Final thesis
Age and size of hollow oaks and their associated
lichen flora and beetle fauna
Niclas Berg
LiTH-IFM- Ex--05/1605--S
zzzzzzzzzzzzzzzzzzzz
Avdelning, Institution
Division, Department
Datum
Date
Avdelningen för biologi
06 06 08
Institutionen för Fysik och Mätteknik
Språk
Language
Svenska/Swedish
Rapporttyp
Report category
ISB
LITH-IFM-Ex--05/1605--SE
Licentiatavhand-ling
X Examensarbete
IS
LiU-Biol-Ex-563
C-uppsats
X D-uppsats
Serietitel och serienummer
Title of series, numrering
-----
X Engelska/English
Övrig rapport
____
ISSN
Handledare: Karl-Olof Bergman
Ort: Linköping
Titel /Title: Age and size of hollow oaks and their associated lichen flora and beetle fauna
Författare /Author Niclas Berg
Sammanfattning/ Abstract:
In Sweden the tree with most associated species is the oak (Quercus robur). More than 200 red-listed beetle species have been found in
oaks. For conservational purposes it is important to understand the succession of associated species and at what age the oaks become
suitable for the more rare species. The age of 77 hollow oaks in the oak landscape south of Linköping, Sweden was determined with tree
ring based methods. In 1994, saproxylic beetles were trapped in window- and pitfall traps placed in the trees. The beetle data was
analysed in relation to age and hollow stage. No significant differences in beetle species richness were found between older and younger
hollow oaks, or between the hollow stages. On younger oaks, the correlation between age and increasing diameter and depth of bark
crevices was good. In older oaks the deviation was greater. Bark crevice depth was a slightly better indicator of age than was diameter.
The mean age of the hollow oaks in this study was 327 years. There were no significant differences between the hollow stages; the mean
age was about the same for all hollow stages. Out of five studied lichen species (Chaenotheca phaecephala, Cliostomum corrugatum,
Calicium adspersum, Schismatomma decolorans and Sclerophora coniophaea) all, except Ch. phaecephala, had higher frequencies in
oaks >300 years old. Two species occurred in higher frequencies in oaks with bark crevices >50 mm (Sch. decolorans and Scl.
coniophaea).
Nyckelord/ Keywords: Quercus robur, tree age, hollows, beetles, lichens, growth rate
1
1. Abstract ..................................................................................................... 1
2. Introduction ............................................................................................... 1
2.1 Aims of the study ................................................................................. 3
3. Materials and methods .............................................................................. 3
3.1 Study areas ........................................................................................... 3
3.2 Determination of oak age..................................................................... 4
3.3 Tree specific parameters and lichen frequencies ................................. 6
3.4 Survey methods.................................................................................... 6
3.5 Analyses ............................................................................................... 7
3.5.1 PCA & RDA .................................................................................. 7
3.5.2 Other statistics ............................................................................... 7
4. Results ....................................................................................................... 8
4.1 Tree age................................................................................................ 8
4.2 Age and size relationships ................................................................... 8
4. 3 Occurrence of lichens ....................................................................... 10
4.4 Growth rate ........................................................................................ 12
4.4.1 Stone coverage and moisture....................................................... 12
4.4.2 Openness ..................................................................................... 13
4.5 Age and hollow stage......................................................................... 14
4.6 Beetle statistics .................................................................................. 15
4.6.1 Osmoderma eremita .................................................................... 15
4.6.2 Species richness ........................................................................... 16
4.6.3 Red-list score ............................................................................... 17
4.6.4 PCA and RDA ............................................................................. 18
5. Discussion ............................................................................................... 19
5.1 Age determination of hollow oaks ..................................................... 19
5.2 Growth rate ........................................................................................ 21
5.3 Lichens ............................................................................................... 22
5.4 Age and hollow stage......................................................................... 23
5.5 Species richness and Osmoderma eremita frequencies ..................... 24
6. Acknowledgements ................................................................................. 25
7. References ............................................................................................... 26
Appendix 1………………………………………………………………..30
1. Abstract
In Sweden the tree with most associated species is the oak (Quercus robur).
More than 200 red-listed beetle species have been found in oaks. For
conservational purposes it is important to understand the succession of
associated species and at what age the oaks become suitable for the more
rare species. The age of 77 hollow oaks in the oak landscape south of
Linköping, Sweden was determined with tree ring based methods. In 1994,
saproxylic beetles were trapped in window- and pitfall traps placed in the
trees. The beetle data was analysed in relation to age and hollow stage. No
significant differences in beetle species richness were found between older
and younger hollow oaks, or between the hollow stages. On younger oaks,
the correlation between age and increasing diameter and depth of bark
crevices was good. In older oaks the deviation was greater. Bark crevice
depth was a slightly better indicator of age than was diameter. The mean
age of the hollow oaks in this study was 327 years. There were no
significant differences between the hollow stages; the mean age was about
the same for all hollow stages. Out of five studied lichen species
(Chaenotheca phaecephala, Cliostomum corrugatum, Calicium adspersum,
Schismatomma decolorans and Sclerophora coniophaea) all, except Ch.
phaecephala, had higher frequencies in oaks >300 years old. Two species
occurred in higher frequencies in oaks with bark crevices >50 mm (Sch.
decolorans and Scl. coniophaea).
Keywords: Quercus robur, tree age, hollows, beetles, lichens, growth rate
2. Introduction
The oak (Quercus robur) is the Swedish tree with most associated species.
Over 500 insect species are associated with oaks (Carlsson 2004). There
are many invertebrates that are specialists on oaks, 58 of them are red-listed
(Jonsell et al. 1998). This is most likely because Q. robur can attain high
ages, probably more than 1000 years, and can live for a long time as a
hollow tree. It is when the tree develops hollows that it becomes an
interesting environment for many red-listed insect species, among them
202 red-listed beetle species (Jonsell et al. 1998). Usually the highest
biological values are connected to large, solitary and hollow oaks in
pastures (Carlsson 2004).
Although much work has been done on saproxylic beetles living in
hollow oaks (i.e Nilsson et al. 1994, Ranius and Jansson 2000, Ranius
2002) little is known about how old these oaks are. Palm (1959) did
extensive work on saproxylic beetles and their habitat requirements. He
concluded that large, hollow oaks sustain the most diverse fauna of
1
saproxylic beetles (Palm 1959). He did not study, however, at what age or
size the tree becomes a suitable environment for the beetle species.
Determination of the age of hollow oaks is a good way to improve
knowledge about the tree and its inhabitants. It is difficult, since the
innermost rings are often missing because the trunks are hollow, due to rot
(Niklasson & Fritz 2003). The rot is caused by saproxylic fungi that
colonize older oaks (e.g. Ranius et al. 2001b). Hollows are believed to
generally develop at age 150-200 years (Ranius et al 2001b), but have been
found in oaks 100 years of age (Carlsson 2004). At what age does the
hollow of an oak become a suitable environment for a given species of
saproxylic beetles, and what does the succession of saproxylic beetles in
the hollow look like? Osmoderma eremita is a red-listed beetle that has
been studied quantitatively (e.g. Ranius 2000, Ranius 2001). Since much is
known about this species it would be interesting to know its occurrence
pattern in relation to age and hollow stage.
