Full text of DOI:10.1093/forestry/73.5.499

The root system s of Fraxinus
excelsior and Fagus sylvatica and
their com petitive relationships
S. RUST1 AND P.S. SAVILL2
*
1
Institut für Forsbotanik der Georg-August-Universität, Göttingen, Germany
Oxford Forestry Institute, Department of Plant Sciences, University of Oxford, Oxford OX1 3RB, England
2
* Present address: Institut für Forstbotanik und Forstzoologie der TU Dresden, Tharandt, Germany
Summary
Following reports from field foresters on so-called ‘beech regeneration dieback’, the root systems and
competitive relationships of sapling-stage mixtures of beech (Fagus sylvatica L.) and ash (Fraxinus
excelsior L.) were investigated in vivo and in vitro. Ash has a plate-root system from an early age,
and beech a heart-root system. The distribution of ash roots is determined by competition for light
under a closed overstorey of beech while the distribution of beech roots changes if there is
competition with ash roots. In a replacement series experiment in a greenhouse, interspecific
competition for water strongly reduced growth and survival of beech saplings. It is concluded that
this competition for water could be the reason for beech regeneration dieback on sites where it
occurs.
Introduction
(Mayer, 1977; von Lüpke, 1989) and consequent
poor establishment, competition for water and
In some parts of Germany foresters have observed light, rodent damage and deer browsing (Wagner,
that on certain sites in naturally regenerated 1990). Too extensive an opening of the canopy
mixed stands of beech and ash, the beech dis- above natural regeneration with too low a pro-
appears before about age 20 years. This phenom- portion of beech seedlings may also be a cause
enon is known as ‘Vereschung’ or beech (Faust, 1963; Conrad, 1966; Wagenhoff, 1975).
regeneration dieback. In pure ash stands grass
Until now, there has been no scientific investi-
cover is dense and tree dry matter increment slow gation into dieback, nor any survey of affected
(Dengler, 1980; Roloff and Peik, 1990). A pro- sites or stands. The reported growth check of
portion of beech is desirable to shade out the beech in pure ash stands has not been supported
grass and to keep the boles of the ash free from by any data, nor have there been any attempts to
epicormic shoots.
explain the death of beech seedlings after heavy
Many factors could cause the dieback of beech. or sudden openings of the canopy.
They include moisture deficits on shallow soils
This paper sets out to describe the root systems
© Institute of Chartered Foresters, 2000
Forestry, Vol. 73, No. 5, 2000

500
FO R EST RY
of beech and ash saplings in two naturally regen- Excavations of roots
erated stands quantitatively, and to evaluate
In order to investigate their structure and distri-
possible reasons for differences between them.
The ecological relevance of differences is exam-
ined with special reference to beech regeneration
dieback. Interactions between the root systems of
the two species, especially in terms of competition
for water, are investigated. More detailed quali-
tative descriptions of the excavated root systems
can be found in Wedler (1991).
butions, whole root systems of saplings as well as
parts of them were excavated. Ten pairs consist-
ing of one beech and one ash in close proximity
(20–30 cm apart) and of equal height (2–2.5 m)
were chosen to ensure as comparable conditions
as possible for the two individuals. The methods
employed for the excavations of entire root
systems and for the profile trenches are similar to
those described in Böhm (1979).
Sites and methods
Core sampling
Most of the excavations and all the soil core sam-
pling described below were carried out in two
adjacent compartments (143 and 145) of Boven-
den forest near Göttingen in Lower Saxony,
Germany. The compartments are situated on a
limestone plateau at 370 m above sea level. The
soils are rendzinas and pseudogley–brown earths
of variable depths (5–60 cm). The nutrient status
is good, but some soils, especially the rendzinas
are liable to drought. The climate is subatlantic.
The vegetation is as variable as the soils. On
sites with loess, the plant community is a species-
rich Melico–Fagetum. Where the loess layer is
shallow or absent it changes to a Lathyro–
Fagetum. The presence of Dechampsia caespitosa
(L.) Beauv. indicates waterlogging on loess soils.
Dactylis glomerata L. and Stellaria holostea L.
indicate a liability to summer drought.
