Sustained carbon uptake in a mixed age southern conifer forest

Abstract

Larger trees accumulate more carbon but have lower growth rates per unit biomass. Nevertheless, large trees are a vital part of the carbon cycle of this mixed age forest. Globally, there is a strong need for detailed information on carbon (C) budgets of native forests, yet New Zealand forests are underrepresented in C inventories. Stem growth and CO2 efflux are two of the major components of forest C budgets, especially in forests with long-lived, large trees. In this paper, we present a breakdown of tree-level C accumulation rates for 26 trees [9.7–176 cm diameter at breast height (DBH)] of one of the world’s largest tree species: the endemic conifer kauri (Agathis australis) over 3 years (2010–2013) in west Auckland. We examined the relationship between C accumulation, tree size, and canopy class (dominant, co-dominant, and intermediate). The total amount of C accumulated was 2.7, 3.5, and 4.2 Mg C ha−2 year−1 in 2011, 2012, and 2013, respectively. Large trees (>70 cm DBH) had the fastest absolute rates of growth (basal area increment; 30–140 cm2 year−1) and C accumulation (0.01–0.048 Mg C year−1), but the lowest rates of relative growth (per unit biomass). Relative growth rates peaked at a diameter at breast height of 25 cm, and consequently, co-dominant trees had the highest annual relative growth rates (1.7–2.2% year−1). The majority of C was accumulated in co-dominant trees (62%), with smaller amounts accumulated in dominant (29%) and intermediate trees (9%). One-third of the stand’s C accumulation was accrued by the largest 6% of trees, which made up 46% of the stand’s basal area. While maintenance and growth respiration rates were higher for larger trees, C use efficiency (CUE) was consistent across tree-size classes. To extend the size of trees analysed, we included data from tree-ring analysis of 13 additional trees up to 243 cm DBH and found larger trees accumulated proportionally less C per unit biomass than smaller trees.