baker et al 2002 diameter change fem paper

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Phenological differences in tree water use and the timing oftropical forest inventories: conclusions from patterns

of dry season diameter change

T.R. Bakera,*, K. Affum-Baffoea,b, D.F.R.P. Burslema, M.D. Swainea

aDepartment of Plant and Soil Science, St. Machar Drive, University of Aberdeen, Aberdeen AB24 3UU, UKb

Forest Management Support Centre, P.O. Box 1415, Kumasi, Ghana

Received 20 November 2000; received in revised form 5 October 2001; accepted 10 October 2001

Abstract

Interspecific variation in water-induced fluctuations in stem girth demonstrates the mechanisms promoting coexistence in

seasonally dry tropical forest. In addition, these fluctuations are a potential, but unevaluated, source of bias in measurements of

annual tree growth rates. To examine diurnal and seasonal patterns of stem diameter change, tree girth was measured over 2 years

(19971999), using dendrometer bands, for three species (Celtis mildbraedii, C. zenkeri and Strombosia glaucescens) in semi-

deciduous forest in Ghana. Soil matric potential was measured concurrently at 15 cm depth. In addition, measurements of all

trees >20 cm dbh on three, 1 ha plots were made at the beginning and middle of the 1998/1999 dry season. During the severe

1997/1998 dry season, soil matric potential declined below 1.5 MPa and two species showed significant stem shrinkage. Forthe evergreen species, C. mildbraedii, there was a significant positive effect of tree diameter on stem shrinkage, and shrinkage

was greater in the second, compared to the first, half of the dry season. For the deciduous species, C. zenkeri, shrinkage was

reduced during the second half of the dry season, following leaf fall. During 1998/1999, soil matric potential, did not decline

below 1.5 MPa, and rates of girth change remained positive for all species. There were no significant effects of size orphenology on the rate of girth change in the plot-based study. Deviations in annual increment calculated over successive monthly

intervals indicate that a 10-fold difference in soil water availability between measurement occasions can lead to a 4% bias in

estimates of annual growth. Measurements of forest plots should be made when inter-annual variation in soil water availability is

low. In this forest, measurements should, therefore, be made during the wet season, contrary to published recommendations.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Dendrometer bands; Ghana; Permanent plots; Tree growth

1. Introduction

Changes in tree diameter are a combination of two

factors: the increase in dry matter from cambial cell

division, and shorter term fluctuations caused by

changes in tree water status (Kozlowski and Winget,

1964; Sheil, 1995). These short term fluctuations

occur over both diurnal and seasonal timescales.

For example, seasonal fluctuations are significant in

seasonal tropical climates, where soil water availabil-

ity is periodically low, and tree diameter may decrease

in the dry season (Boaler, 1963; Daubenmire, 1973;

Lieberman, 1982; Reich and Borchert, 1984; Swaine

Forest Ecology and Management 171 (2002) 261274

* Corresponding author. Present address: School of Geography,

University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK.

Tel.: 44-113-233-3361; fax: 44-113-233-3308.E-mail address: [email protected] (T.R. Baker).

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 7 8 7 - 3


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et al., 1990; Reich, 1995; Baker et al., unpublished).

Changes in tree diameter in response to tree water

status are closely related to changes in soil water

availability, and are also influenced by tree wateruse characteristics. Fluctuations in diameter have been

related to changes in stem water potential (Hinckley

and Bruckerhoff, 1975; Hinckley et al., 1978; Reich

and Borchert, 1982; Garnier and Berger, 1986), and to

the use of stored water (Reich and Borchert, 1984;

Holbrook, 1995; Reich, 1995). Studies of these water-

related changes in tree diameter in seasonal tropical

forests are important for two reasons. Firstly, they

allow hypotheses concerning interspecific differences

in water use to be tested, illustrating mechanisms

that may promote species coexistence. Secondly, they

allow us to test whether inter-annual variation in soil

water availability between years introduces a signi-

ficant bias in measurements of annual diameter

increment. Measurements of tropical forest plots are

increasingly being used to monitor long-term changes

in forest biomass at regional scales (Phillips et al.,

1998). Critiques of this work have focussed on arte-

facts introduced by the methodology itself (Clark, in

press). It is, therefore, important to quantify all the

potential errors and biases in forest inventory proce-

dures (Phillips et al., in press).

