StandLEAP model executable with CRU and TRaCE 21k and paleo example files

The vegetation model StandLEAP is based on the Physiological Principle Predicting Growth (3PG) model. StandLEAP relates absorbed photosynthetically active radiation to Gross Primary Production (GPP) using a radiation use efficiency (RUE) coefficient, adjusted by species-specific multipliers (<i>f</i>₁…<i>f</i>_<em>n</em>). Values below 1.0 indicate increasing environmental limitations (e.g., frost, VPD, ASW), whereas values above 1.0 (e.g., temperature) indicate conditions improving towards an optimum. In StandLEAP, the dynamic interaction between VPD, ASW, and transpiration plays a crucial role in water dynamics, influencing the overall moisture balance in the soil-tree-atmosphere continuum throughout the simulations. Rising VPD amplifies the atmospheric evaporative demand, which in turn extracts more moisture from trees and soil. This increased moisture loss from trees occurs through transpiration, where water is drawn up from the roots and released as vapor through the leaves. Simultaneously, soil moisture decreases due to the higher evaporative demand. ASW is hence closely linked to both VPD and transpiration rates. The model runs on a monthly time step, with canopy light absorption and photosynthesis parameters derived from metadata that were generated using a more detailed multilayer, hourly time-step model of canopy photosynthesis and transpiration. NPP is computed after partitioning respiration into maintenance (Rm) and growth (Rg: a fixed proportion of the difference between GPP and Rm) quantities and subtracting these from GPP. Rm is computed as a function of temperature and biomass.