Scientific Protocols and Team Progress
The scientific research undertaken by the four partner groups has the common goal of evaluating hydrological influences on the germination, growth and competition patterns of certain early successional floodplain woodland species. All partners have established field sites on their respective study rivers and have set up experiments to monitor the establishment and growth of one or more riparian vegetation species. The protocol of these experiments has been adapted to the local conditions, but the instrumentation and data-logging methods are standard (designed and built in Cambridge by A. Hayes); variations in soil-suction, water table levels and temperature levels are monitored at each field site using tensiometers, dipwells and temperature probes, respectively. Greenhouse or laboratory experiments are also being carried out by each research team to test the inferences made from the less controlled conditions in the field experiments. The following sections outline the specific research questions addressed and protocols adopted by the four partner groups.
| I. The Cambridge group. | III. The Toulouse group. | |
| II. The Grenoble group. | IV. The Umeå group. |
I. The Cambridge group.
This group is focusing on the growth responses of the U.K. black poplar (Populus nigra var. betulifolia) to varying soil-moisture conditions, with close attention given to gender and genetic influences on the growth behaviour of this species.
Background: In order to contribute to the debate on setting river flows to benefit ecological communities in floodplain zones, it is important to quantify the flow needs of riparian species. In the U.K., riparian woodlands have largely disappeared, but there is now growing interest in the restoration of floodplain woodland communities in selected locations. One early successional floodplain woodland species which has attracted particular interest in the U.K. is the dioecious black poplar (Populus nigra), a species which is at the northern edge of its range in Britain .
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| Mature black poplar tree (E. Herbert). | Black poplar leaves with female catkins (N. Barsoum, 1995) |
P. nigra is currently considered to be the most endangered native timber tree, with only approximately 4,000 individuals occuring in southern Britain. Of the remaining individuals, the majority are male and although there are a few sites where males and females occur together sexual reproduction by seed does not occur. There is documented evidence that the bias towards males is partly the result of human interference since in the past, farmers preferred to plant male trees over female trees. There is other evidence (from another Salicaceae species - Salix arctica) which further suggests that males and females may have different soil-moisture preferences, leading to a sexual partitioning of habitats on floodplains. This possibility has implications for flow management since flows which favour one sex over the other would have long-term consequences on genetic biodiversity through reduced opportunities for sexual reproduction.
The specific research tasks of the Cambridge group are therefore:
(1) to investigate evidence of sexual partitioning in the spatial distribution of P. nigra in response to water table variations and variations in available soil-moisture,
and;
(2) to determine the ideal sedimentological and hydrological conditions required by P. nigra for regeneration from seed.
These research questions are being addressed through a series of complementary field- and greenhouse-based experiments. GIS techniques have also been used for both mapping and correlating the distribution of P. nigra trees with environmental parameters such as proximity to surface drainage and soil hydrology. In addition, genetic studies of the male and female P. nigra specimens used in the field plot and laboratory experiments have recently been completed using AFLP genetic finger-printing techniques.
Field experimental work: The floodplain topography on the River Great Ouse is very subdued. It has been possible, nevertheless, to establish a series of plots at different heights above the local water table and at different distances from the river bank. These plots are located on an island. The first experiments here, lead by F. Hughes, involved planting male and female cuttings of P. nigra, at three locations (two 3 x 4 m plots at each location); the aim is to investigate their respective growth responses to a variety of soil-moisture conditions, as defined by the variable water table levels and the water retention capacity of different sediment suites at each planting location.
 |  |
| Aerial view of island along River Great Ouse at Hemingford Grey where experimental plots are set up. | Experimental field plots at one of three locations (F. Hughes) |
At each location a hydrological station has been installed to include three tensiometers at different depths within each experimental plot and three tensiometers at the same depths outside the experimental plots to act as controls.
(Lower) Response in near surface and deeper tensiometers in 1998. The surface (30cm) tensiometer shows positive pressure from Julian day 100-107 (10-17 April) during floods.
A rain guage and dipwell have also been installed, the former to record localized rainfall events and the latter to monitor water table levels and to allow abstraction of water for basic water chemistry tests.
| Greenhouse experimental work: N. Barsoum is currently leading experimental greenhouse work which is testing the growth responses of male and female P. nigra cuttings to a range of soil-moisture conditions as defined by different rates of water table decline and differences in the sediment textural properties of the planting substrates used. This experimental work is being achieved using rhizopods (soil-filled planting tubes linked to a central water reservoir which allows for the control of water table levels). Rhizopods at Cambridge University Botanic Garden greenhouses. (N. Barsoum) |  |
Other greenhouse work includes an experiment designed to establish the ideal sedimentological and soil-moisture conditions required by P. nigra seedlings in different age categories (2, 4 and 6 weeks from germination). Fine- and coarse-grained sediments were used as the planting substrates and seedlings were subjected to four experimental regimes; (i) drought stress, (ii) drought stress + regular simulated rainfall events, (iii) complete submergence and (iv) a control treatment.
