Conversion of anthropogenic savanna to production forest through fire-protection of the forest-savanna edge in Gabon, Central Africa

Pores;;;ology Management ELSEVIER

Forest Ecology and Management 94 (1997) 233-247

Conversion of anthropogenic savanna to production forest through fire-protection of the forest-savanna edge in Gabon, Central Africa John King a3* , Jean-Bernard Moutsinga b, Gerard Doufoulon ’ ’ Institute

de Recherches

a Department of Botany Duke Universig Durham, NC 27708 USA Agronomique et Forestibres Centre National de la Recherche Scientifique et Technologique ’ Consultant BP 1888 Ministere des Eaux et For&s Libreville Gabon

Libreville

Gabon

Accepted 1 August 1996

Abstract Tropical moist forest is being destroyed at the rate of 11 million hecares per year, much of which degrades into fire-maintained savanna or low-productivity pasture. Greater economic and ecological benefits can be realized from much of this land if it can be converted into more productive secondary forest ecosystems. This study was conducted to deterimine if anthropogenic savannas could be converted into production forest through relatively inexpensive protection of the forest edge by plowing the soil with an agricultural tractor in highly-degraded (Ancient) and less-degraded (Nascent) savannas on the coastal plain of Gabon, Central Africa. After three years of protection, vegetation surveys revealed rapid colonization of Nascent savannas by 45 species of tree seedlings. Ancient savannas also experienced colonization by tree seedlings, but at a much lower rate. Analysis of soils determined that Nascent savannas have 5 times more calcium and magnesium and higher organic matter than Ancient savannas; indications of their less-degraded nature. Protection of the forest-edge from fire can be an effective, low-cost method of converting anthropogenic savannas into production forest through natural regeneration. The rate of conversion can be maximized by focusing on less-degraded sites to capitalize on more abundant seedling recruitment and higher ecosystem nutrient stocks, but even highly-degraded sites may be reclaimed with additional management. 0 1997 Elsevier Science B.V. Keywords:

Nutrient cycling; Fire ecology: Climate change; Human disturbance;

1. Introduction Humid tropical forests encircle the globe in a latitudinal band centered on the equator and are crucial to the ecological integrity of the planet. These

* Corresponding author: Department of Botany Duke University Durham,

NC 27708 [email protected].

0378-l 127/97/$17.00 Copyright PII SO378I127(96)03925-4

0 1997 Elsevier

Equatorial

rainforest

forests help stabilize the global climate through carbon storagein plant biomassand soils (Woodwell et al., 1983; Myers, 1988, Garcia-Oliva et al., 19951, maintain high biological diversity (Wilson, 1988, Terborgh, 19921, and are important to the global hydrologic cycle (Shukla and Mintz, 1982, Salati and Vose, 1984). Historically, humid tropical forests occupied 25 million (m) km* (Terborgh, 19921, but have experienced increasing rates of deforestation

Science B.V. All rights reserved.

culminating in the current rate of 1 1 m hectares per year (van Uexkiill and Mutert, 1995). This has resulted in the loss of more than 250 m hectares of tropical forest during the second half of this century. leaving behind vast areas of anthropogenic savanna (van Uexkiill and Mutert. 1995). This global trend has been expressed in West Africa, where the world’s highest rate of deforestation (3.7 times the rate of all tropical countries) has exhausted the forests of the region (Gillis, 1988) and will likely result in increased pressure on the largely intact natural forests of central African countries. Gabon is a central African country with an incredibly rich natural heritage in plant and animal life. Largely due to low population pressure, it provides a prime opportunity for the conservation of myriad threatened species in undisturbed tropical ecosystems (McShane, 1990). The country straddles the equator on the west coast of Africa and has a surface area of 267 000 km” (Catinot, 1978), 85% of which is covered in dense tropical rainforest; the remainder (in the southeast and southwest) consists of savannas thought, in part, to be of anthropogenic origin (Fontes, 1978). The Gabonese economy is based on natural resource extraction, mainly oil and minerals, but during the Colonial Period (late 1800s to 1960) timber accounted for 80-90% of all exports (Pourtier, 1989). Selective logging was largely restricted to coastal forests and along major rivers until the importation of modem equipment in the 1950s (Pourtier, 1989). Since that time, technological advances have allowed penetration ever further into interior forests. and the recent completion of the Trans-Gabon Railway has major implications for future forest management (Gillis. 1988. McShane. 1990). Worldwide. much of the humid tropical forest cleared annually degrades into anthropogenic savanna or low productivity “pasture” because the acid, low fertility soils are unable to support low-input agriculture (von Uexkiill and Mute& 1995). Where inputs are too low or too expensive, these areas are more suitable for forest production which not only provides an economic return, but also the ecological “returns” of climate stability, maintenance of higher biodiversity, and efficient cycling of water and nutrients. In addition, secondary forest growth on degraded pastures and savannas may off-

