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Lime (Tilia spp.) genetic resources conservation strategy
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Jan Svejgaard Jensen1
and Sonja Canger2
1Forest and Landscape
Research Institute, Hørsholm, Denmark
2Tree Improvement
Station, The National Forest and Nature Agency, Humlebæk, Denmark
Introduction
Owing to their aesthetic and
cultural value as urban trees and as landscape elements, lime
trees have become increasingly important in Europe in recent
decades. For various reasons, the distribution of Tilia in
Europe has declined over the last 2000 years. Unless protective
measures are taken, the size and constitution of genetic variation
of the Tilia species which occur in Europe will be
significantly reduced.
There is a need for a coordinated
effort to secure the genetic resources of Tilia in Europe. A
coordinated effort can reduce the conservation costs and improve the
quality of the conservation activities. A gene resource conservation
strategy for a particular species must be based on the available
knowledge about the biology of the species and on the basic theories
regarding ecology and population genetics.
An approach for gene conservation
of the species is outlined, based on an overview of the existing
knowledge concerning the geographic distribution, biology and
genetic diversity of Tilia in Europe, and on widely accepted
measures for conservation of forest genetic resources. The approach
described should be regarded as the minimum necessary effort
required in a European context.
Tilia cordata is considered
the most important European lime species. In this paper Tilia
refers primarily to T. cordata except when other species are
mentioned. Tilia cordata has been subject to many more
descriptions and investigations than T. platyphyllos, but
since the biology and morphology of the two species are similar,
information concerning the biology of T. cordata can in many
cases be applied to T. platyphyllos.
Overview of the distribution and
biology of the European Tilia species
Taxonomy and distribution range
The Tilia family includes
400 woody species, among those 30-40 species of Tilia, most
of them found in the tropics. Ten species are found in the temperate
region of the northern hemisphere. In Europe four species are
present: Tilia cordata Miller, Tilia platyphyllos Scop.,
Tilia tomentosa Moench and Tilia dasystyla Stev.
(Maurer 1995).
Tilia cordata is found in
England and its northern distribution limit is at 62?5' in Norway
and 63? in Finland. Tilia cordata is distributed from the
Pyrenees in the west and as far east as Almaty in Kazakstan
(Ksembaev and Dragavtsev 1977; Stepanov 1993). The distribution
range for T. cordata is described in detail in Pigott’s
botanical monograph for the species (Pigott 1991). The core region
for T. cordata is central and eastern Europe and the
distribution has been described in detail by Pochberger (1967).
In France the distribution of T. cordata
is scattered toward the west but more dominant in the eastern
regions and the Pyrenees. In central Europe the species is common in
low mountainous regions (Kleinschmit et al. 1996). Also in
the Alps, T. cordata is mostly absent from lowland valleys,
but more common up in the mountains. The occurrence of Tilia
is limited in Austria (Holzer and Toda 1974) as well as in
Switzerland (Trepp 1947; Surber 1951).
In northern Europe, T. cordata
grows in lowland regions. In Latvia it is reported as a common
species by Baumanis et al. (1996), but it is uncommon in
Estonia and Lithuania. In Scandinavia T. cordata typically
has a scattered distribution with small natural stands. The
distribution of Tilia in Russia is reported by Prokazin et
al. (1998), in Ukraine by Polyakov et al. (1988), in
Poland by Bialobok (1991) and Tomialojc (1991), in the Czech
Republic by Vancura et al. (in Turok et al. 1996) and
Hynek (1996).
In the Mediterranean and in
Georgia and Armenia, T. cordata is scattered and grows mostly
in mountainous regions. It is absent in Portugal, Ireland and
Scotland, and the species is uncommon in Spain, the Netherlands,
Belgium (Bart de Cuyper, pers. comm.), Italy, Greece, Turkey, the
low and dry plains in Belarus, Hungary, Ukraine and lower Volga
region in Russia.
Tilia platyphyllos is rare in
northern Europe, but is more common in mountainous regions of
central and eastern Europe. The distribution range of T.
platyphyllos is quite limited in comparison with T. cordata
and its eastern limit is in Ukraine. However, T. platyphyllos
is more widely distributed geographically in northern Spain and
Italy than T. cordata. Inventories of T. platyphyllos
distribution are sparse in Europe – only a few descriptions exist:
for the Macedonian Republic (Andonoski 1974) and for Great Britain
(Pigott 1981b). Tilia platyphyllos is commonly found on sites
containing T. cordata (Namvar and Spethmann 1986).
