4.1 Shifts in plant phenology drive species dominance
Warming was differentially advantageous to some species and increased
their relative dominance, while warming was disadvantageous to other
species and decreased their relative dominance. Warming-induced shifts
in species-specific full phenological periods and phenological lasts
were significant predictors of warming-induced changes in species
dominance. Specifically, species that increased their relative dominance
experienced a significantly longer full phenological periods or extended
phenological lasts. On the other hand, for species which experienced a
decrease in dominance, all underwent a directional shift in their
phenology, without significantly altering the duration of their growing
season length or flower duration. Our results agree with other studies
that shifts in plant phenology could cause either more differentiation
or similarity among species’ timing
(Nicotra et al. 2010;
Petitpierre et al. 2012), having
cascading impacts on species dominance
(Kraft et al. 2015;
Godoy et al. 2018).
We propose three non-exclusive hypotheses for how shifts in
species-specific plant phenology may impact species dominance. First, a
relatively longer growing season or flower duration could mitigate
potential phenological mismatches between plant growth and optimal
environmental factors (e.g. temperature and moisture)
(Augspurger 2013;
Wheeler et al. 2015). Plants with
the ability to adapt their phenology to capture favorable conditions
could thus have a competitive advantages over others
(Ernakovich et al. 2014). However,
if a longer growing season or flower duration results from a substantial
advance in phenological firsts, this could still leave them vulnerable
to adverse conditions at the shoulders of the growing season. For
example, warming-induced substantial advancement of leaf out and first
flower dates could increase the likelihood of exposure to spring frost
damage or herbivory by spring active insects
(Richardson et al. 2018), which
could decrease species’ dominance in the community.
Second, warming-induced lengthening of the growing season or flower
duration could help species avoid potential trophic mismatches
(CaraDonna et al. 2014;
Fridley et al. 2016;
Renner & Zohner 2018). For mutualistic
plant-animal relationships, for example, phenological mismatches between
flowers and pollinators could have crucial effects on plant community
composition through reduced plant fitness over time
(Elzinga et al. 2007;
Wheeler et al. 2015;
Schmidt et al. 2016). A similar
study conducted in three natural deciduous forests in northern Japan
showed that species with shortened flower duration could experience
pollination failure, leading to lower seed production and consequently
reduced dominance within the community
(Kudo & Ida 2013). Conversely, a
lengthened growing season or flower duration could help plants remain in
sync with their pollinators, despite an advance or delay in the
pollinators’ own phenology.
Third, longer growing season and delayed leaf senescence would allow for
longer periods of photosynthetic activity, nutrient acquisition, and
therefore more resources allocation to growth, fecundity, or survival
mechanisms, which could eventually increase the relative cover and
abundance of a plant species (Ernakovichet al. 2014; Fridley et al.2016). Similarly, species with extended last flower and flower duration
could support increased seed production and reproductive success, which
could also lead to increased abundance
(Craufurd & Wheeler 2009;
CaraDonna et al. 2014). Both
patterns suggest that ongoing climate warming will reshape community
structure towards dominance by species with lengthened phenophases.
Species with lengthened phenophases and increased dominance are
basically the dominant species at our study. This suggests that the
observed shifts in plant phenology with warming scenarios would likely
cause gradual biodiversity losses of non-dominant and rare species, and
thus could move the community towards a biotic homogenization
(McKinney & Lockwood 1999;
Dawson et al. 2011;
Savage & Vellend 2015). For example,
community evenness index was negatively related to dominance value of
species experienced longer growing season and flower duration (Fig. S7).
In addition, there are only eight common species investigated in this
study, which may potentially leave some uncertainties when evaluating
the community level phenology and species composition as a whole.
However, neither the rare species nor the community biodiversity losses
are the objectives of this study. Our goals are to stress the importance
of the phenological lasts and the full phenological periods in
understanding the warming impacts on plant phenology and species
dominance.