set carbon emissions from tropical deforestation (Lug0 and Brown, 1982), although this has recently been disputed (Feamside and Guimarges, 1996)). In areas like Gabon. where highly disturbed ecosystems exist adjacent to as yet pristine foreu. more intcn.4\,c use of secondary forests and conversion of anthropogenic savannas to production forest may facilitate protection of some of the world’s iar;i irue wilderness. Large areas along the coast of Gabon are characterized by a mosaic of secondary for&t rich in the primary timber species. okoume ( .4ltr (r~~~cr: klaineana Pierre), and low-diversity savannas dominated by grasses of the tribe Andropogoneae. The origin and persistence of these savannas has been aberrante>,” (AL&considered “Ccologiquement ville, 1966). It was the goal of this study to examine site physical factors (climate. soils. firef 111relation to plant community distributions to determine ii :I low-cost forest management technique could be rnployed to expand the area of production forest into areas now occupied by savanna. We hypotheszed that protection of the forest-savanna edge from artnual fires would permit the forest to expand through natural regeneration. This was investigated by treating fire breaks parallel to the forest edge and comparing vegetation surveys from protected and unprotected areas several years after implementation A secondary goal of the study was to elucidate some of the factors responsible for the formation of savannas in this zone of equatorial rainforest IO determine iC they actually are of human origin.

2. Methods 2.1. Studv site The study site was located at Oyan (oj 10-N. 9 ’ 30’E) on the coast of Gabon approximately 70 km south of the capital city Libreville (Fig. i). Climate.logical data from the closest meteorological station in Libreville, from 19.51 to 1989, illus.trate the seasonal distribution of precipitation, temperature and pan evaporation in the region (Fig. 2). & in most ot Gabon, the yearly temperature was remarkably con-stant with a monthly mean of 26°C. Average annual rainfall for the period of record was 2,834 mm at-

.I. King et al. / Forest Eco1og.v and Management

Libreville and 1,997 mm at Port Gentil 100 km south of the study site (data not shown). Rainfall was seasonally distributed with a major dry season during the months June, July and August, sometimes extending into September. Potential evapotranspiration calculated by the Thornwaite method greatly exceeded precipitation during these months (Moutsinga, 1991). Soils of this region are classified as ferralic arenosols with a clay content less than 10% and xanthic ferralsols (Anonymous, 1977) which are equivalent to oxic psamments and xanthic-subgroup oxisols in the U.S. System of Soil Taxonomy, respectively (Anonymous, 1987). Both soil types are nutrient poor with low base saturation and few weatherable mineral reserves. In texture, they are sand to clayey sand; clay content gradually increases with depth. The dominant vegetation in the area is a mosaic of gallery forests and savannas. Forests are low in diversity and are dominated by three species: okoumt t Aucoumeu klaine~ana Pierre, Burseraceae), ozouga (Saccoglottis gabonenesis Urb., Humiriaceae) and angoa (Erismadelphus exsul Mildbraed, Vochysiaceae) (Christy et al., 1990). The edge between forest and savanna is usually abrupt due to annual burning of the grasses by natural and anthropogenic fires. The savannas are of two distinct types, termed Ancient and Nascent, and are characterized by very different plant communities (Table 1). Ancient savannas are more extensive and have very low

Fig. 1. Map of Gabon showing

location

94 (1997)

233-247

235

Fig. 2. Monthly averages for Libreville climatological data for 1953 to 1989. Source: Meterologie Nationale. Station 500, Libreville.

diversity, containing 3-5 species per m2. They are dominated by Pobeguinea arrectu (Stapf) Jacq.-Felix (Gramineae), Panicum congoense Franch. (Gramineae), and Bulbostylis Zuniceps C.B. Clarke (Cyperaceae) (Table 1Christy et al., 1990). Nascent savannas have higher diversity (12-25 species per m”) and typically occur as small patches or corridors in forest. The dominant plants are often Gramineae, such as Imperata cylindrica Beauv. and Hyparrhenia diplandra Stapf, but there are many herbs with the following common: Heterotis decumbens (Pal. Beauv. ) Jacq.-Felix (Melastomataceae), Agerutum conzoides L. (Compositae), and Ipomea involucrata Beauv. Fl. Owar. (Convolvulaceae). Small trees and shrubs of the family Connaraceae are also important components of many nascent savannas.

of study site at Oyan with respect to the capital city. Libreville.

236

J. King et al. /Forest

2.2. Forest-edge protection

Ecology

and Management

treatments

the implementation of this study (Chief Prosfire, personal communication). Savane Doufwlon had a treatment area of 3 ha and was also a.former agricultural pIantation of recent origin. Comparison of aerial photos from 1955 and 1982 show the study savannas did not change in size or configuration during. that time. In 1988 fire breaks were installed at each of the study savannas. Fire breaks were strips of exposed earth 10 m wide at a distance of 100 m parallel to the forest-edge, made by plowing the soil with a tractor. The strips were plowed twice a year to inhibit regrowth of vegetation. Distinct regions of equal area within the sEudy savannas were subjected to three treatments: (1) Control -unprotected savanna left to bum with the natural frequency;. (2) Fire protection -protected from fire only: the plant communities were left unchanged except for the

The forest edge protection study was performed in four separate savannas embedded in the forestsavanna mosaic. Two of the study savannas were Ancient: Gmnde Plaine and Savanne Boyau. Grande Plaine was the largest savanna studied, with a total treatment area of 46 ha. This savanna bums annually during the dry season (L. Rivikre, personal communication) and woody vegetation is completely absent. Savane Boyau had a treatment area of 8 ha, is floristically similar to Grande Plaine, and also bums annually. The other two study savannas. Koskos and Doufoulon, were Nascent and burned with a frequency of 3-5 years. Savane Koskos had a treatment area of 2 ha. It was formerly an agricultural plantation for some of the local villagers. The exact time of cultivation is not known, but was within 30 years of