Tilia tomentosa and T.
dasystyla occur in southeastern Europe and the regions around
the Black Sea. The distribution of T. tomentosa in Hungary is
described by Schmidt et al. (1986) and in Romania by Blada
(1998). Seven species related to T. cordata occur in the
Asian region, e.g. T. mongolia and T. japonica
(Maurer and Tabel 1995).
Putative hybrids or intermediate
species between T. cordata and T. platyphyllos have
been found in several places (Tilia × europaea).
However, hybridization between the two species is rarely observed,
maybe because the flowering time differs. Other examples of
hybridization are Tilia × euchlora which is a cross between T.
cordata and T. dasystyla and Tilia × flaccida,
a hybrid between a European species, T. platyphyllos and an
American species, T. americana.
Historical data
The importance of T. cordata in
Europe is well illustrated by the common occurrence of its name as
part of the names of geographic localities throughout Europe.
However, the occurrence of the Tilia name within the area of
distribution does not necessarily refer to large numbers of Tilia
trees, but it may reflect the existence of a marketplace for
lime-bast (Pochberger 1967). Tilia has been used for carving
and almost all parts of the tree can be used for fodder, ropes or
fuelwood. Lime trees are insect-pollinated, and as such are
quite important for honeybees and honey production, especially in
eastern Europe (Endtmann 1990).
Between 6000 BC and 500 AD a
warmer climate favoured Tilia species in northern Europe.
They were dominating species in the forests of England and southern
Scandinavia. Even the rarely found T. platyphyllos was more
commonly distributed in northern Europe than it is now, as shown by
pollen analysis (Huntley and Birks 1983). The occurrences of T.
platyphyllos in northern Europe today are suggested to be
ancient relics.
Climatic conditions and human
impact have been a serious threat to the distribution of Tilia
in most European countries (Turner 1962). In some regions forest
areas were reduced to 2-4% in the year 1800. Another factor is the
competitive effect of beech (Koss 1982). Beech has slowly invaded
Europe after the last glacial period, and the species has become
dominant in central Europe within the last 2000 years. Despite these
conditions, T. cordata and to some extent T. platyphyllos
have managed to survive.
As the present distribution of Tilia
does not seem to be clearly related to climatic or edaphic
conditions, conclusions concerning optimum growing sites for Tilia
in specific regions should be drawn carefully. In many lowland
areas, Tilia may have disappeared when humans included these
areas for agriculture, and the species only remains on marginal
sites. In northern Europe Tilia may have disappeared on many
sites because of low seed fertility. On the other hand, as Tilia
is easily propagated, some stands have evidently been established by
humans.
In recent centuries, Tilia
has been widely spread in landscape and urban areas. Reproductive
material has been moved, especially in northern Europe where it was
imported from the east European countries.
Growth and site requirements
Tilia cordata and T.
platyphyllos are tall trees reaching a maximum height of
35-40 m. Tilias can become very old, more than 500 years. Large
coppice stumps, where the centre core disappeared several hundred
years ago, have been reported from England (Pigott 1991). The
ecological range for T. cordata is wide. The trees favour
good loamy site conditions, but they can be found on sandy infertile
soils as well. Next to oak, T. cordata is supposed to be one
of the most drought-resistant species. Dormant shoots of T.
cordata are reported to resist winter frost temperature down to
–34°C (T. platyphyllos to –25°C) (Till 1956). Tilia
is not troubled by spring frost or autumn frost, as flushing is late
and budset early. Tilia has been found in regions in Russia
with a short frost-free period (105 days) (Pigott 1991).
In some forests in eastern
Europe, T. cordata is found as a dominating species within
the canopy layer, but in most other places in Europe, T. cordata
grows in several types of mixed forest, i.e. commonly in the
Oak-Hornbeam forest type. Tilia is quite shade-tolerant, and
on some locations competitive with oak and ash.
Many insects and fungi may be
associated with Tilia (Pigott 1991), but none of them
seriously affects the vitality and distribution of the species.
