Mapping and evaluating plant phenology in the Qinghai-Tibet Plateau: A digital approach using the plant Phenological Index (PI)

Abstract

A new plant Phenological Index (PI) has been developed based on the visibility of plants, communities, and ecosystems. An evaluation and assessment of Zhang's Phenological Index (PI) were further examined based on the field observations of Alpine Shrub and Alpine Meadow vegetation at Haibei Alpine Ecosystem Research Station in the Northwest Qinghai-Tibetan Plateau.

Zhang's PI is a measure of the plant's phenological status on the time intervals, calculating the accumulation of PI area in two dimensions. Two phenological variables were described as the Phenological Average Ratio (PAR) and the Lasted Days (LD). The LD may overlap when a certain percentage of the plant population has a nutritional stage that lasts longer than one additional phenological stage or occurs between two phenological stages. Correspondingly, the standard deviations of the observed multiple plant species represent plant population variations. After reviewing the PI applications for phenology studies on Alpine Shrub and Alpine Meadow vegetation, we found that they provide a means of measuring and comparing plant phenology at various levels - population, community, and ecosystem.

During the start and end of the season in the Alpine Plateau region, plant phenological changes were significantly constrained by the environmental factors. However, during the summer season, Sunlight Hours (X4), Accumulated Ground Temperature (X3) and Accumulated Air Temperate (X1) above 0 °C had a more uniform impact on plant phenology across the region.

Zhang's PI can be used for climate change research by altering warming temperatures, water conditions, and nutrient levels. We also discussed the concern of applying Zhang's PI to global warming research. Moreover, the Seasonal Phenological Index (SPI) can be described on a regional scale and used with ASOS, AEOS, SPAR, and SLD characters to evaluate the changes in the timing of seasonal events in Eqs. (3)(4)(5).