In one compartment, 123-year-old beech made
up 95 per cent of the growing stock and ash the
remaining 5 per cent. The stand was fully
stocked, having 100 per cent of the yield table
basal area for that age. Natural regeneration of
ash and beech occurred only in small gaps. This
is the ‘shade’ variant.
In the other compartment the beech was
131 years old. The stocking was low, at 20 per
cent of the yield table basal area, and the canopy
consequently open. This allowed an understorey
of naturally regenerated ash (70 per cent) and
beech (30 per cent) to grow. The average age of
the regeneration was 22 years but, as with height
and composition, it was very variable. Because of
the very low overstorey stocking, this is termed
the ‘full light’ variant. The ground flora in this
compartment was relatively dense compared with
that in the shade variant.
Ten sample plots were established in each com-
partment. Each plot was located at random but
was at least one crown diameter in distance from
the closest mature beech, since most roots of
beech do not extend further (Hilf, 1927). Plots
were selected that contained different proportions
of beech to ash saplings, ranging from 10 per cent
beech and 90 per cent ash, 30 per cent beech and
70 per cent ash, up to 90 per cent beech and 10
per cent ash. Each plot was then sampled as
follows.
A beech tree 2–2.5 m tall was chosen as the
centre of the plot. At 10 cm from it and on oppos-
ing sides (north and south), two soil cores were
taken with an 8 cm diameter soil corer to 50 cm
depth, just below the root zone. They were cut
into 5 cm slices. The roots were extracted with a
semi-automatic soil washing system, and then the
living ash and beech roots were separated, by
species, dried at 80ºC and weighed. It is easy to
distinguish between the white ash roots and the
reddish beech roots. Because of the constraints of
time, dry weights were used rather than measur-
ing root length density in the samples. The latter
is often considered to be a better measure of root
activity, and in this case could have been quite sig-
nificant because beech has finer roots than ash.
Greenhouse replacement series experiment
In March 1991 a factorial experiment was laid
out in Oxford to study the competitive interac-
tions of beech and ash grown in mixture. 300
beech (2 years old, 40–80 cm tall) and 300 ash
trees (1 year old, 49–65 cm tall) were arranged in
100, 20 cm diameter pots to give replacement

C O M PET IT IVE R ELAT IO N SH IP BET W EEN BEEC H AN D ASH R O O T SYST EM S
501
series with 0, 25, 50, 75 and 100 per cent beech. primary root is a taproot, which often turns to
In 50 pots the density was four plants per pot, in horizontal growth. No reason for this behaviour
the other 50 it was eight. The densities, equival- was apparent from the soil profiles. The exten-
ent to 0.96 million and 1.92 million trees ha–1, siveness of the root system is in stark contrast to
were well within the range reported for natural beech.
regeneration. Half of the pots were well watered,
and the others were watered (all with the same
amount of water) as soon as the beech showed
Core sampling
indications of wilting to give a ‘drought’ treat- Core sampling gave a much more comprehensive,
ment. The pots were arranged in 10 blocks, the quantitative picture of the rooting of the two
blocking criteria being heights of the plants and species. The analysis concentrated on the distri-
location in the greenhouse. At the end of July bution of roots of less than 2 mm in diameter. The
1991 the trees were harvested and the dry weights saplings only rarely developed roots thicker than
of the roots and leaves were recorded as well as this because of their youth. There was a very high
the current and previous years’ shoots.
variability in the data; coefficients of variation
were in the order of 100–300 per cent. Table 1
compares the distribution of fine roots of beech
and ash in light and shade.
