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Figure 5.2 (cont.). (c) Distribution of relative frequencies of plant and pollen for the main taxa at pollen sampling sites as in (a) and (b) (after Reynaud-Farrera, 1995).

5.3.2 Mixed moist semi-evergreen forest

Mixed moist semi-evergreen forest was sampled in Central Gabon (Jolly et al., 1996) during a preliminary palynological investigation in the forest reserves at Makokou (0°30'N, 12°50') and Belinga (1°06'N, 13° 10'E) (Figure 5.1). Located at 470m elevation, near the Irvindo River, the Makokou forest is dominated by Scoro-dophloeus zenkeri, Baphia, Dialium, Pancovia, Dichostemma glaucescens among the Papilionaceae and Polyalthia among the Annonaceae (Caballe, 1986; Aubreville, 1967). Precipitation amounts to 1,500 to 1,700 mm/yr and mean annual temperature is over 24°C. At Makokou, various sampling strategies (line, diagonals, random, etc.) were tested to collect surface soil samples within 120 x 40 m forestry plots along a 1 km transect. No significant differences were shown between the pollen assemblages obtained with the different collecting methods. It was concluded that a random procedure represents the most parsimonious and the less time-consuming one. Random collection should certainly be made in regions where there is an urgent need to obtain modern pollen data within forests under threats of total disappearance. The pollen counts of 16 pollen assemblages from mixed moist semi-evergreen forest include 82 pollen taxa (Jolly et al., 1996). The highest pollen frequencies come from Mora-ceae and Euphorbiaceae (mainly Alchornea), Celtis, Combretaceae, Pausinystalia (Rubiaceae), Hymenostegia (Caesalpiniaceae)—together with Dacryodes (Bursera-ceae) and Pycnanthus (Myristicaceae), which were not particularly well-represented in the vegetation. Pollen attributed to Papilionaceae has been counted but in much lesser abundance than the corresponding trees. High proportions of Urticaceae and Macaranga have been found within a plot located closer to the river, and were attributed to local disturbance. The pollen spectra from the mixed semi-evergreen forest at Makokou (Gabon) has some common taxa (Celtis, Macaranga, Alchornea, Combretaceae), with samples from Cameroon located in a forest intermediate between the evergreen Biafran and the semi-evergreen "Congolese". Interestingly, the Belinga pollen sample, located at higher elevation (950 m), contains significant percentages of Syzygium pollen. Although Syzygium has swamp species, this finding agrees with modern pollen data from highland forests of Ethiopia (Bonnefille et al., 1993) and South India (Bonnefille et al., 1999).

5.3.3 Drier peripheral semi-evergreen forest 5.3.3.1 North of the equator

Modern pollen data from the drier peripheral semi-evergreen forest (foret semi-caducifoliees), which extends around the 4°N latitude between the mixed moist semi-evergreen and northern savanna, has been provided at two distinct sites: Men-gang (east of Yaounde) and Kandara in Cameroon (Figure 5.1).

The Mengang forest station is located in an area receiving 1,600-mm/yr precipitation interrupted by two dry months (<50 mm)—December, January—and a minimum in July. Atmospheric pollen rain was captured in 1987 (April to October) by an above-ground framed trap, and the results presented at monthly intervals (Fredoux and Maley, 2000). They indicate that the greatest amount of pollen is produced by the forest trees (c. 90% of total counts), greatest monthly flux averaging 1,000 grains/m3. Altogether, 118 tree taxa were identified distributed among 58 families. The Euphorbiaceae show the greatest diversity (21 taxa) and total 27% of the pollen counts dominated by Ulmaceae (33%) and Moraceae (26%). They are associated with Urticaceae (3%)—and Papilionaceae, Caesalpiniaceae, Mimosaceae, Anacardiaceae, Sterculiaceae, Rubiaceae, and Sapindaceae, each accounting for less than 1%. The greatest pollen producers indicated by flux calculations are Macaranga (1 to 1,000/m3 maximum in May), Celtis (1 to 500/m3 in April), Musanga (or Myrianthus) and Combretaceae (10 to 100/m3). The amounts of Pycnanthus, Trema, Urticaceae, and Poaceae pollen are produced in much lower proportion (<1 to 10/m3). Although established for 1 year, such discrepancies have to be remembered while interpreting fossil pollen results. It would have been most interesting to compare this distribution with parallel analysis of surface soil samples within the forest that provide an average of modern pollen rain over several years, minimizing monthly variations.

In southeastern Cameroon—east of Mengang (Figure 5.1), near the Kandara village (4°20'N, 13°43'E, 640 m)—the dry semi-evergreen forest encloses a few square kilometer area of shrub tall-grass savanna. The wet tropical climate is characterized by the same precipitation (1,600 mm/yr) and dry season, with mean annual temperature averaging 24°C. In this area the forest had expanded at a rate of 1 m/yr between 1951 and 1993 (Youta Happi, 1998; Achoundong et al. 2000), in agreement with observations made on aerial photographs in Cameroon (Letouzey, 1968). The forest succession begins with Raphia monbuttorum (Palmae) swamp along the Soukato River, followed by a semi-evergreen Rinorea (Violaceae) forest, a young Albizia forest, and a savanna-forest transition (ecotone) surrounding the savanna dominated by Panicoi-deae grasses with some Albizia clusters (Figure 5.3a). Associated with Rinorea dentata and Rinorea batesii (Achoundong et al., 1996), the most abundant trees of the mature forest are Triplochiton scleroxylon (Sterculiaceae) and Piptadeniastrum africanum (Mimosaceae), good indicators of dry peripheral semi-evergreen forest (Letouzey, 1968). The young Albizia forest includes Albizia adiantifolia (Mimosaceae), Funtumia elastica (Apocynaceae), Canthium (Rubiaceae), Tabernaemontana crassa (Apocyna-ceae), Sterculia rhinopetala (Sterculiaceae), and Myrianthus arboreus (Moraceae). At Kandara, 26 surface soil samples collected in contiguous 20 x 30-m plots, along a 750-m transect across the succession (Figure 5.3a), provided modern pollen rain data. Each pollen sample consists of about 20 sub-samples randomly distributed (Vincens et al., 2000). The relative frequencies of the most abundant ones among the 101 pollen taxa altogether identified are illustrated here (Figure 5.3c). All samples from the dry semi-evergreen forests register percentages of arboreal pollen (AP) ranging from 60 to 80%, in good correspondence with the canopy coverage estimated by field measurements of the leaf area index (Cournac et al., 2002) and abundance of tree phytoliths counted in the same samples (Bremond et al., 2005). The total AP drops abruptly to less than 10% in the nearby savanna, at less than 100 m from the forest limit, indicating very little transport from the forest into the savanna (Figure 5.3b). The pollen assemblages from this transect include some pollen of the trees characterizing dry peripheral semi-evergreen forest. These are Celtis, Chaetachme, Holoptolea among the Ulmaceae, Triplochiton (Sterculiaceae) associated with Piptadeniastrum

| -h -"^-j Raphia swamp [222} Rinorea forest | | Albizia forest | | Ecotone | | Savanna

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