Divergent influences of soil moisture changes on leaf senescence dates of herbaceous plants on the Qinghai–Tibetan Plateau

Understanding of the influences of soil moisture changes on plant phenological shifts on the Qinghai–Tibetan Plateau (QTP) is insufficient mainly because previous studies focused on the climatic factors. We explored the role of soil moisture in regulating plant autumn phenology on the QTP.

Based on long-term ground observations of soil moisture, plant phenology, and meteorology, temporal and spatial changes in soil moisture and leaf senescence dates (LSD) were analyzed using ordinary least squares regression and a meta-analysis procedure. Influences of soil moisture changes on the LSD shifts were assessed through correlation analysis and support vector machine, and also compared with those of air temperature and precipitation.

Nonsignificant interannual changes in soil moisture were observed, and LSD significantly delayed at a rate of 2.7 days/decade. Spatial changes of LSD were more correlated with site elevation and air temperature, and soil moisture and precipitation showed insignificant negative impacts. However, correlations between annual LSD and average soil moisture were mainly positive. Soil moisture and precipitation showed greater importance in regulating the LSD of sedges and grasses, whereas temperature exerted a larger influence on the LSD of forbs. Precipitation showed higher importance in regulating the interannual shifts in LSD, while temperature played a more important role in determining the spatial variations.

Soil moisture had divergent influences on the temporal and spatial shifts in LSD of different plant functional groups on the QTP. Overall, soil moisture was outweighed by temperature and precipitation in regulating autumn phenological shifts. However, soil moisture may become increasingly important in the future and forbs are expected to be more competitive if the QTP becomes warmer and drier, which will bring challenges in grassland management and utilization on the QTP.