The county board of Östergötland has developed a method for
classifying hollow oaks into 4 hollow stages, depending on the diameter of
the entrance of the hollow (Jansson 1995). The method has since been
revised, e.g. three stages without hollows are now included. The hollow
stages are now called stage 4 through 7, where stage 4 oaks have small
hollows (<10 cm) and stage 7 oaks have totally hollow trunks with
openings at ground level (Jansson 1998). This method does not, though,
account for age. For nature conservation purposes, it is important to know
more about hollows and the age of hollow trees. Is it possible to use hollow
stage as a measure of age? This is information that could be crucial in order
to plan the age structure of oaks in nature reserves so that there are always
enough trees of the right age for red-listed species.
On young oaks the lichen flora mainly consists of generalist lichen
species but with increasing age, bark crevice depth and girth, the flora
changes to oak-specific species (e.g. Rydberg 1997). In Östergötland, 25
red-listed lichen species associated with oaks have been found (Ek et al.
2001). The crevices in the rough bark of older oaks are a refuge for several
demanding lichen species (Arup et al. 1997). These lichen species have
commonly been thought to be associated with the large, old oaks of the
pastures but Ek et al. (1995) studied red-listed lichen species on precipices
in Östergötland and found many of the species, among them four of the
lichens in this study, on thin, though old, oaks. Thus, the question is raised
about what determines the lichen flora of oaks and if it is possible to
estimate age from studying the lichens colonizing the trunk.
Most of the trees in this study grow in enclosed pastures, kept open
by cattle and are half-open but some sites are forested and some oaks stand
2
on flat rock. The different rates of sun-exposure and ground conditions
(moisture, the amount of stones on the ground) at these different localities
may greatly affect the growth rate of an oak. This gives an opportunity to
investigate the influence of growing conditions on the growth rate and a
chance to see how diameter and age really affect the beetle fauna, the
lichen flora and the creation of hollows.
2.1 Aims of the study
The aim of this study was to examine the relationship between age and size
of oaks. The study also aimed at testing the theory of hollow stages in
relation to age and beetle fauna. I wanted to know if it could be possible to
estimate the age of oaks from diameter and bark crevice depth as well as
from their associated lichen flora. In order to properly answer the
questions, growth rate, and factors affecting growth rate, was also
examined.
3. Materials and methods
3.1 Study areas
The surveys were all carried out in Bjärka-Säby and its surroundings
(appendix 1), situated in the province of Östergötland, southeastern
Sweden. Bjärka-Säby and its surroundings is today one of the few
remaining areas in Northern Europe where the density of old oaks is high
(Ranius et al. 2001a). Within this area, there are three stands with higher
densities of old oaks than the surroundings. In Sturefors and Bjärka-Säby,
there are core areas of particularly high densities. Brokind, although not as
high in density of old oaks as Sturefors and Bjärka-Säby also stands out. A
total of 16 sites were chosen. A study on saproxylic beetles was performed
in Östergötland in 1994 (Ranius & Jansson 2000, Ranius & Jansson 2002).
Traps were set in around 90 hollow oaks in sites south of Linköping. In this
study I have determined the age of 78 of these trees. Ten of the localities
were wooded pastures grazed by cattle, sheep, horses and deer. Of the other
6, all are now forested and three of the stands grow on hills and flat rock.
The number of hollow trees included from each site is shown in table 1.
3
Table 1: The number of trees on the 16 sites for which it was possible to
determine the age.
Nr of
hollow
Site
trees
1. Bjärka äng
5
2. Brokinds skolhage
8
3. Hjorthägnet N
5
4. Hjorthägnet S
5
5. Kalvhagen
5
6. Labbenäs
7
7. Långvassudde N
3
8. Långvassudde Ö
5
9. Orräng I
5
10. Orräng II
5
11. Orräng III
4
12. Skaggebo
5
13. Sturefors N
4
14. Sturefors S
2
15. Sundsbro
5
16. Sätra-Humpen
6
3.2 Determination of oak age
For determination of the age of the oaks Haglöf increment borer was used.
1-4 cores were taken from each tree, depending on its condition and
diameter. The cores were normally taken 70-80 cm above ground but
sometimes the height differed. As a control, three cores were taken from
each of two diameter classes; 10-20 cm and 20-50 cm. These cores were
used for determination of growth rate of the missing parts of hollow trees.
Cores were taken from one tree of each category of openness in the study.
The categories were named open, half-open and shaded. The tree is open,
or free-standing, if >75 % of the crown is sun exposed, i.e. have no other
trees within two metres shading its crown. Half-open trees have 25-75 % of
the crown sun exposed and the category shaded have <25 % of the crown
sun-exposed. For age determination of the cores, the method of crossdating
was used. This method was first introduced by Douglass (Douglass 1941).
The advantage of cross-dating over mere ring-counting is that one can
overcome the problem of missing rings (partially or completely) or false
(double) rings (Douglass 1941, Stokes & Smiley 1968, Schweingruber
1988) The result is that, without error, every single ring can be exactly
dated to its year of origin. Cross-dating is generally possible if the trees
have the same growth conditions. Thus, they will have a similar growth
pattern and it is possible to compare them. What is done is that you find
4
pointer-years (Schweingruber et al 1990, Schweingruber 1996), years when
most of the trees growing in the same area have developed a ring that
distinctly differs from the ones around it (Niklasson 1998). This could be
due to an extremely cold or warm summer etc. A group of pointer-years
following each other are called signature-years (Schweingruber et al 1990).
These sequences of extreme rings can be very useful when dating
unknown, old or dead material, matching them with known chronologies
(Niklasson 1998). The cross-dating method I used was the original method
of Douglass (1941).
As many of the investigated trees were hollow, and others were too
large for the borer to reach its pith, the missing years were extrapolated
using the controls and other trees with pith from the same site. First, the
length of the missing part (distance to pith) was determined. This was done
by subtracting the length of the core from trunk radius (equation 1).
Equation 1: Distance to pith = Trunk radius - length of core
Since the growth rate of the missing part is unknown, it must be estimated.
This is what the young control trees were used for. The innermost rings of
trees with pith growing in similar conditions can be assumed to be similar
to that of the innermost (missing) rings of the hollow oak. Thus, the growth
rates of hollow and control trees were compared and from that the growth
rate of the missing part was assumed. The number of missing rings was
estimated by division of the growth rate from the distance to pith (equation
2).
Equation 2: Number of missing rings = Distance to pith
Growth rate
The missing rings were subtracted from the oldest ring of the core to get
the year of pith. For reference, the lowest and the highest growth rate
recorded on each site were used to find the highest and the lowest possible
ages of each tree.