One of the major axes of variation allowing speciescoexistence in seasonal tropical forests relates to water

use. The different strategies are exemplified by decid-

uous and evergreen species (Sobrado, 1986, 1993;

Fanjul and Barajas, 1987; Borchert, 1994; Eamus,

1999). In a dry forest in Venezuela, evergreen species

had smaller seasonal changes in leaf water potential

and wood water content than deciduous species

(Sobrado, 1986, 1993), and in a semi-deciduous forest

in Costa Rica dry season stem water potentials

declined to 2.5 MPa in evergreen species, compared

to


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occasion, then diameter increment will be overesti-

mated, as increased hydration will add to the annual

increment. If tree water status is lower, then annual

increment will be underestimated.By studying diurnal and seasonal changes in tree

girth and soil water availability in semi-deciduous

forest in Ghana 19971999, this paper tests two

hypotheses:

1. Dry season stem shrinkage, and hence water

stress, is greater in deciduous compared to ever-

green species, having accounted for differences in

tree size and topographic position.

2. A significant bias is introduced into measurements

of annual increment by inter-annual variation in

soil water availability.

2. Methods

2.1. Study site and species

The study site was Tinte Bepo Forest Reserve

(78040N, 28060W) which is classified as moist semi-

deciduous forest (Hall and Swaine, 1981). Mean

annual rainfall, 19711993, recorded at the nearest

Ghana Meteorological Service station at Bechem,15 km northeast of the site, is 1288 mm. There is a

strong dry season from December to March, when soil

matric potential, at 2060 cm depth, may decline

below 1.5 MPa, and a less severe drier period fromJuly to September (Veenendaal et al., 1996). The site is

at approximately 300 m a.s.l., and in the study area

overlies fine grained, non-micaceous hornblende gran-

ite (Adu, 1993). The topography is undulating with an

elevational difference of 3050 m between summits

and valleys, over horizontal distances of 300500 m.

Maximum slopes are approximately 208. Reddishbrown sandy clay soils are found on the summits,

yellowish brown sandy clay soils on the slopes and

yellowish brown sandy clay loams and sandy loams in

the valleys (Adu, 1993).

There are approximately equal proportions of

individuals of evergreen and deciduous species in

semi-deciduous forest (Hall and Swaine, 1981). Com-

mon canopy species, which may exceed 60 m in

height, are Triplochiton scleroxylon (Sterculiaceae),

Celtis mildbraedii (Ulmaceae) and Nesogordonia

papaverifera (Sterculiaceae). Common understorey

species are Cleidion gabonicum (Euphorbiaceae),

Microdesmis puberula (Pandaceae) and Baphia nitida

(Papilionaceae).The three principal study species were C. mildbraedii

Engl. (Ulmaceae), C. zenkeri Engl. and Strombosia

glaucescens J. Leonard (Olacaceae). C. mildbraedii is

an evergreen, non-pioneer species that is very common

in semi-deciduous forests in Ghana, found throughout

the Guineo-Congolian forest zone and in East African

coastal forests (Hawthorne, 1995). Its abundance

declines markedly as the landscape becomes wetter,

at both regional and local scales (Fig. 1(a), Swaine and

Hall, 1986; Hawthorne, 1995). It is an important fuel

for domestic use and making pestles (N. Gyakari,

pers. comm.). C. zenkeri is a deciduous, non-pioneer

species found across the Guineo-Congolian forest

zone (Hawthorne, 1995). In Ghana, its distribution is

biased towards dry, fertile sites (Fig. 1(b), Swaine,

1996). S. glaucescens is a very common evergreen,

non-pioneer species in Ghana and found throughout the

Guineo-Congolian region. Its distribution in Ghana is

biased toward wet, infertile sites (Fig. 1(c), Swaine,

1996). It is currently important for making telegraph

poles (N. Gyakari, pers. comm.).

