Effects of bamboo expansion on organic matter and nutrient dynamics in mountain streams

Rapid expansion of Moso bamboo (Phyllostachys edulis) has been noted in East Asian countries, and replacement of riparian vegetation by dense bamboo has been observed in many areas. Changes in riparian vegetation have significant effects on stream ecosystems, but the effects of bamboo remain uncertain. In this study, leaf litter breakdown and leaching of dissolved organic matter (DOM) and nutrients in stream were compared between Moso bamboo and other riparian tree species to assess the effects of bamboo expansion on stream organic matter and nutrient dynamics.

Leaf litter of Moso bamboo and two evergreen species in riparian area, Camphor laurel (Cinnamomum camphora) and Japanese cedar (Cryptomeria japonica), that provide leaves to the stream in the same season as bamboo were compared. Litter bags with 5 g of leaf litter were incubated on the streambed in four riffles in July and August. Litter bags were collected 1, 8, 15, 28 and 42 days after the start of incubation to measure breakdown rates and leaching of DOM and nutrients. Macroinvertebrates in the litter bags at day 8 were also examined.  

Breakdown rates of Moso bamboo was significantly lower than those of Camphor laurel and Japanese cedar. Despite the low macroinvertebrate breakdown rates, the number and species richness of macroinvertebrate present were highest in the bamboo litter bags. These results suggest that bamboo leaves were not palatable compared to Camphor and cedar litter, but they functioned as habitat for macroinvertebrate. Leaching of DOM was highest from Moso bamboo leaves at day 0, and it rapidly declined as breakdown progressed. Japanese cedar, on the other hand, released smaller amount of DOM, but maintained the similar rates despite the progress of breakdown. Leaching of bioavailable DOM (BDOM) was lowest from bamboo, 0.64 (± 0.84) mg/g of litter, and highest from Camphor leaves, 5.48 (± 0.99) mg/g, at day 0, and by day 8, leaching of BDOM became similar among species, 3.30 (± 0.67) mg/g, 3.67 (± 0.16) mg/g, 2.37 (± 0.86) mg/g respectively for Moso bamboo, Camphor laurel, and Japanese cedar. Thus, bamboo leaves leached larger amount of DOM, but BDOM was low, and the effects on in-stream nutrient processes may be smaller than other two species.