The biology of kauri (Agathis australis) in New Zealand. Production, biomass, carbon storage, and litter fall in four forest remnants

Abstract

Four kauri forest sites, ranging from pole stands to mature forest, were surveyed for standing biomass and forest floor biomass. Destructive sampling was done at two further sites to obtain data on tree form and biomass. Total dry matter, including that on the forest floor, ranged from 132 to 2290 t ha‐1, with as much as 546 t ha‐1 of forest floor litter and humus in the oldest stand. Total stored carbon above the mineral soil ranged from 64 to 990 t ha‐1. Litter fall was collected monthly in five 1‐m2 litter traps at each site for two years. Total annual litter fall ranged from 4.5 to 13.2 t ha‐1 with leaf litter making up 41% or less, by weight, of the total litter fall. Forest floor accumulation of litter in kauri forest is extremely high with mean residence time of 9–78 years. In general, sites with higher litter fall are accompanied by faster breakdown, but at all sites litter decomposition is slow. Leaf area index (LA) of light‐suppressed saplings and poles ranged from 0.08 to 0.84, whereas young emergent trees had LA of 3.33–3.66. LA of mature trees, estimated from litter fall and mean leafage, was 1.0–3.0, with a new foliage increment of 0.4–0.9 LA per annum. A simple biomass and production model for two mature kauri site shows a wide variation in production. For Trounson Forest, a site with mature trees, the net production is c. 8.5 t ha‐1 yr‐1 with 2.4 t allocated to wood, 2 t to roots, and 4.1 t to replacing litter fall. For a younger stand on farmland at Kaipara Flats, net production is c. 15 t ha‐1 yr‐1 with 11.1 t allocated to replacing litter fall.