DISCUSSION
This study examined the effect of soil warming on a whole-of-life array
of traits (vegetative growth, reproductive output and phenological
response traits including senescence), as well as the germination traits
of the next generation, for populations of O. eriopoda with four
different germination strategies. Our findings suggested that responses
to soil warming and germination strategy are complex, indeed not as
straightforward as we hypothised. In many instances, the responses were
trait-specific. Since we followed the whole life cycle, we were able to
observe that the effect of soil warming on traits changed through
ontogeny and was mostly apparent in the respective active growth stages
of the plant life cycle, demonstrating that care must be taken in
extrapolating from responses of a given trait at any point in time to a
whole-of-life conclusion. We also confirmed that the pattern and
direction of warming effects varied depending on the germination
strategy of the populations of O. eriopoda in question. We
consider the diversity of these responses to warming throughout the life
cycle, and among germination strategies within a species, to be
extraordinary as they highlight the complexity of linkages between the
maternal and offspring environment and make evident that without a
whole-of-life perspective we will struggle to predict impacts of global
change on species. Here we interpret these elements in the context of
impacts of warming on persistence of O. eriopoda and other alpine
species in a novel future climate.
Under rapid climate change, plasticity in phenology and reproductive
traits is likely to have strong fitness consequences (Kozłowski, 1992;
Stinson, 2004), and it is possible that different germination strategies
will have different selective advantages (Hoyle, Cordiner, Good, &
Nicotra, 2014; Willis et al., 2014). We found that variation in
reproductive phenology (Fig. 4) and seed mass (Fig. 3) were inherent to
the germination strategy, whereas adult vegetative traits and
reproductive outputs were more strongly affected by warming than
germination strategy. The ability of this species to adjust reproductive
phenology by shortening seed maturation while maintaining seed quality
(seed mass) may be advantageous for persistence under in changing
climate (Bonser, 2013; Visser & Both, 2005; Willis, Ruhfel, Primack,
Miller-Rushing, & Davis, 2008); even if the total seed number is
reduced the seed are spread through the season. Although warming
substantially reduced lifespan, it was apparent that O. eriopodaindividuals could complete their life cycle and produce healthy,
full-size seed with adequate reserves for early establishment success.
Nearly all of the seeds produced were viable regardless of germination
temperature, maternal conditions, or germination strategy indicating
potential to maintain population regeneration.
The interactions of soil warming and germination strategy were mainly
evident for seedlings or early vegetative traits and not vegetative
traits during the transition to reproductive stage; this corroborates
the finding of a previous study by Hoyle et al. (2015) that germination
strategy of Australian alpine plants does not correlate with adult
vegetative traits. In particular, populations of the immediate
germination strategy (mainly occupy lower <1520 m elevation
sites) exhibited greater plasticity in early leaf increment in response
to warming compared to the staggered, postponed, and postponed deep
strategies (elevations ranging from 1600 m to 2200 m, Supplement Table
1). Previous research on Wahlenbergia ceracea , an alpine herb
that shares the same habitat with O. eriopoda , also found
individuals from higher elevations were less plastic and less likely to
express adaptive plasticity in growth-response to warming (Nicotra et
al., 2015). The positive response of O. eriopoda vegetative
traits to warming that was mainly pronounced during earlier ontogeny,
however, did not lead to greater growth accumulation and earlier
reproductive timing (flowering). The results indicate that there might
be an internal constraint for vegetative growth and maintenance (Starr,
Oberbauer, & Pop, 2000) and thus individuals with an immediate
germination strategy grew and reached the reproductive stage more
quickly but also died earlier than individuals displaying the other
strategies.
By germinating the seeds produced in the warming experiment we confirmed
that the source of variation in timing of germination in the staggered
populations lies within individual plants, indicating a potential
bet-hedging strategy (Starrfelt & Kokko, 2012), which has not been
verified in this species before (Hoyle et al., 2015; Satyanti et al.,
2019). Interestingly, individual plants produced both non-dormant and
dormant seed of varying proportions (Figure S3). This leads to
asynchronous germination within the population, and can reduce the risk
against recruitment failure (Brown & Venable, 1986; Simons Andrew,
2009; Stevens, Seal, Archibald, & Bond, 2014; Venable & Lawlor, 1980).