Reproductive biology
General reproductive and
flowering biology of Tilia is described by Eisenhut (1957),
Anderson (1976), Pigott (1991) and Chistyakova (1982).
Tilia flowers at the end of
June and the beginning of July. Tilia cordata flowers later
than T. platyphyllos. Mast years are not frequent. Tilia
sets flowers at the age of 30, and at least 10 years earlier on
solitary trees. In northern Europe, seed regeneration is sparse,
e.g. Pigott (1981a). The problem of regeneration in England and
northern France is assumed to be caused by low temperatures at the
time of flowering and by too low temperatures to permit complete
development of the embryo (Pigott and Huntley 1981). In other
countries Tilia also shows poor seed regeneration: Finland
(Pigott 1981a; Ranta 1996), Denmark (Skov- og Naturstyrelsen 1994).
At least part of the individuals
within a population are self-sterile (Pigott and Huntley 1981), and Tilia
species are outcrossing. General seed properties have been described
by Suszka et al. (1994). One kilogram contains 25 000-30 000
seeds of T. cordata and 15 000-20 000 seeds of T.
platyphyllos. The germination is around 45% in central Europe.
The fruit is a nut and water can hardly enter it. The maturation of
the seed and germ is slow and normally the seed germinates 2 years
from harvest (Møller 1977). Seed from older trees has a higher
germination rate than seed from younger trees (Eisenhut 1957). Seeds
should be dried and stored at 5°C and can be stored no longer than
2-3 years (Barton 1934; Golosov 1938).
Within the sympatric range,
hybrids between T. cordata and T. platyphyllos have
been observed several times (Pigott 1969; Maurer 1995; Wicksell and
Christensen 1998). Hybridization and introgression are expected to
take place rarely, but possibly to an extent where the genetic
structure of the two species is affected. Like Quercus robur
and Q. petraea, leaf, flower and fruit morphology can be used
for identification of the species. However, no single morphological
trait alone can separate the species owing to large variation within
the populations for the single trait (Maurer 1995). Several traits
need to be investigated at the same time in order to separate the
species and to detect hybrids. Different variants of multivariate
statistics have proven to be efficient tools for such analysis
(Maurer 1995; Wicksell and Christensen 1998).
Tilia cordata and T.
platyphyllos can easily be reproduced by vegetative propagation
through cuttings and root layers. Cuttings can easily be propagated
even from old trees. As Tilia formerly was extensively
used for bast and honey production, this may have been an important
factor in the spread of Tilia and its use as a typical
agroforestry species in the Middle Ages (Pochberger 1967).
Vegetative propagation of T. cordata is reported by Sekowski,
Hrynkiewiecz and Sudenik (Bialobok 1991; Howard 1995). Somatic
embryogenesis has been carried out both for T. platyphyllos
and T. cordata (Chalupa 1990). Grafting and layering of Tilia
in Holland are described by Koster (1977).
Genetic variation
Investigations of morphological
characters of T. cordata have shown little variation between
populations (Pigott 1991). Leaf variability has also been recorded
by Czekalski and Kacz-marczyk (1978). Other studies of morphological
traits have been reported by Scheller (1972).
Descriptions of genetic
properties have rarely been reported. However, general genetic
properties have been described by Giertych (in Bialobok
1991). Further, a small Danish provenance trial from 1954 with two
Danish and three Swiss provenances revealed provenance variation,
but surprisingly no groupwise differences in growth (or quality)
between the provenances. In Germany, several provenance and progeny
trials were established in 1978, 1979 and 1982 including provenances
from several countries (Namvar and Spethman 1986). Results from
these trials have not been published (Kleinschmit, pers. comm.).
However, the trials have been used for provenance recommendations
for the northern German region (Anonymous 1995).
No provenance studies based on
the use of biochemical markers have been reported, but isoenzymes
have been successfully applied on specific genetic material (Maurer
1995), including more than one species and hybrids. Some studies
concerning identification of clones by isoenzymes have been reported
by Maurer and Tabel (1995). Cytology in Tilia has recently
been examined by Jonsson and Eriksson (1989).
