Presence of microbiome decreases fitness and modifies phenotype in the aquatic plant Lemna minor.

Plants live in close association with microbial organisms that inhabit the environment in which they grow. Much recent work has aimed to characterize these plant-microbiome interactions, identifying those associations that increase growth. Although most work has focused on terrestrial plants, Lemna minor, a floating aquatic angiosperm, is increasingly used as a model in host-microbe interactions and many bacterial associations have been shown to play an important role in supporting plant fitness. However, the ubiquity and stability of these interactions as well as their dependence on specific abiotic environmental conditions remain unclear. Here, we assess the impact of a full L. minor microbiome on plant fitness and phenotype by assaying plants from eight natural sites, with and without their microbiomes, over a range of abiotic environmental conditions. We find that the microbiome systematically suppressed plant fitness, although the magnitude of this effect varied among plant genotypes and depended on the abiotic environment. Presence of the microbiome also resulted in phenotypic changes, with plants forming smaller colonies and producing smaller fronds and shorter roots. Differences in phenotype among plant genotypes were reduced when the microbiome was removed, as were genotype by environment interactions, suggesting that the microbiome plays a role in mediating the plant phenotypic response to the environment.

Eco-evolutionary contributions to community trait change in floating aquatic plants.

An entire community of organisms may become modified when its environment changes. These modifications can happen through physiological processes (plasticity), evolutionary processes (adaptation) or shifts in species composition (sorting). The outcome of these three sources of change constitutes the community's phenotypic response, but how they combine to drive community trait dynamics is not currently well understood. We have conducted a community selection experiment in which communities of short-lived floating aquatic plants were grown in a range of stressful conditions, and measured changes in their body size. Determinants of phenotypic change were assessed with a full community reciprocal transplant which led to estimates of the contributions of plasticity, adaptation, and sorting. Species were modified during the experiment by both plasticity and adaptation, but in either case the magnitude and direction of change differed among species. Sorting and adaptation were of equal magnitude but tended to act in opposite directions: in conditions where species with large fronds prevailed, each species evolved smaller fronds, and vice versa. We conclude that community trait dynamics cannot be understood simply by extrapolating the adaptive response of any single species to the whole community.

A basic community dynamics experiment: Disentangling deterministic and stochastic processes in structuring ecological communities.

Community dynamics are governed by two opposed processes: species sorting, which produces deterministic dynamics leading to an equilibrium state, and ecological drift, which produces stochastic dynamics. Despite a great deal of theoretical and empirical work aiming to demonstrate the predominance of one or the other of these processes, the importance of drift in structuring communities and maintaining species diversity remains contested. Here, we present the results of a basic community dynamics experiment using floating aquatic plants, designed to measure the relative contributions of species sorting and ecological drift to community change over about a dozen generations. We found that species sorting became overwhelmingly dominant as the experiment progressed, and directed communities toward a stable equilibrium state maintained by negative frequency-dependent selection. The dynamics of any particular species depended on how far its initial frequency was from its equilibrium frequency, however, and consequently the balance of sorting and drift varied among species.

A traits-based test of the home-field advantage in mixed-species tree litter decomposition.

BACKGROUND AND AIMS: Litter often decomposes faster in its environment of origin (at 'home') than in a foreign environment ('away'), which has become known as the home-field advantage (HFA). However, many studies have highlighted the conditional nature of the HFA, suggesting that current understanding of this phenomenon is not yet sufficient to generalize across systems. METHODS: The HFA hypothesis was tested for mono-specific and mixed-species litter using a tree-based experiment that manipulated the functional identity and diversity of the host tree community. Litter types of varying quality were transplanted between several host tree communities and decomposition rates were measured using litterbags. Since the decomposer community should respond to traits of the litter input and not their taxonomic identity, a traits-based index of litter-tree similarity was developed. KEY RESULTS: Mono-specific litter exhibited HFA, but when the same litter was decomposed in mixture, this trend was not observed. Mixed-species litter decomposed on average no faster or slower than monoculture litter and exhibited both positive and negative species interactions. These non-additive interactions of decomposition rates in mixture were influenced by the degree of similarity between litter and tree traits. Both synergistic and antagonistic interactions decreased in magnitude with increasing litter-tree similarity such that mixture rates were predictable from monocultures. CONCLUSIONS: The HFA occurred more strongly for mono-specific litter than for the litter types mixed together because interactions between species may have masked this effect. However, when expressed as a function of trait similarity between litters and tree communities, the HFA was not detected.