vegetation history of the semi-deciduous rainforest

Charcoal records reveal past occurrences of perturbations in the forests of the Kisangani region(RDC): vegetation history of the semi-deciduous rainforest John Tshibamba Mukendi1,2,3,4, Wannes Hubau4,5, Julie Morin-Rivat4,6, Honorine Ntahobavuka2, Faustin Boyemba Bosela2, Charles De Canniere3, Hans Beeckman4 1

Faculté des Sciences Agronomiques, Université Officielle de Mbuji-Mayi, RDC 2 Faculté de Gestion des Ressources naturelles Renouvelables, Université de Kisangani, RDC 3 Service d’Ecologie du Paysage et Système de Productions Végétales, Université Libre de Bruxelles, Belgique 4 Service de Biologie du bois, Musée Royal de l'Afrique Centrale, Tervuren, Belgique 5 Laboratoire de technologie du bois, Université de Gand, Belgique 6Unité de Gestion des Ressources Forestières et des Milieux Naturels, Agro-Biotech, Université de Liège, Belgique

Introduction

Past disturbances have modified the structure and the floristic composition of Central African rainforests. These perturbations represent a driving force for forest dynamics and they are presumably at the origin of present-day forest mosaics. Fire is a prominent forest disturbance, leaving behind charcoal as a witness of past forest dynamics. The question arises whether quantification, dating and botanical identification of ancient charcoal fragments found in soil layers (pedoanthracology) allows a detailed reconstruction of forest history, including the possible occurrence of past disturbances.

Material and Methods We organized pedoanthracological excavations in 6 regrowth sites and 48 sites of primary forests of Yangambi, Yoko, Masako and Kole in the Kisangani (RDC). We performed a detailed sampling in different vegetation types of a semi-deciduous rainforest (Yoko). Charcoal sampling was conducted in pit intervals of 10 cm. The charcoal was quantified whereby pottery fragments were also registered. A selection of charcoal fragments has been dated through AMS 14C measurement. Floristic identifications were conducted using. former protocols based on wood anatomy, which is largely preserved after charcoalification.  Democratic Republic of Congo map

Pericopsis forest: long lived light demanding species that is a indicator for large openings in the canopy

Maranthaceae forest: Scorodophloeus forest: evergreen species herbaceous plants indicating that is an indicator of a primary mixed forest disturbances semi-deciduous forest

Gilbertiodendron forest: monodominant evergreen climax species

Soil charcoal in a Gilbertiodendron forest, dated layers

Results  Specific anthracomass: charcoal mass (mg)/soil mass (kg)

N° labo

Age 14C Age14C (BP) (2σ calBP) 95,4%

Couche de sol à différentes profondeurs (cm)

Soil layers at different depths Couche de sol à différentes profondeurs (cm)

Anthracomasses spécifiques moyennes (mg/kg) des forêts de la région de Kisangani

Profil de charbon de bois dans la forêt à G. dewevrei et dates 14C (puits IV)

Couche de sol à différentes profondeurs (cm)

Age14C (2σ Age14C (BP) calBP) 95,4%

N° labo

0-10

0-10

10-20

10-20

20-30

20-30 Poz-54202

30-40

115 ± 25BP

254-présent

Poz-54203

430±30

50-60

50-60

508-327

60-70

60-70

10-20

70-80

70-80 Poz-54204

30-20

330±25

452-296

80-90

80-90

90-100

90-100

40-30

0

10

20

40-50

30

40

50

60 70 80 90 100 110 120 Anthracomasse spécifiques (mg/kg)

50-60

130

140

150

160

170

0

180

10

20

30

40

50 60 70 80 90 100 110 Anthracomasses spécifiques (mg/kg)

120

130

140

150

Soil charcoal in a Pericopsis forest, dated layers

foêt de Kole

60-70

30-40

40-50

40-50

0-10

Profil de charbon de bois dans la forêt à G. dewevrei et la date 14C (puits V)

forêt de Masako

70-80

forêt de Yangambi forêt de Yoko

N° labo

80-90 90-100

Age 14C Couche de sol Age14C (BP) (2σ calBP) à différentes profondeurs 95,4% (cm)

Profil de charbon de bois dans le peupelement de P. elata et la date 14C (puits IV)

N° labo

0-10

0

5

10

15

20

25

30

35

40

45

50

Anthracomasses spécifiqueq moyennes (mg/kg)

Poz-54201

Charcoal was found in most pit intervals The anthracomass in the soil of regrowth forests (secondary forests) is much higher than in the primary forest: 27,59 mg/kg for secondary forests et 2,53 mg/kg for primary forests. The specific soil anthracomass of the primary forest of the Yoko reserve is higher (7,7 mg/kg) than in Yangambi (1,9 mg/kg) , Masako (1,7 mg/kg) and Kole (0,8 mg/kg).

505-330

400 ± 30 BP

496-324

Profil de charbon de bois dans le peupelement de P. elata et la date 14C (puits V)

10-20

10-20

20-30

20-30

30-40

400 ± 30 BP 496-324

30-40

Poz-54200 430 ± 25 BP

Age14C Couche de sol Age14C (BP) (2σ calBP) à différentes profondeurs 95,4% (cm) 0-10

40-50

40-50

50-60

50-60

60-70

60-70 70-80

70-80

80-90

80-90

90-100

90-100

0

10 Anthracomasses spécifiques (mg/kg)

0

20

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 260 270 280 290 300 310 320 330

Anthracomasses spécifiques (mg/kg)

No systematic differences have been found between soil charcoal content of the different forest type representing different forest histories. Gilbertiodendron dewevrei (De Wild.) J. Leonard forests showed surprisingly a higher concentration of soil charcoal.

 Charcoal identifications point at a manifest presence of the family of Fabaceae: 25 % of the identified fragments are of this family  C14 dating of charcoal fragments gave evidence of two different forest disturbances Wood anatomy of Garcinia punctata Oliv. Wood anatomy of Afzelia species

Legende Type de forêt daté N° labo 2σ calBP (95,4%) Peuplement à Pericopsis elata Poz-54200 505-330 Peuplement à Pericopsis elata Poz-54201 496-324 Forêt à Gilbertiondendron dewevrei Poz-54202 254-présent Forêt à Gilbertiondendron dewevrei Poz-54203 508-327 Forêt à Gilbertiondendron dewevrei Poz-54204 452-295

A

B

c

A

B

c

A

B

c

A

B

c

Taxon identifié Afzelia Spp Garcinia punctata Bobgunnia fistuloide Scorodophleous zenkeri Scorodophleous zenkeri

Discussion

 Forest disturbances in the Kisangani region appear to be more recent than those in the Mayombe forest in Western RDC ( 3000-2000 calBP (Hubau, 2013)) and those of the Cameroon forest (2300-1300 calBP) (Morin-Rivat, J et al., 2014).  Stratified charcoal conserved in the soil is a useful indicator of past forest disturbances. Références bibliographiques Hubau, W., 2013. Exploring charcoal as a natural archive for palaeofire and vegetation history of the Mayumbe, Democratic Republic of the Congo. PhD Dissertation, Ghent University, 224 p. Morin-Rivat, J et al., 2014. New evidence of human activities during the holocene in the low land forests of the northern congo basin, Radiocarbon, Vol 56, Nr 1, p 209–220

A

= Plan transversal

B

= Plan tangentiel

C = Plan radial