Jacques Moret

Does the invasive species Reynoutria japonica have an impact on soil and flora in urban wastelands

Invasive plants are recognised as a major threat to biodiversity. Although they are well-established in natural areas, the supposed negative impacts of invasive plants upon communities and ecosystems have so far been poorly investigated in urban areas, where invasions are a main issue for ecologists and for urban planners and managers. We propose to assess the effects of an invasive species along an invasion gradient in a typical urban habitat. We focused on the Japanese knotweed (Reynoutria japonica Houtt.), a widespread invasive species in Europe and North America. We considered eight urban wastelands invaded by this species in the heart of the Greater Paris Area, France. On each site, we ran four transects from the centre of the Japanese knotweed patch towards the uninvaded peripheral vegetation. We recorded the flora using the line intercept method, and several soil parameters (thickness of A horizon, abundance of earthworm casts, topsoil Munsell value, pH) every metre along each transect. The A horizon was thicker and the topsoil darker under R. japonica canopy. Thus, this invasive plant species seemed to influence soil organic matter pool. However, our results also steadily showed that R. japonica locally excluded and/or severely reduced the cover of many plant species through competition. Our study clarified the local effects of R. japonica: an influence on the soil organic matter, and a severe negative impact on wasteland plant communities. We suggest implications in both conservation and restoration ecology.

Allee effects within small populations of Aconitum napellus ssp. lusitanicum, a protected subspecies in northern France

Plants growing at low density can suffer from Allee effects as a result of pollen limitation. Previous studies of Allee effects have focused on the effects of variation among populations in size or density on reproduction. Here, the effects of plant distribution within populations on fitness components are explored in a rare plant, Aconitum napellus ssp. lusitanicum, and ecological and genetic mechanisms underlying these effects are identified. To detect pollen limitation, seed production was compared under natural versus hand-supplemented pollinations on inflorescences of different sizes in natural patches differing both in flower density and in isolation from other patches. Germination rate and juvenile survival of seeds produced in low- and high-density patches were also compared. Pollen-supplemented flowers always produced more seeds than open-pollinated flowers, especially among small plants and plants growing at low density. Offspring produced in low-density patches exhibited lower fitness that those produced in high-density patches. This could have been caused by post-fertilization mechanisms, including inbreeding depression or differential maternal resource allocation. These results show that Allee effects on fitness components (ecological and genetic Allee effects) occur within A. napellus populations at different spatial scales. The spatial distribution of plants seems to be a crucial factor affecting reproductive output and fitness.

Impact of ploughing on soil seed bank dynamics in temporary pools

We examined the impact of ploughing on soil seed banks of plant communities living in temporary marshes located in agricultural fields. The quantity, quality and vertical distribution of seeds were quantified under ploughed or unploughed treatment at community level. We also focussed on a typical semi-aquatic species, Damasonium alisma, to investigate the impact of ploughing at population level. We used two complementary techniques to study seed banks: hand sorting and seedling emergence. We found that species richness of seeds, seed abundance and germination ability were strongly affected by ploughing at community level. Concerning D. alisma, most of the seeds (56%) were stored in the two deepest soil layers among the four considered in ploughed pools. Moreover, the germination rate was higher for buried seeds (84%) than for seeds collected at the surface (33.6%). These patterns were almost inverted in unploughed pools. Our results agree with the temporal storage effect generally suggested to describe the seed bank property of plant communities. But in addition, we showed that ploughing induces a spatial storage effect in accumulating species and individuals in the seed banks that favourably influence community dynamics. We conclude that, in contrast to what is usually thought, ploughing disturbance can be of benefit for such ephemeral wetland vegetation.

Genetic variation in the horsetail Equisetum variegatum Schleich., an endangered species in the Parisian region

Equisetum variegatum Schleicher is a circumboreale species of horsetail. In France, it typically grows at high elevations but is very rare in lowlands. The genetic variation of these populations is described using isozyme electrophoresis and PCR-RFLP of chloroplast DNA. Sampled sites were chosen to represent central vs. marginal and/or endangered parts of the distribution area. Extensive clonal multiplication of plants together with the absence of local recruitment by sexual reproduction seem to be responsible for the low genetic diversity observed within populations. Since adaptive response to environmental changes ultimately relies on the presence of genetic variability, clonal populations of E. variegatum may be particularly vulnerable to disturbance. Moreover, in lowland populations, isolation gives no chance to recover new genotypes through migration events. The preservation of the two endangered populations is proposed by propagation by cuttings of all extant genetic individuals. In the case of a disappearance of one genotype in the field, a replacement will be possible. This plan may be sufficient to preserve E. variegatum in the French lowland for several years.

Relationships between channelization structures, environmental characteristics, and plant communities in four French streams in the Seine–Normandy catchment

Abstract We examined the relationships between channelization, environmental characteristics, and plant communities in 4 streams (Betz, Cléry, Lunain, and Ecole Rivers) southeast of Paris, France, with the goal of assessing stream health. Seventeen 100-m-long reaches, each divided into a pool/riffle sequence, were monitored in 2006. Each reach had 0 to 3 channelization structures (i.e., embanking, resectioning, and other man-made structures). Redundancy analysis and mean-comparison tests done at 2 spatial scales (reach scale and pool/riffle sequence scale) indicated that channelization significantly affected flow velocity, depth, substrate type, and number of pools/riffles (4 of 24 variables considered). Physical characteristics of the streams were significantly more affected by channelization than were chemical characteristics. Moreover, different channelization structures affected the streams differently. Resectioning and embanking reduced flow velocity and the number of riffles and increased the number of pools. However, man-made structures either accelerated (downstream) or reduced flow velocity (upstream). Channelization strongly affected floristic richness on the basis of biological type (vascular plants, bryophytes, macroalgae) and ecomorphological type (hydrophytes, helophytes) at the pool/riffle sequence scale. Channelization led to taxonomic shifts and loss of biodiversity. Vascular plant taxa such as Helosciadium nodiflorum and Berula erecta were replaced by opportunist taxa such as Potamogeton crispus in channelized reaches. Combinations of channelization structures affected the stream plant assemblages less than did individual structures, a result that suggested compensatory effects. Our study supports the idea that channelization must be accompanied by measures that preserve the initial physical conditions of streams and the natural plant community composition.

Does channelization alter spatial and temporal dynamics of macrophyte communities and their physical habitat

Channelization is the creation of man-made structures that resect, realign, or enclose aquatic systems in order to prevent flooding or to modify their flows for various land uses. The present study examined the impact of this human pressure on the spatial and temporal dynamics of small streams. The physical and floristic characteristics of seventeen reaches in four French rivers were surveyed six times over the course of two years. Macrophyte communities were divided into three biological groups: vascular plants, macroalgae and bryophytes. We used plant functional traits to understand the effects of channelization on community structure. Our results suggest that channelization affected the spatio-temporal dynamic of physical and floristic composition. Channelized reaches were shallower and narrower than non-channelized reaches, i.e. control reaches. They also exhibited different substrate types and dominant species. Differences were mainly observed at the macrohabitat scale, i.e. the pool/riffle scale, within the selected reaches. Alterations in spatio-temporal dynamics of physical and plant composition could be linked to species biological traits. Vascular plants and macroalgae in channelized reaches used a variety of adaptive strategies (e.g. small versus tall size) which allowed them to persist despite environmental differences, whereas plants in control reaches showed a combination of intermediate strategies. Bryophytes were mainly found in control reaches with the exception of Fontinalis antipyretica. These findings could serve as guidelines for future channelization projects and for conservation measures to preserve the dynamics of natural streams.