Forest Ecology and Management 173 (2003) 105±123
Tree ring analysis reveals age structure, dynamics and wood
production of a natural forest stand in Cameroon
M. Worbesa,*, R. Staschela, A. Roloff b, W.J. Junkc
a
Institute for Forest Botany of the University, BuÈsgenweg 2, D 37077 GoÈttingen, FR Germany
b
Institute for Forest Botany of the TU Dresden, P.O. Box 10, 01735 Tharandt, FR Germany
c
Max-Planck-Institute for Limnology, August-Thienemann-Straûe 2, D 24302 PloÈn, FR Germany
Received 27 December 2000; received in revised form 11 September 2001; accepted 7 December 2001
Abstract
In a semi-deciduous natural forest stand in Cameroon a forest inventory and increment estimations on all trees with a diameter
above 10 cm were carried out in an area of 1 ha. The stand is dominated by Triplochiton scleroxylon and is a part of a forest type
which is widely distributed in West Africa. The existence of annual rings in the wood of trees was proven by radiocarbon dating
and tree ring analysis. The oldest tree (Celtis zenkeri) of the stand was 220 years old. The age class between 41 and 60 years is the
strongest in number of individuals. Trees with an age of more than 120 years were found exclusively in the storey of the
emergents. The age of the trees correlates very weakly with the diameter and the height. The mean diameter growth rates
vary between 0.2 cm per year in understorey tree species and 0.82 cm per year in emergent species. The major timber species
(T. scleroxylon) reaches in mean the minimum felling diameter of 80 cm within 90 years.
According to their age and height distribution together with the wood density, we distinguished three major types of life
strategies of species cohorts. Species with high wood density and low increment rates in all age classes are generally restricted to
the understorey. Species with exclusively old individuals, low or moderate wood densities and high increment rates are restricted
to the upper storey and can be classi®ed as long-living pioneers (T. scleroxylon). Finally, species with moderate or high wood
density, some old individuals in the upper storey and many recruits in the lower canopy can be de®ned as mature forest trees or
trees of the future (Nesogordonia papaverifera, Sterculia rhinopetala). These ®ndings lead to the assumption that the
investigated stand can be classi®ed as a very late secondary stand in transition to a mature forest.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Cameroon; Forest dynamics; Growth rates; Semi-deciduous forest; Tree ring analysis; Tropical trees
1. Introduction
The extensive logging of natural tropical forests has
broadened the discussion and installation of sustainable management systems throughout the world. The
*
Corresponding author. Tel.: ‡49-551-399504;
fax: ‡49-551-392705.
E-mail address: mworbes@gwdg.de (M. Worbes).
success of these systems is bound to an exact knowledge of growth rates of trees, dynamics and productivity of natural forest stands.
The knowledge of growth rates of tropical trees
under natural conditions is rather poor. Estimates are
vague and vary considerably depending on the methods used. Deduction from net primary production to
wood production results in values up to 18 t ha 1 per
year (Bruenig, 1996), three times higher than the wood
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 8 1 4 - 3
106
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
production in a beech forest in the temperate zone
(Ellenberg, 1986). Other calculations derived from
repeated diameter measurements (Jordan, 1983) and
from tree ring analysis in Amazonian ¯oodplain forests indicate a total above ground wood biomass
production of 6±7 t ha 1 per year (Worbes, 1997).
Lowest ®gures were reported by Clark and Clark
(1996) from remeasured trees in La Selva, Costa Rica,
with a zero growth over a period of 14 years.
Little is known about succession in temperate and
tropical forests under natural conditions. There are a
few long-term observations of succession in temperate
forests (Koop, 1989), but forests undisturbed by
humans are rare (Ellenberg, 1986). In the few existing
long-term studies of virgin tropical forests (Manokaran
and Kochumen, 1987; Clark and Clark, 1996), tree
growth rates and stand dynamics were calculated from
repeated diameter measurements by means of statistical methods (Lieberman et al., 1985). Tree ring
analysis has been used to reconstruct the stand history
of temperate forests (Koop, 1989; Worbes et al., 1993;
Abrams et al., 1995; Worbes, 1996), but has not been
applied to tropical forests because of the common, yet
erroneous, assumption that tropical trees lack annual
rings (Lang and Knight, 1983; Lieberman et al., 1985;
Whitmore, 1990). However, many species of tropical
forests with a distinct and predictable dry season have
annual rings (Geiger, 1915; Coster, 1927, 1928; Berlage,
1931; Tschinkel, 1966; Ash, 1983; DeÂtienne, 1989;
Worbes, 1989, 1995, 1999a; Worbes and Junk, 1989).
The global distribution of such forests exceeds that
of equatorial rain forests (Worbes, 1992, 1995). In
West African lowland forests the existence of annual
rings is proven for many trees (Amobi, 1973; Mariaux,
1967a,b, 1969, 1970, 1981). These ®ndings open the
possibility to a broader application of tree ring analysis in tropical forests management.
In the present study, we prove the existence of
annual rings in trees of a natural forest stand in
Cameroon. We will use tree ring analysis for:
revealing stand history and growth dynamics of
important timber species, and
estimating of diameter growth of the trees in this
stand.
With this study, we want to show how tree ring
analysis can help evaluate basic data for the management of tropical forests.
2. Site description
The study site is located in Central Cameroon
100 km north of YaoundeÂ, 15 km northeast of the
village of Biakoa (48400 N, 118320 E) on 600 m AASL.
The forest formation is a closed semi-deciduous forest,
which covers a wide region in that area. A forest
prospection of 20 km2 in the vicinity of Biakoa showed
that our study site is of a similar species composition in
respect of the main commercial timber species. On the
top of steep hills forest is lower and less dense than at
low elevations. Some 30 km northeast the forest is
replaced successively by an anthropogenic savannah.
