Comprehensive description of the carbon cycle of an ancient temperate broadleaved woodland.
K. Fenn, Y. Malhi, M. Morecroft, C. Lloyd, and M. Thomas; 2010
A section of the discussion on productivity
We estimate the annual NPP (Net Primary Productivity) of this system at 7.0±0.8 MgC ha−1 yr −1. Compared to NPP values reported for managed temperate deciduous forests (refs in paper) this demonstrates that relatively unmanaged woodlands, such as Wytham, can also be quite productive.
The clear monthly data on woody productivity demonstrate the value of this scale of dendrometer dbh change measurements. The ability of this simple method to detect monthly changes in the growth of these temperate species enabled us to gain clear insight into the seasonality of wood production at this site. Combining dendrometer data with LAI measurements showed how the production of leaves, concentrated in late spring, was immediately followed by an increase in trunk growth, peaking in August.
The production of leaves in deciduous trees requires the reallocation of resources acquired and stored in the previous growing season. In this system leaf production is the greatest component of total NPP, contributing 38% (where leaf production is allocated to the year in which they appear, rather than the previous year), revealing the high level of internal C-cycling within these trees. Stokes et al. (2010) used carbon isotope (13C:12C) ratios in oak and sycamore leaves at Wytham to show that much of this internal cycling is likely to take place within individual branches, as isotopic signatures of sun and shade branches are maintained.
Here 36% of total NPP was allocated belowground, in the centre of the range of previous findings of 15.2–55.1% for a temperate Quercus and Fraxinus forest, 54% for a regenerated mixed deciduous temperate site and 27% in a Siberian Larix forest. This is a substantial component of NPP (particularly fine root production) that can be missed by traditional studies that have focussed on more easily measurable above-ground components.