Patterns and consequences of plant ageing

As time passes, all organisms age, but not all will also senescence, meaning experience a decrease in one or more fitness components (e.g., survival) as they age. Many theories have been proposed as to why organisms senesce, as it appears to go against natural selection, and it was once believed that no organism could escape senescence. However, it has become clear that this is not true and instead a diverse range of senescence patterns can be found across the tree of life. From actuarial (i.e., survival decreasing with age) and/or reproductive senescence (i.e., reproduction decreasing with age) to no senescence at all or even the complete opposite, called negative senescence (i.e., survival and/or reproduction increasing with age). Though the patterns of senescence are beginning to be known, it remains a question what drives the observed patterns, particularly whether phylogeny and the environment play any part. Historically, the focus has been on mammals and birds, but senescence in plants is a growing field. With this thesis, I aim to expand upon our knowledge on ageing in plants by investigating patterns of senescence in closely related orchids and a clonal moss, as well as exploring how senescence in the latter affects population dynamics and age structure. In manuscript I, we demonstrate the presence of actuarial senescence in ramets of the moss Polytrichastrum formosum, showing how even when accounting for size, ramets still experience decline in survival with age. We also show a potential link between female ramets’ age and the timing of their sexual reproduction. Manuscript II further explores the effect that the ramets’ senescence has on the populations of P. formosum, where we show that young moss ramets are more important for future population growth than older ramets. A size- or stageonly model would not have been able to identify this, thus underlining the need for age to be considered when studying the demography of plants due to ageing’s possible consequences. Lastly, in manuscript III, we show that four closely related orchids (Dactylorhiza incarnata, D. lapponica, D. maculata, and Gymnadenia conopsea) all experience similar patterns of actuarial senescence, indicating that phylogeny can affect the pattern of senescence. We also find differences in age-trajectories of mortality between two study sites; however, the studied environments do not appear to affect their rate of senescence. Collectively, these three manuscripts expand our knowledge of ageing patterns in plants, but also underlines that there are still many questions left to answer.