Climate data were obtained from the nearest climate
station (Biafa) 30 km west of the study site. The
precipitation time series extends from 1951 until the
present. The climate is characterised by a mean annual
precipitation of 1900 mm with a distinct seasonal
distribution (Fig. 1). The dry period, with less than
50 mm precipitation per month, lasts from December
to February. A second, less pronounced ``dry'' period
with precipitation around 100 mm occurs in June or
August. The temperature varies between 22.4 8C in
July and 25.0 8C in February. The soil is a deep latosol
with 50% clay.
The ®eld work was carried out between February
and July 1993. One hectare (100 100 m2) of a nonlogged natural forest was divided into hundred
10 10 m2. Every tree with a DBH > 10 cm was
mapped with its x- and y-co-ordinates, the DBH
was measured with a tape, and the height was measured trigonometrically with a SUUNTO measurement device. The DBH of trees with buttresses was
measured above them.
3. Material and tree ring measurements
In the investigation area from all trees with a
DBH > 10 cm, two wood samples were taken at
breast height along two rectangular radii with an
increment corer (é 5 mm). In trees with buttresses,
cores were taken from the stems portion between two
buttresses. From a nearby logged forest, stem discs
and additional increment cores were taken from
Triplochiton scleroxylon and Terminalia superba for
an additional investigation of the wood and growth
structure and for a radiocarbon analysis.
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
107
Fig. 1. Mean monthly precipitation in Biafa, Cameroon. Time series from 1951 to 1993.
The wood density of the samples was calculated
from size, measured with a sliding caliper and weight
of the oven-dried samples (105 8C). Then the cores
were glued on a wooden support and highly polished
with ®ne sand paper. The ring widths were measured
to the nearest 0.01 mm using a tree ring measurement
device following RINN (unpublished).
In a nearby exploited forest stand, stem discs were
taken from 17 T. scleroxylon trees and four T. superba
trees. The ring widths were measured at four radii, and
the results were compared statistically using the tree
ring program CATRAS (Aniol, 1983) and visually
comparing the outprints of the tree ring curves. This
procedure is usually applied in dendro-chronological
science for the detection of missing rings and is termed
cross-dating. Due to this procedure tree ring curves of
all samples were corrected, including missing rings
and excluding false rings. From the resulting corrected
curves into steps, mean curves for every individual
and, in a second step, mean curves for species were
constructed (Stahle et al., 1999).
The age of the trees was determined on crosssections at breast height. In temperate zones
Schweingruber (1988) adds 7±8 years for the time a
seedling grows to 1.3 m. There are no experiments on
height growth rates of trees in tropical natural forests.
So, we did not add arti®cial ®gures to our estimations.
The real age of trees and forests might therefore be
5±10 years higher than given. In some cases both cores
per tree did not hit the pith and missed it tangentially.
We used a stencil developed in our laboratory for
temperate trees where we can estimate from the width
of the rings and the angle of the ring boundaries the
position of the pith (Bonn and Worbes, 1991).
4. Radiocarbon dating
The radiocarbon dating of individual tree rings is an
independent proof of the annual nature of tree rings
(Worbes and Junk, 1989). The procedure is based on
the atomic weapon effect (Nydal and LoÈvseth, 1983)
and the fact that the radiocarbon content of the wood
re¯ects the radiocarbon content of the atmosphere in a
given year between 1950 and the present in the
temperate zones as well as in the tropics (Worbes
and Junk, 1989). The aim of this radiocarbon measurement is not the pure dating of one growth zone but
the de®nition of an arti®cial time marker in the wood.
With this second date (beneath the felling date), it is
108
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
of the species (67) have less than four individuals in
the study plot. The more frequent species are listed in
Table 1.
The majority of the individuals (76.9%) belong to
the understorey up to 15 m. The 14.8% of the individuals form the main canopy between 15 and 30 m, and
8.3% of all trees can be classi®ed as emergents with
tree heights up to 55 m. The diameter distribution has
an inverse J-shape (Fig. 2) with the majority (63.1% of
all individuals) in the diameter class between 10 and
20 cm and a fast exponential decrease.
Representatives of seven species were found in the
storey above 30 m, with 29 species between 15 and
29 m. The understorey is most diverse with 67 different species.
The total basal area of living trees is 37.93 m2.
T. scleroxylon is dominating the stand in terms of basal
area (9.43 m2) with only 18 individuals, mainly occurring in the highest crown layer with tree heights up to
48 m and a maximum DBH of 125 cm. The tree with
possible to compare the number of growth zones
with the number of years and de®ne the nature of the
periodical growth zones as annually or not. Therefore,
predated individual tree rings from T. scleroxylon were
taken, and their radiocarbon content was measured with
accelerator mass spectrometry by the Physikalisches
Institut der UniversitaÈt Erlangen-NuÈrnberg. The results
were compared with a curve of the radiocarbon content
from the atmosphere (Hua et al., 1999) to the visible
growth structure of the wood.
5. Results
5.1. Species composition, inventory
A total of 516 living trees with a DBH > 10 cm
from 81 species were counted in the investigation area
(1 ha). Ten more trees of unknown species were
standing dead with diameters up to 1 m. The majority
Table 1
Growth features of the most frequent tree species from Biakoa forest
Species
Family
Emergents
E. oblonga
T. scleroxylon
Sterculiaceae
Sterculiaceae
Sterculiaceae
Ulmaceae
Ulmaceae
Rubiaceae
Sterculiaceae
N
Wood density
(g cm 3)
Diameter
increment
(cm per year)
Mean Maximum Minimum
height height (m) age (year)
(m)
Maximum
age (year)
Mean age
(year)
4
18
0.84 0.08
0.49 0.06
0.72 0.06
0.62 0.28
40.7
38.0
45
48
64
56
103
219
85
124
Main canopy
Pterygota sp.
