Abstract
Forest management could potentially affect organisms in all forest habitats. However, aquatic communities in water-filled tree-holes may be especially sensitive because of small population sizes, the risk of drought and potential dispersal limitation. We set up artificial tree holes in forest stands subject to different management intensities in two regions in Germany and assessed the influence of local environmental properties (tree-hole opening type, tree diameter, water volume and water temperature) as well as regional drivers (forest management intensity, tree-hole density) on tree-hole insect communities (not considering other organisms such as nematodes or rotifers), detritus content, oxygen and nutrient concentrations. In addition, we compared data from artificial tree holes with data from natural tree holes in the same area to evaluate the methodological approach of using tree-hole analogues. We found that forest management had strong effects on communities in artificial tree holes in both regions and across the season. Abundance and species richness declined, community composition shifted and detritus content declined with increasing forest management intensity. Environmental variables, such as tree-hole density and tree diameter partly explained these changes. However, dispersal limitation, indicated by effects of tree-hole density, generally showed rather weak impacts on communities. Artificial tree holes had higher water temperatures (on average 2°C higher) and oxygen concentrations (on average 25% higher) than natural tree holes. The abundance of organisms was higher but species richness was lower in artificial tree holes. Community composition differed between artificial and natural tree holes. Negative management effects were detectable in both tree-hole systems, despite their abiotic and biotic differences. Our results indicate that forest management has substantial and pervasive effects on tree-hole communities and may alter their structure and functioning. We furthermore conclude that artificial tree-hole analogues represent a useful experimental alternative to test effects of changes in forest management on natural communities.
Highlights
Human management affects forests in many ways
The abundance of organisms in artificial tree holes was negatively affected by forest management intensity in both regions in June (Fig 2A and 2B, Table 1 and S2 Table)
When we changed the order of the variables in the model and tested the effect of forest management intensity after all other main effects, we found that in the Alb the significant negative effect of forest management on abundance remained significant (F1,25 = 5.01, P = 0.0343) but that it was weaker and partly explained by a significant positive effect of tree-hole density (F1,25 = 8.59, P = 0.0071)
Summary
Human management affects forests in many ways. Direct impacts include changes in forest structure such as increased densities of trees with small crown sizes, more homogeneous tree ages and lower tree diversity in intensively managed forests compared with less intensively managed ones [1, 2]. Arthropods that require aquatic habitats for part of their life cycle colonize these tree holes and typically spend their larval stages there [7, 8]. These are predominantly insect species of Diptera such as Ceratopogonidae, Chironomidae, Culicidae, Cecidomyiidae, Muscidae, Psychodidae and Syrphidae and Coleoptera such as Scirtidae [9]. Tree-hole communities are a suitable system to investigate metacommunity dynamics and, more generally, processes that drive community structure [13, 14], especially because they are small and limited in the species richness and population sizes of the inhabiting organisms [15]. In addition to their use as a model system for testing fundamental ecological questions, they may represent important indicators of subtle forest management effects as they may be more sensitive to changes than larger arthropod communities
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