In reality oaks often have eccentric growth and the pith could be
slightly shifted to one side of the trunk (Niklasson, personal
communication*) but to make age estimation possible it was assumed that
all oaks had had concentric growth, leaving the pith right in the middle of
the trunk. All years of origin mentioned in this paper are pith years and not
the year of germination. This gives an age that is about 5 years too young.
It is an additional measure based on planted trees under favourable
*Mats Niklasson works for the Swedish University of Agricultural Sciences, Alnarp
5
conditions so the tree could in fact be older than that (Niklasson, personal
communication), which is why I have chosen to use pith year.
3.3 Tree specific parameters and lichen frequencies
For all trees, data for girth bark crevice depth, openness and ground
conditions was collected. For hollow trees, the lichen flora was also
examined, looking for occurrence of the species Cliostomum corrugatum,
Chaenotheca phaeocephala, Calicium adspersum, Schismatomma
decolorans and Sclerophora coniophaea. All species live on the trunks of
old oaks where they most often reside in the bark crevices. They are all
listed as indicators for old-growth oaks of high conservation values (Nitare
2000). Four of the species, Cl. Corrugatum, Ch. Phaeocephala, Scl.
coniophaea and Sch. decolorans are red-listed as near threatened (NT) in
the 2000 Swedish Red-list (Gärdenfors 2000). The age of the oak was
determined as is described above. The growth rate of a specific core or tree
was determined from measuring the tree rings. Measurements for the ten
youngest rings, for the ten oldest rings and for the three oldest rings were
taken. In addition the length of the whole core was measured and, for one
or two cores from each tree, the growth rates for ten-year intervals were
also measured. The oaks were classified into hollow stages from the
diameter of their entrance hollows (table 2). An oak is always classified
into stage 7 if they have their entrance hollow at ground level, regardless of
its diameter.
Table 2: Classification of hollow trees into hollow stages. The diameter of the
entrance hollow defines the stage.
Hollow stage
4
5
6
7
diameter (cm)
< 10
10-30
< 30
Entrance at ground level
3.4 Survey methods
Each tree was examined for about half an hour for lichens using a hand
lens. The girth of the tree was measured at breast height (DBH = 130 cm)
on all trees. The diameter was calculated from the girth. The depth of the
bark crevices was determined by placing a ruler across the crevice and then
placing another ruler into the crevice. Only the deepest crevice was
recorded but several measures were made in order to find the deepest one.
The ground conditions beneath the crown were examined for moisture (dry,
fresh, damp, wet) and the ground coverage (stone, dirt, grass etc.). The
coverage was recorded as a percentage, to the nearest 5 %.
6
3.5 Analyses
Statistical analyses were performed in order to clarify the relationships
between different tree parameters. Lichen frequencies were analyzed in
relation to age, diameter, sun exposure and bark crevice depth. Beetle data
was analysed in relation to age and hollow stage. The beetle data used for
the analyses was collected from the studied trees in the summer of 1994
using window- and pitfall traps. In the analysis of beetles on the Swedish
red-list, all red-listed beetle species collected from the tree gave points
(Table 3). A critically endangered species renders more points than a
vulnerable species. The total score of each tree was analyzed in relation to
the age of the tree.
Table 3: Threat categories of the Swedish red list with the red-list score of each
category.
Category
Red-list score
Near Threatened (NT)
1
Vulnerable (Vu)
3
Endangered (EN)
5
Critically endangered (CR)
5
For some of the analyses and graphs an additional number of examined
oaks, from the same localities as the study trees, were used for better
statistical certainty. These trees were studied during the same period as this
study was performed.
3.5.1 PCA & RDA
For multivariate analysis of the beetle fauna the statistical program
CANOCO 4.5 was used (ter Braak & Smilauer 2002). An initial test
showed that beta-diversity was relatively low, and therefore we chose
linear methods: PCA (Principal Component Analysis) and RDA
(Redundancy Analysis). Default options were used throughout, except that
species abundances were square-root transformed to dampen the influence
of a few very abundant species.
To test for the explanatory power of age and hollow stage (four
categorical variables), we conducted a partial RDA (pRDA) using sites as
16 categorical covariables. The ordination results were evaluated with a
Monte Carlo test with 9999 permutations.
3.5.2 Other statistics
For statistical analysis of the results of tree specific parameters, increment
and age, the statistical program MINITAB was used. Frequencies were
tested with a χ2-test adjusted for the different number of trees in each class.
7
Relations with one categorical variable were tested with ANOVA GLM,
and a pairwise comparison (Tukey-test) between classes. Relations where
both parameters were continuous were tested with a regression analysis.
4. Results
4.1 Tree age
The ages of the hollow oaks in the study varied from 214 years to 499 years
when calculated from the most probable growth rate. The mean age of all
trees was 327. If pith age was calculated from the lowest growth rate the
mean was 627 years and when calculated from the highest growth rate it
was 260 years.
Trunk diameter (cm)
Bark crevice depth (mm)
4.2 Age and size relationships
There is a significant linear correlation between the diameter of an oak and
its age (R2 = 0.590, p< 0.001). The depth of the bark crevices too have a
significant (R2 = 0.701, p< 0.001) correlation with age. Hence, the depth of
the bark crevices is better correlated with age than is the diameter. For the
first 200 years the bark crevice points follow each other but after that they
deviate greatly and at about 220 the depths range from less than 20 mm up
to 70 mm. For the first 100 years the diameter-age points follow the trend
line before they begin to deviate greatly. At 130 years the diameters range
from 10 to 120 cm (Figure 1).
250
Depth of bark crevices
Trunk diameter
200
150
100
50
0
0
100
200
300
Age
400
500
600
Figure 1: The relationship between trunk diameter, depth of the bark crevices
and the age of the oak. N= 203 and 187 trees respectively. The equation for the
trend lines are y=0.3327x and y=0.1482x respectively.
Old oaks could have a wide range of diameters when pooled but if pasture
oaks and forest oaks were investigated separately a stronger linear
relationship was found, e.g. in Brokinds skolhage (Figure 2, R2= 0.688, p=
< 0.001). The relationship between bark crevice depth and age was stronger
8
for Brokind than for the pooled data (R2= 0.769, p< 0.001). No hollow trees
were included in Brokind.
bark crevice depth (mm)
Diameter (cm)
140
Bark crevice depth (mm)
120
Diameter (cm)
100
80
60
40
20
0
0
50
100
150
200
250
300
Age Brokind
Figure 2: The relationship between trunk diameter, depth of the bark crevices
and the age of the oak as found in the site Brokinds Skolhage, The equation for
the trend line is y= 0.4406x and y= 0.1573x respectively. Only oaks with pith
were examined, N= 28 trees. Additional trees from the site were used in this
analysis.
The correlation between trunk diameter and depth of bark crevices both
begin to deviate from the trend line as the tree gets older. It seems like the
increase in diameter and the deepening of the bark crevices both level out
with age and thus they are better correlated with each other than with age
(R2= 0.802, p< 0.001 Figure 3).