Since lime trees are
insect-pollinated, relatively large between-population variation is
to be expected (Hamrick et al. 1992), as demonstrated for Acer
saccharum (Perry and Knowles 1989). The extensive fragmentation
and destruction of the biotopes of the species in some European
regions would also be expected to contribute to large variation
between populations. Inbreeding is expected to have an impact on
both T. cordata and T. platyphyllos (Kleinschmit et
al. 1996). In view of their reproductive biology and the
distribution of small populations, a small within-population
variation can be expected. Hybridization and widespread
domestication of foreign species and provenances may very well have
an impact on the existing genepool.
Ongoing gene resource management
of Tilia spp. in Europe
Many countries have adopted gene
resource conservation strategies for Tilia. Table 1 shows
information collected from reports of the first and second meetings
of the EUFORGEN Noble Hardwoods Network (Turok et al. 1996,
1998). The table indicates where actual progress is going on, but it
does not reflect the magnitude of the effort. The records concerning
in situ stands indicate plans for establishment of gene
reserves, nature reserves, seed stands or other in situ
conservation activities. Some data may be missing as several
countries had not concluded their report at that stage. For other
countries in situ efforts are included in other strategies
and reserves. In Denmark, a common strategy for gene resource
conservation has been adopted, but part of it still needs to be
implemented. A similar situation seems to exist in Switzerland
(Rotach 1996). Clonal or seedling seed orchards (ex situ
reserves) have been established in several places in Europe.
Table 1. Existing ex
situ and in situ efforts for gene resource conservation
of Tilia cordata in Europe (data extracted from EUFORGEN
reports and literature studies)
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In situ conservation
activities
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Ex situ conservation
activities
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Austria |
Müller 1996
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Müller 1996
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Switzerland |
Not implemented
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Not implemented (Rotach 1996 †)
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Denmark |
Not implemented
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Yes – Unpublished
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France |
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Germany |
Kleinschmit et al.
1996 †
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Rau et al. 1980, Piper
1981; Maurer and Tabel 1995; Kleinschmit et al. 1996 †
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Slovakia |
Longauer and Hoffmann 1996 †
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Labanc 1992, 1994; Longauer
and Hoffmann 1996 †
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Czech Republic |
Vancura et al. 1996 †
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Lithuania |
Baliuckas et al. 1996 †
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Latvia |
Baumanis et al.1996 †
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Sweden |
Ackzell and Eriksson 1998 ‡
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Ackzell and Eriksson 1998 ‡
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Finland |
Rusanen 1996 †
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Rusanen 1996 †
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Slovenia |
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Pavle et al. 1996 †
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Hungary |
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Tompa 1992
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Croatia |
Gracan 1996 †
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Gracan 1996 †
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Poland |
Korczyk 1998 ‡
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Bialobok et al. 1991;
Czart et al. 1989
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Ukraine |
Mazhula et al. 1998 ‡
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Russia |
Prokazin et al. 1998 ‡
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Romania |
Blada 1998 ‡
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Vancura et al. 1996 †
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† In Turok et
al. 1996.
‡ In Turok et
al. 1998.
In Denmark two clonal seed
orchards have recently been established (Jensen, unpublished) and a
small-scale breeding programme is planned. No specific information
has been available from France, Italy, Spain, Belarus, Estonia,
Norway, Holland and Belgium. Some countries lack necessary
inventories of potential stands. In other countries Tilia cordata
is a low priority species, and for that reason no action has been
taken. In some countries Tilia is not considered to be
endangered, it is of limited economical interest, and as such not
subject to gene resource conservation (i.e. Norway, ICPPGR country
report on forest genetic resources, 1997).
A gene conservation strategy for Tilia
spp. in Europe
Objective of the strategy
Gene conservation of Noble
Hardwoods species should be carried out to secure their ability to
adapt to environmental changes and the conservation activities
should cover the existing genepool, including both the central
distribution area and marginal sites. Conservation should guarantee
the evolutionary potential for the species as the existing genepool
only can be regarded as a transient stage of evolution.
Securing the basis for future
sustainable use of genetic resources is also an important objective.
Gene resource conservation does not per se secure long-term
sustainable forestry. Gene conservation activities should be
combined with ongoing or planned commercial breeding and improvement
programmes to secure the sustainable use of the reproductive
material.
