Single-tree influence of Tectona grandis Linn. f. on plant distribution and soil characteristics in a planted forest
Beckley Ikhajiagbe, Matthew Chidozie Ogwu, Adebayo Emmanuel Lawrence
Abstract
Abstract Background Little is known about the single-tree influence of Tectona grandis Linn. f. on plant distribution and soil characteristics in Benin City, Nigeria. We investigated the possible single-tree effect of T. grandis on understory plants in an 8-year-old teak plantation at the Moist Forest Research Station. An area of 36.57 m by 60.96 m was marked out and divided into 15 equal-sized subplots containing 10 trees per subplot. Marked distances from the base of a randomly selected tree per subplot were made (0–0.5 m, 0.5–1.0 m and 1.0–1.5 m). Results Single-tree influence of T. grandis was observed in the soil total organic carbon, total nitrogen and soluble phosphorus, where concentrations were higher with 1.5-m radius from the tree than beyond. Moreover, the pH of the topsoil within 1.5 m from the base of the tree was lower (pH 4.4) than beyond 1.5 m from the base of the tree (pH 5.4). Species-specific single-tree effect was also observed on the understory plant distribution likely due to diverse ecophysiological interactions. Within 1.5 m from the tree, plant species abundance, especially of Sida garckeana , Reisantia indica , Momordica charantia and Tridax procumbens were negatively affected. However, the distribution of Eleusine indica around the tree was not negatively influenced. Plant abundance was generally suppressed in Cynodon dactylon , Axonopus compressus , Andropogon gayanus , Commelina diffusa and Euphorbia hirta. Generally, there were more plant species beyond the canopy fringes than within the canopy, indicating inhibitory single-tree effects. Conclusion Not all plant species in close proximity to T. grandis are affected. This is important considering that plant-plant associations affect the quality of forest soils. Generally, more plant species were recorded outside the 1.5-m demarcation than within, an increase in soil organic matter may further enhance such plant species abundance. The impact of T. grandis in forest soil quality is possibly a factor of the outcome of its association with neighbouring plant species. Diverse mechanisms at play may be responsible for the observed effects on soil chemistry. However, a reduction in the soil organic matter and variations in other environmental factors also contributed to observed single-tree effect.