Riparian buffers mitigate effects of pine plantation logging on New Zealand streams1. Riparian vegetation structure, stream geomorphology and periphyton

Abstract

Influences of the riparian zone vegetation characteristics on bank erosion, stream geomorphology, irradiance and periphyton were examined at 28 sites in the Whangapoua area of the Coromandel Peninsula, North Island, New Zealand. Riparian buffers were defined as areas alongside streams that were not managed for production forestry, and which contained native indigenous vegetation or mixed indigenous and introduced flora. Five forest and riparian zone types were selected: (i) harvested pine plantation with a clearcut to stream edge, (ii) harvested pine plantation with a vegetated riparian buffer, (iii) mature pre-harvest pine plantation with a riparian buffer dominated by native vegetation, (iv) mature pre-harvest pine plantation with a riparian zone of plantation pines and a native vegetation understorey, and (v) mature native forest reference sites. Up to eight replicates were selected for each management regime. Species composition and canopy cover were measured from bounded plots, and stream bank vegetation was recorded separately. Bank erosion was measured along the full length of each study reach, and stream water width and bankfull width were measured at ten transects within each study reach. Stream and bank lighting were measured using a canopy analyser. Periphyton standing crop was measured as Chlorophyll a and ash-free dry weight. Although channel width increased with catchment area for all of the study reaches, bank erosion and channel widths were greater at harvested sites where plantation pines occurred at the stream edge (i.e. within the riparian zone), than other forest and riparian treatments. Stream lighting was heavily influenced by the presence of riparian vegetation as well as stream size for these small to moderate size streams. Mature radiata pine in riparian areas of pre-harvest sites provided shading which was similar but less variable than that recorded in the native reference sites. Mature pine forest may be shading out the taller growing native broadleaved canopy, and thus creating a denser 5–11.9 m height class, reducing light penetration. The lack of native species >12 m in height is likely to be due to the length of time which native conifers and hardwoods take to develop as these are the species which normally dominate the greater than 12 m height class. Periphyton biomass was lowest at pre-harvest sites with native riparian buffers present and greatest at clearcut harvested sites with clearcut riparian zones. This work shows that riparian vegetation composition and maturity can influence the physical characteristics of afforested and harvested New Zealand streams.