R2 = 0,8015
Bark crevices (mm)
100
80
60
40
20
0
0
50
100
150
200
Trunk diameter (cm)
Figure 3: The relationship between trunk diameter and the depth of the bark
crevices. N= 187 trees. The equation for the trend line is y= 0.4237x.
9
4. 3 Occurrence of lichens
Of the studied lichen species, Cliostomum corrugatum appear at age 214
for the first time (the youngest hollow tree in this study). Schismatomma
decolorans and Chaenotheca phaeocephala appear at age 233 for the first
time. Calicium adspersum was found on a 237 year old oak and
Sclerophora coniophaea was not found until the age of 336. Although all
species had marked patterns only Ca. adspersum (25 findings) showed a
significant distribution for age. It occurred in significantly greater
frequencies on older oaks (χ2= 9.028, p= 0.011, df= 2). It occurred on more
than half of the oaks >400 years (Figure 4). Cl. corrugatum was the most
common species (36) and its highest frequencies was on oaks of age 301400 although the distributions were not significant (χ2= 0.259, p=0.879, df=
2). Chaenotheca phaeocephala (20) seems to be more common on the
youngest trees of the study, 201-300 years (Figure 4). Schismatomma
decolorans (4) and Sclerophora coniophaea (3) had too few findings to be
tested but they seem to mainly occur on older trees. All Scl. coniophaea
findings were made on trees of age 336-352 years and three of the four
findings of Sch. decolorans were on trees older than 390 years old.
This is further indicated by the fact that they occured only on trees
with diameters over 116 cm, Figure 5) and with bark crevices deeper than
50 mm (Figure 6). Cliostomum corrugatum, Ca. adspersum and Ch.
phaeocephala could be found in all classes. Calicium adspersum
occurrence increases five-fold from up to the third class (116-135 cm) and
then decreases. Chaenotheca phaeocephala had a similar pattern while Cl.
corrugatum had almost the same frequency for the three largest classes
(Figure 5). There were no effects of size on the distribution of the lichens
Cl. corrugatum, Ca. adspersum and Ch. phaeocephala (χ2= 2.281, 5.738,
3.117 respectively and p= 0.516, 0.126 and 0.374 respectively. Df= 3 for
all.).
C. corrugatum
C. adspersum
C.phaeocephala
S. decolorans
S. coniophaea
0,6
Frequencies
0,5
0,4
0,3
0,2
0,1
0
201-300
301-400
Age class
>400
Figure 4: The fraction of trees occupied by each lichen species in relation to
age. N= 29, 35 and 13 trees respectively.
10
0,6
C. corrugatum
C. adspersum
C. phaeocephala
S. decolorans
S. coniophaea
Frequencies
0,5
0,4
0,3
0,2
0,1
0
<89
89-115
116-135
Trunk diam eter (cm )
>135
Figure 5: The fraction of trees occupied by each lichen species in relation to the
diameter of the trunk. N= 21, 20, 17 and 19 trees respectively.
0,8
Frequencies
0,7
0,6
0,5
C.
C.
C.
S.
S.
corrugatum
adspersum
phaeocephala
decolorans
coniophaea
0,4
0,3
0,2
0,1
0
<40
40-49
50-59
Bark crevice depth (mm)
≥60
Figure 6: The fraction of trees occupied by each lichen species in relation to the
bark crevice depth. N= 17, 21, 19 and 21 trees respectively.
All five species decline with decreased sun exposure with one exception.
Chaenotheca phaeocephala have higher frequencies again on shaded trees
(>25 % sun exposure, Figure 7). Three species are completely missing from
shaded oaks. The distributions were not significant in a χ2-test.
11
C. corrugatum
C. adspersum
C. phaeocephala
S. decolorans
S. coniophaea
0,7
Frequencies
0,6
0,5
0,4
0,3
0,2
0,1
0
>75
25-75
Openness
0-25
Figure 7: The fraction of trees where each lichen species was found as a
function of openness of the crown. Categories represent the openness in %.
Openness is the sun exposure, i e absence of shading from other trees within 2
m of the crown. N= 19, 53 and 5 trees respectively.
4.4 Growth rate
4.4.1 Stone coverage and moisture
The effect of stones covering the ground beneath the crown was small on
the growth rate of the oaks. Trees with less than 20 % stones did not grow
significantly faster (1.5 mm year-1) than trees with larger amount of stones
(1.3 mm year-1, Figure 8, GLM Tukey-test p= 0.0627).
2,5
Growth rate
2
1,5
1
0,5
0
< 20
20-100
stone coverage (%)
Figure 8: The mean growth rate (±SD) of hollow oaks in relation to the amount
of stone/rock beneath the crown. N= 77 trees.
Oaks growing on dry grounds did not grow significantly slower than oaks
growing on fresh grounds (Figure 9, GLM Tukey-test p= 0.236).
12
2,5
Growth rate
2,0
1,5
1,0
0,5
0,0
dry
Moisture
fresh
Figure 9: The mean growth rate (±SD) of an oak in relation to the moisture of
the ground. Only two different conditions were recorded for the hollow oaks. N=
77 trees.
4.4.2 Openness
The mean growth rate of all oaks in the sites of the study was 1.7±0.9 (SD
n =204) mm year-1 and for the oaks of hollow stages 4 through 7 it was
1.5±0.3 SD (n =2), 1.5±0.6 SD (n =39), 1.3±0.6 SD (n =26) and 1.6±0.6
SD (n =10) mm year-1 respectively. There is a significant decreasing trend
in growth rate with increased shading. (Figure 10, GLM Tukey-test p<
0.001). The oaks growing in open conditions grow significantly faster than
the oaks in the other two categories (P< 0.001 and < 0.001 respectively).
Half-open (25-75%) oaks do not however, grow significantly faster than
shaded oaks (>75%).
4,0
3,5
Growth rate
3,0
2,5
2,0
1,5
1,0
0,5
0,0
>75
25-75
openness
0-25
Figure 10: The mean growth rate (±SD) of oaks in relation to the openness of
the canopy. n(0-25) =15, n(25-75) =81, n(>75) =34 trees.
13
In a comparison of the growth rate during three different time periods, 20
years in each time period, oaks growing in open conditions (n= 20) had
increased their growth rate from the first time period to the third, not
decreased as had the trees that were closed (n = 8) today. Thus, the
difference in growth rate for free-standing and shaded is greater today than
it was during 1850-1870. The oaks growing in open conditions grow
significantly faster than do the oaks growing in shaded conditions for all
three time periods (Figure 11, p< 0.001, < 0.001 and 0.018 respectively).
The growth rate is significantly faster for period 1920-40 than for period
1850-70 (GLM, Tukey-test p= 0.0459) for free-standing trees and then
slow down to 1980-2000 but not significantly (Figure 11). The growth rate
for shaded trees is lower for both time periods following 1850-70. The
differences between time periods were non-significant for shaded trees.
