Introduction
Plant biomass, the amount of carbon captured by photosynthesis (Potteret al. 1999), is an important ecosystem property and an indicator of climatic changes (Wang et al. 2007; Swemmer et al.2007; Ma et al. 2010). In the past decades, significant climate changes (Ma et al. 2010; IPCC 2013; Hansen et al. 2006) have altered plant biomass, especially in the arctic and alpine regions (Walker et al. 1995; Jonas et al. 2018; Kardol et al. 2010; Zhang et al. 2018; Wang et al. 2020). While some studies suggest that plant phenology is a bridge between climate change and plant biomass (Zhang et al. 2018; Walther et al. 2002; Albert et al. 2019), others show that alpine plants will accumulate biomass by accelerating growth rates (Rammig et al. 2019; Wingler et al. 2016) to compensate for limited growing seasons and low temperatures (Suonan et al. 2019).
In the last decades, there has been an increased interest in the impacts of climate change drivers on plant phenology; however, the relationship between climatic changes and plant phenology is still largely unclear (Zhang et al. 2018; Suonan et al. 2019; Shen et al.2015; Reed et al. 2019). For example, in-situ observations and satellite observations show that temperature is the main determinant of phenology, and increased warming in spring can result in advanced spring phenology (Zhang et al. 2018; Wang et al. 2020; Shen et al. 2015; Piao et al. 2011; Fu et al.2014). Conversely, some studies have shown that climate change-related warming reduced temperature sensitivity (Shen et al. 2015; Yuet al. 2010; Fu et al. 2015; Piao et al. 2019) of plants. In contrast to overall warming, daytime warming and night warming are asynchronous processes (Yang et al. 2016; Zhouet al. 2019) and may have different effects on plant phenology (Shen et al. 2015; Piao et al. 2015). We speculate that the effects of air temperature, rather than soil temperature, on plant phenology are more important (Cleland et al. 2012) especially in spring. According to previous studies, precipitation or soil moisture can affect the plant phenology independently or jointly with increasing temperatures (Zhang et al. 2018; Duparc et al. 2013). In addition to temperature and precipitation, relative air humidity (Sparkset al. 2002) and photoperiod (Zhang et al. 2018; Shenet al. 2015; Ernakovich et al. 2014; Fu et al.2019) may also affect plant phenology. However, the impacts of multiple climate factors on plant phenology in alpine regions have rarely been evaluated.
The dynamics of spring phenology of alpine plants against the background of a changing climate have been widely examined (Zhang et al.2018; Shen et al. 2015; Piao et al. 2015; Dong et al. 2013), whereas other while the other phenological processes (i.e., flowering, fruiting, and withering) remain poorly studied (Fridley 2012; Gallinat et al. 2015; Xie et al. 2015). However, plant reproductive phenology of flowering and fruiting of plants (Sherryet al. 2007; Munson et al. 2017) is as important as plant vegetative phenology (Xie et al. 2015), and the trade-off between vegetative and reproductive phenology under climate change (Wanget al. 2014) can affect plant biomass (Millerrushing et al. 2008). Most studies have only focused on the phenology of plant leaves, flowers, and fruits (e.g., green-up, flowering, and fruiting period), whereas little is known about the growth patterns in grasslands (Sun & Frelich 2011).
We assume that there is a trade-off between the length of the fast-growing phase and the intrinsic growth (Brienen et al.2020), especially in areas with a limited growing seasons and low temperatures (Wingler & Hennessy 2019; Suonan et al. 2019; Brienen et al. 2020). The trade-offs in plant phenology or growth patterns with climatic changes can be attributed to the responses of functional groups (Wang et al. 2020; Vitasse et al. 2009; Suonan et al. 2017).
On the Qinghai Tibetan Plateau, alpine grassland is the largest ecosystem, providing key ecological services for a variety of livestock for humans (Li et al. 2019; Dong et al. 2020). As one of the regions which is most sensitive to climatic changes (5 ), the QTP has experienced overall warming and increased precipitation from 1961 to 2010 (Chen et al. 2013). Using data from 21 years (1997 to 2017), with 2-day intervals for phenology dynamics and 10-day intervals for growth patterns, using annual biomass data of four key species in an alpine meadow on the QTP, we conducted this study to address the following questions: (1) How do the phenology and growth patterns of different functional groups of plant species in alpine grassland on the QTP change over time? (2) Can climate change affect the biomass of different functional groups of alpine plant species through altered growth patterns or phenology dynamics? (3) Which environmental factors most significantly affect the key growth and phenological events of different functional groups of alpine plant species?