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?