Celtis adolfi-frederici
C. zenkeri
Mitragyna ciliata
N. papaverifera
``Mbamzok''
T. madagascariense
Indet B
D. crassiflora
Polyalthia suaveolens
Ulmaceae I
S. rhinopetala
Ebenceae
Annonaceae
Ulmaceae
Sterculiaceae
6
6
10
6
30
4
27
14
14
13
11
19
0.73
0.71
0.84
0.66
0.91
0.98
0.67
0.65
0.74
0.88
0.84
0.72
0.05
0.04
0.03
0.06
0.05
0.04
0.04
0.05
0.04
0.04
0.04
0.05
0.81
0.32
0.36
0.70
0.50
0.28
0.46
0.38
0.46
0.34
0.32
0.36
0.22
0.14
0.18
0.16
0.18
0.06
0.16
0.06
0.10
0.10
0.12
0.10
23.2
21.5
20.0
19.8
18.1
15.5
14.8
14.7
14.3
14.2
13.4
11.5
25
35
32
35
45
24
29
22
28
26
26
25
32
47
52
19
22
39
26
42
28
30
49
28
51
146
220
60
124
96
66
100
70
100
104
86
43
91
82
42
50
58
47
69
50
56
61
46
Understorey
``Ofes B''
E. cylindricum
Aningeria robusta
G. perpulchra
``Ofes A''
Indet A
C. preussii
4
5
4
12
27
35
Myristicaceae 102
0.71
0.77
0.80
0.93
0.83
1.04
0.98
0.08
0.10
0.04
0.05
0.07
0.05
0.06
0.48
0.24
0.32
0.24
0.32
0.20
0.26
0.22
0.06
0.10
0.10
0.10
0.06
0.08
11.5
11.4
10.3
9.8
9.3
8.9
8.4
14
18
15
18
13
14
16
33
32
31
50
47
38
31
146
114
64
88
115
94
110
47
66
52
70
64
41
65
Moraceae
Meliaceae
Sapotaceae
Sapotaceae
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
109
Fig. 2. Diameter distribution of all trees with diameter < 10 cm in the Biakoa research site.
greatest dimensions is a Ceiba pentandra with 130 cm
DBH and 55 m height. Coelocaryon preussii is the
most frequent species with 102 individuals, restricted
to the understorey with measured heights between 5
and 16 m and DBH values between 13 and 39 cm. The
relation between diameter and height compared over
all individuals is close (r 2 ˆ 0:82, Table 2).
5.2. Annual tree rings in T. scleroxylon and other
species
On the highly polished stem discs and the prepared
cores, growth zones appear with speci®c differences in
distinctiveness. All Sterculiaceae, Meliaceae and others
show very clear rings, whereas, e.g. the growth zones of
C. pentandra on the cores are not distinguishable,
therefore, we did not include this species which was
only represented with two individuals in the calculations of growth rates. According to our observations this
species and other representatives of Bombacaceae
show indistinct rings in neotropical habits as well
(Worbes, 1989). In this family, the investigation of tree
ring structure usually requires the investigation of stem
discs, because the boundaries of the rings only are
marked by slight density variations. Distinct rings
are delimited by marginal parenchyma as in T. scleroxylon, Entandophragma cylindricum, T. superba and
many others (Fig. 3). Extended descriptions of the wood
Table 2
Correlation coef®cients of structural and growth parameters over all individualsa
Height
Height
DBH
Age
Density
Radial growth
a
±
0.9* (83%)
0.55* (30%)
0.46* (21%)
0.69* (48%)
DBH
±
±
0.63* (38%)
0.42* (18%)
0.72* (53%)
Age
±
±
±
0.1 (0.0%)
0.0 (0.1%)
Wood density
±
±
±
±
0.43* (19%)
Asterisks indicate signi®cance at 99% con®dence level. In brackets R2 are given, explaining the percentage of variability.
110
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
Fig. 3. Cross-sections of E. cylindricum (A) and T. scleroxylon (B). Some boundaries of the growth zones are marked; magni®cation is 20.
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
111
Fig. 4. Ring width curves of four radii together with the mean curve from one stem disc of T. scleroxylon.
anatomy of most of our investigated species are available in DeÂtienne (1989) and Richter and Dallwitz
(2000).
The possibility to cross-date measured ring width
time series is one indication of seasonal growth in
tropical trees (Worbes, 1995; Stahle et al., 1999). We
give examples of individual and interspeci®c crossdating in Figs. 4 and 5. Another strong hint to an
annual growth rhythm is the concurrence between
rain fall and ring width patterns (Worbes, 1999a).
We already showed this for T. scleroxylon (Staschel
et al., 1996), where the width of the annual rings shows
a good correlation with the amount of rainfall in the
transition months between the rainy and dry seasons
(see also Berlage, 1931).
Despite the strong evidence of the described methods that tree rings at the investigated stand are annual in
nature, we applied by means of radiocarbon dating an
additional method independent from the dendro-chronological system. The values of the radiocarbon content
in the wood of the predated rings in T. scleroxylon ®t
the radiocarbon content of the air in the respective
years (Fig. 6). We used for our predictions the period of
increasing atmospheric 14 8C from 1955 until 1965. In
this time the differences in atmospheric radiocarbon
from year to year are highest and provide a clear result.
An often used and sometimes helpful technique is
the comparison of information from local people on
age of trees or speci®c events that can be correlated
with tree age, with number of tree rings. On a logging
road de®nitely not used for 3 years, seedlings of
Ricionodendron heudelotii were found showing three
distinct tree rings.
The knowledge that West African trees and those
from Cameroon show annual rings is not new. Hummel (1946), Lowe (1961), Amobi (1974), DeÂtienne
and Mariaux (1976), DeÂtienne (1989) and above all
Mariaux (1967a,b, 1969, 1981) proved and documented annual wood formation in tree species of our
site as Aucoumea klaineana, C. pentandra, Clorophora excelsea, Guarea cedrata, E. cylindricum,
Mansonia altissima, T. superba, and T. scleroxylon,
which occur in our research site and some additional
30 species.
From these ®ndings and from the ®ndings of numerous publications on tree rings in trees under seasonal
climate conditions in other tropical regions (Worbes,
1995), we made the deduction that all distinct rings of
the investigated species are annual in nature.