Table 1 Ten most abundant

species in Ancient

and Nascent

savanna control

94 f 19971 233-247

transects

(Oyan,

Gabon

199 1)

Species

Family

Density

Ancient savanna Panicum congoense Franch. Pogeguinea arrecta (Stapf.) Jacq.-Fe% Bulbostylis laniceps C.B.Clarke Oldenfundia aflnis D.C. Sauuagesia erecta L. Desmodium sp. Dew. Solenostenom sp. Schumach & Thonn. Schizuchyrium brevifolium (SW.)Nees Clappertoniaficifolia Decne. Dissotis rongolensis (Cogn. ex Buett) J.FiI. Total

Gramineae Gramineae Cyperaceae Rubiaceae Ochnaceae Leguminoseae Lamiaceae Gramineae Tiliaceae Melastomataceae

589 191 56 32 9 8 h 5 : 2

Nascent savanna Agerafum conzoides L. Imperata cylindrica Beauv. Heterotis decumben (P.B.) Jacq.-Ftl. Panicum congoense Franch. Seluginella sp. Beauv. Ipomoea irwolucruta Beauv. FL Owar. Heterunthoeciu guineensis (Franch.) Robyns Hyparrhenia diplandra Stapf Aspilia ufricana (Pers.) CD. Adams Vigna sp. Savi Total

Compositae Gramineae Melastomataceae Gramineae Selaginellaceae Convolvutaceae Gramincae Gramineae Compositae Leguminoseae

a Density = the number of individuals per m2 averaged over all control b Percent = the percentage of the total number of individuals comprised

a

Percent 59.8 23.3 6.8 1.6 i.1 0.9 0.7 0.6 0.3 0.1 96.7

17.0 13.0 7.4 7.1 6.1 4.3 3.3 2.9 2.9 2.R 67.6 transects. by each species.

h

---

J. King

et al. / Forest

Ecology

Table 2 Sampling design for herbaceous and tree seedling data showing the number of randomly located transects within each of the study savannas and treatments a Savanna

Control

Fireprotection

Fireprot. plowed

Total

Nascent Doufoulon Koskos

I 2

2 0

2 3

5 5

Ancient Grande Plaine Boyau

2 2

3 3

3 3

8 8

and

Management

94 (1997)

233-247

237

Agronomiques et Forestibres (IRAF) laboratory in Libreville. Profile descriptions followed the protocol established by the U.S. System of Soil Taxonomy (Anonymous, 1987). At the IRAF laboratory, soil samples were analyzed for particle size, organic matter content, exchangeable bases, total phosphorous, total carbon, total nitrogen and pH.

2.3. Data analysis

a All transects consisted m’ for tree seedlings).

of 5 plots (I m’

for herbaceous

and 100

exclusion of periodic fires; and (3) Fire protection and plowed -protected from fire and the entire protected area plowed before the okoume ( Aucoumea kluineana) seed dispersal (by wind) in January 1988. In January and February, 1991, data on herbaceous plants were collected in replicated transects of five 1 m’ plots spaced at 10 m intervals perpendicular to the forest-edge. The first plot was placed on the “savanna side” of the forest edge and the remaining five plots extended 50 m into the savanna. Within each plot all herbaceous species were identified and the individuals counted. Transects within each treatment were randomly located and replicated (N = 2 to 6) as outlined in Table 2. Data collection for tree seedlings followed a method similar to that for the herbaceous plants in March and April, 1991. The same transects were used, but the plots were enlarged to 100 m2 (10 m to a side). The resulting plots were contiguous and extended 50 m into the savanna. Within each plot all tree seedlings were identified (if possible) and counted. Tree regeneration consisted of both commercial and non-commercial species. Comparisons of soil physical and chemical properties were made between Ancient savannas, Nascent savannas, and forest by digging replicated soil pits (N = 5 to 7) in each community type. Soil pits had dimensions of 1.0 X 0.5 X 0.9 m deep. Soil profiles were described and samples taken from each horizon for analysis at the Institut de Recherches

Analysis of vegetation data involved graphical analysis, analysis of variance, and ordination by detrended correspondence analysis (DCA). Graphical analysis consisted of graphically plotting measures of species richness and abundance against distance from the forest edge. Transects within savanna type and treatment were combined. The species richness, or alpha diversity, was simply the number of species per unit area (Whittaker, 1975). This simple metric of species diversity was used due to its practicality and objectivity (Whittaker, 1972, Peet, 1974) and because the species-area relationship asymptotes almost immediately for Ancient savannas, due to the high degree of evenness and uniform spatial pattern of these communities. Abundance values were the sum of all individuals of all species per plot. A two-factor analysis of variance was used to test for statistically significant differences between treatments for both savanna types on species richness and abundance (Brownlee, 1965, Lapin, 1980). The transect values of these measures were averaged (species richness) or summed (abundance) for each transect thereby giving a random sample since the transects were located randomly. Because the experimental design was unbalanced (Table 2) these hypotheses were tested using Type III estimable functions in a model for unbalanced designs. Detrended correspondence analysis was used for ordination of species with respect to environmental/time gradients (Peet et al., 1988, ter Braak, 1987, Gauch, 1982, Digby and Kempton, 1987). DCA was selected because it is an objective method for ordering species without assuming a linear response and it corrects for the arch effect and compression of axes common with principle components analysis and reciprocal averaging (Gauch, 1982).

