Map Symbol For Evergreen Forest

CD O

ds ms me we1 we2 we 3 hw forest type

Figure 4.6 Comparison of forest types with respect to (A) rainfall, (B) water holding capacity and (C) cation availability.

Comparison with existing national and local vegetation maps

We compared our forest types with other ones that were made for the separate countries Ghana, Côte d'Ivoire, Liberia and Sierra Leone (Table 4.5). Our vegetation map is comparable to the map for Ghana (Hall & Swaine 1981). Their gradient from wet evergreen via moist evergreen to moist semi-deciduous and further to dry semi-deciduous nicely fits into the longer gradient as developed here. The gradient here is longer however, and resulted in an extra wet forest type, that we called the hyperwet evergreen forest. Hall & Swaine (1976, 1981) distinguished four forest types that do not show in our analysis and that are important for biodiversity in Ghana:

upland evergreen, the northwest moist semi-deciduous type (MSNW), the southern marginal type and the southeast outlier type. The upland evergreen did not show up in our analysis as we have only a few sites that fall into one of the other classes (based on our 40 species). As this forest type is important for the whole of Upper Guinea we have put an extra class on the map, based on altitude. Their distinction within the moist semi-deciduous forests does not show in our classification because the most distinguishing species for their MSNW have been excluded (Pericopsis) or merged with relatives (Khaya

T. tubmaniana

T. tubmaniana

N. papaverifera

N. papaverifera

Figure 4.7 Abundance and response curves to rainfall for the three characteristic species Tetraberlinia tubmaniana, Nesogordonia papaverifera and Petersianthus macrocarpus. (A) Spatial interpolation of the abundance values for three selected species for each of the sites. The size of the symbols indicates the abundance. A dark colour indicates a high abundance, a light colour a low abundance. (B) Response curves of species abundance on rainfall.

Figure 4.7 Abundance and response curves to rainfall for the three characteristic species Tetraberlinia tubmaniana, Nesogordonia papaverifera and Petersianthus macrocarpus. (A) Spatial interpolation of the abundance values for three selected species for each of the sites. The size of the symbols indicates the abundance. A dark colour indicates a high abundance, a light colour a low abundance. (B) Response curves of species abundance on rainfall.

anthotheca). The remaining two types do not occur elsewhere in sufficiently large areas (in fact neither in Ghana, cf Hawthorne & Abu Juam 1995).

Waterman et al. (1978) made a general vegetation map for Ghana and Côte d'Ivoire. The map for Ghana is based on Hall & Swaine (1976) and they extended that system into Côte d'Ivoire. Wet evergreen forest was found in the southeast and in the southwest of Côte d'Ivoire, which is comparable to our results.

Guillaumet & Adjanohoun (1971) recognised four main forest areas, the moist closed evergreen forests, the moist closed semi-deciduous forests, the littoral forest and the moist closed montane forest. The evergreen forests (their "secteur ombrophile") are found in areas with more than 1700 mm of rainfall and a dry period of between 2

and 5 months, while the semi-deciduous forests (their "secteur mesophile") are found in areas with beween 1200 and 1600 mm rainfall and a 4-6 months dry period. Within the evergreen forests they distinguish five types, based on specific species. Ecologically they are differentiated on a combination of soil characteristics (mainly related to water retention capacity in the soil, rainfall and the length of the dry season).

Within the semi-deciduous forest (generally between 1200 and 1600 mm of rain) they recognised three forest types, based on characteristic species, and not on soil or climate: a group with Aubrevillea kerstingii and Khaya grandifoliola, a group with Celtis spp. and Triplochiton scleroxylon (1200-1600 mm/yr), and a group with Nesogordonia papaverifera and Khaya ivorensis. This last group is intermediate between semi-deciduous and evergreen forest and mainly located in the east of Côte d'Ivoire, with between 1400 and 1500 mm rainfall per year.

In Côte d'Ivoire, the ecotone between the evergreen and the semi-deciduous forest is south of Man in the west, via south of Gagnoa in the centre to south of Abengourou in the east, roughly following the distribution overlap of the species Uapaca guineensis, U. esculenta, Lophira alata, Sacoglottis gabonensis (wet end), Mansonia altissima (dry end) and Celtis species. The northern limit of forest is marked by the distribution limits of Aubrevillea kerstingii, which is north of Man, north of Daloa, north of Gagnoa to just south of Bondokou.