Average growth rate
3
2,5
2
shaded
1,5
Open
1
0,5
0
1980-00
1920-40
1850-70
Tim e period
Figure 11: The mean growth rate (±SD) during three 20-year periods for trees
that are shaded today (openness 0, N= 8) and for trees which free-standing
today (openness 2, N= 20 trees).
4.5 Age and hollow stage
Oaks of the four hollow stages, as defined by the county board of
Östergötland, showed no significant differences in age (GLM, Tukey-test
p= 1.00, 0.921 and 0.988 respectively). The mean age of the hollow oaks of
the four stages was in this study over 300 years (334, 333, 322 and 313
respectively). The two oaks of stage 4 were both 334 years old. The oldest
oaks of stage 5, 6 and 7 were 499, 494 and 412 years respectively and the
youngest trees were 214, 214 and 224 years respectively. Tree age was
significantly higher if the pith age of a tree was calculated from the highest
growth rate than when calculated from the lowest growth rate recorded on
each locality. The estimated age was closer to the youngest possible age
(Figure 12).
14
Mean age
1000
900
800
Oldest possible age
Estimated age
Youngest possible age
700
600
500
400
300
200
100
0
4
5
6
7
Hollow stage
Figure 12: The correlation (±SD) between the age and the life stage of oaks of
this study. A comparison between the average ages of hollow oaks with
estimated pith according to the highest, the lowest and the assumed growth rate
of the hollow space. N = 2, 39, 26 and 11 trees respectively.
4.6 Beetle statistics
4.6.1 Osmoderma eremita
Osmoderma eremita inhabited no trees of stage 4. The fraction of trees of
stage 5-7 with O. eremita was 40, 32 and 33 percent respectively. The
average numbers of individuals collected in inhabited oaks of hollow stages
5-7 were 4, 3 and 1 respectively (Figure 13). The number of individuals
was not significantly greater for stage 5 than for stages 6 and 7. Twice as
many individuals were found in oaks over 400 years of age, than in oaks of
class 201-300 and 301-400. The mean occurrences were 5.4, 2.6 and 2.8
individuals tree-1 respectively (Figure 14).
Average nr of O.
eremita/inhabited tree
10
8
6
4
2
0
4
5
6
7
Hollow stage
Figure 13: The average number (±SD) of Osmoderma eremita inhabiting a
colonized tree in relation to hollow stage. In hollow stage 4 no O. eremita
inhabited the examined oaks. N= 6, 30, 34 and 3 trees respectively.
15
Average nr of individuals
16
14
12
10
8
6
4
2
0
201-300
301-400
>400
Age class
Figure 14: The average number (±SD) of Osmoderma eremita in an inhabited
tree in relation to age. N is 9, 10 and 5 trees respectively.
4.6.2 Species richness
There were no significant differences, with regard to species richness,
between the four hollow stages (GLM, Tukey-test p= 0.180) and no
significance for age (p= 0.435). There was no correlation between species
richness and age (Figure 15). In relation to hollow stage, the species
richness is approximately the same in the three latter stages and a little,
though not significantly, higher for stage 4 (Figure 16, p = 0.475)
Number of
species
25
20
15
10
5
0
0
100
200
300
Age
400
500
600
Figure 15: The species richness of saproxylic beetles in relation to age. N= 73
trees..
16
Average number of species
25
20
15
10
5
0
4
5
6
7
Stage
Figure 16: The average number of species (±SD) of saproxylic beetles in
hollow oaks in relation to hollow stage. N is 6, 34, 30 and 3 trees respectively.
4.6.3 Red-list score
There was no correlation between red-list score and age. A 200-year old
tree had a score of 23 while 300-500 year old trees could have scores of
three or four (Figure 17). No correlation with age was found for pitfall
traps, window traps or for both kinds of traps joint together (p= 0.097,
0.354 and 0.097 respectively). When compared with the life stage of the
tree the red list score showed p-values over 0.5 in most cases and there
were no significant differences between the stages in any of the three
comparisons.
Redlist score
25
20
15
10
5
0
0
100
200
300
Age
400
500
600
Figure 17: The red list score for beetles caught in window traps in relation to
the age of the tree. N= 71 trees
17
25
Age of tree
20
15
10
5
0
0
100
200
300
400
Red list score
500
600
Age of tree
Figure 18: The red list score for beetles caught in pitfall traps in relation to the
age of the tree. N= 73 trees
40
35
30
25
20
15
10
5
0
0
100
200
300
Redlist score
400
500
600
Figure 19: The total red list score for beetles caught in trees with both kinds of
traps in relation to the age of the tree. N= 73 trees
4.6.4 PCA and RDA
The PCA revealed a gradient from species-rich sites (right part of Figure
20) to relatively species-poor sites.
18
1.0
labbenäs
Gna b/n
Ato mor
Din pan Scy hel
Nem col Hist sp
orräng1
Cry con Cry que
sturefor
Ste god
långv-V
hjorth-S
Pse cer
orräng3
långv-.
Den cor
sturefor
skaggebo
fornh
brokind
Ten lar
bjärka ä
Eup kar
Lar lat
Cry bad
Pti ruf
hjorth-N
Pti fur
Xes ruf
Ten mol
Pti sub
orräng2
-1.0
sundsbro
sätrah
-0.8
1.0
Figure 20: Principal component analysis of saproxylic beetle assemblages from
oaks in central Östergötland, Sweden. Triangles indicate the centroids of trees
growing at particular sites. Eigenvalues for PC1 and PC2 was 0.219 and
0.0937, respectively. Species names are abbreviated.
There was no significant relationship between insect assemblages and the
age and hollow stage of the oaks (pRDA with 9999 permutations; p=
0.2060).
5. Discussion
5.1 Age determination of hollow oaks
Age determination of old oaks is a challenge. But information about ages of
old oaks are an important part of conservation work when trying to
conserve the fauna and flora connected to them. Most of the oaks in this
study had hollow trunks. Only for the control trees and a few of the older
trees was it possible to reach the pith with the borer. This presented a
problem since there is no conventional method for dating old trees (Ek et
al. 1995). The possible age of one tree can differ with hundreds of years
depending on what growth rate is implemented for the missing part (Figure
19
12). Sometimes linking of the chronology of the examined tree with known
chronologies from stumps and drywood of trees that died a long time ago
can be used for dating stands (e.g. Andersson & Niklasson 2004). This
should be possible to do in order to get a better knowledge about the
growth rate of the missing part of the trunk and thereby a more certain age
of hollow oaks. The application of the slowest growth rate of a tree from
the same locality as the hollow oak caused, in almost all cases, an
overestimation of the age since it is generally much slower than that of the
core taken from the hollow oak. The mean estimated age of the trees is
generally a lot nearer the lowest possible age than the highest possible age,
since oaks with really slow growth rate are most often suppressed by
overgrowth or grow on flat rock or precipices. The fast grown trees in most
cases have growth condition more similar to those of the hollow oaks.