6. Wood density
The wood density of a tree species is one indication
of its life strategy. Pioneer trees generally have soft
wood and trees of the mature forest show a high wood
112
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
Fig. 5. Mean curves of E. cylindricum with (a) T. scleroxylon; (b) S. rhinopetala; (c) T. superba. The x-axis scale is logarithmic to point the
characteristic minima.
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
113
Fig. 5. (Continued ).
density (Swaine and Whitmore, 1988) often combined
with low increment rates (Worbes, 1989). In the
Biakoa forest the wood densities vary between
0.26 g cm 3 in R. heudelotii to 1.1 g cm 3 in Garciana
kola. Most trees of the upper storey have wood
densities below 0.6 g cm 3 (e.g. C. pentandra with
0.45 g cm 3). The majority of trees with high wood
density are found in the understorey. Some of these are
restricted to the understorey (e.g. Coeloracyon preusii:
0.98 g cm 3), whereas others like Nesogordonia
papaverifa (0.91 g cm 3) have representatives in all
height classes (Table 1). The mean for all individuals
is 0.83 g cm 3, while the mean for species mean is
0.8 g cm 3. This almost equals the value for a primary
forest in the Amazonian inundation forest (VaÂrzea:
0.86, Worbes et al., 1992).
7. The age of the trees
The diameter of a tree is dependent on its age, but in
a number of randomly selected individuals (as in
a diverse tropical forest site) it is not possible to
correlate both parameters with suf®cient accuracy.
The correlation between diameter and age is weak
(r 2 ˆ 0:37, Table 2). Trees of the same age can have a
diameter of 10 or 120 cm (Fig. 7a). There is also no
signi®cant correlation between height and age of the
trees calculated for all individuals. Examples in Fig. 7b
show that 35 m tall trees might be 60 or 220 years old.
The same is true within one species (T. scleroxylon,
Fig. 8). Consequently, the pattern of the age class
distribution (Fig. 8) differs considerably from the
diameter class distribution (Fig. 2). The division into
20-year step age classes shows a normal distribution
with its maximum between 41 and 60 years. The
37.1% of all individuals fall into this age range.
The mean age for all individuals is 61 years.
The age of the trees at the time of investigation
varies considerably between species and within species between individuals (Table 1). The oldest tree is a
C. zenkeri with 220 years, followed by a T. scleroxlon
with 183 years. The youngest recorded tree was
Staudatia kamerunensis with 14 years. Maximum,
114
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
Fig. 6. Radiocarbon concentration (d14 C in % modern, what is the relation to the pre-bomb period) of selected growth zones of T. scleroxylon
in x-position as predated. Radiocarbon concentration of the air is derived from Hua et al. (1999).
minimum, and mean age of the most frequent species
are given in Table 1.
Trees with an age of more than 120 years were
found exclusively in the storey of the emergents
(above 30 m), the youngest tree of the upper storey
is 56 years old (T. scleroxylon), and the mean age of
the upper storey is 113 years.
Species cohorts are unevenly aged. The greatest
difference in age between the youngest and the oldest
individual within one species is found in C. zenkeri
(168 years) and T. scleroxylon (127 years). The smallest difference is found in the unidenti®ed ``Faut
Koto'' with 19 years.
8. Diameter growth rates
The diameter growth rates of trees with more than
four individuals are listed in Table 1. The lowest values
show the species of the understorey (between 0.20 and
0.58 cm per year). Highest values occur in the main
canopy and in emergent species (0.36±0.82 cm per
year). The mean for all individuals is 0.19 cm per year.
The highest individual values were also found in upperstorey trees (Eribroma oblonga: 1.86 cm per year, T.
scleroxylon: 1.42 cm per year, R. heudelotii: 1.38 cm
per year). Lowest values were shown by Gambeya
perpulchra and Coelocaryon preusii with 0.14 cm
per year in the understorey.
The growth rates are correlated over all individuals
positively with tree height (0.69) and the diameter at
breast height (0.72) and not at all with a tree's age. A
weak negative correlation between mean growth rate
and mean density of species is observable ( 0.43).
That means simply that a large tree due to its better
exposition to the light in comparison with a small tree
has higher growth rates and can thus attain a high trunk
diameter. This is independent from a tree's age,
because young trees with high growth rates may also
have a thick stem.
In contrast to the statement of Swaine and Putz
(1987) that trees' ``growth rates are fairly conservative
over time'', measured tree ring curves show a high
variation from year to year (Figs. 4 and 5). This can be
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
115
Fig. 7. (a) Diameter±age relation of all age dated trees over 10 cm DBH in 1 ha of Biakoa forest; (b) height±age relation of all age dated trees
over 10 cm DBH in 1 ha of Biakoa forest.
116
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
Fig. 8. Age class distribution of all trees above 10 cm DBH in 1 ha of Biakoa forest.
traced back to climate variations, in particular the
variation of precipitation patterns between consecutive years as we proved for T. scleroxylon in the
investigation area (Staschel et al., 1996). Similar
®ndings derived from tree ring analysis are shown
for Tectona grandis on Java (Berlage, 1931) and in
Thailand (Pumijumnong et al., 1995). In Costa Rica,
Clark and Clark (1994) found those increment patterns
using annually repeated diameter measurements. The
year by year variation in tropical trees is obviously
higher than the variations observed in trees from the
temperate zones.
Additionally, different patterns of long-term growth
trends can be observed in the investigated species.
Typical examples are:
Relatively constant growth over the entire life span
(Fig. 9a).
A fast increase from the very young to a maximum
followed by a decrease to a low level in the mature
tree. This is called an age trend (Schweingruber,
1988) and is typical for trees in the artificial
temperate forests or trees in secondary forests in
natural stands (Fig. 9b).
A more or less constant increase from the past to the
present.
Trees with one or more abrupt changes of the
growth curve during their life span (Fig. 9c).
Within one species several patterns as de®ned above
may occur. They are the result of the individual life
histories in respect of the given light conditions in any
period of their life.
