PRELUED (Mäkelä et al. 2008) is an empirical model for daily gross primary production (GPP) in boreal and temperate forest canopies, constructed with eddy covariance (EC) data. We developed it as a practical alternative to physiologically-based photosynthesis models (such as SPP) that require too detailed input information to be applicable on a large scale.
The model is based on the widely used light use efficiency (LUE) approach. In theoretical optimal conditions, where no environmental factors restrict photosynthesis, the daily GPP of a canopy is assumed to depend linearly on the daily photosynthetically active radiation (PAR) absorbed by the canopy (APAR). The proportionality constant is referred to as the potential LUE of the canopy. For real suboptimal conditions, this potential LUE need be modified by the effects of restricting environmental factors. This is done with multiplicative modifier functions assuming values in [0, 1]. For the time being, the model contains the modifiers describing (i) photosynthesis saturation at high daily PAR, (ii) effect of mean daily temperature on photosynthesis through annual cycle, and (iii) photosynthesis decline at high mean daily water vapour pressure deficit (VPD) or low mean daily soil water content (SWC). The actual daily LUE of the canopy is the product of the potential LUE and the daily values of the modifier functions, and multiplying this with the daily APAR finally results in the daily GPP of the canopy.
Originally, the model was fitted in EC data from five European conifer sites and tested in two independent American conifer sites (Mäkelä et al. 2008). Since, it has been re-estimated in the data from 14 sites around the world (Peltoniemi et al. 2011).
In theory, potential LUE is supposed to be constant across sites and species (as the biochemical mechanism of photosynthetic production is universal and largely independent of species), and the day-to-day and site-to-site variation in GPP is thought to be explained by the variation in environmental factors (PAR, temperature, VPD, SWC), on one hand, and by the variation in leaf area and canopy structure (through APAR), on the other. We have found, however, that while the same response to the environmental factors can be assumed across different sites and species, the level of GPP still remains site-specific. The variation in potential LUE appears to be associated with foliar nitrogen content, but we could not significantly improve the model performance by incorporating a modifier to account for this (Peltoniemi et al. 2011).
PRELUED has been applied as one component of a summary model system for predicting yearly GPP, NPP and stem volume growth in a large number of Finnish National Forest Inventory plots (Härkönen et al. 2010), with results of similar accuracy as those given by a well-established empirical tree-level growth model.
Härkönen, S., Pulkkinen, M., Duursma,R. & Mäkelä, A. 2010. Estimating annual GPP, NPP and stem growth in Finland using summary models. Forest Ecology and Management 259:524–533.
Mäkelä, A., Pulkkinen, M., Kolari, P., Lagergren, F., Berbigier, P., Lindroth, A., Loustau, D., Nikinmaa, E., Vesala, T. & Hari, P.2008. Developing an empirical model of stand GPP with the LUE approach: analysis of eddy covariance data at five contrasting conifer sites in Europe. Global Change Biology 14(1):92–108.
Peltoniemi, M., Pulkkinen, M., Kolari, P., Duursma, R., Montagnani, L., Wharton, S., Lagergren, F., Takagi, K., Verbeeck, H., Christensen, T., Vesala, T., Falk, M., Loustau, D. & Mäkelä, A.2011. Does canopy mean N concentration explain differences in light use efficiencies of canopies in 14 contrasting forest sites? Submitted to Global Change Biology.