Carbon and nitrogen distribution and accumulation in a New Zealand scrubland ecosystem

Reversion of agricultural land to native woody vegetation can sequester carbon (C), influencing regional and global C budgets. We examined whole-ecosystem differences in C and nitrogen (N) storage and distribution, and sapwood - leaf area relationships in a scrubland vegetation chronosequence in New Zealand dominated by manuka (Leptospermum scoparium J.R. et G. Forst) and kanuka (Kunzea ericoides var. ericoides (A. Rich.) J. Thompson). At 25 years, manuka dominated, and vegetation C was 6.5 kg C·m-2. In the 55-year-old stand, stem density was similar for the two species, and vegetation C storage was 15.1 kg C·m-2, similar to the 35-year-old stand (p = 0.9). Foliar biomass comprised 3-5% of vegetation C stock but contained 26%-37% of vegetation N. Root biomass was 10-15% of total and varied little with stand age. The sapwood - leaf area relationship differed significantly for the two species (p < 0.05). Mineral soil C and N (to 0.30 m) did not vary with stand age, but forest floor C and N were highest in the 55-year-old stand (2 kg C·m-2; p < 0.01). Soil and forest floor C/N ratios were significantly higher in the 35-year-old stand (p < 0.04), possibly because of high interspecific competition for N. While the sampling intensity was too limited to allow spatial extrapolation, our results suggest that carbon accumulation in this scrubland is rapid and similar to plantation forests, suggesting that land abandonment could significantly impact New Zealand's C budget.