In terms of dry weights, the root biomass of
beech decreases relatively slowly with depth,
whereas that of ash in full light drops very
Results
Root systems in the field
Beech The roots of beech are much finer than quickly. These are the typical features of ‘heart-
those of ash and whereas the main direction of root’ and ‘plate-root’ systems, respectively. The
growth is horizontal in ash, it is downwards, at root biomass of beech is always much greater
an angle of roughly 45º in beech. The beech roots than that of ash, and only in the top 5 cm are they
divide early into increasingly fine rootlets and end of similar magnitude, contrasting the very exten-
in fine tips. By contrast, the fine roots of ash sive surface rooting of ash and the intensive
usually end suddenly and bluntly. A hemispheri- rooting of beech. The data showing relative root
cal volume under the base of each tree is distributions in Table 1 illustrate the different
extremely intensively rooted. Beech roots do not strategies. In the ‘light’ variant almost 60 per cent
extend very far and the rooting, especially in the of ash roots are close to the surface. The beech,
top 5–10 cm is very intensive, although where however, roots much more uniformly with depth
there is a high proportion of ash and sufficient and has only 30 per cent in the top 5 cm.
light, the soil is dominated by tough ash rootlets.
The distribution of beech roots in the different
soil levels, and the relative distributions of the
Ash The ash root systems are superficial but far- roots are the same in both the ‘shade’ and ‘light’
reaching, with tough horizontal roots that send variants (Table 1). The ash, however, is strongly
laterals vertically downwards. The main hori- affected by light. In shade its root mass in the top
zontal roots decrease rapidly in diameter to 5 cm is only a quarter of that in full light. Biomass
2–4 cm and then extend rope-like through the at lower levels in shade does not differ signifi-
upper soil horizons. The fine lateral roots are con- cantly to that at similar depths in the light. In
centrated in clumps and between them are root- addition, the relative distribution of ash roots in
free zones. The clumps often originate directly shade is very similar to that of the beech.
from much thicker ‘ropes’ and are concentrated
Further analysis of the data (see Table 1)
in the upper horizon. The superficial rooting is reveals that the rooting patterns are not only
very intensive and dominates the upper 0–5 cm of affected by physical factors like light and those
the soil profile (0–5 cm). Single roots of trees as which are integrated by the effect of ‘depth’, but
small as 2–2.5 m tall extend for several metres also by the presence or absence of competing trees
and end in a tuft in the litter layer after travelling and their vigour.
at a depth of down to 20 cm, only rarely branch-
Under ‘full light’ conditions ash, a pioneer
ing and with an almost constant diameter. The species, is much more vigorous than in the

502
FO R EST RY
Table 1: Absolute (g per 250 cm3 soil) and relative (percentage of total root dry weight) distributions of fine
root biomass by soil depth
Ash
—————————————————–—— ———————————————————
Shade Light Shade Light
—————————– —————————– —————————— ————————–
Beech
Soil
depth
(cm)
g
%
G
%
g
%
g
%
0–5
5–10
86
81
34
111
65
13
22
21
9
28
17
3
392
107
45
63
20
59
16
7
10
3
414
285
262
236
102
139
1438
29
20
18
16
7
429
294
260
209
166
151
1059
29
19
17
14
11
10–15
15–20
20–25
25–30
Total
34
661
5
100
10
100
10
100
390
100
Each figure is a mean of 20 core samples over the whole range of beech and ash saplings selected.
compartment fully stocked with beech, and con- 0–30 cm depth). In the 5–10 cm horizon this
sequently has a larger root biomass.
relationship is shown in Figure 1.
There were significant correlations between the
In the 5–10 cm horizon the proportion of beech
percentage of beech saplings in the ‘full light’ roots diminished as numbers of ash trees in the
compartment (i.e. low mature beech overstorey) sample plot increased, but in the 10–15 cm
and the ratio of its root biomass in any one horizon (not shown) the proportion of beech
horizon to the total beech root biomass found in roots increased as the proportion of ash above-
the whole soil profile sampled (i.e. between ground decreased. No such patterns were found
Figure 1. Light variant compartment. Ratio of beech root biomass at 5–10 cm depth to the total biomass
2
between 0 and 30 cm soil depth plotted against the proportion of beech saplings in the sample plot. The r
value is 0.90. Each point represents the results from 10 sample trees.

C O M PET IT IVE R ELAT IO N SH IP BET W EEN BEEC H AN D ASH R O O T SYST EM S
503
in the top 0–5 cm. No similar correlation was planting density affected the production of new
found for ash root growth in the ‘full light’ com- shoots, leaves and roots of both species though
partment.