.I. King

238

et 01. / Forest

Ecologv

and

of

distance

,from ,forest-edge

on tree

In Ancient savannas, tree seedling species richnessand abundanceplot meanswere essentially zero in all control plots at distances greater than 10 m from the forest edge (Fig. 3. panels A and 0. Fire protection and fire protection plus plowing enabled tree seedlingsto become establishedin Ancient savannas up to SO m from the edge. Differences between these latter treatments were not statistically significant, however differences between control plots and the fire protection treatments (both) were significant. Nascent savannashad much higher tree seedling species richness and abundance than Ancient savannas (Fig. 3, panels B and D). Control plots at the forest-edge had much higher values than in the Ancient savannas, and tree seedlings were present at all distances from the forest edge. Fire protection resulted in maintaining or increasing tree speciesrichnessand abundance.and the fire protec-

14.0

OFire-Protectm !S Fire-Protection

to.0 -

10-20

Distance 150.0

20-30

fmm forest

Speciesrichnesswas substantially higher for both herbaceousplants and tree seedlingtransect meansin the Nascent savannasfor all treatments (,Table 3). In Ancient savannas,herbaceousabundancevalues were generally much higher and tree seedling abundance lower than for Nascent sites. Mean transect tree seedling abundance was greatest in the fire protection-plowed treatment in Ancient savannasand in the fire protection treatment in the Nascent savannas. Herbaceous speciesrichness (Table 4) was significantly affected by savanna type (P == 0.006) and individual savannawithin savannatype ( P = 0.009). Herbaceous plant abundance was not affected by either factor nor were effects of the treatmentsstatistically significant. As discussedabove, speciesrichnessof tree seedlingswas significantly affected by the savanna type by treatment interaction (Table 4).

14.0

.controi

A _ Ancient

12D 3 A

94 (19971233-247

tion-plowed treatment generally causeda decreasein both variables.

3. Results 3.1. .ECfect seedlings

Manapnent

12.0 sod Plowed

~ to.0

30-40

150.0

1O-20

20-30

from forest

I

edge (m)

C - Ancient

Distance

‘3 - Nascent

30-40

edge (m)

Fig. 3. Tree seedling species richness (panels A and B) and abundance Units are in species per plot for species richness or number of individuals

I

to-20

20-30

Distance

from forest

30-40

edge-(m)

D - Nascent

o-to

1 O-20

m-30

Distance

from forest

30-40

JO-50

@p(m)

(panels C and D) plot means for Ancient and Nascent per plot for abundance; each plot had an area of lf3Om’.

savatmas. N = 2-6.

J. King ei al. /Forest Table 3 Species richness Savanna type

Ancient Control Fire protection Fire prot-plowed Nascent Control Fire protection Fire prot-plowed ’ Transect

and abundance

a (Oyan,

Gabon

Ecology

and Management

94 f 19971233-247

19911

Species richness (no. of species/plot)

Abundance

Herbaceous

Herbaceous

6.5 + 1.05 7.8 + 1.12 9.0 + 1.08

Tree seedling (12) (12)

0.3 kO.25 1.6 k-O.57

(12)

1.9+0.44(10)

12.6 + 0.85 (12) 10.4 * 1.15 (5) 10.5 + 0.67 (10)

means rt standard

error in Ancient

239

4.5 * 0.74 6.6 + 0.69 2.7 + 0.37

and Nascent

637.8

(IO) (10)

552.5 616.2

(no. of individuals/plot) Tree seedling

+ 197.12 + 172.99 _+ I Il.39

(12)

4.2 + 2.92 12.5 f 5.54

(IO) (10) 19.3 f 4.84 (101

(12)

(12)

i 36.43 (12) f 238.95 (5) 101.5 + 17.07 (IO)

202.7 778.2

(10) (5)

(10) savannas by treatment,

number

of observations

(N)

50.2 62.12

k15.18 (10) + 2.3 (5)

13.2

t3.19

(101

in parentheses.

Additionally, the increase in tree species richness due to the treatments (Fig. 3, panels A and B) was statistically significant when combined with savanna type (P = 0.02). The response of tree seedling abundance was qualitatively similar to that of species richness. An unquantified but potentially important response to the fire protection treatment in Ancient savannas was an accumulation of large amounts of dead grass foliage which completely obscured the soil surface.

dinea gabonensis (JAR GAB), Afromomum subsericeum (AFR SUB), and Paspalum paniculatum (PAS PAN). The second DCA axis appears to group species according to their response to the treatments. This is most evident at the right-hand side of the first axis (Fig. 4). At the upper end of the second axis, species that benefited from the fire protection plus soil

3.3. CommuniQ composition and environmental gradients

Table 4 Analysis of variance data ‘, (Oyan, Gabon

for herbaceous 1991)