GIS project: A GIS study carried out as an MPhil project by Esther Kamau involved plotting grid references for up to 300 sexed P. nigra trees (data obtained from the Biological Records Office at the Institute of Terrestrial Ecology - Huntingdon) onto overlays of soil hydrology and drainage network databases.
Genetic studies: Cuttings for one female parent tree used in the field experiments consistently grew faster than all other individuals and was suspected of being a hybrid. An AFLP analysis (by Dr. Mark Winfield - Cambridge University Botanic Gardens) revealed this to be the case illustrating the significant influence just one hybrid can have on experimental growth data and the need for caution when selecting experimental specimens.
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II. The Grenoble group.
This group is investigating the relationship between patterns of suspended sediment transport in the Isère River and the density and species composition of riparian vegetation on instream islands. The research interests of this group also extend to studies of the influence of rates of sediment deposition, nutrient fluxes in groundwater and different rates of water table decline on the germination, growth and survival of seedlings of five Salicaceae species.
Background: Alpine hydrosystems have undergone significant changes in the last two centuries as the result of river management activities. Along the Isère River, there has been a substantial increase over the past 30 years in vegetation coverage on river banks and on instream islands. The increased density of vegetation is attributed to an abatement of flooding events which has lead to a decrease in bedload transport and an increase in the stability of bedforms.
Flood risks are consequently replaced today by new risks; i.e. high vegetation densities are thought to accelerate embankment degradation and during floods, log jams can cause serious structural damage. The encroachment of vegetation onto the floodplain has further acted to reduce floodplain biodiversity and has particularly affected regeneration of early successional woody riparian species which require open clearings for successful recruitment.
In order to consider the feasibility of restoring these important European floodplains and the structure of floodplain woodlands, it is important to assess and understand the impacts of human activities on floodplain dynamics and separate these artificial modifications from natural changes to these systems. This is being achieved by the Grenoble team on three different spatial scales:
(i) catchment level: This involves studying long-term climatic variation in the catchment and also the evolution of management activities on a number of study reaches along the Isère River in the Gresivaudan Valley,
(ii) site level: Detailed quantitative and qualitative information has been collected over the past five years from a number of instream islands to provide information on vegetation development as related to specific (and measured) variations in hydrological, geomorphological and hydrochemical conditions
and;
(iii) species level: Greenhouse experiments and field surveys are designed to document the response of individual riparian tree species to a range of abiotic constraints (e.g. an excess or deficit in water or nutrient supplies) during the first phases of their development.
Catchment and site level research: G. Pautou, J. Girel and J. L-. Peiry have studied and documented the long-term evolution of hydrological conditions along the Isère River and related changes to floodplain vegetation. Changing environmental conditions are linked to changes in vegetation coverage using a variety of methods (quantitative vegetation surveys, aerial photographs, satellite imagery, archive documents and hydrological records).
A statistical study relating the morphometric and sedimentological characteristics of up to 144 islands in the Isère River with vegetation coverage has been undertaken by J. L-. Peiry and F. Vautier thoughout the Gresivaudan valley. A typology of the study islands has been collated based on (i) biogeographical features (i.e. percentage vegetation coverage and species composition), (ii) morphometric parameters such as island surface area, perimeter length, topographic elevation and a shape index and (iii) sedimentological parameters such as the thickness of fine sediment deposits, gravel bar volume, surface and subsurface texture. An automated monitoring site installed at the University Campus along the Isère River, provides detailed data on suspended sediment and total dissolved loads.
Finer scale research of this kind has been pursued on an island known as Brignoud Island, where repeat topographic surveys (1994-1999) and a comparison of digital elevation models provide estimates of deposition rates and their relationship to changes in vegetation coverage and species composition.
Groundwater surveys across the island using piezometers, are pursued to understand temporal and spatial fluxes of nutrients in groundwater as compared to nutrient concentrations in the river water. Three sets of tensiometers installed along a transect, provide further information on fluctuations in soil-water.
Species level research: R. Foussadier is leading species-specific research which involves field survey and greenhouse/botanical garden experimental work.
Field survey work: The survival and growth of Salix alba, S. eleagnos, S. purpurea, S. triandra and P. nigra seedlings were monitored at fortnightly intervals along an altitudinal gradient on the Brignoud Island. The seedlings of each of these Salicaceae species were found in distinct, but occasionally overlapping elevational bands and were coincident with specific sediment textural conditions.