The diameter of trees is often used as an indicator of tree age. It has
been showed that there is a significant relationship between tree girth
(diameter) and age (e.g. Johannesson 1996, Edelfelt 1997, Carlsson 2004).
In most studies of age, diameter and growth rate of oak, the trees have
fairly uniform site and growth conditions (e.g. Carlsson 2004) and when
comparing trees in the same site, trunk diameter was good as an indicator
of age. However, if one wishes to have one single method for
approximation of age on all sites, the pooled results of this study show that
diameter would not be as good an indicator. The oldest trees are often
found on precipices but these old oaks generally have very thin trunks for
their age (Ek et al. 1995, Andersson & Niklasson 2004) which would lead
to a decreased certainty in age approximation from girth of the tree.
An alternative to the conventional approximation of tree age,
diameter, was also studied, the depth of bark crevices. The results of this
study indicate that bark crevice depth can give us a better estimate of the
actual age of a hollow oak than the diameter of the oak when oaks of
pastures with high increment are pooled with forested and rocky sites of
poor increment (Figure 1). In a study of age and growth rate for Quercus
robur in pastures, Carlsson (2004) showed a significant linear relationship
with age for both the bark crevice depth and for the diameter. However,
girth increment will decrease with the age of the oak. Thus, a tree will stay
longer in latter diameter classes than in younger. This study shows that the
same applies for the depth of bark crevices. Carlsson (2004) also showed
that crevice depth and diameter have a significant linear relationship. This
is supported by the results of this study. Both parameters seem to level out
at similar ages. In conclusion, Bark crevice depth was a good indicator of
age, indifferent of site conditions but if growth conditions are known and
20
the relationship for a specific site is established, girth can be a good
indicator as well.
5.2 Growth rate
The growth rate of a tree is dependent on many factors, such as ground
conditions and sun exposure. Temperature and precipitation is often growth
delimiting factors and are important in the formation of pointer years
(Niklasson 1998). Shaded trees have lower growth than free-standing or
overstorey trees (Worbes et al. 2003) but if they are released from shading
the growth rate increases (Dwyer et al 1995, Niklasson & Fritz 2003). Free
standing oaks had a mean growth rate in this study, 2.3 mm year-1
significantly higher than shaded trees (1.2 mm year-1) and half-open trees
(1.5 mm year-1).
Carlsson (2004) found the average growth rate of pasture oaks to be
2.69 mm year-1. In this study average growth was lower but that is to expect
since growth conditions are harsher for many trees and the trees are
generally older. The study of diameter and age showed that girth increment
decreased with age, which means the growth rate decreases with age.
Carlsson (2004) noticed compared to the highest growth rate there could be
up to 50 % decreases in oaks over 200 years age, thus many hollow trees of
this study could show lower average growth rates than they would have if
they were not hollow. When comparing growth rates during three 20-year
periods there were no significant changes found for oaks that are shaded
today. Thus, most trees were probably growing in the same conditions 150
years ago as they are today. The 20 oaks which are free-standing today
could have had slightly more shaded growth conditions in 1850-70 because
growt rates were significantly higher in 1920-40.
Growth rate was observed to be higher, although not significantly,
for oaks on fresh grounds compared to oaks growing on dry grounds. This
is in compliance with climatic studies saying that higher precipitation
yields higher growth rates (e.g. Dittmar et al. 2003). Ollinger & Smith
(2005) found that growth rates are higher on higher elevations and argued
that it could be a consequence of ground moisture being higher.
The effect of stones covering the ground beneath the canopy was
non-significant. Oaks on precipices generally grow slower than do pasture
oaks (Ek et al 1995, Rydberg 1997) but apparently there is only a weak
correlation between the amounts of stones on the ground near pasture oaks
and the growth rate.
21
5.3 Lichens
The lichen flora is different on old trees than it is on younger trees (Hilmo
1994). Acid sensitive lichens are absent at low pH (Wolseley 1999). Young
oaks produce chemical substances and acids and the bark pH is low.
Because of these substances the old oak specialists of lichens studied may
be unable to colonize the smooth, thin bark of younger oaks (Rydberg
1997, Berg et al. 2002). As the bark gets rougher, the crevices grow deeper
and the levels of the substances decrease. This makes colonization possible
(Carlsson 2004).
The results of this study indicate that species diversity increases with
increasing size, bark crevice depth and age. Cliostomum corrugatum was
found on hollow oaks of the smallest diameters. The smallest was 43 cm in
diameter and it was found on a few trees with diameters around 60 cm.
Similar notations have been made before (e.g. Ek et al. 1995, Rydberg
1997, Carlsson 2004). The oak of smallest diameter grew on a hill side and
was 214 years old despite the small diameter. Ek et al. (1995) found Cl.
corrugatum on old oaks with even smaller diameters on precipices in
Östergötland. Håkan Lättman (unpublished data) found Cl. corrugatum on
oaks of diameter down to 65 cm when investigating stands with large, old
oaks in the county of Östergötland. Cliostomum corrugatum increase in
frequency with increasing diameter but seem to stabilize at 0.55 in the
higher classes. All other species except for Chaenotheca phaeocephala
show approximately the same pattern with increasing frequencies with
increasing girth (diameter), bark crevice depth and age.
Chaenotheca phaeocephala has previously been noted to be the first
of the five species to appear on old oaks. It has a peak frequency on oaks of
1 m diameter and does not occur on oaks with diameters > 1.5 m (Carlsson
2004), which could be the reason why it decreases in frequency in the latter
classes in this study. Ek et al. (1995) concluded in their study on precipices
that the popular view of many lichen species as being girth-dependent and
appearing only on oaks with large diameters might be wrong and further
studies may prove that age is most important for these species. Age is an
important factor for colonization. However, it may not be the most
important. Bark roughness has been outlined as a factor in determining
lichen communities (van Dobben & de Bakker 1996, Rydberg 1997). When
the bark gets thicker, crevices grow deeper and antagonistic substances
become less abundant (Rydberg 1997). This could be the reason why
sometimes old trees with thin trunks have species traditionally connected to
large pasture oaks. However, I have found in this study that bark crevice
depth is closely correlated with the age of oaks. Thus, it is not possible to
state if age or bark crevice depth is the most important factor for lichens.
22
One thing that has to be kept in mind when evaluating the results of
this study is that all of the examined oaks are more than 200 years old.
Carlsson (2004) found both Cl. corrugatum and Ch. phaeocephala on
younger oaks, thus the frequency pattern of this study does not picture the
whole truth of the lichen flora in these stands. As an example, in Carlssons
(2004) study, both species had their peaks in the class 201-250 years.
All lichen species decrease in frequency with decreasing sun
exposure except for Ch. phaeocephala but the increase in frequency on
shaded trees might be because this class only contained five oaks. Only two
findings of the species were made on shaded oaks and only one finding
totally for the other five species. Chaenotheca phaeocephala has earlier
been noted to prefer the most sun exposed side of the trunk (Tibell 1978).