Source

Species richness

Abundance

F-value

P-value

F-value

P-value

10.050

0.006 0.009

4.120

0.060

6.530 0.560 0.830

0.581 0.457

0.360 0.690

0.704 0.517

1.980

0.172

1.000

0.422

2.1 IO

0.141

0.910

0.0001 0.426

7.160 0.770

3.050

0.079

4.890

0.024

0.920 3.250

0.018 0.481 0.419 0.069

0.680

0.579

0.560

0.650

Species scores were plotted on the first two DCA axes, which described most of the variation in the data (Table 5). The DCA ordination arrayed the herbaceous species along the first two axes in associations similar to those found in the field (Fig. 4). Along the first axis, the species appear to be grouped along a gradient from species-poor Ancient savannas to species-rich Nascent savannas and the forest edge. Lower values of the axis are characterized by Oldenlandia capensis and 0. a&finis (OLD CAP and OLD AFF, respectively), Pobeguinea arrecta (POB GUI), Punicum congoense (PAN CON), and Bulbostylis laniceps (U50 SPP). These species occur in Ancient savannas of Oyan and were strongly associated in the field. On the right-hand side of the first axis were species found in Nascent savannas or at the forest edge. including Lantana camara (LAT CAM), Jar-

Herbaceous Savanna type Savannaftype) Treatment Treatment * savanna type Treatment (type)

* savanna

Tree seedlings Savanna type Savanna(type1 Treatment Treatment *savanna type Treatment * savanna (type)

27.530

plants and tree regeneration

a Type III sums of squares, 0.05 level of confidence.

240

J. King et al. / Forest Ecology

Table 5 Detrended correspondence seedling data *

Herbaceous Eigenvalue Tree seedling Eigenvalue

analysis

on

herbaceous

and

DCA axis 1

DCA axis 2

DCA axis 3

DCA axis 4

0.81

0.64

0.42

0.45

0.61

0.52

0.42

0.32

* Eigenvalues are for species and samples, showing tion of variation explained by each axis independently.

tree

the propor-

preparation treatment are grouped together. Important species in this group include: Heterotis decumhens (BET DEC), Zmperufa cylindricu (IMP CYL). Walthetia indica (WAL IND), Urena lobata (URE LOB), and Borreria spp. (BOR SPP). These are lianescent or woody shrubs (except for I. cylindricd which compete aggressively with tree seedlings. The lower end of the second DCA axis contains species +

and Managemenf

94 (1997)

233-247

present in Nascent savannas in the control and fireprotection treatments. Afromomum subserieeum, Heteranthoecia guineensis (BET GUI), Solenostenom sp. (SOL SPP), and Clappertonia jicijolia (CLA l?IC) are characteristic of these areas. The ordination of species scores for tree seedlings matched the pattern for herbaceous species very closely (data not shown). Due to small size and lack of diagnostic characters (fruits or flowers), many .tree seedlings could not be identified thereby limiting our interpretation of the response of individual species (known species encountered are listed in Appendix A. Collectively, however, the overall pattern suggests that tree seedlings were responding to environmental gradients similarly to the herbs. Ordination of sample scores for both herbaceous plants and tree seedlings produced patterns similar to-those of species scores and confirm that the majority of the variation in the data was accounted for by the first two DCA axes (King, 1991).

I I DCAAXIS

2

a

* Fig. 4. Species scores for the first two axes of the detrended correspondence corresponding to the codes in this figure are listed in Appendix A.

analysis

of the herbaceous

data. Oyan,

Gabon.

f99l.

Species

J. King et al. / Forest Table 6 Physical and chemical

properties

Property Particle Clay Silt Sand

from the Al Ancient

horizon

Ecology

of savanna

savanna (n = 7)

and Management

94 (1997)

and forest soils a (Oyan, Nascent

241

233-247

Gabon

savanna (n = 5)

1991) Forest ( n = 6)

size analysis (%) 11.3 (0.92) 7.9 (0.63) 75.7 (3.50)

10.9 (2.54) 7.6 (0.37) 77.1 (2.60)

6.9 (0.86) 12.2 (1.33) 77.2 (2.35)

Organic matter (%) Organic matter Total C Total N C/N ratio

3.0 1.4 0.1 14.9

(0.83) (0.441 (0.03) (0.78)

3.40.09) 2.MO.62) O.l(O.03) 16.4c1.07)

3.0 1.7 0.1 15.7

Exchangeable Ca Mg K Na

0.12 0.10 0.02 0.02

(0.02) (0.04) (0.004) (0.005)

2.54 0.52 0.03 0.017

(0.16) (0.16) (0.01) (0.006)

0.90 (0.33) 0.16 (0.05) 0.04 (0.01) 0.009 (0.003)

(12.7) (0.11) (0.09) (6.51)

176.4 5.5 4.5 25.0

(46.1) (0.12) (0.04) (5.77)

(0.54) (0.31) (0.01) (1.07)

bases (cmol,/kg)

Total P (kg/g) pH water pH KCL Depth of Al (cm) a Means + standard

176.7 5.2 4.2 21.7

170.8 4.5 3.7 10.0

(29.1) (0.22) (0.21) (1.06)

errors.

