6. Process-Based Simulation and Prediction of Plant Phenology Spatiotemporal Variations

Using tree first leaf unfolding and grass green-up data, and daily air temperature and precipitation data, local and regional unified phenology models were fitted and validated in northern China and the Inner Mongolian Grassland, respectively. Based on the regional phenology models, spatiotemporal patterns of first leaf unfolding dates and green-up dates were reconstructed over the continuous geographic coverage. Within the 250 optimum local first leaf unfolding models for the four tree species at 136 stations, the unified forcing and chilling models account for 83 and 17 %, respectively. Thus, forcing temperature predominantly influence first leaf unfolding dates in most parts of northern China, while the affect of chilling temperature was stronger for earlier than later first leaf unfolding species. Spatial and temporal validation confirmed the capability and reliability of the 16 regional unified species-specific models in predicting leaf unfolding dates in the four climate regions. The reconstructed leaf unfolding dates show a significant advancement in most parts of northern China over 1960–2009, which is stronger for earlier than later first leaf unfolding species. For grass green-up modeling, previous temperature accumulation controls green-up dates of the three grass species at three stations, while both previous temperature and precipitation accumulations control green-up dates of these grass species at another three stations. The accumulated precipitation plays a more important role as the precondition of forcing temperature than as the supplementary condition of forcing temperature in triggering green-up of grasses. The accuracy of the regional unified models in simulating and predicting green-up dates of grasses at internal and external stations is at acceptable levels overall. The reconstructed mean green-up dates for the three grass species represented a similar spatial pattern across the Inner Mongolian Grassland, which is aligned approximately along the thermal and moisture gradient. Spatial patterns of green-up date linear trends indicate a significant advancement at 40.3–71.4 % of all grids over the Inner Mongolian Grassland during 1983–2009.