In the fully stocked ‘shade’ compartment no watering treatment had less well-marked effects.
correlations of the kind shown in Figure 1 existed The percentage of beech present in each pot
the effects were particularly strong with ash. The
for beech, but the proportion of fine root biomass had significant effects on the production of roots
of ash in the 0–5 cm layer diminished as the pro- per tree of both ash and beech – the greater the
portion of beech saplings increased (r2 = 0.52, percentage of beech present the more were pro-
P > 99 per cent).
duced. The percentage of beech also significantly
affected new shoot and leaf production in ash,
but not in beech. With beech, dry weights of
leaves, shoots and roots per tree were respectively
Greenhouse replacement series experiment
Establishment and survival The ash flushed 42 per cent, 39 per cent and 31 per cent greater
early in April and all trees survived to the end of when comparing pots with 100 per cent beech
the experiment, but this was not so for beech. In with those with 25 per cent beech. In ash, the con-
May 80 per cent of the beech had flushed but trasts were much bigger, all being around 200 per
some of the remainder produced only small cent.
shoots and their growth was minimal (hereafter
Planting density (four or eight plants per pot)
referred to as ‘dead’). During late July 20 per cent also had significant but contrasting effects on the
of the surviving beech became desiccated during weights of new shoots, leaves and roots. In ash,
a particularly hot period that occurred immedi- the effect of increasing the density was to reduce
ately before the harvest (the maximum tempera- production per tree, whereas in beech it
ture in the greenhouse was 35ºC). The leaves of increased. For example, mean leaf weight of ash
entire trees wilted and dried out rapidly, often per tree decreased by 30 per cent at the higher
within a matter of hours. Both treatments had density (5.72 g per tree to 3.96 g), while in beech
been watered well in an attempt to prevent the it increased by 49 per cent (0.74 g to 1.10 g).
trees from becoming dehydrated.
Increasing proportions of ash in the pots sig-
nificantly reduced survival of the beech (P > 99.9
per cent using arcsine-transformed data). Mortal-
ity was 10 per cent in the pure beech pots and 82
per cent where beech was only 25 per cent of the
ash/beech mixture. There were no other signifi-
cant effects or interactions.
The proportion of beech that became desic-
cated in late July was mainly affected by the pro-
portion of ash in the pots: the more ash the higher
the risk of wilting, the rate significantly increas-
ing from 0 per cent desiccation in pots with 100
per cent beech to 43 per cent in those with 75 per
cent ash.
Replacement series Replacement diagrams such
as those shown in Figure 2 provide a method for
analysing and displaying competitive relation-
ships of two species grown in replacement series.
They are constructed by dividing the biomass
attained by a species in one treatment by the
biomass attained in the pots with 100 per cent of
that species. Adding the relative yields of the two
species gives the relative yield total (RYT). A RYT
of near unity indicates competition for the same
resource whereas a RYT below it points to mutu-
ally damaging effects and a RYT greater than
unity shows that competition is less severe
between species than within species. A concave
response such as that for beech in both parts of
Biomass growth per tree The effects of the Figure 2 indicates that the species is suffering
treatments on individual trees are summarized in interspecific competition, since the yield-density
Table 2 for the 5-month period of the experiment. response in monoculture can only be linear or
Statistical analyses were carried out with the convex. The convex response found for ash in this
Minitab General Linear Model, using the dry experiment indicates that ash suffers more from
weights of old shoots (i.e. the shoots that existed intraspecific than from interspecific competition.
at the time of planting) as covariates where they This is also confirmed by the figures shown in
improved the significance of results. In general, Table 2 when calculating the effect of substituting
the percentage of beech in each pot and the one beech for one ash.

504
FO R EST RY
Figure 2. Replacement diagrams for leaf dry weight. For explanation see text.
Discussion
far, though in comparison with mature trees the
rooting is very shallow.
