DISCUSSION
Our overall results demonstrate rapid evolution in three out of ten
traits under in situ climate manipulations in natural plant
communities after merely 10 years, i.e. at most 10 generations of our
annual study species. This is a remarkably short time span, given that
numerous interacting factors may hamper evolution in natural communities
(Hoffmann & Sgró 2011; Shaw & Etterson 2012). The fact that this
evolutionary response was consistent in two independent sites renders
chance effects, e.g. genetic drift, unlikely to have affected these
results and underpins that the evolutionary response was directly driven
by manipulated rainfall. Intriguingly, our multiple independent lines of
evidence corroborate that these changes were adaptive.
After 10 years of artificial drought, phenology had evolved in
chronological time (days to flowering) and ontogenetic time (leaf number
at flowering). Theory suggests accelerated life-cycles as a drought
avoidance strategy that reduces the risk of mortality before
reproduction (Cohen 1976; Kigel et al. 2011). Yet, early
reproduction comes at the cost of smaller plant size and hence possibly
lower competitive ability (Liancourt & Tielbörger 2009; Kigel et
al. 2011). In line with theory, plants from dry-manipulated plots
flowered earlier and with fewer leaves than plants from control and wet
plots. Moreover, this rapid evolutionary response paralleled the
long-term evolutionary response of B. didyma along the natural
rainfall gradient where plants from more arid sites flowered earlier; a
trend found in many other annuals along natural rainfall gradients
(Stinchcombe et al. 2004; Kigel et al . 2011; Wolfe &
Tonsor 2014; Kurze et al . 2017). Interestingly, the observed 3-4
days acceleration in phenology corresponds to an ecological distance ofc. 100 mm lower rainfall at origin for annuals along our study
gradient (Kigel et al . 2011; Kurze et al. 2017). Given the
magnitude of rainfall reduction in dry plots (-90 mm in SA, -160 mm in
M), this suggests that a substantial part of the ‘required’ acceleration
in phenology could be realized within ten years. The adaptivity of
accelerated phenology under drought was furthermore corroborated by our
selection analyses under controlled watering conditions in the
greenhouse, which eliminated potentially confounding factors along
natural environmental gradients (Mitchell-Olds & Schmitt 2006, De
Frenne et al. 2013). Here, earlier flowering with fewer leaves was
stronger favored under low than under high water availability. These
multiple lines of evidence – theory, natural rainfall gradient,
selection analyses, and consistency in both sites – provide compelling
evidence that the observed rapid evolution in phenology was adaptive.
Rapid evolution of earlier flowering under drought was found also in
other climate change studies; it is thus far the trait most often
reported to evolve under drought in natural conditions (Franks et
al. 2007; Vigouroux et al . 2011; Nevo et al. 2012; Nguyenet al. 2016). Accelerated phenology therefore emerges as a key
pathway for rapid evolutionary adaptation to drier climates. While this
underpins the central role of phenology for drought adaptation in
annuals (Cohen 1976; Kigel et al. 2011; Kurze et al.2017), it may also signpost that phenology evolves easier than other,
possibly more complex traits. However, this conclusion is still hampered
by the few tests beyond our study reporting multiple traits besides
phenology (Ravenscroft et al. 2014; Nguyen et al. 2016).
Here, we also observed rapid evolution in reproductive allocation. As
competition is reduced in drier sites along our gradient (Schiffers &
Tielbörger 2006), theory suggests reduced investment in vegetative
tissue for outgrowing neighbors and increased allocation to reproduction
(Aronson et al. 1990; 1993). In line with theory and in both
sites, plants from dry manipulated plots produced 10-15% more seeds per
vegetative biomass than control plants. Although reproductive allocation
was rarely assessed in climate manipulation studies, a similar
evolutionary tendency was reported for a perennial herb (Ravenscroftet al . 2014). This evolutionary response was again congruent with
our selection analyses in the greenhouse, and it matched the clinal
trend in reproductive allocation along our natural rainfall gradient,
and parallel clines in other species (summarized in Kurze et al.2017). Thus, in all traits showing rapid evolution in the field, our
independent lines of evidence demonstrate that these changes were
adaptive. Intriguingly, parallel studies had reported remarkable
resistance in many plant community parameters to imposed climate
manipulations (Tielbörger et al . 2014; Bilton et al .
