of 11 there may be no stemfow at all because it occurs only Such dispersal could afect the genefow, population when the storage capacity of the tree is exceeded. Te stability, population dynamic, and thus, the organismal texture of the tree bark is a crucial determinant of diversity in soil. stemfow. Rough bark has a higher water storage capac- Terefore, in this study we examined the stemfow of ity than smooth bark such that a larger amount of pre- three species of middle European trees (Quercus robur, cipitation is necessary before the bark is saturated and Fagus sylvatica, and Carpinus betulus). Our main goals stemfow is generated [5]. For example, in one study, were: (1) to document the abundances of metazoan taxa stemfow along the trunk of rough-barked Quercus transported by stemfow, focusing on nematode diver- robur (rough bark) was generated following 5.4 mm sity, and (2) to document stemfow-mediated transport of precipitation whereas in smooth-barked Fagus syl- by diferent middle European broad-leaved tree species. vatica only 2.8 mm was required [6]. Additional factors As stemfow may be a possible vector for the pas- infuencing the stemfow yield are wind, snowfall, and sive dispersal of organisms we hypothesized that it the morphometry of the tree (e.g., stem diameter and contains a diverse composition of typical colonizers of number of branches) [2]. bark, moss, lichens, and water-flled tree holes, espe- For soil, stemfow provides a concentrated input of cially, nematodes and rotifers (mainly bdelloidea) but rainwater containing solutes and microorganisms. Hence, also of other metazoans (< 2 mm) (hypothesis H1). We around the trunk both the amount of moisture and the also expected that trees difering in the texture of their concentrations of, e.g., Na+, K+, Ca++, and NH4+ will be bark would difer in the abundances and composition higher [1]. In addition, fungi and bacteria transported by of the associated organisms transported by stemfow. stemfow contribute to shaping the soil microfaunal com- Specifcally, trees with rough bark should generate less munity [1]. stemfow such that fewer organisms are washed from Over the last 30 years, there have been > 900 studies of the stem surface (hypothesis H2.1). Nonetheless, by stemfow, according to the Web of Science (September ofering a more structured habitat for organisms, more 2018; search term: stemfow). However, to our knowl- nematode taxa would be contained in the stemfow of edge, none of them examined the stemfow-mediated rough-barked (Q. robur) than of smooth-barked (F. syl- transport of multicellular organisms (metazoans) and the vatica and C. betulus) trees (hypothesis H2.2). Based ecological value of this process. Te stem surface is colo- on their predominance in water flled tree holes we nized by numerous metazoans, with bark, moss, lichens, expected especially bacterial and hyphal feeding nema- and water-flled tree holes serving as habitats for rotifers, todes in stemfow (hypothesis H2.3). nematodes, tardigrades, mites, and collembolans [7–10]. Tardigrades, mites, and collembolans can actively dis- perse along the stem [11–13]. For the reginal transport Methods of small organisms (< 2 mm), the wind is a crucial vec- Study setup tor [14]. Especially nematodes can represent > 44% of the Te investigation was conducted in the vicinity of aeroplancton and show dispersal rates of > 3000 individu- Bielefeld University for 8 weeks beginning in April als m−2 in 4 weeks [15]. Rough barked trees are impor- 2017. Te study site is part of the Teutoburg Forest, tant traps for such wind transported organisms [16]. Germany (52°02′N, 8°29′E), a lime-beech forest with a According to this, trees with a pronounced bark texture mean annual precipitation of 832 mm. Stemfow was and covered to varying extents by moss and lichens host collected from trees of three diferent species (Fagus a greater number and diversity of arthropods than trees sylvatica, Carpinus betulus, and Quercus robur). For with a smooth bark [8]. Ptatscheck and Traunspurger [17] each species, three trees located next to each other previously demonstrated that trees can be hotspots for (< 8 m) and whose crowns were in contact were chosen. the occurrence of rotifers (mainly bdelloidea) and nema- Te basal areas of the stems were 0.07–0.09 m2 for F. todes, whose densities may reach 500 individual’s cm−2. sylvatica, 0.09–0.13 m2 for C. betulus, and 0.26–0.46 m2 Te authors identifed several species occupying water- for Q. robur. Te trees were leafess at the beginning of flled tree holes at abundances more than twice as high as the investigation. those in forest soils [18–20]. Nematodes collected from Stemfow collectors were made of hoses (4 cm diam- the tree holes are typical colonizers of soil or connected eter) from which a section was cut lengthwise. Tese gut- ecosystems (e.g., moss, bark, and dead wood) and were ters were wound 1.5 times around the stem, beginning dominated by bacterial and hyphal feeders [10, 21]. at a height of ~ 1.3 m, and fxed with aquarium silicone. Tese observations suggested that, by linking stem and At the lower end of the hose, a smaller, intact hose was soil stemfow may be a vector for the passive dispersal of installed that drained the stemfow into a covered 10-L organisms between these two habitats. plastic bucket. Ptatscheck et al. BMC Ecol (2018) 18:43 Page 3 of 11 Data collection Results Depending on the duration of rain events, the buckets Stemfow funneling were sampled at least once a day, with the volume of During the study, the weekly rain volume was between the contained stemfow determined using a measur- 4 and 20 mm (Fig.