Forestry Commission, Alice Holt Lodge Station, Wrecclesham-Farnham, Surrey, UK

This paper reviews existing research on floodplain ecosystems and considers the research needs of genetic conservation, contrasting the need for studies of genetic diversity for ex situ conservation with the need for studies of genetic adaptation in relation to ecosystem dynamics to guide in situ conservation strategies.

In setting about the genetic conservation of forest trees, it is perhaps surprising to find that black poplar has been seen as a good place to start. Reasons for seeking to conserve it are:

• its natural habitat – the floodplain forest – has been lost to agriculture over large areas of Europe, and is still being eroded

• it is easy to propagate by cuttings

• it is one parent of the commercially important hybrid Populus × canadensis (syn. P. euramericana) (P. deltoides × P. nigra)

• it is highly resistant to bacterial canker (Xanthomonas populi) and shows some resistance to other important diseases of poplar

• on a European scale, poplar is highly significant economically.

Existing populations therefore certainly contain genes of commercial value, but there is also a wealth of genetic variation of which we are unaware, and which may be used one day by tree breeders to counter adverse environmental changes, or new pests and diseases.

Relict populations which no longer reproduce sexually will contain less genetic wealth than populations which continue to evolve naturally and continue to be subject to selective pressure. The natural habitat of poplar is in the highly dynamic riparian ecosystem in which successful genotypes will have developed strategies to deal with fluctuating water tables, silting and flooding. If such systems could be conserved, they would offer the best place to look for adaptive traits as a source for tree breeding.

Relict populations remain threatened by competition for land use (White 1993). The introduction of cultivars and hybrids, over almost two centuries, means that young specimens may contain genes which are foreign to the local type. Therefore, one strategy is to take cuttings from older individuals to keep in ex situ genebanks either as stool-beds, or to grow into mature trees.

Where actively breeding populations still exist, relatively free from genetic pollution from imported stock, it may be possible to establish an in situ reserve of much greater value, in that new successful combinations of genes through natural selection and new mutations are still emerging.

In the absence of clear information about the genetic make-up of a given population, and about the origins of these genes, it is difficult to be clear about whether the population is "native" or not. Using analysis of chloroplast DNA, Ferris et al. (1997) were able to distinguish an East Anglian population of pedunculate oak from the many introduced specimens, and also found a marker which distinguished eastern and western European populations of both species of oak. This also showed that a number of very old oaks in Britain were in fact introductions from eastern Europe. Thus it is difficult to define the boundaries of a block of genetic material which is to be conserved, unless we can be clear about its taxonomic boundaries based upon molecular genetic information.

Riparian systems are characterized by enormous ecological gradients along their length, and by rapid and dramatic changes in environment caused by flooding or by changes in the course of the river, by snow melts and by debris carried down by the river. This is the habitat to which poplars are specifically adapted, regenerating on elevated shingle-bars and mudflats in the braided streams of meandering rivers. The intense selection pressures of this environment have moulded the genetic structures of the poplars which ultimately the river leaves behind in the relatively stable environment of the floodplain forest.

An excellent description of this progression is given in Peterken and Hughes (1995). These authors subdivide floodplain woodlands into four types:

1. Pioneer stands of fast-growing poplar and willow on recently deposited sand and shingle.

2. Alder-dominated mixtures in peaty depressions in extinct channels and back-swamps.

3. Mixed elm, oak, ash and alder with scattered poplar and willow on well-drained mineral soil, sometimes flooded in winter.

4. Oak, hornbeam and lime woodland on the floodplain margins.

Evolution of diseases, disease resistance and biocontrol agents, and adaptations to site and climate make these systems a rich source of genetic material for biological study or as a source for tree breeding.

"River margin ecosystems offer a dynamic interface between terrestrial and aquatic systems where biological processes and biodiversity tend to be maximised. Recent evidence suggests that river margin ecosystems are highly sensitive to global, regional and local environmental changes" (introduction to ERMAS II). If we learn how to monitor dynamic processes in riparian ecosystems, this might give us a sensitive barometer with which to assess the effects of climate change.

The priority for conservation will be to conserve actively reproducing populations, in a habitat which is as 'natural' as possible, that is an in situ population subject to the selection pressures of an uncontrolled river system. In the following table, value to genetic conservation declines from types 1 to 4:

We begin with a relatively poor understanding of the genetic diversity of black poplar. The origins of the taxon are unclear, and we have little understanding of how the subspecies have differentiated, and hence where to look for the extreme genotypes within the species. Did populations of black poplar become isolated during the last ice age? Is subspecies betulifolia in Britain distinct from the same subspecies on the continent? To what extent is betulifolia distinct from subspecies typica?

The dynamics of poplar populations has been considered by Robert Farmer in Chapter 2 of "The Biology of Populus" (Stettler et al. 1996). He contrasts opposing processes of genetic drift and genetic 'communication' over long distances. Poplar is specialized for rapid genetic adaptation because of the following traits:

• dioecious sexual system

• small, effectively wind-distributed seed/pollen borne in high crowns

• early sexual maturity.