2.2. Topographic definitions

Within areas of uniform soil parent material, the

water regime is an important determinant of colour in

tropical soils due to its influence on the content and

form of the predominant iron oxide present. Haemi-

tite, which gives the soil a bright red colour, is the form

typical of well-drained areas whilst the formation of

goethite is favoured under wetter conditions (Schwert-

mann and Taylor, 1989). As the particular focus of this

study was to examine the influence of topographic

variation on water supply, soil colour was used todefine landscape position (cf. Clark et al., 1998). Soil

colour was assessed using a Munsell Soil Colour

chart at 15 cm depth, 2 m away on the western side

of each selected tree (see below). There were clear

and consistent changes in soil colour downslope,

and different landscape positions were defined on

the basis of the hue (Oyama and Takehara, 1991).

Summit soils were red, hue 5 YR, slope soils yellow/

red, hue 7.5 YR and valley soils pale brown/grey,

hue 10 YR.

T.R. Baker et al. / Forest Ecology and Management 171 (2002) 261274 263


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2.3. Dendrometer bands

Adult trees greater than 20 cm dbh whose crowns

received at least some direct side light, i.e. Dawkinscrown classification of 4 or 5 (Dawkins, 1958), were

selected for study, over an area of approximately

12 ha, over several watersheds. The large study area

indicates that the study trees were typically well

dispersed (>20 m) and were considered independent

data points for the purposes of statistical analysis.Seasonal changes in tree girth were monitored using

dendrometer bands constructed from 20 mm width,

Fig. 1. Distributions of: (a) C. mildbraedii, (b) C. zenkeri and (c) S. glaucescens in Ghana. The location of Tinte Bepo Forest Reserve, 250 mm

isohyets and extent of the forest zone (shaded area) are also shown.

264 T.R. Baker et al. / Forest Ecology and Management 171 (2002) 261274


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150 mm thickness, hard-tempered aluminium and held

in place with a stainless steel spring. Shoe eyelets were

used to strengthen the holes where the spring held the

band. Measurements to 0.01 mm were made using

Vernier calipers of the distance between scribe markson the overlapping front and rear ends of the band.

Bands were placed at 1.3 m height or 50 cm above the

top of any fluting or buttress. Trunk climbers were

cut prior to fixing the band. From 23 July 1997 to 6

August 1997, bands were applied to 28 C. mildbraedii,

five C. zenkeri and three S. glaucescens trees; one

S. glaucescens was added to the sample on 22 August

1997, two C. mildbraedii on 1 October 1997 and two

C. zenkeri and one S. glaucescens on 17 October 1997.

The total sample sizes were, therefore, 30, 7 and 5 for

the three species, respectively, which reflects thespecies relative abundance at the landscape scale in

this forest type.

Measurements of tree girth were made at approxi-

mately monthly intervals until May 1999. In addition,

on four occasions (18 September 1997, 11 October

1997, 22 October 1997, and 21 November 1997)

measurements were made of diurnal fluctuations for

C. mildbraedii, by comparing tree girth between

morning (08:0011:00 h) and late afternoon (15:30

17:30 h) measurements. Weekly measurements at the

start of the study indicated that a short period was

required for the bands to settle (Keeland and Sharitz,

1993). The results are not influenced by long-term

settling of the bands, as a control band placed on one

individual ofC. mildbraedii that had a badly damagedcrown, showed no change in girth over the 2 years

of the study. Therefore, to obtain reliable readings,

those taken less than 30 days following the attachment

of a band were discarded (cf. Pelissier and Pascal,

2000). On seven occasions, out of a total of 837

measurements, slipped or damaged bands had to be

replaced. For calculations of the rate of girth change,

data from these individuals were discarded for at

least 30 days, and annual increments were only calcu-

lated for trees and periods with continuous, 12 month,

records.

2.4. Soil matric potential

Measurements of soil matric potential were made

concurrently with measurements of tree girth. The

filter paper method of Deka et al. (1995) was used

as it gives reliable results down to low matric poten-

tials of approximately 2 MPa, sampling is rapid andit is inexpensive. A standard filter paper is equilibrated

over 6 days with a soil sample, the water content of the

Fig. 1. (Continued).



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filter paper determined and then converted to matric

potential using calibration equations given by Deka

et al. (1995). From 11 October 1997, samples at 15 cm

depth were taken 2 m from the southern side of 14 C.mildbraedii across the range of landscape positions.

On the second and subsequent measurement occa-

sions, samples were taken 50 cm from either side of

the original sample point, and then three sets of three

sample points, each 50 cm behind the previous set.

