NET ECOSYSTEM PRODUCTIVITY, NET PRIMARY PRODUCTIVITY AND ECOSYSTEM CARBON SEQUESTRATION IN A PINUS-RADIATA PLANTATION SUBJECT TO SOIL-WATERDEFICIT

Tree carbon (C) uptake (net primary productivity excluding fine root t urnover, NPP') in a New Zealand Pinus radiata D. Don plantation (42 de grees 52' S, 172 degrees 45' E) growing in a region subject to summer soil water deficit was investigated jointly with canopy assimilation ( A(c)) and ecosystem-atmosphere C exchange rate (net ecosystem producti vity, NEP). Net primary productivity was derived from biweekly stem di ameter growth measurements using allometric relations, established aft er selective tree harvesting, and a litterfall model. Estimates of A(c ) and NEP were used to drive a biochemically based and environmentally constrained model validated by seasonal eddy covariance measurements. Over three years with variable rainfall, NPP' varied between 8.8 and 10.6 Mg C ha(-1) year(-1) whereas A(c) and NEP were 16.9 to 18.4 Mg C ha(-1) year(-1) and 5.0-7.2 Mg C ha(-1) year(-1), respectively. At the end of the grow ing season, C was mostly allocated to wood, with near ly half (47%) to stems and 27% to coarse roots. On an annual basis, th e ratio of NEP to stand stem volume growth rate was 0.24 +/- 0.02 Mg C m(-3). The conservative nature of this ratio suggests that annual NEP can be estimated from forest yield tables. On a biweekly basis, NPP' repeatedly lagged A(c), suggesting the occurrence of intermediate C st orage. Seasonal NPP'/A(c) thus varied between nearly zero and one. On an annual basis, however, NPP'/A(c) was 0.54 +/- 0.03, indicating a co nservative allocation of C to autotrophic respiration. In the water-li mited environment, variation in C sequestration rate was largely accou nted for by a parameter integrative for changes in soil water content. The combination of mensurational data with canopy and ecosystem C flu xes yielded an estimate of heterotrophic respiration (NPP' - NEP) appr oximately 30% of NPP' and approximately 50% of NEP. The estimation of fine-root turnover rate is discussed.