9. Discussion
9.1. Forest dynamics
The Biakoa forest belongs to a forest type which is
widely distributed at the West African coast, dominated on the family level by Sterculiaceae and on
species level by T. scleroxylon an important representative of this family. The region of the investigation
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
117
Fig. 9. Tree ring curves of individual T. scleroxylon trees together with their trend curves (moving average) showing different long-term
trends. For easier comparison the respective upper borderline of a curve is the 1 cm, the lower border is the 1 mm line. The scaling is
logarithmic.
site belongs to northern part of the eastern distribution
centre of T. scleroxylon (Hall and Bada, 1979). In the
vicinity of the investigation area T. scleroxylon is the
most frequent large tree species. Several other species
of large trees which are closely associated with
T. scleroxylon in Ghana and Nigeria (Hall and Bada,
1979) also occur in the Biakoa forest (C. pentandra,
Chlorophora excelsea, N. papaverifera and others).
Therefore, our investigation on the forest dynamics
in Biakoa may have some evidence for many forest
stands in West Africa.
The principal mechanisms of successional processes in tropical forests were described by the model
of silvigenetic cycles (Halle et al., 1978) as a
sequence of alternating dynamical and homoeostatical periods. During dynamical stages pioneer species
are replaced by mature forest species. During
homoeostatical periods tree mature and reach their
maximum age. The question on the length of certain
sequences especially of older successional stages
remains unsolved. Many attempts have been made
to solve this question. The most conservative but
closest to the truth is the long-term observation of
forest stands in permanent plots. However, even the
longest observation period in tropical forests, e.g. 35
years in Luquillo Experimental Forest, Puerto Rico
(Crow, 1980; McCormick, 1995), is short in comparison with the supposed age of mature forest trees.
Assumptions on maximum ages of trees in tropical
lowland forests reach up to 2000 years (Condit et al.,
1995). The ages of the trees in Biakoa forest are much
lower than these. We will discuss some possible
causes for the differences below (see also Worbes
and Junk, 1999).
In the analysis of dynamical processes in tropical
forests an often disregarded feature of Budowski's
table (Budowski, 1961) is the wood density of the
canopy trees, where pioneers are species with low
118
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
Fig. 10. Height±age relation of six selected tree species pointed out with trend curves. The wood density is shown in detail in each graph.
wood density and a high density is typical for a tree
from the mature forest (Swaine and Whitmore, 1988).
Together with the age and the development of height
we used this feature to classify three major types of life
strategies of species cohorts (Fig. 10):
Trees within 100 years usually reach a maximum
height of 15 m and are obviously restricted to the
understorey. This behaviour is combined with a
high wood density and low radial increments (e.g.
C. preussii).
Trees which occur in high age classes up to 200
years mainly in the upper storey and have few or no
younger recruits in the stand. These trees show
generally low or moderate wood densities at high
increment rates (mainly T. scleroxylon, but also C.
pentandra) and can be classified as long-living
pioneers (Halle et al., 1978).
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
Trees which have the capacity to reach the main
canopy or to establish as emergents, shown by a few
individuals of a given species and with the majority
of the cohort as younger individuals being recruits
in the lower storeys. The wood of these trees shows
moderate or high density (Trilepsium madagascariense, N. papaverifera, Sterculia rhinopetala).
These species can be defined as mature forest trees
or trees of the future (Halle et al., 1978).
In respect of the composition of these species groups
the Biakoa forest has a lot of features which classify it as
a very late secondary forest. The highest trees are those
with high increment rates and low wood density. Typical pioneer trees of the region (Okali and Ola-Adams,
1987) as reminds to former silvigenetic stages as
Chlorophera excelsea, T. superba, C. pentandra and
Alstonia boonei occur with few individuals in the upper
storey and do not have recruits. The dominating species
T. scleroxylon has only one about 60 years old suppressed recruit in the understorey showing very slow
growth. On the other hand, slow-growing, shade-tolerant species with a high wood density like N. papaverifera, S. rhinopetala and C. zenkerii, classi®ed in
Nigerian forest as mature forest trees (Okali and OlaAdams, 1987), already attained with some representatives the upper canopy within about 150 years and have
many recruits in all size and age classes in the lower
storeys. In total in Biakoa the long-living pioneer
species are larger and in majority older than the mature
forest species.
The duration of a certain successional stage is not a
®xed value. In the central Amazonian ¯ood plain
(VaÂrzea), which has fertile soils, a late secondary stage
is obviously reached when the dominating long-living
pioneer species (here Pseudobombax munguba) has
reached the maximum age of about 80±120 years due
to its very low wood density of about 0.2 g cm 3
(Worbes, 1989). Dominating species (Piranhea trifoliata) of the mature forest reach an age of about 400
years (Worbes et al., 1992). The time span from a
pioneer stage until the beginning of a mature forest
stage may, therefore, vary from about 100 years under
good growth conditions until about 200 years as in the
Biakoa forest under moderate growth conditions.
In this context, age estimations from other tropical
forests with a mean of projected life spans for all
individuals of 230 years in a Costa Rican rain forest
119
(Lieberman et al., 1985) or maximum tree ages until
442 years for Carapa guianensis and 529 years for
Neea divaricata, a small stemmed mid-canopy species
in a rain forest in Amazonian Ecuador (Korning and
Balslev, 1994) seem to be very high. On the basis of
the estimated growth rates, a mathematical model
(Lieberman et al., 1985) calculates the time the slowest growing tree needs to grow from 10 cm (DBH) to
the largest observed trunk diameter of a given species.