It exhibits strong clinal patterns of variation for photoperiodism with latitude, for frost hardiness and for drought tolerance, for example. Farmer concludes:

• geneflow has been sufficient to prevent genetic drift – geographic variation is not usually the result of geographic isolation

• adaptations are rapid and large

• studies such as that showing the adaptation of aspen to damaging low-level ozone (Berrang et al. 1986, 1989) point to dynamic processes that could not be elucidated simply by assessing variation in genebanks or by molecular genetic surveys. It is necessary to study dynamic processes in action in sexually reproducing populations.

Existing research

Molecular genetic studies

Modern molecular methods for analyzing genetic material now offer means for examining genetic diversity and defining relationships between individual taxa. Legionnet (1997) used isoenzymes to examine genetic diversity and population biology in P. nigra growing in France and found that there was more genetic diversity within rather than between stands. As a consequence, it would be more efficient to conserve more individuals from a small number of stands than vice versa. Cottrell et al. (1997) used RAPD markers to study 36 accessions of black poplar broadly sampled within Great Britain and found only 17 distinct genotypes. Genotypes were local in their distribution and genetic diversity was low. These authors also concluded that there had been so much interference by humans that there are unlikely to be distinct eastern and western types. In a more concentrated study of black poplar in the Upper Severn area, Winfield et al. (in prep.) used AFLP analysis to examine genetic diversity in 146 individuals and 3 individuals considered to be non-betulifolia poplars. Genetic diversity was low, confirming the results of Cottrell et al. (1997). There was a general correlation between geographic proximity and genetic similarity. They concluded that it was possible to identify a small number of individuals exhibiting maximum diversity for inclusion in a replanting/conservation programme.

Of the 36 trees sampled by Cottrell et al. (1997), only 6 were female and DNA analysis of these revealed that there were only two distinct genotypes, despite the fact that they were sampled from a wide geographic range.

The ERMAS I programme was established in 1992 to increase our understanding of the processes controlling the structure and function of European river margin ecosystems through a Europe-wide network. ERMAS II focuses on the role of biodiversity in determining the sensitivity of river margin ecosystems to environmental conditions, particularly temperature and hydrology. There are three main tasks:

• to understand the role of biodiversity in maintaining the structure, function and stability of ecosystems

• to analyze ecosystem processes with particular reference to the organic matter cycle

• to determine and compare interactions and links between ecosystem processes and physical processes in contrasting situations, defined on two scales: climatic region and patch.

The study sites range from 64°N to 43°N, with a climatic gradient from subpolar to maritime temperate, Mediterranean and temperate continental. Partnership details for this and the following EU shared-cost project can be obtained from the Internet via CORDIS (on-line information service about the activities of the European Community concerning research and development).

The full title of this project is "Floodplain biodiversity and restoration: hydrological and geomorphological mechanisms influencing flooplain diversity and their application to the restoration of European floodplains." The main objectives are:

• to contribute to the development of a scientific methodology for determining the flow needs of riparian plant communities on selected European floodplains

• to create effective links between the scientific understanding of the functioning of riparian ecosystems and the institutional mechanisms by which river management for conservation and restoration occur.

The main activities are:

1. To identify and quantify hydrological and sedimentological conditions favoured by riparian species for their establishment and growth. Using black poplar in the UK and France, and alder and willow in Sweden, field and laboratory experiments are investigating species' response, e.g. to waterlogging and drought.

2. To link contemporary floodplain patterns to our understanding of past climatic and land-use changes at a catchment scale and over a range of time scales. Archival studies of river flows and management practices will be linked to studies of catchment-scale riparian vegetation patterns and land-use practices.

3. To investigate the institutional framework within which river restoration projects take place and the degree to which knowledge of the functioning of floodplain ecosystems influences their implementation. An inventory of restoration projects across the European Community will be made, and the institutional frameworks involved and their knowledge of the functioning of floodplain ecosystems will be studied in the UK, Sweden and France.

Against this background, it is possible to identify two broad areas of research need:

1. Studies of genetic diversity and genetic origins of black poplar, designed to guide ex situ conservation strategies, and in particular to increase the efficiency of those strategies by identifying individuals to conserve.

2. Studies of adaptation and ecological dynamics in in situ populations, concentrating on breeding populations subject to the selection pressures in dynamic river systems. For instance, it will be important to decide how large such populations need to be to conserve the ecological and genetic processes which give rise to valuable new genetic combinations.

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Cottrell, J.E., G.I. Forrest and I.M.S. White. 1997. The use of RAPD analysis to study diversity in British black poplar (Populus nigra L. subsp. betulifolia (Pursh.) W. Wettst. (Salicaceae)) in Great Britain. Watsonia 21:305-312.

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White, J. 1993. Black Poplar: The most endangered native timber tree in Britain. For. Comm. Res. Inf. Note 239.