4 Discussion
A growing body of studies have highlighted phenological shifts in plants
in response to global change factors (Cleland et al., 2006; Zhou et al.,
2023; Yang et al., 2023). Nevertheless, how species-specific responses
impact plant community dynamics remains poorly understood (CaraDonna &
Inouye, 2015). Our study explored how long-term warming and nitrogen
addition affected plant flowering phenology and community dynamics in a
temperate desert steppe. Our results documented the previously
unrecognized responses of plant flowering phenology and species
dominance to warming over ten consecutive years. We find that warming
advanced plant flowering time by 4.3 days for C3 plants
and 2.8 days for C4 plants, respectively. Warming
resulted in reductions in the duration of flowering and consequent
declines in the dominance of C3 plants. In contrast,
warming did not change the duration of flowering of C4plants, but enhanced their dominance. The increased dominance of
C4 plants was further favored by N deposition which
extended the duration of flowering of C4 but not
C3 plants. To our knowledge, our study is the first to
report temporal variation in C3 and C4plants flowering phenology in a desert steppe. Importantly, these
warming-induced changes in flowering phenology led to changes in
community composition via plant dominance, providing empirical evidence
that environmental change can alter community dynamics via modifying
plant phenology.
An important question is what caused the variation in the warming effect
on flowering phenology of C3 and C4plants across ten years of the experiment. Our results revealed that
warming significantly advanced the flowering time of five species and
shortened the duration of flowering of most plants. This may be
attributed to accumulated temperature for plant growth and development
(Zhang et al., 2007; Forrest, 2015), whereby an increase in temperature
could induce a shift from vegetative to reproductive growth of plants
(Zhang et al., 2003; Liu et al., 2021). Moreover, the results of SEM
analysis indicated that soil temperature warming affects flowering
phenology. Specifically, warming increased the 0-30cm soil temperature
and shortened the duration of flowering of C3 plants.
This agrees with a previous study that reported that soil surface
warming shortened the flowering phenology of the dominant
C3 grass, S. breviflora (Bai et al., 2022).
Several mechanisms can potentially explain the observed changes in
flowering phenology with warming. First, warming can alter plant
physiology through the mediation of plant enzyme and hormone activity,
which in turn influences plant phenology (Weih & Karlsson, 2001;
Crimmins et al., 2010; Fanin et al., 2022). For instance, increased
temperature has been shown to prolong the period of vegetative phenology
by reducing production of ethylene and enhancing leaf lifespan (Moore et
al., 2021; Ren et al., 2021), but shortened the period of reproductive
phenology (Fracheboud et al., 2009; Zettlemoyer et al., 2019). Second,
warming can alter flowering phenology by affecting leaf water loss (Weih
et al., 2001). Increased temperature can accelerate vegetative growth by
affecting the rate of leaf water loss and reduced the period of
flowering phenology (Pallas et al., 1967; Sadok et al., 2021). Third,
warming can influence pollinators that consequently result in
phenological changes (Shivanna & Tandon, 2014). Insect-pollinated
species that flower early have been found to be more sensitive to
warming than those that flower later (Fitter & Fitter, 2002). Finally,
warming can affect flowering phenology by regulating plant traits
associated with resource acquisition. For example, warming-induced soil
surface water loss has been shown to limit the growth and development of
shallow rooted plants and shorten their duration of flowering (Wang et
al., 2021).
Our results show that N addition prolonged the duration of flowering of
C4 plants, but did not affect that of C3plants. SEM showed that the N effect on flowering phenology was mediated
by soil water availability. High N supply has been shown to affect soil
porosity and soil structure by increasing litter content, promote the
retention and migration of soil water and nutrients, and thereby enhance
soil water availability (Prevéy & Seastedt, 2014; Richardson et al.,
2018). In parallel, N addition also resulted in increased N uptake by
plant roots. This suggests that, from a resource allocation perspective,
more resources will be allocated to reproductive growth, leading to an
extension of the time needed for the development of reproductive organs
and consequent increases in duration of flowering (Tilman & Wedin,
1991; Xia & Wan, 2013).
We further find that duration of flowering had a consistently positive
effect on species dominance under warming and N deposition. Several
potential factors were likely to explain the relationship between plant
phenology and community dynamics, such as height, density and coverage
of plants. First, warming significantly reduced height of
C3 plants (Figure S6), which probably led to decreases
in light availability (Sun & Frelich, 2011; Liu et al., 2022), and thus
shortened duration of flowering and decreased dominance of
C3 plants. In contrast to the response of
C3 plants to warming, a longer duration of flowering of
C4 plants could allow more time for photosynthetic
activity and nutrient acquisition (CaraDonna & Inouye, 2015; Chen et
al., 2020), and consequently enhance the dominance of C4plants in a community. Second, N addition increased duration of
flowering, coverage and density of C4 plants(Figure S6),
which could contribute to the increase of dominance of
C4 plants in a community (Myneni et al.,1997; Jiang et
al., 2016). An extended duration of flowering can enhance species’
competitive ability, seed yield and reproductive success (Craufurd &
Wheeler, 2009; Ernakovich et al., 2014; Godoy & Levine, 2014), and thus
increase species dominance in a community.
Current attempts to link plant phenological responses to global change
have primarily focused on shifts in ecosystem functioning (Wolf et al.,
2017; Wang et al., 2020; Liu et al., 2022), but often neglect possible
effects on plant community dynamics. Our results emphasize the
importance of flowering phenology in mediating community dynamics under
climate change. We found that warming-induced changes in the duration of
flowering have a large effect on plant dominance, with duration of
flowering extensions of C4 plants increasing their
dominance, while decreases in duration of flowerings contribute to
declines in the dominance of C3 plants. These findings
suggest that plant flowering phenology plays an important role in
modulating plant dominance and community dynamics in response to global
change. The present study provides empirical evidence that variation in
phenological responses among plants influences species dominance,
ultimately driving shifts from C3 to C4plants under increased temperature and N deposition. These changes in
plant phenology and dominance are likely regulated by soil temperature
and water availability. Our results highlight the importance of
including plant phenology to better understand the mechanisms that drive
changes in community structure and ecosystem function under climate
change.