Their montane forests are delimited at above 1000 m above sea level, mainly located in the west, near Mount Nimba, and in the massive of Dans. Parinari excelsa is a characteristic species. The littoral forest extends only to a few kilometres from the coast and is slightly more dry than further inland, mainly due to a longer dry season and a less water-holding soil (see also Kouamé et al., chapter 5).

Our gradient coincides with the forest types in Liberia (Voorhoeve 1965). Voorhoeve distinguished two major types, the evergreen forests (with a mixed type and a mono-dominant type) and the moist semi-deciduous forests, where the evergreen forests are determined by high annual rainfall and an absence of a dry period. This coincides with the wetter part of our gradient (the higher axis 1 scores).

The forests of Sierra Leone were classified into two broad groups (Savill & Fox 1967), the moist semi-deciduous forests (consisting of two types) and the tropical rainforest. The difference is mainly based on the length of the dry season, without clear differences in total yearly rainfall.

In a forest classification for Nigeria, Hall (1977) found that the major forest division was based on soil characteristics. Only the second level classification was based on rainfall, but that may have been due to the slightly biased sample selection: one of the soil/rainfall categories was effectively missed out (M.D. Swaine personal communication). In our analysis, in contrast, soil is not important compared to rainfall. Probably rainfall is really the overruling factor as long as a large rainfall gradient is covered. At more local scales, with shorter rainfall gradients, soil may become more important.

We expected to find a large variation in nomenclature, even in a relatively small area as Upper Guinea. However, we did not expect the cut-off limits between the forest types to be that variable (Table 4.5). Of course this

Table 4.5 The present classification compared to the most recent ones for the separate countries.

Rainfall (mm)

1000 1200 1400 1600 1800 2000 2200 2600

Sierra Leone 1

Moist semi-deciduous forest (Tonkoli type and Kasewe type) (>2500, 4 months of dry period)

Tropical rain forest (>2500, <3 month dry period)

- mixed forest type

- mono-dominant type Moist semi-deciduous (ecotone)

Semi-deciduous (1600-2000)

Cote dIvoire

Semi-deciduous (1200-1600) 3 types (dry period 6-8 months, deficit >600 mm)

Evergreen (1700 till over 2000) 5 types

(dry period 2-5 months)

Ghana 4

Southeast Outliers (<1000)

Southern Marginal (1000-1250)

Dry semi-deciduous (1250-1500)

Moist semi-deciduous NW type ( 1250-1500)

Moist semi-deciduous SE type (1500-1750) Moist evergreen (1500-1750)

Wet evergreen (>1750)

Upper Guinea 5

Dry semi-deciduous (1200-1600) Moist semi-deciduous (1250-1750)

Moist evergreen (1500-1800)

Wet evergreen1 (1400-2300)

Wet evergreen 2 (1600-1700)

Wet evergreen 3 (1700-2100)

Hyper wet evergreen (2000-3500)

Upper evergreen forest (above 500m altitude)

1) Savill & Fox 1967, 2) Voorhoeve 1965, 3) Guillaumet & Adjanohoun 1971, 4) Hall & Swaine 1981, 5) This chapter.

1) Savill & Fox 1967, 2) Voorhoeve 1965, 3) Guillaumet & Adjanohoun 1971, 4) Hall & Swaine 1981, 5) This chapter.

depends on the available environmental variation in the area under consideration. For instance, the wet forests of Ghana are the wettest forests found in the country but compared to other countries like Liberia and Sierra Leone the Ghanaian forests are relatively dry. Although the local situation sometimes is different, the present classification of forests thus provides a framework for forest classification in the whole forest zone of Upper Guinea.

Conclusions

We classified the forests of Upper Guinea into eight different types: seven types resulted from a combined classification and ordination of 176 forest sites, one extra class is for the montane evergreen forests. This Guinea-wide classification was similar in the resulting forest types to earlier smaller-scale forest classifications (mostly for one country or more local situations). The classification was based on a selection of 40 large tree species only.

There is a good similarity between a classification based on abundance and one based on presence/absence only.

Based on this classification and a spatial interpolation we produced a vegetation map for whole Upper Guinea. Although this regional map is less detailed compared to earlier produced maps for more local situations, the new map provides a nice integration for Upper Guinea as a whole.

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