3.4. Soils The A horizon had a sandy to clayey sand texture, was dark brown in color and varied in depth from 0.10 m in the forest to 0.25 m in the savannas (Table 6). The B horizon was yellowish in color and had a clayey sand texture to sandy clay texture. Below this the C horizon exhibited a deep red color, had a sandy clay to clayey texture, and was undifferentiated with depth. During profile description an extensive stratum of charcoal and brick fragments were found under Savane Boyau at depths of 10 to 30 cm. The depth of the Al horizon in the Ancient and Nascent savannas was 21.7 and 25 cm, respectively, but only 10 cm in the forest soils. Forest soils had less clay (6.9%) and more silt (12.2%) than either Ancient or Nascent savannas, which had about 11.O% clay and only 7% silt (Table 6). Organic matter averaged 3.05% in Ancient savannas, 3.44% in Nascent savannas, and 3.09% in forests. Total carbon was somewhat higher in Nascent savannas (2.050/o), as was total nitrogen (0.124%), yielding a carbon to nitrogen ratio of 16.4. This is slightly higher than in either Ancient savannas or forest soils and reflects a

somewhat higher accumulation of carbon in Nascent Soils. Calcium levels were five-fold greater in Nascent savannas than in Ancient savannas and almost three-fold greater than in forest soils (Table 6). Likewise, levels of magnesium were approximately five-fold greater than in either Ancient soils or forest soils. Levels of potassium and sodium were very low for all vegetation community types, as were the levels of total phosphorous. All soils had low pH, with those of the forests being the most acid.

4. Discussion 4.1. Short-tern exclusion

response of the forest-edge

to $re

Protection of the edge between forest and savanna has resulted in the rapid establishment of tree seedlings in the savannas, including the commercially valuable okoume, and suggests that maintenance of fire breaks is a viable management strategy to convert degraded savannas into production forest. This appears to have been due to the short-term

242

J. King et al. / Forest

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dynamics between the herbaceous and woody plant communities and how they interacted with savanna type and the fire exclusion treatments. At Ancient sites protected from fire, herbaceousspeciescomposition was unchanged but the quantity of dead biomass(in the form of dead grassfoliage) increased manyfold. The lower abundanceof herbaceousplants in Ancient savannas(Table 3) reflects the increased mortality of live grasstillers due to the accumulation of dead biomass. In these sites it was difficult for tree seedlingsto becomeestablishedin the densemat of dead grass that completely covered the soil surface. When Ancient savannasunderwent soil preparation the mat of accumulated biomass was broken up, exposing the soil to light and allowing the establishment of tree seedlings. In Nascent sites, tree seedling regeneration was naturally more abundant (control transects, Table 3). This is attributed to reduced fire frequency and potentially better nutrition at these sites (Eden and McGregor, 1992). Protection from fire allowed tree seedlingsto continue growing and compete with herbaceousplants. Soil preparation in Nascent savannas, however, effectively destroyed pre-existing tree regeneration decreasing tree seedling abundance(Table 3). 4.2. Factors maintaining the @rest-savanna mosaic The major physical factors influencing the forest-savanna mosaic at Oyan are the seasonaldistribution of rainfall, soil physical and chemical properties, and fire. The point at which humid forest is rainfall limited in tropical ecosystemsis 1,400- 1,500 mm per year; however, this limit may be reduced to 1,250 mm per year if the rainfall is evenly distributed (Aubreville, 1962). The Oyan area receives between 1,900 and 2,500 mm of rain a year, but there are 3-4 months with a rainfall index of about 30 mm, which is low enough to be considered “ecologically dry” (Avenard, 1969). Severe water stressto plants can develop becausethe sandy soil has low water holding capacity. Therefore, even though the total annual rainfall is well above the lower limit necessaryfor forests, the dry seasoncan be a major stressfor tree seedlingsand increasesthe potential for fire in the savannas. Low soil fertility and periodic fires also strongly influence the formation and persistence of the for-

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est-savanna mosaic at Oyan. The major pod of nutrients in many forest ecosystemson poor soils is in the biomass of the vegetation and forest floor (Kimmins, 1987). Such systemsrequire long periods of time to recover after disturbance due to the IOw nutrient supply rate of the soils (Ingestad, 1987, lngestad and Agren, 1992) and depend on efficient nutrient cycling for continued growth (Mifler et al., 1978. Waring and Schlesinger, 1985). When the vegetation is burned. export of nutrients occurs through volatilization (Halt and Coventry, l990, Cook, 1994) and leaching fNye and Greenland, 1964. Kellman et al., 1985, Holt and Coventry, 1990). Although many of the volatilized nutrients are replaced through atmosphericdeposition (Hingston and Galbraith, 19891,N and K lossescan be significant (Cook. 1994), as can leaching lossesof’ Mg, K anrJ Na (Nye and Greenland. 1964, Kellman et al.. 1985). Cook ( 1994) determined that under a regime of annual burning. N reservesin a savannain Northern Australia were being depleted. Associative N-fixation might help replace some of the lost N, but in Lamto. west Africa, only 60% of the N lost in fires was replaced by this means(Balandreau. 1976). At Oyan, Nascent savannasbum with a frequency of 3-5 years. while Ancient savannasbum one or more times per year (Rivi&re, personal communication) With every cycle of repeated burning, tree regeneration at the forest edge is destroyed and &henutrients accumulated in plant biomassare volatilized. Atmospheric deposition of the volatilized nutrients (except perhaps N and K) is likely and.may serve to redistribute nutrients from the burned savannas to the forests (which represent a growing “sink” for de-. posited nutrients). This could be a mechanism for maintaining savanna nutrient capital at a low level, further hindering tree regeneration in Ancient savannas. 4.3. Human disturbance, climate change and kmgten ecosystemdynamics At longer time scales, it appears the forests and savannasin the vegetation mosaic at Qyan are stable endpoints in a system which was long age perturbed by humans. The coast of Gabon was inhabited asearly as 540 B.C. and became a thriving center of trade with the arrival of Portu&ese explorers in the

