The timing of different life-history events is often correlated, and selection might only rarely be exerted independently on the timing of a single event. In plants, phenotypic selection has often been shown to favor earlier flowering. However, little is known about to what extent this selection acts directly versus indirectly via vegetative phenology, and if selection on the two traits is correlational. We estimated direct, indirect, and correlational phenotypic selection on vegetative and reproductive phenology over 3 years for flowering individuals of the perennial herb Lathyrus vernus. Direct selection favored earlier flowering and shorter timespans between leaf-out and flowering in all years. However, early flowering was associated with early leaf-out, and the direction of selection on leaf-out day varied among years. As a result, selection on leaf-out weakened selection for early flowering in one of the study years. We found no evidence of correlational selection. Our results highlight the importance of including temporally correlated traits when exploring selection on the phenology of seasonal events. 

Premise: Climate warming has altered the start and end of growing seasons in temperate regions. Ultimately, these changes occur at the individual level, but little is known about how previous seasonal life-history events, temperature, and plant-resource state simultaneously influence the spring and autumn phenology of plant individuals.

Methods: We studied the relationships between the timing of leaf-out and shoot senescence over 3 years in a natural population of the long-lived understory herb Lathyrus vernus and investigated the effects of spring temperature, plant size, reproductive status, and grazing on spring and autumn phenology.

Results: The timing of leaf-out and senescence were consistent within individuals among years. Leaf-out and senescence were not correlated with each other within years. Larger plants leafed out and senesced later, and size had no effect on growing season length. Reproductive plants leafed out earlier and had longer growing seasons than nonreproductive plants. Grazing had no detectable effects on phenology. Colder spring temperatures delayed senescence in two of three study years.

Conclusions: The timing of seasonal events, such as leaf-out and senescence in plants can be expressed largely independently within and among seasons and are influenced by different factors. Growing season start and length can often be dependent on plant condition and reproductive status. Knowledge about the drivers of growing season length of individuals is essential to more accurately predict species and community responses to environmental variation.

1. Timing of reproduction affects the outcome of interactions between plants and their pollinators, grazers and seed predators, as well as with their local abiotic environment. In seasonal environments, phenotypic selection has often been shown to favour early flowering. Yet, we still know little about the agents driving selection in natural populations and whether observed phenotypic selection corresponds to genotypic selection – a prerequisite for evolutionary change.

2. In this study, we experimentally assessed phenotypic and genotypic selection for flowering time in a natural population of the perennial herb Lathyrus vernus. We transplanted sibling individuals, obtained through controlled crosses, to their source population and found net phenotypic selection for earlier flowering in the field.

3. Despite a higher susceptibility to roe deer grazing, early‐flowering plants had higher fruit set and more seeds per fruit than late‐flowering plants. We found no support for genotypic selection on flowering time, and heritability for first flowering day was very low.

4. Synthesis: Our results suggest that commonly observed patterns of higher fitness in early‐flowering plants do not always correspond to selection on genotypic values and are thus not necessarily expected to result in evolutionary change even if the relationship between flowering time and fitness is causal. This finding should be important to understand how species phenology might respond to changing environmental conditions.

Phenology, or the seasonal timing life-history events such as emergence, reproduction and senescence will determine the outcome of interactions between plants and both abiotic and biotic aspects of the environment. Such timing is therefore of utmost importance for plants in seasonal environments. In this thesis, I first investigated the factors determining the start, end and length of the growing season for a perennial herb. Secondly, I estimated phenotypic selection on flowering time and investigated to which extent it corresponded to genotypic selection in a natural field setting. Thirdly, I estimated population differentiation in flowering time in a common garden and in the field. Lastly, I experimentally manipulated the synchrony of a perennial herb and its main herbivore to investigate the effects of herbivore phenological preference and plant-herbivore synchrony on the direction of selection on flowering time.

I found that flowering individuals emerged earlier in spring than non-flowering individuals and that large individuals senesced later in autumn, suggesting that the length of the growing season is linked to individual condition and resource demands. Phenotypic selection favoured early-flowering individuals, but there was no genotypic selection. I found evidence for genetic population differentiation in flowering time in a common garden but not in the field. This suggests that, although flowering time has a genetic component, the observed variation in flowering time was mainly plastic under natural field conditions. Lastly, I show that constant herbivore preferences of plant phenology, in combination with environmentally driven variation in relative synchrony of the plant and the herbivore, leads to among-year variation in natural selection on flowering time. With this thesis, I contribute to identifying the factors affecting plant phenology as well as of the mechanisms shaping selection on flowering time in perennial plants. Such knowledge is essential for predicting species responses to climate change.

Temporal variation in natural selection has profound effects on the evolutionary trajectories of populations. One potential source of variation in selection is that differences in thermal reaction norms and temperature influence the relative phenology of interacting species. We manipulated the phenology of the butterfly herbivore Anthocharis cardamines relative to genetically identical populations of its host plant, Cardamine pratensis, and examined the effects on butterfly preferences and selection acting on the host plant. We found that butterflies preferred plants at an intermediate flowering stage, regardless of the timing of butterfly flight relative to flowering onset of the population. Consequently, the probability that plant genotypes differing in timing of flowering should experience a butterfly attack depended strongly on relative phenology. These results suggest that differences in spring temperature influence the direction of herbivore-mediated selection on flowering phenology, and that climatic conditions can influence natural selection also when phenotypic preferences remain constant.

Our knowledge of how plant seasonal development is related to local versus larger-scale environmental variation is limited. We investigated differentiation in the timing of vegetative and reproductive development among populations of the forest herb Lathyrus vernus over different spatial scales. We predicted earlier development and shorter development time for populations from a colder, northern region compared to populations from a warmer, southern region. Also, we predicted earlier and shorter development within regions to be associated with colder temperatures and higher proportions of deciduous trees at their sites of origin. Lastly, we predicted that earlier flowering is strongly correlated with earlier start of development. To examine these predictions, we conducted a common garden study, and compared the development of 10 northern and 10 southern Swedish L. vernus populations. Start of development, development time and start of flowering did not differ between populations from the two regions in contrast to our prediction. Within the southern region, start of flowering was earlier in populations from colder sites, while start of development was earlier with colder temperatures within the northern region. Start of flowering occurred earlier in southern populations from sites with higher proportion of deciduous trees. Thus, the prediction for the timing of development within regions was partly confirmed. However, vegetative and reproductive development was not simultaneously influenced by temperature and proportion of deciduous trees within regions, possibly due to the negative correlation between vegetative growth and development time. This implies that earlier start of development or shorter development time not necessarily correspond to earlier start of flowering or vice versa. Overall, the results suggest that smaller scale effects within region, such as temperature and interspecific competition for light, was more important for the timing of development than the larger scale gradients between regions. Lastly, the population differentiation across gradients of temperature and proportion of deciduous trees implies that populations may adapt to long-term changes in light or climatic conditions, and differ in their short-term response to climate change.