A similar calculation of trees in Biakoa gives 340
years of ``life expectancy'' of the thickest tree of
N. papaverifa, which is in reality 148 years old. For
T. scleroxylon the oldest tree with 224 years could
have an age of about 370 years. The misleading
assumption is that slow-growing trees of the understorey could reach dimensions of the emergents of the
same species. Usually, slow-growing trees are suppressed in the shade of competitors and die early
(Swaine and Putz, 1987). In Biakoa, the upperstorey
trees with a great diameter always have a much higher
increment than the recruits in the understorey of the
same species. Nevertheless, the discussed estimations
from repeated diameter measurements are much
closer to our results than the 2000 years from mathematical calculations on the basis of mortality rates
(Condit et al., 1995). In general, the comparison
of traditional age dating in tropical forests and our
results lead to the assumption that the age and the
longevity of tropical trees and forests often were
overestimated. Especially, in ecosystems frequently
stroked by catastrophic events like hurricanes in
Central America (Basnet et al., 1992; Zimmerman
et al., 1994), extreme pluriannual ¯ood and drought
events in the great ¯oodplains (Junk, 1989), ®res in
regions with low precipitation (Abrams et al., 1995;
Worbes, 1999b) or human-impact silvigenetic cycles
often are interrupted and a mature stage of a forest is of
a theoretical nature.
9.2. Growth rates and growth trends
Since the concept of a general lack of annual rings
in trees of tropical regions is still widely accepted in
tropical ecology and forestry (Lieberman et al., 1985;
Whitmore, 1990; Bruenig, 1996), growth rate and age
estimations of tropical forest trees were carried out by
repeating diameter measurements (Veillon, 1985;
Lieberman et al., 1985; Manokaran and Kochumen,
120
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
Fig. 11. (a) Individual and mean cumulative increment curves of 17 T. scleroxylon stem discs. The box shows the time to pass 20 cm DBH for
a mean stem. The minimum felling diameter is 80 cm; (b) cumulative increment curves from T. scleroxylon (n ˆ 17), Diospyros crassi¯ora
(n ˆ 15). S. rhinopetala (n ˆ 20), N. papaverifera …n ˆ 30† and C. preussii …n ˆ 101†.
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
1987; Swaine and Putz, 1987; Clark and Clark, 1992;
Korning and Balslev, 1994). Often these investigations
cover only a short time period in relation to the age of a
tree in natural forests. In the observed period either
poor or good growth conditions may be present as
a result of varying climate behaviour. Additionally,
unidenti®ed or dynamic reasons may in¯uence the
growth considerably. For example, Clark and Clark
(1994) report on trees in La Selva, Costa Rica, without
measurable growth over a period of 14 years. These
trees are emergents and must have been grown faster in
former times or be in®nitely old. Our results, however,
represent the radial increment over the entire life span
of the trees. These data are of interest for foresters, who
want to plan the yield for the entire tree, a species and a
forest site.
The mean measured diameter growth rate of
3.8 mm per year in the Cameroon forest exceeds
the results from other growth rate estimations in
tropical forests. Manokaran and Kochumen (1987)
found in a Malaysian Dipterocarp forest a mean radial
increment over all species of 2.6 mm per year. The
mean for the published data in an Ecuadorian rain
forest is 2.6 mm per year (Korning and Balslev, 1994).
The comparison of these with our results must consider that we include in our estimation not only the
increments of the mature forest but juvenile phases
with higher growth rates. The values for canopy
species in a Costa Rican rain forest with mean increment of 4.2 mm per year (Lieberman et al., 1985)
equal our data from the main canopy in Biakoa.
Probably due to the long observation period of 24
years the data from Veillon (1985), who measured in
Venezuela 3.8 mm per year in a semidry and 4.5 mm
per year in a moist forest, are in the range of our
results. The ®nding of all reports that overstorey trees
grow faster than understorey trees is con®rmed by our
results. This is doubtless the result of decreasing light
saturation from the top to the ground of the forest.
In general the mean diameter increment of a forest
stand says little about the in¯uence of abiotic site
factors, when the successional stage of the stand is
not considered. In a young pioneer stand with a high
percentage of fast-growing, light-demanding trees
with a low wood density, wood increment rates are
generally higher than in an old multistoried forest
(Jordan and Farnworth, 1980). In old grown forests
the low increment rates of many understorey individuals
121
equal higher values from trees of the higher canopies
(Worbes et al., 1992; Worbes, 1996).
In Cameroon the minimum harvestable diameter for
T. scleroxylon is laid down with 80 cm (Obam, 1992).
In the investigated stand trees need 50±100 years to
grow into this diameter class; the mean is close to 80
years, corresponding to a mean annual diameter increment rate of 1 cm (Fig. 11a). Trees need about 20 years
to pass from 60 to 80 cm DBH. These results con®rm
assumptions of Hall and Bada (1979) on the growth
rates of these species. T. scleroxylon belongs to the
fastest-growing timber species in the stand (C. pentandra is not considered but with certainty is also
among this group). Other species (Diospyros, Sterculia) grow much more slowly and would not reach
80 cm in diameter before an age of 150 years
(Fig. 11b). In Zimbabwe, Stahle et al. (1999) measured with tree ring analysis the growth rates for the
hardwood species Pterocarpus angolensis and found,
due to the drier environment, growth rates much lower
than ours. In total the growth rates in tropical forests
seem to be low. Plans for a sustainable management of
these forests must consider that even the fastest growing timber species needs almost 90 years before it can
be used commercially.
10. Conclusions
The use of tree ring analysis in tropical forest
ecology is a valuable tool for the interpretation of
forest dynamics and for growth rate estimations. Tropical trees show a high variation in life histories and a
sensitive reaction to changing growth conditions. The
increasing use of forests in the tropics and the strong
demand for a reliable database as the precondition for
sustainable management planning require further
development of tree ring analysis to provide an increase
of ecological knowledge of these unique ecosystems.
Acknowledgements
The study was funded by the Deutsche Forschungsgemeinschaft (DFG). We thank the Ministry of Environment and Forest in Cameroon and Dr. H.L. Stoll
from the Feldmeyer KG, who enabled the ®eld studies
in the Biakoa forest.
122
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
References
Abrams, M.D., Orwig, D.A., Demeo, T.E., 1995. Dendroecological
analysis of successional dynamics for a presettlement-origin
white-pine-mixed-oak forest in the southern Appalachians,
USA. J. Ecol. 83, 123±133.
Amobi, C.C., 1973. Periodicity of wood formation in some trees of
lowland rainforest in Nigeria. Ann. Bot. 37, 211±218.