2016). The present findings highlight that rapid adaptive evolution
played an important role for climate change responses in annual species,
and probably contributed significantly to community resistance.
Notably, all evolutionary changes occurred solely in the dry manipulated
plots, i.e. the treatment which increased, rather than decreased stress
for resident plants. This appears intuitive because drought may directly
lead to rapid exclusion of drought-sensitive and late-flowering
genotypes, especially in dry study years. In wet plots, selection among
genotypes was probably driven by competition for additional resources
(Schiffers & Tielbörger 2006), resulting in weaker, more gradual
fitness differences, as was shown for B. didyma in a
cross-transplant with and without competition (Ariza & Tielbörger
2011), and hence slower progression of exclusion and adaptive evolution.
However, seven further candidate traits did not evolve. This is
surprising because five of them exhibited clinal shifts along the
natural rainfall gradient, suggesting that they contribute to B.
didyma’ s long-term evolutionary response to drier climates: germination
fraction, stomata density, height, vegetative biomass and seed number.
In conjunction with existing theory we had expected corresponding
evolution of these traits under climate manipulations (Westoby 1998; Liuet al. 2012; Tielbörger et al. 2012; ten Brink et
al. 2020). Selection analyses supported this expectation for vegetative
biomass, although not for stomata density and height, and they were not
possible for germination fraction (no differential watering) and seed
number (response variable in selection analyses). One possible
explanation for the lack of evolution in other candidate traits is that
sufficient adaptation was ensured by those traits that did evolve, and
hence evolution of further traits was unnecessary. Alternatively, the
multiple potential constraints for evolution under natural conditions
hindered adaptation in other traits (Hoffmann & Sgró 2011; Shaw &
Etterson 2012). In that case, the observed rapid evolution in a subset
of traits may indicate incomplete adaptation to new conditions,
cautioning that climate change may imperil species despite rapid
evolution. Unfortunately, almost all available evidence for rapid
evolution under natural conditions was restricted to very few traits
(e.g. Franks et al. 2018; Nguyen et al . 2016; Grossmanet al. 2014; but see Frachon et al. 2017), i.e. there is
lacking information and focus on the importance of non-evolving traits
for adaptation to climate change. Our results thus highlight that
studies relying on few traits might be misleading.
High trait plasticity was intensely debated as a mechanism retarding
adaptive evolution to climate change, yet rarely tested in nature
(Merilä & Hendry 2014; Kelly 2019; Fox et al. 2019). Here, each
of the three rapidly evolving traits had a completely different degree
of plasticity. This first assessment under field conditions suggests
that traits may evolve rapidly irrespective of their degree of
plasticity, and that rapid evolution appears rather restricted by other
mechanisms such as the genetic architecture of traits, potential
underlying trade-offs, or limited genetic variation within populations
(Barrett & Schluter 2007; Hoffmann & Sgró 2011; Shaw & Etterson
2012). Our findings furthermore give little support for the idea that
climate change leads to increased plasticity as a means to rapidly
adjust the phenotype to novel conditions (Crispo 2007; Lande 2009;
Merilä & Hendry 2014; Kelly 2019, Fox et al. 2019), because only a
single trait (diaspore weight) showed increased plasticity under
drought. Evolution of increased plasticity is therefore unlikely to be a
pathway for climate change adaptation in our system, corroborating the
few existing climate change studies which found plasticity either
unchanged (Franks 2011) or lower (Grossman & Rice 2014).
Overall, our study demonstrates
that rapid evolution plays an important role for climate change
adaptation in natural annual plant communities. The novel setup of our
study – combining for the first time in situ climate
manipulations with a natural climatic gradient and selection analyses
under controlled conditions – provided independent, compelling lines of
evidence that observed evolutionary shifts were adaptive. However, with
rapid evolution in merely a subset of candidate traits, our study
emphasizes the importance of multi-trait studies for assessing whether
rapid in situ evolution may safeguard species under climate
change.