Grass and cropland phenology

[luyssaert]

Grassland Phenology

(1) The senescence of grassland is now triggered by the ratio of maintenance respiration to GPP. The current threshold is 0.6, same as forest PFTs. This threshold is subject to further adjustment when refining the comparison with observations. Global simulation shows that this new approach to determine senescence has influences on both dates of start and end of growing season. The effect on the start date of growing season is likely through the influence on soil moisture. Simulation results using the new approach seems at least as good as the old climatic approach. For several regions, the new approach agrees better with a rough comparison against satellite observations and shows more promise for parameter tuning.

(2) The simulated grassland phenology shows a continuum from being deciduous to evergreen. In arid and semiarid regions such as Mediterranean region and central Asia, the grassland shows mainly deciduous behavior. In regions of western Europe, Southeast Asia, Japan, New Zealand and Southeast US, grassland shows largely evergreen behavior. This seems like a desirable model capability but it needs to be further tested.

(3) The moigdd phenology makes use of the shortest day to reset the gdd counter. This is causing a abrupt change by 180 days in the start of the growing season along the equator. This is not easy to fix. Along the equator two crop cycles could fit within a calendar year. This is one of the many phenology-related issues that justify having a closer look at how phenology should be treated in ORCHIDEE.

(4) Evegreen phenology shows clear seasonal pattern in LAI and GPP for temperate regions. However, the seasonal patters of LAI and GPP do not correspond well with each other and the two are sometimes even reverse. For some pixels (Japan), the trough of LAI corresponds to the peak of GPP. For a pixel in New Zealand, the trough of GPP corresponds to the midpoint when LAI decreases from seasonal peak. The timing of GPP seems reasonable, while LAI first decreases when the growing season starts and reach its maximum about 2-3 months before the end of growing season. This seasonal pattern of GPP is likely well driven by climate. The decrease in LAI at the beginning of growing is likely because leaf turnover is too fast. The reasons need to be investigated.