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15th century (Clist, 1987, Clist et al., 1985). Direct evidence of ancient human activity at Oyan, the charcoal and brick stratum, was found during this study. We can be sure this stratum is anthropogenic because the brick fragments in the soil unequivocally demonstrate past human activity. Similar evidence from the late Neolithic-early Iron Age has been found in the interior of Gabon, and interestingly, the authors were unable to explain the presence of a 30-40 cm thick layer of soil over the charcoal deposits (Oslisly and Dechamps, 1994). Interior forests were able to recover from the activities of early iron-smelting peoples (Oslisly and Dechamps, 1994), but at Oyan disruption of the forest canopy and the use of fire may have been the seminal events leading to the formation of coastal savannas. Correlation of the establishment of savannas with the activities of aboriginal people in zones of tropical rainforest has also been reported in Australia (Kershaw, 1992). Drier paleo-climatic conditions have been invoked as having possibly contributed to the formation of the savannas in central Africa (Aubreville, 1966) and researchers have established that a period of drier climate prior to 1500 BP did occur at similar latitudes in the New World (Kjellmark, 1996 and references therein). If drier paleo-climatic conditions coincided with fire use by humans in this part of Africa, then the potential for conversion of moist forest to fire-adapted savanna would have been much greater than at present. Analysis of aerial photographs (195 1 to 1989) shows that the forests and savannas appear to be in equilibrium under current climatic conditions. However, recent Global Circulation Model projections of future climate conditions indicate that the fringe areas of moist tropical forest in Africa and South America will be drier (I. Woodward, personal communication), and therefore may converge towards savanna dominated ecosystems. 4.4. Forest management The importance of understanding the history of human disturbance and the interaction between plant community distributions and the physical conditions at Oyan, is that Nascent savannas represent more recently disturbed, less degraded ecosystems in which forest management is more likely to succeed. Evi-

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dence of this is the elevated levels of calcium and magnesium in Nascent savanna soils (Table 6) and the continuum of species from Ancient to Nascent communities in the ordination of herbaceous species (Fig. 4). Higher levels of soil calcium and magnesium in Nascent savannas probably represent residual nutrients from former agricultural activity (slash and bum agriculture) and may directly contribute to higher tree seedling survival in species which accumulate these elements (Eden, 1986, Richter et al., 1994). The ordination of herbaceous species shows that Ancient and Nascent savannas occur along a continuum of community types, and given the evidence of ancient and recent human disturbance, strongly suggests that this is a gradient of time since disturbance. As a result of many centuries in a recurrent burning cycle, Ancient savannas have no natural tree regeneration and low ecosystem nutrient capital. The right-hand side of the first DCA axis (Fig. 4) represents Nascent savannas and conditions of the forest edge. These areas were cleared approximately 30 years ago for agriculture and have been in a 3-5 year burning cycle for a much shorter time. As a result, tree regeneration is much more abundant, microsite conditions are more favorable, and total ecosystem nutrient capital is higher (in both the vegetation and soils). At present there appears to be a balance between the forest and savanna; neither is expanding at the expense of the other. This study has shown that the use of relatively inexpensive fire breaks parallel to the forest edge can facilitate the conversion of antbropogenic savannas back to forest. At Oyan, the rate of conversion under fire protection is highest in Nascent savannas which have higher nutrient capital and naturally abundant tree recruitment. Therefore, efforts to expand production forest should begin in these areas and in savannas intermediate between Nascent and Ancient. It is probable that even Ancient savannas, or analagous systems in other parts of the tropics, could be recovered by fire protection. However, these systems would have the added costs of tillage and perhaps an initial application of fertilizer. Finally, we should realize that if paleo-climatic conditions favored formation of anthropogenic savannas in some areas of tropical moist forest in the past, global climate change could predispose these systems toward savannas in the future.

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Ackno~ments This study was made possible by the contributions of people too numerous to mention, but the following especially deserve our thanks. Malcolm Gillis and the Student International Discussion Group at Duke generously provided financial support which helped defray the costs of the trips to Africa. Mr. Memvie Boniface, Adjoint General Director of the Ministry of Eaux et For&s, graciously facilitated the official procedures allowing the senior author to work in Gabon. Laurent Riviere and his family made the stay at Oyan most comfortable and became good friends during the short time we had together. The

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vegetation sampling would not have been possible without the unflagging help of Ngbwa Fidel and the assistance of all of the employees at the field site is greatly appreciated. Dr. Art Louis of the National Herbarium of Gabon helped immensely in the identification of the many plants encountered. Chief Prospere provided hospitality and an invaluable oral history of recent human habitation at Oyan. Thanks also go to the many reviewers whose eomments greatly improved the quality of this manuscript. Finally, thanks go to the members of my @SK) Master’s committee: Dan Richter, John Terborgh, and Norm Christensen.

