Background Functional diversity is vital for forest ecosystem resilience in times of climate-induced forest diebacks. Admixing drought resistant non-native Douglas fir, as a partial replacement of climate-sensitive Norway spruce, to native beech forests in Europe appears promising for forest management, but possible consequences for associated biota and ecosystem functioning are poorly understood. To better link forest management and functional diversity of associated biota, we investigated the trophic niches (∆13C, ∆15N) of epigeic generalist predators (spiders and ground beetles) in mixed and pure stands of European beech, Norway spruce and non-native Douglas fir in north-west Germany. We assessed the multidimensional niche structure of arthropod predator communities using community-based isotopic metrics. Results Whilst arthropod ∆13C differed most between beech (high ∆13C) and coniferous stands (low ∆13C), ∆15N was lowest in non-native Douglas fir. Tree mixtures mitigated these effects. Further, conifers increased isotopic ranges and isotopic richness, which is linked to higher canopy openness and herb complexity. Isotopic divergence of ground beetles decreased with Douglas fir presence, and isotopic evenness of spiders in Douglas fir stands was lower in loamy sites with higher precipitation than in sandy, drier sites. Conclusions We conclude that tree species and particularly non-native trees alter the trophic niche structure of generalist arthropod predators. Resource use and feeding niche breadth in non-native Douglas fir and native spruce differed significantly from native beech, with more decomposer-fueled and narrower feeding niches in beech stands (∆13C, isotopic ranges and richness). Arthropod predators in non-native Douglas fir, however, had shorter (∆15N) and simplified (isotopic divergence) food chains compared to native forest stands; especially under beneficial abiotic conditions (isotopic evenness). These findings indicate potential adverse effects of Douglas fir on functional diversity of generalist arthropod predators. As tree mixtures mitigated differences between beech and conifers, mixed stands including (non-native) conifers constitute a promising compromise between economic and conservational interests.

In many tropical ecosystems, termites are important ecosystem engineers. Mound-building termite species often contribute to topographical landscape heterogeneity, and the associated accumulation of soil moisture and nutrients affects the spatial distribution of plant communities. Plants that grow next to the termite mound are known to benefit from the provided nutrients and water storage. The other way around, protection against erosion and cooling effects on the microclimate of the mounds imposed by associated trees have been described. However, little is known regarding the influence of tree parameters and the long-term effects on colony development. In an interval of 12 yr, we studied all Macrotermes michaelseni mounds occurring within one square kilometer of Namibian thornbush savanna in terms of morphology, life history, and associated plants, with a focus on the role of associated trees which grow directly adjacent to the termite mounds. Here, we present the results of a combined analysis of the character states recorded at the termite mound and the associated trees. More than 50% of all 345 recorded termite mounds were associated with a tree. Mounds with associated trees showed increased activity, growth, and height. Mound growth within the study period was correlated positively with the canopy coverage by the associated tree, while total mound heights were correlated non-linear with medium canopy coverage of about 40%. Mounds with a tree covering the sun's average zenith (north) were taller, grew most within 12 yr, and were more likely to be active than those with no northern canopy coverage. Mounds associated with the evergreen Boscia albitrunca attained highest average height (195.06 AE 11.76 cm). We conclude that vital parameters like growth, size, and activity of the termite colonies are facilitated by shading associated trees. These benefits were shown to take significant effect within the span of 12 yr. This indicates a proper longterm mutualism, not a loose alliance and not an obligative eu-symbiosis. Therefore, we propose to use the term "partner tree" for trees associated with M. michaelseni mounds.

Termite mounds contribute to the heterogeneity and productivity of many semi-arid ecosystems worldwide. Regular spatial patterns of termite mounds are well documented but the underlying pattern-building mechanisms remain to be clarified.This study analysed a comprehensive data set of Macrotermes michaelseni mound metrics, spatial patterns and dynamics, recorded 12 years apart within 1 km 2 of Namibian thornbush savanna. We used both unmarked and marked point pattern analyses to assess spatial distributions of termite mounds.We recorded 485 termite mounds in 2007 and 2019. Out of the investigated 142 active termite mounds in 2007, 81 were active 12 years later. Only 2% (n = 3/129) of remnant mound sites in 2007 were newly colonised in 2019. All mounds were distributed at regular distances. Mounds higher than 2 m and long-term active mounds (active in 2007 and 2019) showed high regularity. Mounds up to 0.7 m height and new mounds occurred in clusters and showed a spatial repulsion to large and long-term active colonies. The spatial distribution of Boscia albitrunca trees generally clustered around termite mounds, but did not cluster around young colonies.The results contradict the hypothesis of preferred colony settlement on previously inhabited mound sites. Remnant mounds are relatively unlikely to be recolonised when recently abandoned. The observed spatial patterns show that intraspecific competition between large colonies is the decisive factor for regular spatial patterns of termite colonies, whereas small and young colonies are limited to unoccupied patches. Surrounding trees are a pattern-building factor of only secondary importance.The spatio-temporal scale in our study reveals general trends of termite mound life histories and spatial pattern-building mechanisms. Availability of not previously inhabited sites form the patterns of founding new colonies. Well-established mounds are highly regulated by intraspecific competition. This highlights that termites contribute to ecosystem heterogeneity through self-organised spatial regularity.

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