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.