Thomas Kiprotich Kiptoo^{1, 2,}, James L. Kiyiapi¹, Francis K. Sang¹

Alien species invasion of the forest ecosystem may lead to structuring of the plant community structure. Invasive Acacia is one of the alien tree species in forests. The ecological relationship between Acacia melanoxylon invasion and forest ecology is not well understood in tropical humid forest ecosystems. Therefore, the main objective of this study was to determine the influence of A. melanoxylon on two commercial forest tree species (Cupressus lusitanica and Pinus patula) in a humid tropical forest (North Tinderet Forest, in Kenya). Transects measuring 500 m long were used for sampling the uninvaded and invaded sites. Along the transect, three 10 m × 10 m plots were systematically placed at 235 m intervals to enable counting and recording of trees (density), measuring diameter at breast height (DBH) > 1.3 m) as well as tree heights. The study established that the tree density, DBH and height of Pinus patula and Cupressus lusitanica, were significantly (P < 0.05) higher at the non-invaded sites compared to the invaded sites. These findings suggest that A. melanoxylon invasion reduced the growths and establishment of commercial tree species. It is thus recommended that future studies on ecological conditions for growth of A. melanoxylon should be conducted in controlled environment through growth response measurements which was not possible under the current study. Invasion by A. melanoxylon reduce the growth of commercial plantation tree species (Pinus patula and Cupressus lusitanica) therefore strategies for controlling growth of the A. melanoxylon should be done. Future studies should investigate how the nutrient uptake by the commercial plantation tree species is influenced by Acacia melanoxylon density.

ecological conditions, acacia, growth, invasiveness, tropical environment, forest growth