Influence of Canopy Cover, Ground Disturbance and Soil Characteristics on Understory Vegetation in Forest Reserves, Southwestern Nigeria

Abstract

Biodiversity research in tropical forests has mostly focused on the overstory components of the forests with little attention accorded to understory species. However, the forest understory plays a crucial role in the functioning of forest ecosystem. This study investigated the influence of canopy cover, anthropogenic ground disturbances and soil characteristics on understory vegetation in selected forest reserves in southwestern Nigeria. The forest reserves purposively selected for this study were: Akure (primary forest), Eda (highly degraded forest) and Shasha (degraded forest). Each forest reserve was categorized into three canopy cover classes (open, partial and closed canopy covers). Data were collected from various anthropogenic disturbed areas of the forest reserves (Farm lands, Logged areas and Tractor/foot paths). The canopy cover classes were marked out on a digitized map and three of each class were randomly selected from each forest reserve. Data were collected from a total of fifty-four temporary sample plots of 25m × 25m in the three forest reserves. Within each sample plot, trees with dbh ≥ 10 cm (overstory trees) were identified and measured. Also, data were collected from forest understory layer categorized into saplings and seedlings. Soil samples were collected at any two edges of 4m2 sub-plot, laid at the centre of the main plot, at 0–15 cm, 15–30 cm and 30–45 cm depths and analyzed for physical and chemical properties. Results revealed that number of tree species ranged from 11–34 and 12–22 for the different canopy cover classes and anthropogenic ground disturbances of the three forest reserves respectively. The Shannon-Wiener diversity index ranged from 2.48-3.11 (canopy cover classes); 1.89-2.90 (anthropogenic ground disturbances) while species evenness varied from 0.88–0.97 (canopy cover classes) and 0.76–0.97 (anthropogenic ground disturbances) for the three forest reserves. For canopy cover classes, the range of species richness, diversity index and evenness for the saplings were: 6–15; 1.68–2.55 and 0.87–0.95, respectively and 3–13; 1.08–2.49 and 0.90–0.99, respectively for seedlings. For anthropogenic disturbed areas, species richness, diversity index and evenness were 0-12; 0-2.43; 0-1.00 for saplings and 0-8; 0-2.07; 0-0.99 for seedlings. Generally, soil chemical properties decreased with increasing soil depth in the three forest reserves except exchangeable Magnesium, Calcium and soil pH which increased with soil depth, while in the anthropogenic disturbed areas, there was a general decrease with increase in soil depth in all the forest reserves, except exchangeable sodium and soil pH which increased with increasing soil depth. The result revealed that anthropogenic ground disturbances and soil characteristics had significant influence on forest understory with strong effect from the canopy covers in the three forest reserves. All the closed canopy layers of the three forest reserves had high regeneration potentials considering the high number of tree species present in their understories. Therefore, anthropogenic disturbances pose great risk to understory species diversity and regenerations, which leads to biodiversity loss. It is therefore recommended that relevant policies and laws be enforced to protect the forests against anthropogenic disturbances and minimize their adverse effects on forest biodiversity.