Appendix A. Key to kntwn species Wed in Fig. 4, Oyan, Gabon Code ACR ZIZ AFR SUB ASP AFR AX0 FLE BOR SPP CAL MUC CAS FIL CEP SPP CLA FIC CYN SPP DES SAL DES SP3 DES SPP ELE MOL GE0 SPP GLE POL HET DEC HET GUI HYP DIP HYP SPP IMP CYL IND PAR IPO INV JAR GAB LAT CAM LOU FLA MOR MOR NEP BIS

Species Acrocerus zizanoides (HBK.) Dandy Afromomum subsericeum(Oliv. and Hanb.) K. Schum. Aspilia africana (Pers.) C.D. Adams Axonopusflenuosus C.E. Hubbard Borreria sp. G.F.W. Mey Calopogonium mucunoidesDesv. Cassythafiliformis Mill. Cephaelis sp. Swartz Clappertonia ficgolia Decne. Cynodon sp. Rich. Desmodiumsalicifolium DC. Desmodiumsp. Desv. Desmodiumsp. Desv. Elephantopusmollis H.B.K. Geophila sp. Berger Gleichenia ploypodiodes Heterotis decumberis(Pal. Beauv.) Jacq.-Felix Heteranthoecia guineensis(Franch.) Robyns Hyparrhenia diplandra Stapf Hyptis sp. (Jacq.) lmperata cylindrica Beauv. Zndigoferu parvtjlora Heyne Zpomoeainuolucrata Beauv. Fl. Owar. Jardinea gabonensisSteud. Lantana camara L. LoudetiajZammida (Trin) C.E. Hubbard Morinda morindoides(Baker) Milne-Redhead Nephrolepis biserrata (SW.) Schott

Family Gramineae Zingiberaceae Compositae Gramineae Rubiaceae Leguminoseae Lauraceae Rubiaceae Tiliaceae Gramineae Leguminoseae Leguminoseae Legtmrinoseae Compositae Rubiaceae Gleicheniaceae Melastomataceae Gramineae Gramineae Lamiaceae Gramineae Leguminoseae Convolvulaceae Gramineae Verbenaceae Gramineae Rubiaceae Davalliaceae

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NEU LOE Neurothecu loeselioides Oliver OLD AFF Oldenlandia a&finis D.C. Oldenlandia cape&s L. Rubiaceae OLD CAP Panicum brevifolium Jahn, ex. S&rank Gramineae PAN BRE PAN CON Panicum congoense Franch. PAS PAN Paspalum paniculatum L. PAS SCR Paspalum scrobiculatum L. PLA AFR Platostoma africanum Beauv. Fl. Owar. PLE GOS Pleiadelphia gossweileri Stapf POB GUI Pobeguinea arrectu (Stapf) Jacq.-Felix SAB SPP Sabicea sp. Aubl SCH AME Schwenckia americana (Klotzsch) L.A.F. de Carvalho SCL BAR Scleria barteri Boecke. SCL SPP Scleria sp. Berg. Cyperaceae Selaginella sp. Beauv. SEL SPP Sida linofolia Cav. Malvaceae SID LIN Solenostemon sp. Schumach and Thonn. SOL SPP Tristemma hit-turn Beauv. Melastomataceae TRI HIR Bulbostylis laniceps C.B. Clarke us0 SPP Urena lobata L. Malvaceae URE LOB Vigna sp. Savi VIG SPP Waltheria indica L. WAL IND Known * tree seedling species encountered. Alchornea cordifolia Meull. Arg. Anthocleista nobilis G. Don Aucoumea klaineana Pierre Barteria nigritana Hook f. Elaeis guineensis Jacq. Harungana madagascarensis Poir. Irvingia grandifolia Engl Irvingia sp. F. Muell. Macaranga sp. Thou. Manilkara lacera Dubard Maprounea membranacea Pax & K. Hoffm. Milletia sp. Meissn. Psychofria sp. L. (2 species) Rauvoljia manii Stapf Rhabdophyllum sp. Van Tiegh. Sacoglottis gubonensis Urb Uapaca sp. Baill. Xylopia aethiopica A. Rich.

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Gentianaceae Rubiaceae

Gramineae Gramineae Gramineae Lamiacea Grarnineae Gramineae Rubiaceae Solanaceae Cyperaceae Selaginellaceae Lamiaceae Cyperaceae Legurninoseae Sterculiaceae Euphorbiaceae Loganiaceae Burseraceae Passifloraceae Palmae Hypericaceae Irvingaceae Irvingaceae Euphorbiaceae Sapotaceae Euphorbiaceae Leguminoseae Rubiaceae Apocynaceae Ochnaceae Humiriaceae Euphorbiaceae Annonaceae

* There was a total of 45 species of tree seedlings encountered in the study savannas, 26 of which could not be identified due to lack of diagnostic characters.

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