Soil moisture controls on patterns of grass green-up in Inner Mongolia: an index based approach

Abstract

Abstract. Water availability is one of the most important environmental controls on vegetation phenology, especially in semi-arid regions. It is often represented in terms of soil moisture in small-scale studies, whereas it tends to be represented by precipitation in large-scale (e.g., regional) studies. Clearly, soil moisture is the more appropriate indicator for root water uptake and vegetation growth/phenology. Its potential advantage and applicability needs to be demonstrated at regional scales. The paper presents a data-based regional study of the effectiveness of novel water and temperature-based indices to predict spring vegetation greenup dates based on the Normalized Difference Vegetation Index (NDVI) observations in the grasslands of Inner Mongolia, China. The macro-scale hydrological model, VIC (Variable Infiltration Capacity), is employed to generate a soil moisture database across the region. In addition to a standard index based on temperature, two potential hydrologybased indices for prediction of spring onset dates are defined, based on the simulated soil moisture data as well as on observed precipitation data. Results indicate that the correspondence between the NDVI-derived green-up onset date and the soil-moisture-derived potential onset date exhibits a significantly better correlation as a function of increasing aridity compared to that based on precipitation. In this way the soil-moisture-based index is demonstrated to be superior to the precipitation-based index in terms of capturing grassland spring phenology. The results also showed that both of the hydrological (water-based) indices were superior to the thermal (temperature-based) index in determining the patterns of grass green-up in the Inner Mongolia region, indicating water availability to be the dominant control on average. The understanding about the relative controls on grassland phenology and the effectiveness of alternative indices to capture these controls are important for future studies of vegetation phenology change under climate change.

Diverse Responses of Remotely Sensed Grassland Phenology to Interannual Climate Variability over Frozen Ground Regions in Mongolia

“…As shown in Figures 4 and 5, our study shows that spring phenology of grasslands is mainly controlled by precipitation in Mongolia, particularly by the precipitation of the previous autumn. Most existing vegetation phenological models based on dominant driving factors and their thresholds ignore the contribution of the precipitation of the previous autumn and the role of frozen ground in dry and cold regions [12,18,20]. The findings in this study may also help to target adaptation strategies for herders to schedule the movement of their livestock.…”

“…By means of long-term phenological data, these thresholds could be determined by averaging long term climatic data. For example, Shen et al [20] and Liu et al [18] determined the thresholds of cumulative temperature and precipitation using daily weather data before SOS over the span of 25 years in Inner Mongolia and on the Tibetan Plateau, respectively. Alternatively, empirical investigations of relationships between vegetation phenology and climatic factors could be undertaken by reviewing monthly or seasonal means of climatic factors.…”

“…Such an area is perfect for understanding the response of vegetation phenology to climate change under extremely dry and cold climatic conditions. Previous studies show that spring phenology of grasslands is mainly controlled by spring precipitation in arid regions on the Tibetan Plateau and in the north of China [9,15,16,18,20]. These study areas are mainly located in seasonal frozen ground regions.…”

“…Air temperature before SOS usually triggers vegetation to leaf out if there is no water deficit [2]. However, soil water content is a key limiting factor for this event in arid areas [12,16,18], which mainly originates from precipitation over dry grasslands. Then, precipitation may become a trigger for drier environments.…”

“…In humid regions, it is mainly controlled by the degree of winter chilling, photoperiod, and temperature [2,10,11]. In arid regions, it is mainly controlled by air temperature and precipitation (or soil moisture) [12][13][14][15][16][17][18].…”