Shaping Plant Community Composition Research Articles

Underground evolution: New roots for the old tree of lumbricid earthworms

Earthworms belonging to the family Lumbricidae are extremely abundant in terrestrial temperate regions. They affect soil properties and nutrient cycling, thus shaping plant community composition and aboveground food webs. Some lumbricids are also model organisms in ecology and toxicology. Despite the intense research efforts dedicated to lumbricids over the last 130years, the evolutionary relationships and taxonomic classification of these organisms are still subject to great debate. Resolution of their systematics is hampered by the structural simplicity of the earthworm body plan and the existence of cryptic species. We sampled 160 earthworm specimens belonging to 84 lumbricid species (28 genera) and 22 Lumbricoidea outgroups, sequenced two nuclear genes, four mitochondrial genes and seven mitochondrial tRNAs and examined 22 morphological characters. We then applied a combination of phylogenetic methods to generate the first robust genus-level phylogeny of the Lumbricidae. Our results show that the current Lumbricidae classification and the underlying hypotheses of character evolution must be revised. Our chronogram suggests that lumbricids emerged in the Lower Cretaceous in the holarctic region and that their diversification has been driven by tectonic processes (e.g. Laurasia split) and geographical isolation. Our chronogram and character reconstruction analysis reveal that spermathecae number does not follow a gradual pattern of reduction and that parthenogenesis arose from sexual relatives multiple times in the group; the same analysis also indicates that both epigeic and anecic earthworms evolved from endogeic ancestors. These findings emphasize the strong and multiple changes to which morphological and ecological characters are subjected, challenging the hypothesis of character stasis in Lumbricidae.

Soil organisms shape the competition between grassland plant species

Decomposers and arbuscular mycorrhizal fungi (AMF) both determine plant nutrition; however, little is known about their interactive effects on plant communities. We set up a greenhouse experiment to study effects of plant competition (one- and two-species treatments), Collembola (Heteromurus nitidus and Protaphorura armata), and AMF (Glomus intraradices) on the performance (above- and belowground productivity and nutrient uptake) of three grassland plant species (Lolium perenne, Trifolium pratense, and Plantago lanceolata) belonging to three dominant plant functional groups (grasses, legumes, and herbs). Generally, L. perenne benefited from being released from intraspecific competition in the presence of T. pratense and P. lanceolata. However, the presence of AMF increased the competitive strength of P. lanceolata and T. pratense against L. perenne and also modified the effects of Collembola on plant productivity. The colonization of roots by AMF was reduced in treatments with two plant species suggesting that plant infection by AMF was modified by interspecific plant interactions. Collembola did not affect total colonization of roots by AMF, but increased the number of mycorrhizal vesicles in P. lanceolata. AMF and Collembola both enhanced the amount of N and P in plant shoot tissue, but impacts of Collembola were less pronounced in the presence of AMF. Overall, the results suggest that, by differentially affecting the nutrient acquisition and performance of plant species, AMF and Collembola interactively modify plant competition and shape the composition of grassland plant communities. The results suggest that mechanisms shaping plant community composition can only be understood when complex belowground interactions are considered.

The role of dispersal in shaping plant community composition of wetlands within an old‐growth forest