The character of ash root systems changes dra-
Ash root systems
All results from this study indicate that the plate matically in the shade of tree canopies where
root system of ash is developed from an early age, rooting patterns cannot be distinguished from
when it grows in open conditions. Saplings those of beech. In the surface horizon (0–5 cm)
exhibit most of the features reported for the few rooting is reduced by 78 per cent compared to ash
mature specimens that have been excavated so trees growing in the open. A likely cause of the

C O M PET IT IVE R ELAT IO N SH IP BET W EEN BEEC H AN D ASH R O O T SYST EM S
505
Table 2: Mean weights (g per tree) of new shoots, leaves and roots
Ash
Beech
————————————————————–– ———————————————————––
Treatment
New shoots
Leaves
Roots
New shoots
Leaves
Roots
Species proportion (% beech)
0
1.59
1.90
2.52
3.29
–
3.56
4.20
5.19
6.42
–
21.07
25.65
29.52
40.99
–
–
–
–
25
50
75
100
SEd
P*
0.28
0.20
0.38
0.39
0.11
0.238
0.79
0.79
0.98
1.12
0.19
0.200
5.83
7.17
5.85
7.64
1.10
0.017
0.88
0.000
1.40
0.000
6.80
0.000
Planting density (plants per pot)
4
8
SEd
P*
2.96
1.69
1.08
0.000
5.72
3.96
1.50
0.000
33.73
24.88
2.21
0.23
0.40
0.14
0.034
0.74
1.10
0.30
0.006
5.68
7.56
1.57
0.016
0.002
Watering
‘Drought’
Well watered
SEd
2.11
2.54
0.37
0.063
4.47
5.22
0.63
0.052
28.23
30.39
2.43
0.25
0.38
0.11
0.087
0.80
1.04
0.20
0.045
6.16
7.09
0.78
0.220
P*
0.438
Overall mean
2.32
4.84
29.31
0.31
0.068
6.62
* P = probability of a significant difference between means.
difference is the comparatively low level of light. in the surface layer and become more of a heart-
In shade, mortality of ash is high and young trees root system.
often have an etiolated appearance. Apparently
the root:shoot ratio changes in favour of the
shoot. Helliwell and Harrison (1979) found that
Beech root systems
root: shoot ratios in 2-year-old ash seedlings fell The root system of beech in the naturally regen-
from 2.21 at 82 per cent full light to just 0.76 at erated stands that were studied is a typical heart-
7 per cent.
root system. The root distribution and biomass
A further reason for the change is indicated are independent of the light conditions within the
from the core sampling data, which showed that range observed. This is surprising, because it is in
the proportion of ash roots in the top 5 cm is contrast to the findings of Burschel and Huss
reduced where there is more beech (Table 1). This (1964) and Burschel and Schmaltz (1965a, b).
could either be caused by shade from the beech They found that beech root biomass reacts to
saplings, or, more likely, from competition with only small reductions in light and that the reduc-
the beech roots.
tion in root growth is more pronounced than that
Although the shade tolerance of ash seedlings of the shoot. Since sapling densities in this study
(e.g. Gia, 1927; Dengler, 1944; Miegroet, 1956) were similar in both light and shade, the findings
and the inflexibility of the ash root system presented here seem to provide a clear indication
(Köstler et al., 1968) have been much empha- of the shade tolerance of beech saplings, which
sized, the changes in root, and certainly shoot retain their root biomass in shade.
morphology are great. The root systems of ash
The shift downward in the root distribution of
saplings growing in shade lose their plate-root beech, which is correlated with an increasing pro-
structure with the typical concentration of roots portion of ash above ground (see Figure 1) could

506
FO R EST RY
be an indication of root competition with ash (Eastham et al., 1990). Situations are rare where
and/or with grasses that develop where ash is deep reserves dry up and only small precipitation
common. If ash is a more successful competitor events occur, thus making a shallow and exten-
for water and can deplete the soil moisture to sive root system of greater adaptive value than a
levels below those accessible to beech roots, ash deeply penetrating one (Schlichter et al., 1983).
could reduce soil water potential to an extent that The fact that ash saplings survive droughts better
causes death of fine roots of beech and reduced than beech (Faust, 1963; Röhrig, 1966a, b),
root extension growth. Göttsche (1972) has despite their shallow rooting, combined with the
shown that in dry soils the root biomass of beech results of the greenhouse experiment, indicate
can decline sharply. That interspecific compe- that ash is relatively drought resistant, because it
tition can reduce root biomass and change its is more likely to experience dry soil throughout
distribution has been reported by Richardson the soil volume exploited by its roots.
