Patterns of Tropical Tree Species Richness along Elevational Gradients of Mountain Afadjato, Ghana

Eurasscience Journals Eurasian Journal of Forest Science (2014) 2(2): 18-23 PATTERNS OF TROPICAL TREE SPECIES RICHNESS ALONG ELEVATIONAL GRADIENTS OF MOUNTAIN AFADJATO, GHANA Edward D. Wiafe1* 1* Department of Environmental and Natural Resources, Presbyterian University College, P. O. Box 393, Akropong-Akuapem, Ghana. Abstract This paper evaluates the effect of altitude on tree species richness, diversity and tree abundance to assess the responses of tree species to environmental changes. The highest mountain in Ghana, Afadjato, was categorized into three namely, lower elevation (200m -400m ASL), middle elevation (400m-600m ASL) and from 600m ASL and above the mountain as upper elevation. On five transects running from the bottom to the top of the mountain, two 20m by 20m squared plots were laid in each category on any 20m rise in altitude along the gradient of the mountain. All trees greater than 5cm in diameter at 1.3m above ground were identified and measured. The species richness, basal areas of trees and diversity differed in all the three levels of elevation. Tree abundance was found to decrease with increasing altitude. Recommendations have been made for future studies to establish the various relationships between the factors contributing to the variations along the elevation gradient. Keywords: Elevation, Ghana, Mountain, Species richness, Tropical trees. Özet Bu çalışma ile ağaç türlerinin çevresel değişimlere gösterdiği tepkiyi ortaya koymak maksadıyla yükseltinin ağaç türü zenginliği, tür çeşitliliği ve rastlanma sıklığı üzerine etkileri araştırılmıştır. Gana’nın en yüksek dağı olan Afadjato dağı denizden yüksekliklerine göre 200-400 m, 400-600 m ve 600 m’nin üzeri olmak üzere üç yükselti basamağına ayrılmıştır. Kütlenin eteklerinden zirvesine doğru 5 kesit alınmış ve her bir 20 m yükselti artışında her bir kesitten 20 x 20 m2’lik 2’şer tane örnek alan alınmıştır. Göğüs yüksekliği çapı 5 cm’nin üzerinde olan bütün ağaçlar teşhis edilmiş ve ölçümleri alınmıştır. Ağaç türü zenginliği, göğüs yüzeyi alanı ve tür çeşitliliği bütün yükselti kademelerinde farklı bulunmuştur. Ağaçların rastlanma sıklığının yükselti arttıkça azaldığı tespit edilmiştir. Çalışmamızdan çıkarılan sonuçlara göre yükselti ile gerçekleşen değişimlerin daha iyi anlaşılması için daha farklı değişkenlerin denendiği çalışmaların yapılması gerektiği anlaşılmaktadır. Anahtar kelimeler: Yükselti, Ghana, Dağ, Tür zenginliği, Tropikal ağaç türleri. INTRODUCTION Mountain ecosystems provide a vast array of goods and services to humanity, both to people living in the eurasscience.com mountains and to people living outside mountains (MA, 2005). Mountains represent an enormous compression of life zones with a wide diversity of climate combined with local differences owing to geomorphological, edaphic and plant cover features 18 Eurasian Journal of Forest Science 2(2): 18-23 (2014) have given rise to a range of micro-climate which has become habitat for different species which adapt and making them their specific niches. According to Heywood (1995), about 3% of the terrestrial surface of the earth is covered by high mountain ecosystems, where about 4% of the earth’s flora is found. Mountain ecosystems provide a fundamental characteristic to the drastic change in vegetation and in climate from the base to the top of mountain (Yu, 2004). Elevation gradients create varied climates, along with resultant soil differentiation; promote the diversification of plant and animal species (Brown, 2001; Lomolino, 2001). Many studies have investigated species richness along elevation gradient across habitats and taxa (Rahbek, 1995; Austrheim, 2002; Vetaas and Gerytnes, 2002; Sanders et al., 2003) and as part of efforts to understand ecosystem effects on biodiversity and conservation of biodiversity (Vetaas and Gerytnes, 2002). Strong local segregation occurs between similar species, such as the masked shrew (Sorex cinereus) and the southeastern shrew (Sorex longirostris) (Pagels and Handley, 1989; Ford et al., 2001). Species distribution variation along the gradient has been demonstrated by early forestry researchers on tree distribution such as Whitteker (1956) and relied on the assumption that increasing elevation is analogous to decreasing moisture. However, the distribution response of species across an elevation gradient has been found to be related to the latitudinal location of the study site. For example, Acer pensylvanicum occurs at high elevations (800+m) in west Virginia and Virginia, but occurs at middle elevations (400m600m) in North Carolina and Tennessee (Whittaker, 1956; Mills and Stephenson, 1999). But Acer rubrum shows a reversal of the pattern occurring at high elevations (800+m) in South Carolina, but low to middle (200 to 600m) elevations in areas of north of South Carolina (Whitney and Johnson, 1984; Harrison et al., 1989; Davis et al., 2003). However, some species show little or no variation in their elevation distribution across a geographic area. Further, elevation has been found to be merely a surrogate for a suite of biotic and abiotic factors that influence species richness (Rahbek, 1995). The intention of this research is to establish the variations that occur in the community of tree species along the gradient of mountain ecosystems. The question is at what magnitude of altitude can tree species change in richness, diversity and in abundance? Therefore the main goal of the study was eurasscience.com to assess the variations in distribution of trees along different elevations on Afadjato mountain located in the southeast of Ghana. Materials and Methods Study Area Mountain Afadjato is part of Akwapim-Togo range which constitutes the highest hill in Ghana with the Afadjato itself being the highest mountain in Ghana at 885m above sea level which runs in the northeast and southwest direction between the Volta River and the Ghana-Togo border (Ntiamoah-Baidu et al., 2001) (Figure 1). The hill lies within longitude 0o15’E and 0o45' E and latitude 6o 45' N and 7o 15' N and covers an area of 1172km2 (Owusu, 2010). The mountain is endowed with two major waterfalls - Wli and Tagbo waterfalls at the northern and southern borders, respectively (Ntiamoah-Baidu et al., 2001). Mountain Afadjato has been declared as an Important Bird Area (IBA) by Fispool and Evans (2001). Climate and Vegetation The annual rainfall total ranges between 1100mm and 1500mm, averaging 1300mm. The rainfall pattern is bimodal with two distinct rainy seasons. The major rains start from April through to July while the minor season covers the period from September through November. Occasionally the Afadjato area bimodal pattern gives way to continuous rain from April through to November. On the average, the major season receives about 43% of the total annual rainfall as compared to about 40% for the minor season. Comparative figures however establish a greater reliability of the rain both in quantity and distribution du (...truncated)