Greenhouse experimental work: The germination rates of the Salicaceae species listed above were tested across a wide range of temperature and soil-moisture gradients. It was possible to set and control a range of experimental temperatures in growth chambers. Soil-moisture gradients were created by altering the osmotic pressure of the irrigation treatment.
Another greenhouse experiment involved the construction of an apparatus known as a rhizotron; a transparent growing vessel made of perspex sheets which displays the roots of experimental plants.
 | Seedlings of the four Salix species and P. nigra were planted in rhizotrons and their growth responses were studied under different conditions of soil-moisture, nutrient availability and soil-grain size. Water table levels were set to decline at rates of 10 cm, or 50 cm per day. Root development in response to the range of experimental treatments was measured fortnightly by tracing the root system of each experimental plant onto plastic sheets which were then digitised using a GIS system. Preliminary results indicate that in the first few weeks of seedling establishment, under favourable soil-moisture conditions, availability of soil-nutrients has no influence on the development of roots. In general, greatest productivity correlates with deeper water tables although results vary between the 5 species studied. |
Rhizotrons in Grenoble University
Botanic Garden greenhouses. (N. Barsoum)
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III. The Toulouse group.
This group is concentrating on temporal and spatial variations in plant-available water on the floodplain and how these variations affect the regeneration potential and health of two Salicaceae species commonly found along the Garonne River; i.e. white willow (Salix alba) and black poplar (Populus nigra).
Background: The Garonne River has been subject to an intensification of river management activities over the past 20 years. Today river flow is controlled by reservoirs (total storage capacity of approximately 1 billion m³) constructed for both hydroelectric power generation and the supply of water for irrigation. These river management schemes have significantly restricted natural flows and channel dynamics with potentially important negative implications for the health and natural regeneration of some of the remaining extensive tracts of riparian woodland along the Garonne. The focus of this group's research is therefore, to understand the minimum flow requirements of certain early successional riparian woodland species for regeneration with particular attention attributed to biological responses to temporal and spatial variations in hydrological processes.
The specific research tasks of the Toulouse group are:
(1) To explore the importance of the timing of seed release for successful recruitment of P. nigra and S. alba.
(2) To assess elevation- and substrate- related controls over germination and seedling growth performance and the susceptibility of seedlings of different ages to drought and flood stresses,
and;
(3) To understand the sources of water being exploited by various riparian tree species over time.
These research questions are being addressed through complementary field survey and greenhouse experimental work.
Field survey research: Five permanent transects, perpendicular to the main channel, have been established by E. Muller and H. Guilloy at two different field sites on the Garonne River (Monbequi and Mauvers) to monitor the establishment and growth of P. nigra and S. alba along an elevational gradient. The densities and heights of seedlings are being recorded three times a year (in March, June and October) over a period of two years and patterns of seedling establishment are related to the timing of seed release. Tensiometers and piezometers have been installed to monitor fluctuations in the moisture status of floodplain sediments along the transects. In conjunction with these measurements, repeat microtopographic surveys at fixed locations at the Monbequi and Mauvers field sites are being used to generate digital elevation models and provide information on sediment deposition following flooding events.
Studies by L. Lambs are also in progress to investigate temporal variations in sap flow and identify the sources of water used by trees on the floodplain (particularly during periods of drought); this is achieved using stable isotope measurements of the groundwater, river water and sap.
Greenhouse experimental research: A series of greenhouse experiments, lead by H. Guilloy and E. Muller, have been set up to study temporal variations in the physiological responses of P. nigra and S. alba at various early stages of regeneration to specific and changing hydrological conditions. In one experiment the relationship between the germination success of P. nigra and S. alba and the timing of seed release is assessed.
Another experiment is testing the survival and growth responses of P. nigra and S. alba seedlings of different ages to a range of sedimentological and hydrological conditions comparable to those encountered on the natural floodplain (i.e. periodic flooding, or droughts). A study has also been completed investigating the effects of moisture gradients on the growth and survival of P. nigra and S. alba seedlings of different ages using an apparatus known as a rhizotron.
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IV. The Umeå group.
This group is interested in studying the occurrence of mycorrhizal fungi in riverbank sediments and their influence on the structure of plant communities in riparian environments. The survival and growth responses of two riparian tree species (Salix triandra and Alnus incana) to hydrological variations and changing species composition along a river bank height gradient is also investigated.