The decrease in frequencies with increased shading is consistent with
previous studies on the species concluding that they are favoured by sun
exposure (e.g. Ek et al. 1995). Calicium adspersum has about the same
frequencies for free-standing and half-open oaks but is absent from shaded
oaks. This could verify the view that it is found both in solitary, sun
exposed trees and on overgrown oaks in more closed forests (Tibell 1977,
Rydberg 1997). Scl. coniophaea prefers damper, shadier conditions than
the other species in this study and in southern Sweden findings have been
made mainly in deep crevices near the base of the tree (Rydberg 1997,
Tibell 1984). This could be one reason for the low amount of findings of
the species in this study. Schismatomma decolorans is mainly found on
really old oaks with very rough bark (Arup et al. 1997) which could be the
reason for the low frequencies in this study; the trees could be too young.
5.4 Age and hollow stage
There were no significant differences in ages between different hollow
stages. Apparently the actual age of the oak is not what defines at what
stage in life the tree is. The oaks of the four hollow stages of Quercus
robur all have high ages of mean, >300 years age, and regeneration takes a
long time. Thus, we have to take care of the remaining localities with large,
hollow trees since they have decreased dramatically (Jonsell et al. 1998).
Minimum and maximum ages of the oaks differ greatly in each stage, but
the fact that the min and max ages are approximately the same for all
stages, except for stage 4, incline that something other than mere age is
what makes oaks perceptive to rotting fungi. Pasture oaks on fertile
grounds have broad crowns with large branches originating in the lower
part of the trunk. These trunks are prone to break when the tree gets older
because of their heavy weight. A branch of that size breaking creates a
damage which the oak has a hard time repairing and is a major opportunity
23
for saproxylic fungi and beetles to inhabit the oak (Mats Niklasson,
personal communication*). Oaks on poorer grounds have lower growth rate
and might be older before their branches become too heavy for the tree to
bear. Oaks that grow with competition for light from other trees grow
thinner trunks and smaller branches and are thus less affected by this
problem and might be able to resist fungi longer.
It is a known fact that pasture oaks suppressed by forest regrowth
often die prematurely due to outshading (e.g. Rydberg 1997). When the
large, lower trunks are outshaded they die and fall off, creating the same
scenario as above with openings for rot. This fact could be one reason for
the differences in age of oaks in the same hollow stage. Another possible
reason for this result is that pure stochastic events are the major factors
determining when a tree is colonized by saproxylic fungi causing hollow
formation. Oaks growing on fertilized grounds have higher growth rates but
perhaps the increased amount of nourishment also makes them more
susceptible to fungi and insects.
5.5 Species richness and Osmoderma eremita frequencies
There was no significant relationship between oak age or hollow stage and
the insect assemblage of the trees. It has been proposed that, up to a certain
age, the older an oak is the more diverse is its beetle fauna. Ranius &
Jansson (2000) found that some saproxylic species only occur in trees with
large dimensions. When the county board of Östergötland developed the
method for classifying hollow trees in stages they thought that the beetle
fauna would be more diverse and there would be significantly more
individuals of each species in trees of stages 5 and 6 than in trees of stage 4
and 7 (Jansson, personal communication**). Since no significant
differences were recorded, this view, that species richness would be
different within different hollow stages and higher with age, is not
supported by the results of this study. In fact species richness is the highest
for hollow stage 4, though not significantly higher than the other three
hollow stages, and a decrease in diversity with increasing hollow stage,
although non-significant, can be observed. These results indicate that beetle
communities are more or less indifferent to hollow stages or that there are
weaknesses in the hollow stage classification.
The definition of the hollow stages includes the amount of wood
mould in each stage (Jansson & Antonsson 1998). Maybe if the amount of
wood mould in a tree is determined, the classes would be possible to
*Mats Niklasson works for the Swedish University of Agricultural Sciences, Alnarp
**Nicklas Jansson works for the County Board of Östergötland and is a PhD-student at
Linköping University.
24
distinguish between. In this study, however, this was not done. Of the 86
beetle species in this study 53 % are on the 2000 Swedish Red-list
(Gärdenfors 2000). No relationship between the red-list score (table 2) and
age or hollow stage was found. This is contradictory to the hypothesis that
older trees sustain more threatened species. Few quantitative studies had
been made on saproxylic beetles (Ranius 2002). Hence, it is difficult to
validate the theory. The youngest tree in this study was 214 years and thus
all species could already be present at that age since I have shown that trees
of different sizes and hollow stages exist in all age classes.
Osmoderma eremita has been observed to occur in higher
frequencies in hollows with a lot of wood mould (Ranius 2000) and from
this study this seems to be true if Jansson & Antonsson’s assumption about
the hollow stages is correct. Hollow stage 4 and 7 is assumed to have less
wood mould according to the definitions (Jansson & Antonsson 1998) and
the quality would be better with age of the hollow (Ranius 2000). However,
in this study the amount of wood mould was not measured and hence the
theory could not be tested. Another possible source of errors is that pitfall
traps does not allow sampling in the smallest hollows, which could
influence the results of beetle studies (Ranius 2001). Osmoderma eremita
had higher frequencies of occurrence and had greater average numbers of
individuals in hollow oaks of stage 5 and 6. Oaks over 400 years old had
higher numbers of inhabitants than younger oaks. The tree with the highest
number of individuals was in hollow stage 5. There were large fluctuations
in the number of individuals between trees, consistent with what Ranius
(2000) found. However, populations are often pretty constant and the same
trees often sustain the highest numbers of individuals between years
(Ranius 2001).
6. Acknowledgements
I would like to thank my supervisor Karl-Olof Bergman for support and
advice. Thanks to Nicklas Jansson for beetle data, help with finding the
right trees and for help with maps, statistics and advice. Thanks to Per
Milberg for help with statistics. Thanks to Mats Niklasson for teaching me
how to use the increment borer, for lending me his lab in Alnarp for the
preparation and crossdating of the samples and for the help with
crossdating the most difficult samples. Thanks to Thomas Ranius of SLU
Uppsala for tree data and advice. I thank Larsénska fonden for finacial
support (to Thomas Ranius).Thanks Håkan Lättman for photos and help
with lichens in Brokind. And last but not least, a great big thank you to the
land-owners for allowing me to work on their grounds. Thank you count
Thure Gabriel Bielke of Sturefors Gods, count Henric Falkenberg of
25
Brokinds Gods, Oscar Ekman of Bjärka-Säby Gods, Robert Ekman of
Stavsätter and Gustaf Ekman of Sätra-Humpen.
7. References
Andersson M & Niklasson M (2004) Rekordgammal tall på Hornslandet i
Hälsingland. Svensk Botanisk Tidskrift 98, 333-339.
Arup U, Ekman S, Kärnefelt I & Mattson J-E (ed.) (1997) Skyddsvärda
lavar i sydvästra Sverige. SBF, Lund.
Berg Å, Gärdenfors U, Hallingbäck T & Norén M (2002) Habitat
preferences of red-listed fungi and bryophytes in woodland key habitats in
southern Sweden – analyses of data from a national survey. Biodiversity
and Conservation 11, 1479-1503.
