Net primary production of forests: a constant fraction of gross primary production?

TLDR

Progress has been made in modeling and measuring annual gross primary production, yet estimating maintenance respiration and net primary production remains challenging. The study aimed to identify common relationships by assembling annual carbon budgets from twelve diverse forest sites across North America and Australasia. Annual NPP, respiration, and synthesis costs for foliage, branches, stems, and roots were estimated at each site using a standardized procedure. Across the sites, the NPP/GPP ratio averaged 0.47 ± 0.04, remained constant, and ranged from 0.22 to 0.63 for root allocation, suggesting that forest growth models could be simplified and highlighting the need to investigate environmental drivers of NPP partitioning.

Abstract

Considerable progress has been made in our ability to model and measure annual gross primary production (GPP) by terrestrial vegetation. But challenges remain in estimating maintenance respiration (Rm) and net primary production (NPP). To search for possible common relationships, we assembled annual carbon budgets from six evergreen and one deciduous forest in Oregon, USA, three pine plantations in New South Wales, Australia, a deciduous forest in Massachusetts, USA, and a Nothofagus forest on the South Island of New Zealand. At all 12 sites, a standard procedure was followed to estimate annual NPP of foliage, branches, stems, and roots, the carbon expended in synthesis of these organs (Rg), their Rm, and that of previously produced foliage and sapwood in boles, branches, and large roots. In the survey, total NPP ranged from 120 to 1660 g C m−2 year−1, whereas the calculated fraction allocated to roots varied from 0.22 to 0.63. Comparative analysis indicated that the total NPP/GPP ratio was conservative (0.47 ± 0.04 SD). This finding supports the possibility of greatly simplifying forest growth models. The constancy of the NPP/GPP ratio also provides an incentive to renew efforts to understand the environmental factors affecting partitioning of NPP above and belowground.