This systematic sampling pattern was then repeated,

moving clockwise, and assumes there is no systematic

variation in soil matric potential for a 3.5 m radius

around each tree. From 5 November 1997 until 8 April

1999, samples were taken at 15 cm depth around all

42 trees, at the same time as measurements of the

dendrometer bands.

2.5. Forest plot study

To test whether size or phenology-related patterns

of dry season stem shrinkage could be detected at the

stand level, measurements were also made of three

1 ha permanent sample plots (PSPs) in the same forest

reserve, during the 1998/1999 dry season. Measure-

ments were made at the start (November) and end

(January) of the dry season. The plots showed no

evidence of recent fire damage, and are part of theon-going programme of permanent sample plot enu-

meration run by the Ghana Forest Service (Bird and

Sackey, 1991). A full account of the field procedures

are given in Wong (1997) and Affum-Baffoe (1999).

On each measurement occasion in this study, the

girth of each tree >20 cm dbh was recorded, at a

height of 1.3 m, or 50 cm above the top of any buttress

(diameter at reference height, drh).

2.6. Data analysis: tree water relations

The relationships between patterns of dry season

stem diameter change, phenology, tree size and topo-

graphy were examined in two ways. Firstly, the effect

of these factors on dry season diameter change was

tested during the severe 1997/1998 dry season using

the dendrometer band data. Three repeated measures

ANOVA were performed, comparing the first and

second half of the dry season, 21 November 1997

to 14 January 1998, and 14 January 1998 to 18 March

1998, respectively.

1. Dry season diameter change ofC. mildbraedii was

analysed with topographic position as a factor, and

tree diameter and annual increment (calculated 18

September 1997 to 18 September 1998) ascovariates.

2. Dry season diameter change ofC. mildbraedii and

C. zenkeri were compared in summit positions

with species as a factor, and diameter and annual

diameter increment included as covariates.

3. Dry season diameter change ofC. mildbraedii and

S. glaucescens were compared in slope positions

with species as a factor, and diameter and annual

diameter increment included as covariates.

Secondly, the plot-based study allowed the influ-

ence of the same factors on dry season diameterchange to be assessed at the stand level. Trees were

grouped into size and phenology classes. The median

diameter was used as a threshold in a two-way classi-

fication of tree size, with individuals less than or

equal to 30.5 cm diameter classified as small. Species

were classified as deciduous or evergreen, following

Hawthorne (1995); species with variable vegetative

phenology, such as Cylicodiscus gabonensis (Mimo-

saceae), were omitted from the analysis. The resulting

dataset comprised 465 trees in four categories. As the

residuals from parametric analysis were not normallydistributed, differences between groups were tested

for using KruskalWallis tests (Sokal and Rohlf,

1995).

2.7. Data analysis: bias in estimates of tree growth

The effect of inter-annual variation in soil water

availability on measurements of annual increment was

examined for individuals of C. mildbraedii growing

in summit positions. Annual increments were calcu-

lated from successive measurement occasions, withvalues for exact years obtained by linear interpolation

between measurement dates in the second year of

study. To remove the influence of temporal change

in actual growth over the course of the study, the data

were detrended using linear regression. The residuals

of this regression were then compared with the change

in mean soil matric potential between years. Similar

to the calculation of successive annual increments,

exact values for soil matric potential on equivalent

dates during the second year of study were calculated



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by linear interpolation of mean logged soil matric

potential data.

3. Results

3.1. Soil matric potential regime

The soil matric potential at 15 cm depth was higher

in valley than slope and summit positions, apart from

during the late dry season of 1997/1998, when all sites

became very dry, and periodically during the wet

season when water availability was similarly high

in all sites (Fig. 2). Less negative minimum soil matric

potentials during November 1998 to March 1999,

indicate that the dry season was less severe during

the second year of the study (Fig. 2).

3.2. Magnitude of fluctuations

No significant diurnal girth fluctuations were found

for C. mildbraedii, either on the four occasions when

they were measured, or when the data were combined

(Table 1). However, significant dry season stem

shrinkage occurred for two of the three species, and

for C. mildbraedii, the magnitude of the mean dry

Fig. 2. Rates of change of girth for C. mildbraedii in summit and valley positions (slope position not shown), C. zenkeri and S. glaucescens,

and the soil matric potential regime in summit, slope and valley positions, at 15 cm depth, in semi-deciduous forest in Ghana, from October

1997 to April 1999. For the rate of girth change, significant differences between species, tested using univariate F-tests, are indicated,p < 0:05, p < 0:01, p < 0:005.