(1953), quoted in Messenger (1976) for Acer
On the sites investigated, roots had no access
pseudoplatanus L., by Farré (1979) quoted in to groundwater. On the sites threatened by beech
Atkinson (1983) for fruit trees, by Nielsen and regeneration dieback, shallow soils on limestone
Mackenthun (1991) for Fagus sylvatica L. and predominate. The water storage capacity in the
Picea abies (L.) Karsten and by Noordwijk et al. rooting space is low at 80–120 mm though the
(1996) for many other trees and crops. They water storage capacity per unit volume of soil is
emphasize the importance of the top 5 cm of soil high, about 25 mm per 10 cm. The roots of beech
for nutrient procurement and obtaining moisture and ash are largely confined to the same space
from light showers in summer.
because of the shallow soils (rendzinas). Compe-
tition for soil resources, especially water, is
necessarily intense. The low water storage capac-
ity is a threat to the beech. Beech roots are turned
Greenhouse replacement series experiment
The greenhouse experiment supports the con- over rapidly and in dry horizons their biomass
tention that ash is more drought tolerant than declines quickly (Göttsche, 1972). The green-
beech (Faust, 1963; Röhrig, 1966a, b), and that house experiment has shown the strong potential
it has the potential to reduce severely the chances of ash to reduce the viability of beech seedlings,
of survival of beech seedlings. It has been clearly where their roots are confined to the same space.
demonstrated that the likelihood of mortality of The possible trend towards hotter and drier
beech is strongly correlated with the proportion summers could make the situation worse. A
of ash in the mixture. A. Roloff and M. Wedler drought year occurs about once every 10 years in
(personal communication) found substantial much of Europe.
numbers of recently dead beech in mixture with
ash after prolonged periods of low rainfall in the ance of ash and weaker competition by beech for
late summers of 1989 and 1990. water with vigorous ash saplings are likely to be
It is postulated that the greater drought resist-
It seems possible that beech seedlings can tol- important causes of beech regeneration dieback.
erate strong competition for water without reduc- The latter is supported by observations by
tion in growth rate up to the point where it is Conrad (1966), Wagenhoff (1975) and Beck and
lethal. Ash is adapted to regeneration in larger Göttsche (1976), who found pure groups of ash
gaps and therefore copes with competition with mostly where the canopy had been opened too
plants of similar size where resource pre-emption fast and too early. This allows ash to develop a
is possible and of adaptive value. It might there- dense and superficial root system that is not only
fore be able to deplete soil resources rapidly.
better adapted to water procurement in drought
periods but also outcompetes the beech saplings.
Ecological implications of root distributions
Silvicultural implications
In temperate regions, the availability of water in
periods of drought increases with depth and the Over a whole rotation, beech and ash compete on
proportion of water drawn from deep horizons unequal terms. During the first 60 years or so, ash
increases with the duration of
a drought dominates the upper storey of the regeneration

C O M PET IT IVE R ELAT IO N SH IP BET W EEN BEEC H AN D ASH R O O T SYST EM S
507
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phase entails risks because ash is very likely to
lack the necessary growth advantage to survive
later in the rotation without a high management
input. A rather dense beech overstorey has the
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Talsma, T. and Charles-Edwards, D.A. (1990)
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seedlings for water and light but also provide a
habitat for rodents that can severely damage
beech seedlings.
A mixture by groups reduces the interface
between the two species. It should therefore be
possible to maintain the growth advantage of ash,
without risking the failure of beech. Thus, it is
necessary to direct the regeneration towards a
sufficient density of beech saplings by opening the
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problem, since ash reproduces regularly and
abundantly and the saplings grow rapidly as soon
as the canopy is opened. Then groups of pure
beech and pure ash of the size of at least one
crown diameter width of a mature tree of the
species can be created.
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Received 30 April 2000