Rare alleles should hardly be
targets for a common gene conservation strategy for Tilia:
conservation of rare alleles requires very large population sizes,
which give rise to practical problems. Further, rare alleles are not
likely to contribute to the evolutionary potential of the species in
question (Graudal et al. 1995).
The Multiple Population Breeding
System (MPBS)
The overall principle for gene
conservation, within both the central core region of the present
distribution and the marginal areas, should be the Multiple
Population Breeding System (MPBS). In order to cover the genetic
variation of a target species, where genetically different
populations have evolved through adaptation to different ecological
and environmental conditions, it is necessary to establish a network
of conservation stands (Graudal et al. 1995). The network
should cover the spectrum of ecological variability within the area
of distribution.
Within each country a national
strategy should be implemented. Programmes related to conservation
and breeding in all countries can contribute to conservation of the
total genepool. Relevant programmes include nature protected areas
and reserves, gene conservation areas, improvement programmes,
genetic trials etc. As the objective for conservation is dynamic,
the strategies should be flexible and be geared to cope with
adaptation and evolution (Eriksson et al. 1993).
Practical application of the MPBS
concept has been proposed for Quercus suber by Varela and
Eriksson (1995). A strategy of this type could also be developed for
Tilia spp. in Europe where it could efficiently protect
existing genetic variation and secure adaptive and evolutionary
processes.
Sampling strategies for gene
conservation of Tilia
The different patterns of
distribution and biology of Tilia in Europe call for
different gene conservation actions. In some areas Tilia is a
dominating species, but on many other sites Tilia is rare and
threatened. Both T. cordata and T. platyphyllos vary
in terms of rareness, distribution pattern, reproductive biology and
in vulnerability with regard to threats from pollen pollution from
exotic seed sources. In general Tilia occurs in mixed forests
and can even be an indicator for ancient forest. A number of
different tree species, e.g. Quercus spp., Carpinus
betulus, Ulmus spp., Acer spp. and several shrubs and
herbaceous plants, coexist with Tilia. Hence, in situ
conservation of Tilia may in many areas be carried out
together with associated species and gain from other ongoing or
planned conservation activities.
A preliminary step in preparing a
sampling strategy is the definition of regions in which there is no
ranking change between genotype with respect to fitness (Eriksson
1998). In other words there should be no genotype by environment
interaction for a sample of populations and sites within the region.
These regions should ideally be defined on the basis of tests
concerning several traits, but for most Noble Hardwoods species,
that type of research would be out of proportion with the economic
importance of the species. For practical purposes, the regions can
instead be identified on the basis of ecogeographic variation which
should be modified to take into account either expectations of
geneflow or general knowledge about regional genetic variation
within different species (Graudal et al. 1995).
In some marginal regions Tilia
species are rare, and conservation activities have to be targeted
toward single trees. This is the case in northern Europe for T.
platyphyllos.
Central core regions
Large gene reserves within the
central core of distribution will be very efficient for gene
conservation purposes and should be a first priority goal, as large
genetic variation is expected to be present in the core distribution
area.
Existing nature protected areas
(e.g. national parks) will only partly serve as gene conservation
areas, because national gene reserves often consist of
noncommercial, high-elevation forest (Ledig 1986) and are normally
not selected at random. Further, Tilia may in some cases have
disappeared from national gene reserves. Establishment of additional
conservation stands will therefore be necessary.
Marginal regions
In some regions, large gene
reserves of Tilia are not present at all, and the source may
be strongly fragmented. At the same time the occurrences may also be
subject to pollen contamination from new plantations originating
from non-local seed sources. In situ conservation per se
may not be effective or desirable. In some of the marginal regions
the regeneration of Tilia is missing or weak. The existing
genepool of Tilia is slowly but constantly declining. The
genepool is plastic and varying and the existing genetic variance
should generally be regarded as a transient stage.
In areas where Tilia is no
longer adapted to the prevailing climate, the relevance of gene
conservation efforts can be questioned. Still most countries have
decided to carry out gene conservation, presumably because Tilia
has significant historical value and the climate may change again
(Ledig 1993).
In the marginal regions where
situations with many small populations and risk for genetic drift
are typical, ex situ conservation of the genetic resources of
Tilia is recommended. Preferably ex situ conservation
stands should be established on the basis of reproductive material
from within the local regions, and as such the conservation efforts
will be related to in situ principles. In situ
conservation in marginal regions may include a larger number of
populations.
