Introduction
In seasonal environments, timing of events such as emergence, growth,
reproduction and seasonal senescence is often temporally and
developmentally correlated (Rathcke and Lacey 1985, Kelly 1992, O’Neil
1997, Aizen 2003, Sola and Ehrlén 2007, Keenan and Richardson 2015).
Such correlations imply that selection might only rarely be exerted
independently on the timing of a single event. Instead, selection on a
focal phenological trait is often a combination of direct selection, and
indirect selection acting via other phenological traits (Rathcke and
Lacey 1985, Kelly 1992, Ehrlén 2015, Galloway et al. 2018). Selection
could also target the relative timing of phenological traits. Such
selection has been found for development time to maturity and
reproduction in both plants and animals (Kingsolver and Pfennig 2004).
Lastly, the strength or direction of selection on a focal trait might
often depend on the level of other phenological traits (correlational
selection; Lande and Arnold 1983, Phillips and Arnold 1989, Kelly 1992).
Taken together, this implies that to accurately estimate selection
acting on a focal phenological trait, it is necessary to account for
indirect selection via temporally correlated traits, as well as for
correlational selection.
In temperate plants, the timing of reproduction is a particularly
important life-history trait, as it influences interactions with the
biotic and abiotic environment and is often strongly linked to fitness
(e.g. Elzinga et al. 2007, Ehrlén 2015, Austen et al. 2017). Phenotypic
selection has been found to favour early flowering in many temperate
plant species (Harder and Johnson 2009, Munguía-Rosas et al. 2011), and
the consistency of this pattern has raised questions as to why plants do
not appear to respond to this selection (Austen et al. 2017). One
suggested explanation for the observed pattern is that selection on
flowering time is exerted indirectly via correlated life-history traits
and that this indirect selection is not accounted for in analyses (cf.
Rathcke and Lacey 1985, Austen et al. 2017). For example, several
studies have documented correlations between flowering time and
vegetative phenology (e.g. Kelly 1992, Diggle 1999, Yao and Mehlenbacher
2000, Sola and Ehrlén 2007, Brachi et al. 2012). If vegetative and
reproductive spring phenology are correlated, evolution of flowering
time is influenced also by selection on timing of vegetative phenology,
and selection will act simultaneously on both traits. In these cases, we
expect direct and indirect selection on both traits, as well as on their
relationship. Yet, very few studies have simultaneously examined
selection on vegetative and reproductive phenology, and quantified
direct, indirect and correlational
selection (but see
Kelly 1992).
In this study, we explored the relationship between vegetative spring
phenology and flowering phenology in a natural population of the
perennial understorey herb Lathyrus vernus in Sweden, and used
multiple regressions to estimate phenotypic selection on these two
traits. Previous studies with this system have found that selection
favours earlier flowering in most years (Ehrlén and Münzbergová 2009,
Ehrlén and Valdés 2020), but also that flowering time is correlated with
start of shoot growth and leaf development (Sola and Ehrlén 2007). It is
thus possible that some of the observed selection on flowering time is
exerted indirectly, via selection on vegetative phenology. We expected
that most of the among-year differences in the distributions of
phenological traits in L. vernus are driven by climatic
conditions (Ehrlén and Valdés 2020), while within-year differences among
individuals in these traits are caused by other factors, including
genetic differences. We also expected the causes of selection to both
depend on the density and frequency of phenological trait values within
years (soft selection, e.g. in terms of among-individual competition for
pollinators, and avoidance of seed-predation and vertebrate grazing),
and to be density-independent (hard selection, e.g. in terms of effects
of weather conditions) (cf. De Lisle and Svensson 2017). To assess
selection on vegetative and reproductive spring phenology, we monitored
leaf-out day and first flowering day, and recorded individual fitness in
terms of seed production, during three years. We addressed four specific
questions: 1) How closely are vegetative and reproductive phenology
correlated? 2) Is there phenotypic selection acting on vegetative and
reproductive phenology? 3) If so, to what extent is selection on
vegetative and reproductive phenology direct vs. indirect? 4) Is there
selection on the relationship between vegetative and reproductive
phenology, in terms of selection for time of development between
leaf-out and flowering or correlational selection?