GOTILWA+ model simulates carbon and water uptake and fluxes through forests in different environments (from North boreal Europe to Mediterranean), for different single tree species stands (coniferous or broad-leaved, evergreen or deciduous) and in changing environmental conditions, either due to climate or to management regimes.
The input data includes: climate (max. and min. temperatures, rainfall, VPD, wind speed, global radiation); stand characteristics (tree structure; DBH class distribution); tree physiology (photosynthetic and stomatal conductance parameters), site conditions including soil characteristics and hydrological parameters and also forest management criteria. Results of GOTILWA+ are computed for each DBH class and they are integrated at the stand level. The processes are described with different sub-models that interact and integrate the results of simulated growth and evolution of the whole tree stand through time (hourly calculations integrated at a daily time step). Horizontal space is assumed homogeneous and vertical profile distinguishes two canopy layers (sun and shade conditions).
Light extinction coefficient is estimated by Campbell's approach (1986), based on an ellipsoidal leaf angle distribution. The photosynthesis equations are based on Farquhar and co-workers approach (Farquhar and Von Caemmerer 1982). Stomatal conductance uses Leuning's approach that modifies Ball, Woodrow and Berry model (Leuning 1995). Leaf temperature is determined based on leaf energy balance (Gates 1962, 1980) and transpiration is estimated according to the Penman-Monteith equation (Monteith 1965, Jarvis and Mcnaughton 1986). Autotrophic respiration is separated in maintenance and growth respiration. Maintenance respiration is calculated as a proportion of total respiring biomass (structural and non-structural components distinguished), with rates that depend on temperature according to a Q10 approach. Growth respiration is a fraction of available carbohydrates for growth consumed when transformed into new tissues. A constant efficiency of 0.68 is assumed (g of new tissue / g of carbohydrate). NPP is allocated first to form new leaves and fine roots to compensate their turnover. The remaining is allocated to the pool of mobile carbon in leaves and woody tissues. The surplus is invested in new tissues (leaves, fine roots and sapwood) according to the pipe model (Shinozaki et al. 1964). Soil is divided in two layers, organic and inorganic horizons. Soil organic matter (OM) is originated by plant litter: leaves, branches, stems and reproductive organs aboveground and coarse and fine roots belowground. OM is decomposed depending on soil temperature (according to a Q10 approach) and soil moisture (optimal at 60% of the maximum soil water-filled porosity). Soil moisture is calculated based on water inputs and outputs and soil traits. Temperature also affects leaf shedding through a Q10 approach. Root mortality that is also dependent on temperature (Q10 approach), soil moisture and the length of the growing period.