1. Introduction
Shifts in plant phenology under climate warming alter the competitive environment experienced by the individuals and species (Cleland et al. 2007), potentially affecting species dominance and reshaping plant community composition (Smith & Knapp 2003; Parmesan 2006; Forrest & Miller-Rushing 2010). Whereas rising temperatures may have substantial impacts on plant phenology and species dominance (Root et al.2003; Thuiller et al. 2005; Cleland et al. 2007), predicting how warming-induced shifts in plant species-specific phenology affect species dominance remains challenging (Rudolf 2019). Our current understanding of warming impacts on plant phenology and its linkages with species dominance mainly stems from the focus on the ‘phenological firsts’ (e.g., leaf out and first flower) (Dunneet al. 2003; Sherry et al.2007; Leblans et al. 2017). However, several recent studies have shown that ‘phenological lasts’ (e.g., leaf senescence and last flower) respond asymmetrically or even contrastingly to climate warming, as compared to the phenological firsts (CaraDonna et al. 2014; Gallinat et al. 2015; Prevéy et al. 2019). The impacts of shifts in ‘phenological lasts’ on species dominance, however, remained unresolved. Indeed, a better understanding of the underlying drivers for shifts in species-specific phenological firsts and lasts will help determine the effects of warming on the full phenological periods, as well as on the implications for variations in species dominance.
Plant phenology is highly sensitive to climate warming and finely tuned to the changing environment (Parmesan 2006; Cleland et al. 2007). However, the underlying driving factors and their importance for shifts in species-specific phenology to climate warming remain unclear (Tang et al. 2016; Chmura et al. 2019), hindering an improved understanding of the potential links between plant phenology and species dominance. Rising temperatures could advance leaf out date of some species due to faster accumulation of growing-degree days (Cayton et al. 2015; Suonan et al. 2017), or could delay leaf out date for other species due to delayed or even failed fulfillment of winter chilling requirements (Marchin et al. 2015; Guo et al. 2019).
Apart from the direct effects of rising temperatures, warming-induced changes in soil moisture and soil nutrient availability could also have significant indirect effects on plant phenology (Estiarte & Peñuelas 2015; Gill et al. 2015; Marchin et al. 2015). For example, warming-induced reductions in soil moisture could potentially cause delayed reproductive phenology (Sherryet al. 2007; Dorji et al.2013) or declines in flower duration (de Valpine & Harte 2001). Furthermore, phenological firsts and lasts are likely controlled by different environmental factors due to niche differentiation among various plant growth stages (Ernakovich et al. 2014; Bahuguna & Jagadish 2015; Gill et al. 2015), further heightening the challenges to predict the impacts of warming on species-specific phenology, species dominance, and ecosystem structure.
Assessments of shifting plant phenology and species-specific dominance have largely proceeded independently (Diezet al. 2012; Rudolf 2019). This is, in part, because larger-scale phenology assessments, primarily derived from satellite remote sensing, have limited power in representing species-specific phenological patterns (Zhang et al. 2003). However, species have consistently shown divergent movements in their phenological patterns to climate warming, rather than shifting unidirectionally (Sherry et al.2007). These highly differed species-specific phenological patters may have substantial but underexplored impacts on species dominance, invasion, and community composition (Fridley et al. 2016; Post et al. 2016; Zohner et al. 2018), as the timing of phenological events often determines the competitive conditions experienced during each developmental phase (Parmesan 2006; Forrest & Miller-Rushing 2010; Augspurger 2013). Therefore, it is critical to integrate species-specific phenological firsts and lasts to better understand the phenological responses to climate warming, and the consequences this may have for plant species dominance.
To close this knowledge gap, a three-year field-manipulative warming experiment using open top chambers (OTCs) was conducted in an alpine meadow grassland on the Tibetan Plateau to study the responses of various plant phenological events and the consequent impacts on species dominance. To assess and compare the species-specific responses of plant phenology and species dominance, eight common plant species were monitored across the three growing seasons, which were the only common species observed in all experimental plots at the study site. Furthermore, the Tibetan Plateau is warming at a faster rate than the global average due to its relative high altitude (Deutsch et al. 2008; You et al. 2016). Studies of plant phenology within temperature-limited regions, such as the Tibetan Plateau, are especially valuable, given that species within these regions are highly sensitive to climate change and may respond to climate warming in unexpected ways (Arftet al. 1999; Khorsand Rosaet al. 2016; Prevéy et al.2017). Two key questions motivated our work: (1) what are the species-level impacts of warming on plant phenology? and (2) do species-level impacts of warming on plant phenology scale up to affect species dominance, and if so, what are the underlying mechanisms for such changes?