Afromontane Forest Diversity and the Role of Grassland-Forest Transition in Tree Species Distribution

Iveren Abiem,David Kenfack,Hazel Chapman,Gabriel Arellano

doi:10.3390/d12010030

Abstract

Local factors can play an important role in defining tree species distributions in species rich tropical forests. To what extent the same applies to relatively small, species poor West African montane forests is unknown. Here, forests survive in a grassland matrix and fire has played a key role in their spatial and temporal dynamics since the Miocene. To what extent these dynamics influence local species distributions, as compared with other environmental variables such as altitude and moisture remain unknown. Here, we use data from the 20.28 ha montane forest plot in Ngel Nyaki Forest Reserve, South-East Nigeria to explore these questions. The plot features a gradient from grassland to core forest, with significant edges. Within the plot, we determined tree stand structure and species diversity and identified all trees ≥1 cm in diameter. We recorded species guild (pioneer vs. shade tolerant), seed size, and dispersal mode. We analyzed and identified to what extent species showed a preference for forest edges/grasslands or core forest. Similarly, we looked for associations with elevation, distance to streams and forest versus grassland. We recorded 41,031 individuals belonging to 105 morphospecies in 87 genera and 47 families. Around 40% of all tree species, and 50% of the abundant species, showed a clear preference for either the edge/grassland habitat or the forest core. However, we found no obvious association between species guild, seed size or dispersal mode, and distance to edge, so what leads to this sorting remains unclear. Few species distributions were influenced by distance to streams or altitude.

Highlights

Afromontane forests are widespread across the African highlands [1,2,3]
The most abundant species was Garcinia smeathmannii, an understory tree of the Clusiaceae family with 11,960 individuals (607.63/ha), representing approximately 30% of individuals ≥1 cm dbh (Table 2). This was followed by Deinbollia pinnata (Sapindaceae) with 3077 individuals (158.68/ha) and Pleiocarpa pycnantha (Apocynaceae) with 2142 individuals (107.51/ha)
That unlike the situation in the South African temperate montane forests where small seeds are obviously key to forest success, we found that in West Africa, larger-seeded and fire-adapted species seem to dominate the edges of each forest patch

Summary

Introduction

Afromontane forests are widespread across the African highlands [1,2,3]. Typically occurring above1500 m in elevation, they extend from the Arabian Peninsula south along the rift to the DrakensbergMountains in the east. In Western Africa, they are restricted to the Cameroon volcanic line and the Guinea highlands [1,4] (Figure 1). This disjunct distribution is ancient, pre-dating the Miocene doming that resulted in the Rift Valleys and the aridification of the East African interior [5]. Despite this distribution, Afromontane forests have a distinctive fauna and flora and harbor a high proportion of endemic species [6,7,8]. They have substantial carbon stocks [9], provide watershed protection [10]

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