Should variation in snowmelt patterns compromise recruitment in either
season, populations with a staggered strategy could be advantaged and
this raises a question about how germination strategy is established and
controlled in the species; its high variability indicates it is highly
labile on some timeframe.
We examined the transgenerational effect of different seed development
(maternal) conditions on germination traits since this has been proposed
as a mechanism that may help species to tolerate future climates (Herman
& Sultan, 2011). Given how variable germination strategy is we
hypothesized that it may be highly plastic and reflect developmental
conditions. But, contrary to expectation, we found no evidence of
phenotypic plasticity in seed dormancy, i.e. germination strategy acrossO. eriopoda populations was constant regardless of seed
development temperature. The warming impact was imposed when the plants
were 18-20 weeks old, not the earliest seedling stage, but given that
warming was imposed before the transition to reproductive meristems,
this delay seems unlikely to have impeded the response. Our results
suggest that seed dormancy variation is not dependent on seed maturation
environment. Seed development temperatures have been shown to control
seed dormancy induction and cycling in other species (Bernareggi et al.,
2016; Donohue et al., 2005; Footitt & Finch-Savage, 2017; Steadman,
Ellery, Chapman, Moore, & Turner, 2004) but we do not know the relative
contributions of air or soil temperature. Further, the degree of seed
dormancy, the important determinant of germination strategy, and
dormancy cycling may indeed depend on the variation not just the mean of
temperature (Satyanti et al., 2019; Topham et al., 2017).
Alternatively, it may be the case that germination strategy is plastic
but has a ‘half-life’. It is possible that more than one cycle of
warming is required to change the dormancy degree or fraction of
non-dormant seed of the respective population. Previous studies suggest
that the warming effects on seed dormancy and germination traits may be
gradual rather than instant, just like phenological trait responses to
warming (Franks, Sim, & Weis, 2007; Hoffmann et al., 2010). Such a
delay effect may indicate that the mechanism underlying this shift is
epigenetic. Dormancy patterns have been shown to be under epigenetic
control in other systems (Nonogaki, 2017; Richards et al., 2017), but
further research would be needed to demonstrate that here.
Diversification of germination strategies across population,s as
exhibited by O. eriopoda , could still be an advantage that
assists species’ persistence (Cochrane, Yates, Hoyle, & Nicotra, 2015).
The potential for plants to respond to warming not only plastically but
also as a function of genetic (or epigenetic) variation within the
species demonstrates that species’ response to warming will often be
manifested as a combination of rapid plastic responses and long-term
evolutionary responses (Valdés et al., 2019). Considering the spatial
and temporal heterogeneity of the alpine environment it is intuitive to
expect variations in seed trait, particularly in dormancy, and hence we
suggest that variation in germination phenology may be quite common
within alpine species (Venable & Brown, 1988). However, very few
studies have documented intraspecific variations in seed dormancy and
germination strategy at the level exhibited by O. eriopoda (from
populations with non-dormant, dormant, to deeply dormant seeds) and
hence, we could not really conclude whether other alpine species would
be as resilient under future warming.
An important implication of the results of our study is that predicting
species’ responses and fate under global warming as either positive,
negative or neutral could be a gross oversimplification when such
assessments consider only one or few populations or are based only on a
limited number of life-stages or traits (Saatkamp et al., 2018). The
effects of warming vary not only among populations and individuals but
as a function of ontogeny and hence, when assessing response to climate
change at both species and community levels, within-species variations
in germination strategy should be considered as important as
between-species variation, and impacts must be assessed on a
whole-of-life scale, not just at a single life stage. Our results imply
that variability in seed dormancy and germination strategy could be key
to assisting a species to persist under unpredictable conditions. Thus,
further studies on how common these extraordinary intraspecific
variations in seed and germination traits are across species, and the
source of variation among individuals, are fundamental to predict the
risk of species’ survival in a warmer world.