Etienne Muller

Recent Changes in the Riparian Forest of a Large Regulated Mediterranean River: Implications for Management

The structure of the floodplain forests of the Middle Ebro River (NE Spain) was examined at patch and landscape scales along a three-step chronosequence defined according to the extent of flow regulation-induced hydrogeomorphic changes, with the ultimate purpose of producing baseline information to guide through management and restoration plans. At patch scale, a total of 6,891 stems within 39 plots were registered for species, diameter and health status. The stem density, size class distribution, canopy dieback and mortality were further compared by means of non-parametric tests. At landscape scale, the temporal evolution of the area occupied by forest stands of different ages in the floodplain along the chronosequence was evaluated using four sets of aerial photographs dated in 1927, 1957, 1981 and 2003. The within-patch structure of pioneer forests (<25–30xa0years old) was characterized by dense and healthy populations of pioneer species (Populusxa0nigra, Salixxa0alba and Tamarix spp.), but the area occupied by these forest types has progressively decreased (up to 37%) since the intensification of river regulation (ca. 1957). In contrast, non-pioneer forests (>25–30xa0years old) were characterized by declining and sparse P.xa0nigra–S.xa0alba–Tamarix spp. stands, where late-seral species such as Ulmusxa0minor and Fraxinusxa0angustifolia were frequent, but only as small-size stems. At landscape scale, these type of senescent forests have doubled their surface after river regulation was intensified. Populusxa0alba only appeared in the oldest plots recorded (colonized before 1957), suggesting sexual regeneration failure during the last five decades, but usually as healthy and dense stands. Based on these findings, measures principally aimed at recovering some hydrogeomorphic dynamism are recommended to guarantee the self-sustainability of the floodplain forest ecosystem.

Seed dispersal, germination and early seedling establishment of Populus alba L. under simulated water table declines in different substrates

Populus alba L. is an autochthonous species dominating the overstory of the floodplain forests across the Mediterranean region. In contrast to some other Populus spp., very little is known about its regeneration strategies. Poplars yearly disperse huge amounts of wind- and water-dispersed, non-dormant, short-lived tiny seeds that need the bare, open and moist substrates created by fluvial-geomorphic events to germinate and establish. To survive, the growing roots must keep pace with the falling water table and associated soil moisture zone. Using a greenhouse experimental facility, 9-day-old P. alba seedlings were subjected to five hydrological treatments (permanent saturation, drawdown rates of 1, 2.5, 5xa0cmxa0day−1 and immediate drainage) in two different substrates (coarse and sandy), and their survival and growth were evaluated. Also, P. alba seed dispersal was monitored in the field, and seed germinability and longevity were tested in the laboratory. No seedlings survived the water table declines in the coarse substrate although survival was high (85%) under saturated conditions. In the sandy soil, survival was significantly greater in the permanent saturation (87%) and 1xa0cmxa0day−1 (88%) treatments than in the 2.5xa0cmxa0day−1 (58%), 5xa0cmxa0day−1 (25%) and immediate drainage (22%) treatments. The lowest root and shoot growth rates occurred under the saturated and immediate drainage conditions. Seed dispersal lasted 6–8xa0weeks and peaked in mid-April, initial seed germinability was high (92%) and seed longevity was relatively long (half viability period, 30xa0days). The creation of nursery sites and release of controlled floods in mid-April, followed by water table declines of less than 1xa0cmxa0day−1 in coarse substrates and less than 5xa0cmxa0day−1 in sandy substrates are recommended for enhancing the initial establishment of P. alba seedlings.

Mangrove trees growing in a very saline condition but not using seawater

Mangrove trees, which develop along tropical coasts, are known to use saline water uptake. In French Guiana, the high salinity condition is the result of seawater evaporation on mud banks formed from the Amazon sediment flumes. In the back mangrove a few kilometres inland, groundwater, soil water and the xylem sap uptake in the trees remain highly salty, and only very tolerant plants like Avicennia germinans can flourish, whereas the less salt-tolerant Rhizophora mangle is more difficult to find. Curiously, the same Avicennia trees propagate on the seafront. However, stable isotope ratio mass spectrometry (IRMS) measurements and ion analysis (high-performance liquid chromatography (HPLC) and inductively coupled plasma atomic emission (ICP-AES) spectroscopy reveal that the origin of the water in the back mangrove is not seawater. It is freshwater percolating into the sand bars from the inland marshes and rainwater during the wet season that redissolves a marine evaporite and gives a saline groundwater. The absence of barren saltine areas (tanne) in French Guiana could be explained by this freshwater inflow, the aquifer being no longer linked with the ocean.

Abrupt Drops in Water Table Level Influence the Development of Populus nigra and Salix alba Seedlings of Different Ages

Early seedling establishment is a demographic bottleneck for the sexual regeneration of many riparian tree species such as Populus nigra and Salix alba, two dominant species of European floodplains. Immediately after spring germination, seedling roots must track receding water tables to avoid desiccation. This may be especially difficult if the groundwater recession is too abrupt. In this study, seedling survival and growth under different abrupt drops in water table levels were examined using an experimental facility that allowed continuous and non-destructive measurements of above- and below-ground seedling development. A Cox proportional hazards regression model showed that the risk of mortality for a seedling was significantly lower in P. nigra than in S. alba, was lower for older than for younger seedlings, and increased with greater drops in water table level. In fact, almost no seedlings survived abrupt water level drops ≥40xa0cm. However, no substantial differences in any of the growth parameters were observed between species, age, and water table treatments, as shown by R-ANOVA and paired tests. This suggests that abrupt drops in water table level did not stimulate plant growth but only favoured the survival of the most resistant individuals. These findings should be considered when designing improved water flow prescriptions along managed river courses in Europe.

Electrospray and chemical ionization mass spectrometry of di‐n‐butyl sulfate. Unimolecular chemistry of its protonated form and quantification method by liquid chromatography/electrospray ionization tandem mass spectrometry

Di-n-butyl sulfate (DNBS) has been studied by electrospray (ESI) and chemical (CI) ionization mass spectrometry. The use of methanol as solvent in electrospray ionization allows observation of relatively abundant [DNBS + CH(3)OH + H](+) ions (m/z 243) which upon collision dissociate to [DNBS + H](+) ions (m/z 211). In both ESI and CI experiments, it is found that [DNBS + H](+) ions lead to m/z 113 daughter ions. The composition of this m/z 113 fragment ion and its mechanism of formation have been established by high resolution measurements and CID-MIKE experiments. An internal substitution reaction involving an ion-neutral intermediate is proposed to explain the formation of a [C(8)H(17)](+) ion (m/z 113) by loss of a H(2)SO(4) molecule. Finally, a LC/ESI-MS/MS quantification method is proposed in which a detection limit of di-n-butyl sulfate in the ppm range is obtained. It is suggested that the quantification method might be extended to higher dialkyl sulfates. Copyright 2000 John Wiley & Sons, Ltd.