Summary 1. Dispersal ability can influence the importance of dispersal relative to other processes organizing metacommunities, such as species sorting among habitats along environmental gradients. 2. We compare plants with different dispersal modes and habitat affinities, evaluating the roles of environmental and spatial controls on plant community composition in 128 wetlands within 10 km2 of old‐growth maple–beech forest in southern Québec, Canada. 3. We address two hypotheses. First, we ask whether species with short‐distance dispersal mechanisms are more dispersal‐limited than species with adaptations for long‐distance dispersal. Second, because wetland habitats are more fragmented than upland habitats in this forested area, we test the hypothesis that wetland species are more dispersal‐limited than upland species growing in the same wetlands (e.g. on hummocks within a swamp). 4. Variation partitioning based on constrained ordinations showed that the distributions of species with short‐distance dispersal mechanisms related more strongly to spatial factors than the distributions of long‐distance dispersers, supporting the interpretation that these species are more dispersal‐limited. The distributions of short‐distance dispersers also showed finer‐scale spatial patterns than the distributions of long‐distance dispersers. 5. Distributions of wetland species related more strongly to environmental conditions than the distributions of upland species growing in the same wetlands, suggesting that wetland species are actually less dispersal‐limited than upland species. Wetland and upland species had similar patterns of spatial variation in community composition. 6. Synthesis. The processes of dispersal limitation and species sorting along environmental gradients have differential importance to plants that grow within the same communities, but differ in dispersal ability and habitat affinity. This result emphasizes the impact of dispersal ability on the organization of metacommunities.

Distribution, species composition and management implications of seed banks in southern New England coastal plain ponds

Buried seeds that germinate during periods of low water or water level drawdown can play important roles in shaping plant community composition, community dynamics and species richness in ecosystems with fluctuating water levels. Northeastern US coastal plain ponds have fluctuating water levels and contain a characteristic shoreline flora that contains many rare plants. The objectives of this study were to: (1) test whether geographically distant ponds in Cape Cod and Martha’s Vineyard had distinct seed banks, (2) determine if hydrologic status as permanent and ephemeral ponds led to differences in seed banks, and (3) examine seed diversity and seed abundance across gradients of shoreline elevations and sediment characteristics. Viable seeds of 45 plant species were identified from nine ponds. Native species dominated pond-shore seed banks and made up 89–100% of all species. There was high overlap in seed bank composition across hydrological classes and geographic regions. One hydrological class captured 73–76% of total species and one geographical region captured 69–78% of the total species recovered from the entire suite of seed bank samples. Seeds were relatively evenly distributed along the shorelines of ephemeral ponds but seed diversity and abundance were lower at low elevations in permanent ponds. Results suggest that strategies to protect pond shorelines to capture maximum diversity of coastal plain pond plants contained in pond sediment seed banks should be implemented across pond hydrologic classes and across a wide geographic area. Shoreline seed distributions indicate that ground-water withdrawals or climate changes that lower pond water levels in permanent ponds will reduce the diversity and abundance of plants recovered from seed banks by shifting water levels to a shoreline zone of high sediment organic matter where seed densities are lower. This effect will be much less in ephemeral ponds where seed diversity and abundance on pond bottoms was high.

Responses of an old-field plant community to interacting factors of elevated [CO2], warming, and soil moisture

The direct effects of atmospheric and climatic change factors—atmospheric [CO2], air temperature and changes in precipitation—can shape plant community composition and alter ecosystem function. It is essential to understand how these factors interact to make better predictions about how ecosystems may respond to change. We investigated the direct and interactive effects of [CO2], warming and altered soil moisture in open-top chambers (OTCs) enclosing a constructed old-field community to test how these factors shape plant communities. The experimental facility in Oak Ridge, TN, USA, made use of 4-m diameter OTCs and rain shelters to manipulate [CO2] (ambient, ambient + 300 ppm), air temperature (ambient, ambient + 3.5°C) and soil moisture (wet, dry). The plant communities within the chambers comprised seven common old-field species, including grasses, forbs and legumes. We tracked foliar cover for each species and calculated community richness, evenness and diversity from 2003 to 2005. This work resulted in three main findings: (1) warming had species-specific effects on foliar cover that varied through time and were altered by soil moisture treatments; (2) [CO2] had little effect on individual species or the community; (3) diversity, evenness and richness were influenced most by soil moisture, primarily reflecting the response of one dominant species. We conclude that individualistic species responses to atmospheric and climatic change can alter community composition and that plant populations and communities should be considered as part of analyses of terrestrial ecosystem response to climate change. However, prediction of plant community responses may be difficult given interactions between factors and changes in response through time.