Other conservation activities
A precise overview of the status
for gene conservation of Tilia in Europe is lacking.
The further work of the EUFORGEN
Noble Hardwoods Network should include exchange of knowledge about
existing work being carried out and promotion of new initiatives and
projects. Limited research on genetic properties of Tilia is
also a constraint for the preparation of an efficient gene
conservation strategy. It is especially important that the different
ecological zones for the Tilia species be identified.
Ecogeographical zones may cross
borders between countries and many breeding and gene conservation
projects should therefore be carried out in close cooperation
between neighboring countries. Coordination and optimum use of
resources will allow for more conservation activities to be
implemented. These considerations are particularly relevant for Tilia
species, and the Network is an important tool to promote such common
actions.
Use and management of genetic
resources of Tilia
Breeding, improvement and
management of genetic resources of Tilia should be combined
with gene conservation. Combining use and conservation is especially
necessary for species of low economic interest, as it is unlikely
that sufficient resources will be made available for conservation
alone. Furthermore, the combination of conservation and use would
allow the evolutionary forces to work. In situ stands should
be maintained until they are able to regenerate. Some stands,
especially within the marginal parts of the distribution area, will
probably be very low in effective population size, and as the
geneflow between stands of Tilia will be limited, the next
generations could possibly be affected by inbreeding.
At some locations the lime trees
will be eradicated, if expensive precautions are not taken.
Alternatively, resources could be used to promote establishment of
new populations constructed from local seed collections. At other
sites, Tilia may be regenerated through coppice forest
management, and a large number of individuals can be kept alive this
way. However, this approach will allow some directional selection to
occur (for successful vegetative reproduction), and the conservation
is therefore quite static by nature.
A policy for sustained management
of areas with Tilia is complicated, as the species generally
occur in mixed stands. Choice of species which should be removed or
promoted is one problem. Another problem is the timing and strength
of thinning and general management practices, which balance gene
resource management and low cost.
Stands used for seed production
can contribute to conservation of genetic variation. However, the
stands will often be based on progenies from selected trees, which
in turn will influence gene conservation.
In special situations where
ecological regions cross country borders, seed transfers may be
necessary. International cooperation will be very valuable. Eriksson
(1996) has suggested international cooperation regarding gene
conservation of T. platyphyllos. This is also proposed by De
Vries (1996), as the species is very rare in the Netherlands.
Public awareness regarding
management of Tilia resources may promote local or national
gene conservation programmes. Forest owners may be quite active in
participating in such programmes, but often they are neither aware
of the biological value of their forest, nor of the possibilities
for economic support for conservation activities.
Legislation and seed transfer
rules
It should be ensured that
reproductive material for forestry and open landscape plantings is
used in a proper way, and the origin of the material well
documented. Tilia cordata has recently become part of the
OECD scheme, which has improved this important aspect of gene
conservation. Furthermore, national rules have been implemented in
different countries. Tilia cordata was enrolled in the German
regulations in 1979, and before that time most plantings are assumed
to have been established with material from southeastern Europe
(Namvar and Spethman 1986). Still many countries have adopted a
rather liberal legislation in this field and reproductive material
of T. cordata has evidently been widely moved from one
country to another.
Concerning certification in
general, there is an increasing public awareness, and many people
require reproductive material of local origin. It is obviously an
important task for each country to promote the use of the most
suitable genetic material.
Conclusions
In many places in Europe, Tilia
species are vulnerable to human impact and their genetic variation
may be endangered unless gene conservation activities are
undertaken. In most European countries, gene conservation activities
are taking place, and Tilia species are often considered as
very valuable - mostly for landscaping or cultural and historical
reasons. However, international cooperation is important because the
distribution of the Tilia species is fragmented, and because
borders of the distribution areas do not follow country borders. The
use of the Multiple Population Breeding System (MPBS) is
recommended.
Acknowledgements
We would like to thank Alfas
Pliura and Mari Rusanen for excellent inspiration from their
respective strategies for Fraxinus and Acer. We also
thank Mari Rusanen, Gösta Eriksson, Jochen Kleinschmit and Erik
Kjær for constructive remarks on the draft of this document.
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