The phenology module of tree and grass PFTs is based on the growing season index (GSI) approach (Jolly et al., 2005). Thereby the continuous development of canopy greenness is modelled based on empirical relations to temperature, day length and drought conditions. The GSI approach was modified for its use in LPJmL (Forkel et al., 2014) so that it accounts for the limiting effects of cold temperature, light, water availability and heat stress on the daily phenology status phen_PFT:

Each limiting function can range between 0 (full limitation of leaf development) and 1 (no limitation of leaf development). The limiting functions are defined as logistic functions and depend also on the previous day's value:

where x is daily air temperature for the cold and heat stress-limiting functions f_cold and f_heat, respectively, and stands for, short-wave downward radiation in the light-limiting function f_light, and water availability for the water-limiting function f_water. The parameters b_x and sl_x are the inflection point and slope of the respective logistic function; tau_x is a change rate parameter that introduces a time-lagged response of the canopy development to the daily meteorological conditions. The empirical parameters were estimated by optimizing LPJmL simulations of FAPAR against 30 years of satellite-derived time series of FAPAR (Forkel et al., 2014).

• Forkel, M., Carvalhais, N., Schaphoff, S., v. Bloh, W., Migliavacca, M., Thurner, M., and Thonicke, K.: Identifying environmental controls on vegetation greenness phenology through model–data integration, Biogeosciences, 11, 7025-7050, https://doi.org/10.5194/bg-11-7025-2014, 2014
• Jolly, W. M., Nemani, R., and Running, S. W.: A generalized, bio-climatic index to predict foliar phenology in response to climate, Glob. Change Biol., 11, 619–632, https://doi.org/10.1111/j.1365-2486.2005.00930.x, 2005