Which role can arbuscular mycorrhizal fungi play in the facilitation of Ambrosia artemisiifolia L. invasion in France?

Ambrosia artemisiifolia L. (common ragweed), an annual invasive plant, was introduced more than 100 years ago from North America to Europe. Like the majority of other invasive plants in Europe, it develops in open, disturbed areas such as fields, wastelands, roadsides, and riverbanks. Recently, arbuscular mycorrhizal fungi (AMF) have been suspected to play a role in some plant invasion processes. As the common ragweed is known to be colonized by AMF in its native range, the intensity of mycorrhizal root colonization was studied in 35 natural populations in eastern France. About 94% of the A. artemisiifolia populations sampled were mycorrhizal. Root colonization levels varied from 1 to 40% depending on the ecological sites, with lower levels for agricultural habitats and higher levels in disturbed sites, such as wastelands or roadsides. A subsequent greenhouse experiment showed positive impacts of AMF on the growth and development of A. artemisiifolia. It is proposed that the spread of this invasive plant species could be facilitated by AMF, underlining the need to integrate symbiotic interactions in future work on invasive plant processes.     

Do arbuscular mycorrhizal fungi play a role in the ability of rare plant species to colonize abandoned fields?

While some plant species colonize abandoned agricultural fields and dry grasslands with similar frequency (generalists), others are absent or underrepresented in abandoned fields (specialists). We tested if inoculation with dry grassland or abandoned field soil could improve specialist performance in an abandoned field and compared the effects of inoculation in the stage of sown seeds and transplanted seedlings. Arbuscular mycorrhizal fungi from abandoned field had higher root colonization potential. This could explain the higher performance of the sown specialists inoculated with the abandoned field inoculum compared to those inoculated with dry grassland inoculum. This difference disappeared when specialists were transplanted instead of sown. The results do not provide any support for higher performance of specialists inoculated with dry grassland inoculum. Transplantation, however, seems to be an efficient way to introduce specialists into the abandoned fields.     

The role of human disturbance in the local Late Holocene establishment of Fagus and Picea forests at Flahult, Western Småland, Southern Sweden

A pollen record from a small alder carr located in the centre of aFagus stand near the hamlet of Flahult in southern Småland has shown thatFagus became established in a semi open cultural landscape about 900 B.P. Human disturbance seems to have controlled the local establishment ofFagus at this site through an expansion of pastoral farming. TheFagus dominance in the present stand seems to be of recent origin, asFagus pollen percentages and influx values have increased considerably only during the last 50 years. The modern composition and structure of theFagus stand are probably an effect of changes in land-use and decreased human activity at the end of the last century. Today, only occasionalPicea individuals occur in the studied stand, andPicea does not appear to have been more abundant in the recent past. The regional expansion ofPicea has probably occurred during this century and has been favoured by modern forestry during the last 50 years.     

Transfer seeds,hay, or soil blocks? The importance of the completeness of biological inputs to address dispersal and establishment limitations during the restoration of plant assemblages in floodplain grasslands

This study investigated different techniques of grassland restoration to overcome dispersal or establishment limitation, which are key processes influencing early-successional plant community assembly. A fully randomized in situ experiment was set up in a former arable land in a floodplain along the Garonne river (south-western France) to test for the effect of (1) the type and completeness of the biological input (any biological material—seed, hay, and soil—transferred to the site under restoration), (2) soil disturbance by deep tillage, and (3) their interaction on plant community dynamics for 5 years. All inputs influenced the plant community structure and composition. The effects depended on the type of the input but not necessarily in link with its completeness. High density and diversity seed mixture led to high levels of richness and relative abundance of target species like soil blocks, the input considered as the most complete. During the first year, hay transfer mostly influenced community assembly through negative litter effects. Delayed germination of several species contributed to buffer these early effects. This study supports the importance of dispersal limitation during early succession in degraded grassland ecosystems whereas soil disturbance had only subtle effects on the seed bank and standing plant community, indicating that competition and establishment limitation were of secondary importance in our study system. Our results suggest that even low-frequency immigration events (e.g. when a few seeds are transferred within hay) can be of great importance providing that a sufficiently long time period is considered for grassland restoration.     

The ‘organizing’ role of the D quadrant in an equal-cleaving spiralian,Lymnaea stagnalis as studied by UV laser deletion of macromeres at intervals between third and fourth quartet formation

Summary

In the spiralian embryos studied which display unequal-cleavage at the first two cleavages (either by a polar lobe or an asymmetric cleavage mechanism) the D quadrant is determined at the four cell stage by an unequal segregation of cytoplasmic stuffs. The normal formation of eyes, foot, and shell by overlying micromeres in these forms requires the inductive interaction with the D quadrant before the formation of the third quartet of micromeres. In equal-cleaving spiralians the D quadrant (3D macromere) becomes determined as a result of inductive interactions with first quartet derivatives (animal-vegetal interaction) sometime after the production of the third quartet of micromeres. This paper investigates the exact timing of D quadrant determination and the inductive role of third-order macromeres on the development of micromere derived structures in an equal-cleaving spiralian. Deletions of third-order macromeres, and their derivatives, were performed without rupturing the egg capsule membrane of the Lymnaea embryo with a UV laser microbeam. Virtually normal snails were produced when the 3A, 3B, 3C, or 4D macromere was irradiated. Juvenile snails lacking all mesodermal structures but possessing eyes, foot, and shell were obtained when the mesentoblast (4d) or its progenitor (3D) were deleted. Furthermore, ‘mesoderm-less’ snails were produced by deleting one of the two possible 3D candidates (cross furrow macromeres) as early as 20 min after third quartet formation. These results indicate that the 3D macromere begins to become determined at, or soon after, animal-vegetal interaction; before the 3D macromere becomes visibly distinguishable from the 3B macromere. The results also demonstrate that normal pattern formation in the overlying micromeres does not require the ‘prolonged’ interaction with an asymmetrically positioned 3D macromere. Possible adhesive differences between the 3D macromere and the remaining three macromeres are also revealed.