Carlsson J (2004) Betydelsen av ålder och tillväxt hos ek (Quercus robur)
för förekomst av rödlistade arter i Östergötland. Master of science thesis,
Linköping University, Linköping.
Dittmar C, Zech W & Elling W (2003) Growth variation of common beech
(Fagus sylvatica L.) under different climatic and environmental conditions
in Europe – a dendroecological study. Forest Ecology and Management
173, 63-78.
Dwyer JP, Cutter BE & Wetteroff J (1995) A dendrochronological study of
black and acarlet oak decline in the Missouri Ozarks. Forest Ecology and
Management 75, 69-75.
Douglass AE (1941) Crossdating in dendrochronology. Journal of Forestry
39, 825-831.
Edelfelt S (1997) Demografi och naturvärdeni fyra ekpopulationer I
eklandskapet söder om Linköping. Master of science thesis, Linköping
University, Linköping
Ek T, Wadstein M & Johannesson J (1995) Varifrån kommer lavar knutna
till ekar? Svensk Botanisk Tidskrift 89, 335-343.
Ek T, Fransen M, Hagström M & Wadstein M (2001) Sällsynta lavar i
Östergötland 2000 –nationellt och internationellt rödlistade arter. The
County Board of Östergötland, report 2001:1.
26
Gärdenfors U (Ed.) (2000) The 2000 Red List of Swedish species. Swedish
threatened species unit, SLU, Uppsala.
Hilmo O (1994) Distribution and succession of epiphytic lichens on Picea
abies branches in a boreal forest, Central Norway. Lichenologist 26, 149169.
Jansson (1995) Eklandskapet som miljöövervakningsobjekt. The County
board of Östergötland, Linköping.
Jansson N (1998) Vedskalbaggsfaunan kring gamla ekar på militära
övningsfältet söder om Linköping i Östergötlands län. The County board of
Östergötland, report 1998:5.
Jansson N & Antonsson K (1998) Miljöövervakning av biotoper med
gamla ekar i Östergötland. The County board of Östergötland, report
1998:1.
Johannesson J (1996) Sällsynta lavar knutna till ekar. Utbredning och
miljökrav i S:t Annas skärgård. Master of science thesis, Linköping
University, Linköping.
Jonsell M, Weslien J &Ehnström B (1998) Substrate requirements of
saproxylic invertebrates in Sweden. Biodiversity and Conservation 7, 749764.
Nilsson SG, Arup U, Baranowski R and Ekman S (1994) Trädbundna lavar
och skalbaggar I ålderdomliga kulturlandskap. Svensk Botanisk Tidskrift
88, 1-12.
Niklasson M (1998) Dendroecological studies in forest and fire history.
Silvestria 52, Swedish University of Agricultural Sciences.
Niklasson M & Fritz Ö (2003) Hur gammal kan en bok bli? 400-åring
upptäckt i Småland. Svensk Botanisk Tidskrift 97, 150-156.
Nitare J (Ed) (2000) Signalarter – indikatorer på skyddsvärd skog.
Skogsstyrelsen, Jönköping.
27
Palm T (1959) Die Holz- und Rindenkäfer der Süd- und Mittelschwedische
Laubbäume (The wood- and bark coleopteran of deciduous trees in
southern and central Sweden). Opuscula Entomologica Suppl. 16 [in
German, English summary].
Ollinger SV & Smith M-L (2005) Net Primary Production and Canopy
Nitrogen in a Temperate Forest Landscape: An Analysis Using Imaging
Spectroscopy, Modeling and Field Data. Ecosystems 8, 760-778.
Ranius T (2000) Minimum viable metapopulation size of a beetle
Osmoderma eremita, living in tree hollows. Animal Conservation 3, 37-43.
Ranius T (2001) Constancy and asynchrony of Osmoderma eremita
populations in tree hollows. Oecologia 126, 208-215.
Ranius T (2002) Population ecology and conservation of beetles and
pseudoscorpions living in hollow oaks in Sweden. Animal Biodiversity and
Conservation 25.1, 53-68.
Ranius T & Jansson N (2000) The influence of forest regrowth, original
canopy cover and tree size on saproxylic beetles associated with old oaks.
Biological Conservation 95, 85-94.
Ranius T & Jansson N (2002) A comparison of three methods to survey
saproxylic beetles in hollow oaks. Biodiversity and Conservation 11, 17591771.
Ranius T, Antonsson K, Jansson N & Johannesson J (2001a) Inventering
och skötsel av gamla ekar I Eklandskapet söder om Linköping. Fauna &
Flora 96, 97-107.
Ranius T, Antonsson K, Jansson N & Johannesson J (2001b) Fauna och
Flora i Eklandskapet söder om Linköping. Fauna & Flora 96, 177-189.
Rydberg H (1997) Knappnålslavar på gamla ekar i Södermanland – status
och naturvårdsåtgärder. Svensk Botanisk Tidskrift 91, 39-57.
Schweingruber FH (1988) Tree rings. Basics and applications in
dendrochronology. Dordrecht, Kluwer.
28
Schweingruber FH (1996) Tree rings and environment. Dendroecology.
Birmensdorf, Swiss Federal Institute for Forest, Snow and Landscape
Research. Berne, Stuttgart, Vienna, Haupt.
Schweingruber FH, Eckstein D, Serre-Bachet F & Bräker O U (1990)
Identification, presentation and interpretation of event years and pointer
years in dendrochronology. Dendrochronologia 8, 9-39.
Stokes MA & Smiley TL (1968) An introduction to tree ring dating.
University of Chicago press, Chicago.
ter Braak CJF & Šmilauer P (2002) CANOCO, Software for Canonical
Community Ordination (version 4.5). Microcomputer Power, Ithaca.
Tibell L (1977) Lavordningen Caliciales i Sverige. Inledning och släktet
Calicium. Svensk Botanisk Tidskrift 71, 239-259.
Tibell L (1978) Lavordningen Caliciales i Sverige. Släktena Chaenotheca
och Coniocybe. Svensk Botanisk Tidskrift 72, 171-188.
Tibell L (1984) Faktablad: Sclerophora coniophaea – rödbrun blekspik.
Swedish threatened species unit, SLU, Uppsala.
van Dobben H F & de Bakker A J (1996) Re-mapping epiphytic lichen
biodiversity in The Netherlands: the effects of decreasing SO2 and
increasing NH3. Acta Botanica Neerlandica 45, 55–71.
Wolseley PA (Reprint) & Pryor KV (Author) (1999) The potential of
epiphytic twig communities on Quercus petraea in a Welsh woodland site
(Tycanol) for evaluating environmental changes. Lichenologist 31, 41-61.
Worbes M, Straschel R, Roloff A & Junk WJ (2003) Tree ring analysis
reveals age structure, dynamics and wood production of a natural forest
stand in Cameroon. Forest ecology and Management 173, 105-123.
29
30