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season change was more than 30 times greater than

mean daily fluctuation (Table 1).

3.3. Dendrometer study: tree water relations

For C. mildbraedii, tree diameter had a significant

main effect on the degree of dry season diameter

change, indicating that larger trees shrank more than

smaller trees during the severe 1997/1998 dry season

(Fig. 3, Table 2). Annual diameter increment did

not have a significant main effect, but there was a

significant time/increment interaction (Table 2).

Although trees better supplied with water in valley

Table 1

Diurnal and dry season stem girth change in C. mildbraedii, C. zenkeri and S. glaucescens in semi-deciduous forest in Ghana, 1997/1998a

Diurnal Dry season

Mean S:E:(mm)

n Significance Mean S:E:(mm)

% of

diameter

% of annual

increment

n Significance

C. mildbraedii 0:02 0:02 59 ns 0:68 0:13 0.18 15.2 30


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evergreen species, in the plot-based study, during the

1998/1999 dry season (H 2:73, d:f: 3,p 0:434,overall mean diameter change 0:03 0:01 cm,median change 0:00 cm). The effects of size andphenology may only be important during particularly

dry years. In the dendrometer study, higher rates of

girth change during the 1998/1999 dry season com-

pared to 1997/1998, reflected the less severe dry

season and resulted in similar patterns in all three

species, in contrast to the previous year (Fig. 2).

3.5. Dendrometer study: hypothesis 2

As there was no evidence for significant diurnal

fluctuations in tree girth, only the impact of seasonal

changes on the measurement of annual increment was

evaluated further. Annual increments calculated from

successive measurement occasions show a steady

significant increase over the study (Fig. 5). However,

there is also substantial variation around this relation-

ship, particularly for annual increments initiated dur-

ing the course of the dry season (Fig. 5). There was a

significant positive correlation between the difference,

and mean, of soil matric potential measured in suc-

cessive years (Fig. 6). This result demonstrates that

soil matric potential was more variable at generally

dry times of year. In addition, the difference in soil

matric potential between years is correlated with the

residual error in the calculation of annual increment

(Fig. 7). The significant regression indicates that a 10-

Fig. 5. Successive mean annual increments, S.E., for C. mildbraedii in summit positions in semi-deciduous forest in Ghana, 1997/1999.Annual increments are calculated from successive measurement occasions during the first year of study. Linear regression: y 11:8 0:026x,

F 263, p < 0:005.

Fig. 6. Relationship between the mean, and variation, in soil matric potential between 1997/1998 and 1998/1999. Linear regression:

logy 0:45logx 0:013, F 9:6, p < 0:05.



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fold increase in soil matric potential between mea-

surement occasions leads to an overestimate of annual

increment by 0.5 mm, or approximately 4%. In this

study, the most extreme errors in the calculation of

annual increment were found in measurements at the

height of the dry season, when annual growth is

overestimated by more than 9% (Fig. 5).

4. Discussion

4.1. Tree water relations

As dry season diameter change only represents an

indirect measure of tree water status, inferring species

and size effects on tree water relations from these data

must be done with caution. Seasonal decreases in stem

diameter are caused by water loss from elastic storage

elements such as the phloem, cambium and sapwood

parenchyma (Holbrook, 1995). Close correlationshave been obtained between xylem water potential

and stem diameter over diurnal timescales (Worrall,

1966; Hinckley and Bruckerhoff, 1975; Hinckley et al.,

1978; Reich and Borchert, 1982; Garnier and Berger,

1986), but extending this relationship over longer,

seasonal timescales is more complex. Specifically,

inferring species and size effects on seasonal changes

in plant water potential from patterns of stem diameter

change assumes that tissue capacitance does not vary

across species and size classes. Tissue capacitance,

defined as the change in water content per unit change

in water potential, per unit volume (Holbrook, 1995),

does, however, vary between trees, due to differences

in tissue structure or the relative proportions of dif-

ferent tissues. For instance, variation in bark thickness

is important (Borchert, 1994). As phloem is an elastic

tissue, absolute declines in stem diameter, for a given

reduction in plant water potential, will be greater in

trees with thicker bark. In addition, differences ingrowth rate may be important, as fast growing trees