Amobi, C.C., 1974. Periodicity of wood formation in twigs of some
tropical trees in Nigeria. Ann. Bot. 38, 931±936.
Aniol, R.W., 1983. Tree-ring analysis using CATRAS. Dendrochronologia 1, 45±53.
Ash, J., 1983. Growth rings in Agathis robusta and Araucaria
cunninghamii from tropical Australia. Aust. J. Bot. 31, 269±
275.
Basnet, K., Likens, G.E., Scatena, F.N., Lugo, A.E., 1992.
Hurricane Hugo: damage to a tropical rain forest in Puerto
Rico. J. Trop. Ecol. 8, 47±55.
Berlage, H.P., 1931. Over het verband tusschen de dikte der
jaarringen van djatiboomen (Tectona grandis L.f.) en den
regenval op Java. Tectona 24, 939±953.
Bonn, S., Worbes, M., 1991. Klimaein¯uû und abrupte
Zuwachsschwankungen von Fichten und Kiefern verschiedener
HoÈhenstufen Niedersachsens. Berichte des Forschungszentrum
WaldoÈkosysteme. Reihe B 30, 1±68.
Bruenig, E.F., 1996. Conservation and Management of Tropical
Rain Forests. CAB International, Wallingford, UK.
Budowski, G., 1961. Studies on forest succession in Costa Rica
and Panama. Ph.D. Thesis. Yale University, New Haven,
p. 189.
Clark, D.A., Clark, D.B., 1992. Life history diversity of canopy and
emergent trees in a Neotropical rainforest. Ecol. Monogr. 62,
315±344.
Clark, D.A., Clark, D.B., 1994. Climate-induced annual variation
in canopy tree growth in a Costa Rican tropical rain forests.
J. Ecol. 82, 865±872.
Clark, D.B., Clark, D.A., 1996. Abundance, growth and mortality
of very large tree in neotropical lowland rain forest. For. Ecol.
Manage. 80, 235±244.
Condit, R., Hubbel, S.P., Foster, R.B., 1995. Mortality rates of 205
Neotropical tree and shrub species and the impact of a severe
drought. Ecol. Monogr. 65, 418±439.
Coster, C., 1927. Zur Anatomie und Physiologie der Zuwachszonen
und Jahresringbildung in den Tropen I. Ann. Jardim Bot.
Buitenzorg 37, 49±161.
Coster, C., 1928. Zur Anatomie und Physiologie der Zuwachszonen
und Jahresringbildung in den Tropen II. Ann. Jardim Bot.
Buitenzorg 38, 1±114.
Crow, T.R., 1980. A rainforest chronicle: a 30-year record of
change in structure and composition at El Verde, Puerto Rico.
Biotropica 12, 42±45.
DeÂtienne, P., 1989. Appearance and periodicity of growth rings in
tropical woods. IAWA Bull. n.s. 10, 123±132.
DeÂtienne, P., Mariaux, A., 1976. Nature et peÂriodicite des cernes
dans le bois de Samba. Bois For. Trop. 169, 29±35.
Ellenberg, H., 1986. Vegetation Mitteleuropas mit den Alpen.
Ulmer, Stuttgart, p. 989S.
Geiger, F., 1915. Anatomische Untersuchungen uÈber die Jahresringbildung von Tectona grandis. Jahrbuch fuÈr Wissenschaftliche Botanik 55, 521±607.
Hall, J.B., Bada, S.O., 1979. The distribution and ecology of
Oboche (Triplochiton scleroxylon). J. Ecol. 67, 543±564.
HalleÂ, F., Oldeman, R.A.A., Tomlinson, P.B., 1978. Tropical Trees
and Forests. An Architectural Analysis. Springer, Berlin, p. 441.
Hua, Q., Barbetti, M., Worbes, M., Head, J., Levchenko, V.A.,
1999. Review of radiocarbon data from atmospheric and tree
ring samples for the period 1950±1977 A.D.. IAWA J. 20 (3),
261±284.
Hummel, F.C., 1946. The formation of growth rings in Entandophragma macrophyllum A. Chev., and Khaya gradifoliola
C.DC. Empire For. Rev. 25, 103±107.
Jordan, C.F., 1983. Productivity of tropical rain forest ecosystems
and the implications for their use as future wood and energy
sources. In: Golley, F.B. (Ed.), Tropical Rain Forest Ecosystems. Ecosystems of the World, Vol. 14A. Elsevier, Amsterdam,
pp. 117±136.
Jordan, C.F., Farnworth, E.G., 1980. A rainforest chronicle.
Perpetuation of a myth. Biotropica 12, 233±234.
Junk, W.J., 1989. Flood tolerance and tree distribution in central
Amazonian ¯oodplains. In: Holm-Nielsen, et al. (Eds.), Tropical
Forests: Botanical Dynamics, Speciation and Diversity. Academic
Press, London, pp. 47±64.
Koop, H., 1989. Forest Dynamics: SILVI-STARÐA Comprehensive Monitoring System. Springer, Berlin, 242 pp.
Korning, J., Balslev, H., 1994. Growth rates and mortality patterns
of topical lowland tree species and the relation to forest
structure in Amazonian Ecuador. J. Trop. Ecol. 10, 151±166.
Lang, G.E., Knight, D.H., 1983. Tree growth, mortality, recruitment, and canopy gap formation during a 10-year period in a
tropical moist forest. Ecology 64, 1075±1080.
Lieberman, D., Lieberman, M., Hartshorn, G., Peralta, R., 1985.
Growth rates and age-size relationships of tropical wet forest
trees in Cost Rica. J. Trop. Ecol. 1, 97±109.
Lowe, R.G., 1961. Periodic growth in Triplochiton scleroxylon K.
Schum. Federation of Nigeria Department Forest Research
Technical Note No. 13.
Manokaran, N., Kochumen, K.M., 1987. Recruitment, growth and
mortality of tree species in a lowland dipterocarp forest in
Peninsular Malaysia. J. Trop. Ecol. 3, 315±330.