Background: In order to develop a scientific basis for floodplain restoration strategies, we need more basic information on how river-bank vegetation is structured under natural conditions. In a European context, the remaining free-flowing rivers with catchment areas of low anthropogenic impact are situated in the northern parts of Europe, e.g. Sweden. The general research task of the Umeå team is therefore focused on the natural interaction between abiotic and biotic factors on river banks, and how they control the structure of riparian vegetation. The fundamental abiotic factors determining the distribution of different plant species on river banks are the hydrology of the river and the river-bank soils. Physiological constraints to flooding and desiccation in different species will set their ecological limits; this will in turn influence biotic factors, such as competition from other plant species and mycorrhizal relationships.
The specific research tasks of the Umeå group are therefore:
(1) To relate growth and survival of seedlings of the pioneer tree species Alnus incana and Salix triandra to hydrological variation along a river-bank gradient,
(2) To evaluate the effects of competition from established river-bank vegetation on the establishment and growth of Alnus incana and Salix triandra along this gradient.
and;
(3) To evaluate the role of mycorrhiza in relation to the distribution of plant species on river banks.
Mycorrhizal fungi are thought to play an important role in the composition and structure of plant communities. These interactions are poorly understood, however, in riverbank soils where fluctuating water table levels and nutrient fluxes create dynamic and unstable soil-environmental conditions. The Umeå group is thus, also concerned with the following specific research questions:
(i) To what degree are mycorrhizae present in riverbank soils?
(ii) How do the distribution of different groups of mycorrhiza and their associated plants vary along a riverbank height gradient?
(iii) To what extent do the hydrological conditions in river-bank soils affect mycorrhizal relationships?
Field survey and experimental research: A river bank has been selected along the River Öre where field survey and experimental research is being pursued along an elevational gradient. Floodwaters on the River Öre can inundate areas of floodplain up to 4 m above the low flow water level making it necessary to adopt a transect design to cover the range of floodplain heights (i.e. from the mean high water level to the mean low water level).
 | Experimental plots: 24 plots (cleared of vegetation) have been established at height intervals of between 6 and 50 cm on the river bank transect, with tensiometers installed at two soil depths (30 and 90 cms) in 13 of these plots. Seedlings of Alnus incana and cuttings of Salix triandra have been planted in these plots and in adjacent uncleared plots. Field plots along riverbank transect. (F. Hughes) |
Growth statistics are being collected every two weeks throughout the growing season; some end-of-season growth results (for S. triandra) are plotted in figure 9.
Mycorrhizal field experimental research: An experimental study on the relative importance of mycorrhiza for different plant species has been undertaken along the tensiometer-recorded gradient.
Installation of tensiometers and links to data logger along riverbank transect.
Five species (split into 3 categories) were chosen for the study: (a) non-mycorrhizal plants: Carex acuta, Silene dioica, (b) arbuscular mycorrhizal plants: Filipendula ulmaria, Alnus incana, and (c) ecto-mycorrhizal plants: Alnus incana, Betula pubescens. The plants were grown from seeds in sterile sand in the greenhouse, and were transplanted into the field in mid- June with the sterile sand still surrounding the roots. Ten individuals of each species were planted at four levels on the river bank. The levels were selected according to their vegetation composition: (1) At summer low-water level; vegetation dominated by Carex spp., (2) 0.3 m above level 1 and dominated by Calamagrostis canescens, (3) 1.2 m above level 1 and dominated by Alnus incana, Filipendula ulmaria, Calamagrostis spp., and (4) 3 m above level 1 and dominated by Betula pubescens, Calamagrostis spp., Convallaria majalis. The variation in growth of the plants at the different levels is compared to the colonisation rate of mycorrhiza. Preliminary results show that the abundance of plants and the abundance of their associated mycorrhiza generally covary along the river bank height gradient; i.e. a decrease in the abundance host plants is associated with a decrease in the degree of mycorrhizal infection.
Mycorrhizal field survey work: In a field screening study, the degree of mycorrhizal infection has been determined for eight plant species (categorized into four groups) along the river bank gradient; these include (a) non-mycorrhizal plants: Carex acuta, (b) arbuscular mycorrhizal plants: Calamagrostis canescens, Solidago virgaurea, Alnus incana, Salix caprea, (c) ecto-mycorrhizal plants: Alnus incana, Salix caprea, and (d) Ericaceae-mycorrhizal plants: Vaccinium vitis-idaea. Roots from 30 individuals of each species have been sampled along the length of the transect. The occurrence and type of mycorrhizal infection have been determined by dying roots and examining them under the microscope. The degree of infection is also being assessed by measuring concentrations of the chemical markers ergosterol and chitin, using HPLC-techniques.
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