Effects of stream size and forest type on leaf litter decomposition and macroinvertebrate diversity in Kamweti area, Kenya

Abstract

Streams draining the Southern part of Mt. Kenya are currently heavily impacted by human encroachment and large patches of riparian forests have been converted from indigenous to plantation forests. Modification of riparian vegetation was hypothesized to have influence on streams. This study aimed at investigating the effect of forest type and stream size on leaf litter decomposition, macroinvertebrate diversity and water quality in streams draining Mt. Kenya catchment. Riparian tree species were identified along both banks of the studied streams. About 2.5g dry leaves of Syzygium cordatum were incubated in streams for 56 days to estimate leaf-processing rates. In total, 150 litter-bags measuring 11 x 11cm, mesh size 5 x 5 mm were prepared and exposed on 1st February 2015 in six studied streams. Five bags were retrieved after 0, 7, 14, 28, 42, and 56 days of exposure per stream. Macroinvertebrates were separated from the leaves, counted, enumerated and identified up to the family level before assigning them to the major functional feeding groups. Plants were more diverse along the streams draining native tree dominated forests (H´ = 2.9) than in exotic tree dominated forests (H´ = 1.9). The most dominant riparian species were Tabernaemontania stapfiana, Neubotonia macrocalyx, and Syzygium guineense in native tree dominated forests while Eucalyptus species dominated the riparian riverbanks of the streams draining exotic tree dominated forests. Decomposition of S. cordatum leaves was slower in streams draining exotic tree dominated forests (– k = 0.0172) than in streams draining native tree dominated forests (– k = 0.0213). Processing rates were significantly lower in small streams (– k = 0.0174) in comparison to big (– k = 0.021). There was a significant difference in mean decomposition rates between small and big streams (Two-way ANOVA df = 1, F = 5.026; P < 0.05) while no significant difference was evident in mean decomposition rates between streams in native and exotic tree dominated forests. Macroinvertebrate abundance and diversity associated with decomposing litter were lower in streams draining exotic tree dominated forests than in streams draining native tree dominated forests and in small streams than in big streams while no significant differences of macroinvertebrate abundance were found between forest types (Two-way ANOVA d. f = 1, F = 2.423; P > 0.05). The study concluded that depletion and modification of native riparian vegetation through successive land use activities and replacement with exotic tree species has negative impacts in altering stream ecosystem functioning and small streams are more impacted than big streams.