Abstract
Water-filled tree holes are abundant microhabitats in forests worldwide and are inhabited by specialized communities of invertebrates. Despite their importance, the temporal dynamics of communities within and between years are largely unknown. Here, I present a case study on the phenology of insect larvae in two holes in a beech tree (lower and upper canopy) located in southern Germany over a period of three years. I asked whether water temperature and the characteristics of insect larvae at the community and population levels are similar in periodicity every year and whether they differ in the lower and upper canopy. The water temperature in tree holes differed greatly from air temperature, and this effect was more pronounced in the lower than in the upper canopy, which resulted in a lower probability of drying out occurring in the lower canopy. This was associated with a higher species richness in the lower canopy and greater abundance of drought tolerant species in the upper canopy. There was a significant periodicity in larval abundance, biomass, species richness and body size distribution of abundant species in both tree holes, but it was not seasonal. This result indicates that unpredictable drying out of tree holes are more important drivers of tree hole community dynamics than changes in water temperature. The community of larvae in the tree hole in the upper canopy lagged behind that in the lower canopy, which indicates that most species mainly colonize the more stable microhabitats in the lower canopy. Hopefully this case study will encourage future larger-scale phenological studies to test (1) whether the patterns observed in this study can be generalized over larger spatial scales and (2) the relative importance of abiotic and biotic drivers of the dynamics of communities in tree holes.
Highlights
Ephemeral water bodies associated with living plants occur worldwide and are inhabited by a high diversity of organisms, from bacteria and protists to invertebrates and vertebrates (Kitching, 2000)
Water-filled tree holes occur between buttress roots up to a height of over 30 m in the canopy, where they can be found in stem and branch forks and holes formed when a branch breaks (Gossner et al, 2016)
Twenty four tree holes per hectare were recorded in the area of forest studied, which is an average value when compared with counts recorded in a large-scale study of tree holes in beech forests in two regions of Germany (Fig. S1-2) (Gossner et al, 2016)
Summary
Ephemeral water bodies associated with living plants occur worldwide and are inhabited by a high diversity of organisms, from bacteria and protists to invertebrates and vertebrates (Kitching, 2000). In pans, which are created by stem and branch forks as well as between buttress roots, the hole is lined with bark (Kitching, 1971) Both types of water-filled tree holes occur in a many broadleaved and coniferous species (Rohnert, 1951; Schmidl et al, 2008; Rotheray, 2013; Gossner et al, 2016), but the frequency of such structures varies between different species of trees. Water-filled tree holes occur between buttress roots up to a height of over 30 m in the canopy, where they can be found in stem and branch forks and holes formed when a branch breaks (Gossner et al, 2016) They fill with rain water, either directly or by stem flow. The varying nutritional conditions among tree holes affect the reproduction of individual species of
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