Preliminary estimates of mass-loss rates, changes in stable isotope composition, and invertebrate colonisation of evergreen and deciduous leaves in aWaikato, New Zealand, stream

Rates of mass loss are important in the choice of tree species used in ripa rian rehabilitation because leaves that break down fast should contribute t o stream food-webs more rapidly than leaves that break down more slowly. To examine comparative mass-loss rates of some native evergreen and introduce d deciduous trees in a New Zealand stream, fallen leaves were incubated in bags with 2 x 3 mm mesh openings. The native trees were mahoe (Melicytus ra miflorus), kahikatea (Dacrycarpus dacrydioides), silver beech (Nothofagus m enziesii), rewarewa (Knightia excelsa), tawa (Beilschmiedia tawa), and the introduced trees were silver birch (Betula pendula) and alder (Alnus glutin osa). The leaf bags were left in the Mangaotama Stream for 28 days from mid April to mid May 1995 when mean water temperature was 14.5 degrees C, givi ng a total of 406 degree days. Rates of mass loss followed the sequence: ma hoe > silver birch > alder > kahikatea > silver beech > rewarewa > tawa. Me an mass-loss rate for mahoe, assuming a negative exponential model, was 0.0 507 k day(-1) (0.00350 k (degree day)(-1)), and for tawa was 0.0036 k day(- 1) (0.00025 k (degree day)(-1)). C:N ratio decreased on average hem 45:1 to 35:1, and delta(15)N increased between 0.7 and 3.0 parts per thousand (1.8 +/- 0.41 parts per thousand, mean fl standard error), excluding kahikatea. Changes in delta(13)C were smaller and not consistent in direction. Biomas s of invertebrates was greatest in bags that had lost 25-45% of their initi al leaf biomass.