Mariaux, A., 1967a. Les cernes dans les bois tropicaux africains,
nature et peÂriodiciteÂ. Bois For. Trop. 113, 3±14.
Mariaux, A., 1967b. Les cernes dans les bois tropicaux africains,
nature et peÂriodiciteÂ. Bois For. Trop. 114, 23±37.
Mariaux, A., 1969. La peÂriodicite de formation des cernes dans le
bois de Limba. Rev. Bois For. Trop. 128, 39±54.
Mariaux, A., 1970. La peÂriodicite de formation des cernes dans le
bois de l'OkoumeÂ. Bois For. Trop. 131, 37±50.
Mariaux, A., 1981. Past effects in measuring age and annual growth
in tropical trees. In: Bormann, F.H., Berlyn, G. (Eds.), Age and
Growth Rate of Tropical Trees: New Directions for Research.
Yale University Press, New Haven, pp. 20±30.
McCormick, J.F., 1995. A review of the population dynamics
of selected tree species in the Luquillo experimental forest,
Puerto Rico. In: Lugo, A.E., Lowe, C. (Eds.), Tropical Forests:
M. Worbes et al. / Forest Ecology and Management 173 (2003) 105±123
Management and Ecology. Ecological Studies, Vol. 112,
pp. 224±257.
Nydal, R., LoÈvseth, K., 1983. Tracing bomb 14C in the atmosphere
1962±1980. J. Geophys. Res. 88, 3621±3642.
Obam, A., 1992. Conservation et mise en valeur des forets au
Cameroun. YaundeÂ, p. 285.
Okali, D.U.U., Ola-Adams, B.A., 1987. Tree population changes in
treated rainforests at Omo Forest Reserve, southwestern
Nigeria. J. Trop. Ecol. 3, 291±314.
Pumijumnong, N., Eckstein, D., Sass, U., 1995. Tree ring research
in Tectona grandis in northern Thailand. IAWA J. 16, 385±
392.
Richter, H.G., Dallwitz, M.J., 2000. Commercial Timbers:
Descriptions, Illustrations, Identi®cation, and Information
Retrieval, Version 4, May 2000 (in English, French, German,
and Spanish).http://biodiversity.uno.edu/delta/.
Schweingruber, F.H., 1988. Tree Rings. Reidel, Dordrecht, p. 246.
Stahle, D.W., Mushove, P.T., Cleaveland, M.K., Roig, F., Haynes,
G.A., 1999. Management implications of annual growth rings
in Pterocarpus angolensis from Zimbabwe. For. Ecol. Manage.
124, 217±229.
Staschel, R., Worbes, M., Roloff, A., 1996. Wachstumsdynamik
von Triplochiton scleroxylon (K. Schum.) in einem halbimmergruÈnen Naturwald in Kamerun. Verhandlungen der Gesellschaft
È kologie 26, 183±188.
fuÈr O
Swaine, M.D., Putz, F.E., 1987. The dynamics of tree populations in
tropical forests: a review. J. Trop. Ecol. Bois For. Trop. 3, 359±
369.
Swaine, M.D., Whitmore, T.C., 1988. On the de®nition of
ecological species groups in tropical rain forests. Vegetatio 75,
81±86.
Tschinkel, H.M., 1966. Annual growth rings in Cordia alliodora.
Turrialba 16, 73±80.
Veillon, J.P., 1985. El crecimiento de algunos bosques naturales de
Venezuela en relacioÂn con los paraÂmetros del medio ambiente.
Rev. For. Venezolana 29, 1±120.
123
Whitmore, T.C., 1990. An Introduction to Tropical Rain Forests.
Oxford University Press, Oxford, p. 296.
Worbes, M., 1989. Growth rings, increment and age of trees in
inundation forests, savannas and a mountain forest in the
Neotropics. IAWA Bull. n.s. 10, 109±122.
Worbes, M., 1992. Occurrence of seasonal climate and tree ring
research in the tropics. Lundqua Rep. 34, 338±342.
Worbes, M., 1995. How to measure growth dynamics in tropical
treesÐa review. IAWA J. 16, 337±351.
Worbes, M., 1996. Untersuchungen zur Besiedlungsgeschichte und
Sukzessionsdynamik von GebuÈschen auf ehemaligen HalbÈ kologie 26,
trockenrasen. Verhandlungen der Gesellschaft fuÈr O
189±196.
Worbes, M., 1997. The forest ecosystems of the Amazonian
¯oodplains. In: Junk, W.J. (Ed.), The Amazonian Floodplains:
Ecology of a Pulsing System. Ecological Studies, Vol. 126.
Springer, Berlin, pp. 223±265.
Worbes, M., 1999a. Annual growth rings, rainfall dependent
growth and long-term growth patterns of tropical trees from the
Forest Reserve Caparo in Venezuela. J. Ecol. 87, 391±403.
Worbes, M., 1999b. La degradacioÂn del bosque de la Gran Sabana.
Scientia Guaiana, pp. 87±104.
Worbes, M., Junk, W.J., 1989. Dating tropical trees by means of
14
C from bomb tests. Ecology 70 (2), 503±507.
Worbes, M., Junk, W.J., 1999. How old are Tropical Trees? The
persistence of a myth. IAWA J. 20 (3), 255±260.
Worbes, M., Klinge, H., Revilla, J.D., Martius, C., 1992. On the
dynamics, ¯oristic subdivision and geographical distribution of
vaÂrzea forests in Central Amazonia. J. Veg. Sci. 3, 553±564.
Worbes, M., Hofmann, M., Roloff, A., 1993. Wuchsdynamik der
Baumschicht in einem Seggen-Kalkbuchenwald in Nordwestdeutschland (Huckstein). Dendrochronologia 10, 91±106.
Zimmerman, J.K., Everham, E.M., Waide, R.B., Lodge, D.J., Taylor,
C.M., Brokaw, N.V.L., 1994. Responses of tree species to
hurricane winds in subtropical wet forest in Puerto RicoÐ
implications for tropical tree life-histories. J. Ecol. 82, 911±922.