Document: NEA-3-66-21

Factors controlling carbon accumulation in New Zealand secondary scrublands: Implications for the national carbon budget.

SCOTT, N.A.* ¹, J.D.WHITE ^1,2, C.M.TROTTER ¹, D.WHITEHEAD ³ and J.A.TOWNSEND ¹

Landcare Research,Palmerston North, NEW ZEALAND ¹
Baylor University, Waco, TX 76798 USA ²
Landcare Research, Lincoln, NEW ZEALAND ³

Abstract:
In New Zealand, reversion of marginal agricultural land to secondary native woody vegetation is an important land-use change, and could influence the national carbon (C) budget. However, while several studies have examined factors controlling productivity and the C balance of mature forests, few studies have addressed these questions in native ecosystems accumulating C rapidly. We examined C and nitrogen (N) distribution in four age-sequences of vegetation dominated by manuka ( Leptospermum scoparium) and kanuka (Kunzea ericoides) at sites with varying climate and soil type. We also examined age-related variation in key factors controlling forest productivity and the whole-ecosystem C balance at one intensive site (stands of 25, 35, and 55-years-old) including N cycling, root C production/allocation, and leaf area index. Across sites, maximum biomass C accumulation ranged from 10 kg C m^-2 to 19 kg C m^-2 in stands 50-55 years old. Root biomass allocation varied little across sites, averaging about 15% of total biomass in spite of site differences in precipitation, temperature, and solar radiation. At our intensive site, leaf area index and canopy N concentration were lowest in the 25-year-old stand and highest in the 35- and 55-year-old stands, which did not differ (P=0.4). Root production and soil N availability did not vary significantly with stand age. We used these data to parameterize the BIOME-BGC model, which was then used (with national-scale climate and soils data) to predict C accumulation in these scrublands nationally. The BIOME-BGC model assesses carbon and nitrogen allocation to foliage, stems, roots, litter, and mineral soil, and was used to examine the major factors constraining C and N fluxes in these ecosystems. From the modeling work, nutrient availability appears to limit C accumulation early in stand development more than climatic factors over larger spatial scales. Assuming a normal age-class distribution and about 1 million ha of area, these scrublands could accumulate about 1.8 Mt C y^-1, or about 20% of New Zealand's annual energy-derived CO₂ emissions.

Keywords: carbon budgets, shrublands, age-related productivity changes, scaling

This abstract is being presented at: 2:00 PM in session:
Oral Session #52: Carbon Storage in Ecosystems.