PARENT SESSION
Oral Session #62: Forest Carbon Dynamics.
Presiding: R. Norby
Wednesday, August 7. 1:00 PM to 4:45 PM. Greenlee Meeting Room, TCC.

Resource manipulation of the carbon budget of a Eucalyptus plantation.

Stape, Jose¹, Binkley, Dan^*,1, Ryan, Michael¹, ¹ Universidade de Sao Paulo, ESALQ, Piracicaba, Sao Paulo, Brazil

ABSTRACT- Monoculture stands of Eucalyptus species dominate the plantation forestry in the Tropics, covering millions of hectares and producing billions of dollars in revenues annually. The rate of wood production depends on resource supply, gross primary production, and biomass allocation among components of the trees. We evaluated these components in a 3.5 to 5.5 year-old plantation of clonal Eucalypus grandis x urophylla in northeastern of Brazil, with experimental manipulations of water and nutrient supplies in replicate plots. All plots averaged 4 kg of woody mass/m² at age 3.5 years, and the irrigated plots reached 14 kg wood/m² at age 5.5 years. Averaged across a drought year (1200 mm rain) and an average year (1770 mm rain), gross primary production (GPP) was 4.9 kg C/m²/yr for control plots, and 6.7 kg C/m²/yr for irrigated plots. Fertilization had no effect on production. Control plots allocated 34% of GPP (1.1 kg C/m²/yr) belowground, compared with 28% of GPP (1.9 kg C/m²/yr) for irrigated plots. Drought lowered GPP in control plots by more than 50%, with the largest relative allocation reduction in wood growth (60%). The drought year also lowered GPP by about 25% in the irrigated plots, probably as a response of canopy conductance to high vapor pressure deficits. Quantum efficiency ranged from 0.027 mol C of GPP per mol of intercepted photosynthetically active radiation (control plots, drought year) to 0.060 mol C/mol PAR (irrigated plots, non-drought year). Net ecosystem production (or net ecosystem exchange) was moderate in control plots during the drought year (790 g C/m²/yr), and very high during the year of normal precipitation (2300 g C/m²/yr). The NEP was comprised primarily of wood increment; change in soil storage of C was on the order of 2% of NEP.

KEY WORDS: Gross primary production, belowground production, net ecosystem production