Assessing forest productivity in Australia and New Zealand using a physiologically-based model driven with averaged monthly weather data and satellite-derived estimates of canopy photosynthetic capacity

To evaluate the effects of spatial variation in climate and soils on forest productivity across broad regions requires an approach that can be widely applied and tested. Detailed physiological and micro-meteorological studies have recently lead to new insights that greatly simplify the prediction of above-ground net primary production (NPP). a variable closely related to conventional measures of forest growth, such as Mean Annual Increment (MAI) of stemwood. We applied these simplifications in a monthly time-step model driven by estimates of the fraction of light intercepted by green canopies, derived fr om near-infrared and red reflectances monitored from National Oceanographic and Atmospheric Administration (NOAA) weather satellites. and fi om equations utilising local temperature and rainfall records. Absorbed photosynthetically active radiation (APAR) was estimated fr om global solar radiation, derived from an established empirical relationship based on average maximum and minimum temperatures, and from a linear relation with the satellite-derived normalised difference vegetation index (NDVI) which represents the photosynthetic capacity of all vegetation within a cell for a given month and is often correlated with the fraction of PAR absorbed (fPAR). Monthly values of environmental constraints on productivity were expressed by modifiers calculated from the vapour pressure deficit (VPD) of the atmosphere, soil water deficit, or frost. This procedure leads to estimates of utilisable radiation (APARu). Gross primary production (GPP) was calculated by multiplying APARu by a constant canopy quantum efficiency (1.8 g C MJ(-1)) and total NPP has been shown, in a number of studies, to approximate 0.45 +/- 0.05 of GPP. The model partitions NPP into root and above-ground foliage and stem mass. The fraction of total NPP allocated to root growth increases from 0.2 to 0.6 as the ratio APARu:APAR decreases from 1.0 to 0.2. Above ground NPP (NPPA) predicted by the model was compared with estimated above-ground NPP derived at eight contrasting forested sites in Australia and New Zealand. There was a linear relation between predicted NPP, and measured wood production (r(2) = 0.82). The analysis also provided an assessment of the relative importance of various climatic variables upon production which varied extensively from site to site. (C) 1998 Elsevier Science B.V.