produce larger cells that may show a greater reduction

in volume for the same change in water potential. The

result of these effects is that a given reduction in water

content (stem diameter) in two trees, that are different

species or sizes, may not reflect similar changes in

plant water potential. In this study, the effect of

differences in growth rate can be discounted, as sig-

nificant effects of tree size and species on dry season

diameter change were found, independently of varia-

tion in annual increment (Table 2). However, differ-ences in bark thickness are important for interpreting

the relationships between dry season diameter shrink-

age and tree water relations.

The greater shrinkage of larger trees of C. mild-

braedii during the 1997/1998 dry season is consistent

with the suggestion that large trees experience greater

water deficits than small trees during the dry season.

The greater transpirational demand of a larger canopy

apparently exceeds the ability of a larger root system

to supply water. Research on the sources of dry season

Fig. 7. Residual error in linear increase of annual increment through time, plotted against the difference in soil matric potential between years.Linear regression: y 0:57x 0:31, F 5:1, p < 0:005.



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water and patterns of tree mortality has also suggested

that large trees are more susceptible to seasonal

drought. For example, by relating the isotopic signa-

ture of xylem sap to groundwater values, large treeswere found to tap shallower sources of water than

small trees during the dry season in a seasonally dry

forest in Panama (Meinzer et al., 1999). In addition,

drought related mortality was 50% higher in trees

>60 cm dbh, compared to individuals


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Significant water-related changes in stem diameter

do result in errors in estimates of growth. The steady

increase in growth measured over successive, over-

lapping, time periods is caused by the decreasingcontribution of the 1997/1998 dry season to the cal-

culation of annual increment. Rates of wet season

expansion of these trees remained constant between

years, whereas dry season diameter change was sig-

nificantly higher in the second year of study (Baker

et al., unpublished). The hypothesis that increased soil

water availability on successive measurement occa-

sions leads to an overestimate of growth is supported,

as the deviations in annual increment about this trend

are negatively correlated with the difference in soil

matric potential between years (Fig. 7). Although the

potential error is small, it will be important where

intercensus intervals are short, in forests with a

strongly seasonal climate, and where inventories take

several months to complete.

Published recommendations on methods of tropical

forest inventory suggest that the optimum time to

measure forest plots is during the dry season, when

inter-annual variation in water-induced changes in tree

diameter are assumed to be minimal (Sheil, 1995).

However, this study indicates that variation in soil

water availability is greatest during the dry season

(Fig. 6). Although this study only compares 2 years,one of which is strongly influenced by an El Nino

event, it is likely that the patterns described here are

representative of West African forests, and will be

reflected over longer timescales. Climatic fluctuations

during El Nino events are likely to be the main

determinant of inter-annual variation in climate, and

in this region, El Nino events typically lead to more

severe dry seasons (Swaine et al., 1997). Less bias

would be introduced into estimates of growth if enu-

merations were carried out during the wet season, to

avoid the large inter-annual variation in dry seasonpatterns of stem diameter change, caused by variation

in dry season soil water availability. This study pro-

vides direct evidence to support a similar recommen-

dation made for plots in wet evergreen forests with a

monsoon climate in the Western Ghats of India (Pelis-

sier and Pascal, 2000). Finally, in large inventory

programmes that maintain plots in a variety of forest

types, with a full-time schedule of remeasurements,

care should be taken in the timing of plot enumeration.

Measurements of plots in the climatically wettest

forest type should be made during the driest time of

year, and of the driest forest plots at the wettest time of

year, to minimise the effect of inter-annual variation in

dryseasonsoilwateravailabilityonestimatesofgrowth.

Acknowledgements

This work was funded by a University of Aberdeen

Faculty Studentship (TB) and the Department for

International Development (TB/KAB). Yaw Atua-

hene, William Asante and Raymond Votere provided

invaluable assistance with the fieldwork, and Doug

Sheil and Rolf Borchert gave very helpful comments

on a previous version of the manuscript.

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