Tree species diversity of forested ecosystems control the diversity of leaf litter inputs to the soil, with cascading effects on the microbial communities colonizing decomposing litter. However, the extent to which bacterial and fungal communities inhabiting the litter layer are affected by shifts in tree species diversity is not well understood. To investigate the role of litter species diversity, litter species identity and litter functional traits on bacterial and fungal communities of a typical Mediterranean oak forest, we set up a yearly field litterbag experiment that considered leaf litter mixtures of four abundant species: Quercus pubescens, Acer monspessulanum, Cotinus coggygria and Pinus halepensis. We found that both bacterial and fungal communities varied strongly during decomposition but showed distinct succession patterns. Both communities were also strongly influenced by litter species diversity, litter identity and litter functional traits. The intensity and the direction of these effects varied during decomposition. Litter diversity effects were mediated by litter species composition rather than litter species richness, highlighting the importance of litter species identity-and associated litter traits-as drivers of microbial communities. Both the "mass-ratio hypothesis", measured through the community weighted mean (CWM) litter traits, and the "niche complementarity hypothesis", measured through the functional dissimilarity (FD) of litter traits, contributed to litter diversity effects, with a greater relative importance of FD compared to CWM, and with an overall stronger impact on fungal than on bacterial communities. Interestingly, increasing FD was related to decreasing bacterial diversity, but increasing fungal diversity. Our findings provide clear evidence that any alteration of plant species diversity produces strong cascading effects on microbial communities inhabiting the litter layer in the studied Mediterranean oak forest.
Evolve and Resquence (E&R) studies are a useful tool to study genomic processes during rapid adaptation, e.g., in the framework of adaptive responses to global climate change. We applied different thermal regimes to a natural Chironomus riparius (Diptera) population in an E&R framework to infer its evolutionary potential for rapid thermal adaptation. We exposed two replicates to three temperatures each (14°C, 20°C and 26°C) for more than two years, the experiment thus lasting 22, 44 or 65 generations, respectively. The two higher temperatures presented a priori moderate, respectively strong selection pressures. Life‐cycle fitness tests revealed no appreciable adaptation to thermal regimes but a common adaptation of all six replicates probably due to the rearing conditions, presumably increased larval density and water quality. Genomic analyses showed a strong, genome‐wide selective response in all replicates (mean s of selected SNPs = 0.305). This genomic response was significantly similar at all genomic levels among all replicates (SNPs, 10 kb windows, genes, exons, regions of elevated allele frequency change [REA]). The intersections among the replicates exposed to the same temperature were either insignificant or underrepresented. This confirmed a selective response to identical selection pressure(s), however, not to thermal regime. Genes closest to the SNP with the highest selection coefficient per REA were enriched for GO terms related to ion transport, regulation of transcription and signal transduction, which supported the presumed acting selection pressures. Our study showed the evolutionary potential for rapid adaptation by genome‐wide and probably polygenic selection on standing genetic variation in C. riparius. However, because of the impossibility to accurately predict the acting selective regime in evolutionary experiments, we discuss the sobering perspectives for inferring the evolutionary potential of natural populations with this approach.
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