Actuarial senescence progresses similarly across sites and species in four boreal orchids

Whole-plant senescence, defined as a decrease in individual fitness as an organism grows older, has often been assumed to not occur in plants; however, it has now been detected in a range of plant taxa. Still, reported senescence patterns vary substantially, and it remains unknown how consistent patterns are within phylogenetic groups and how they may be affected by environmental factors. Plants show a high diversity in life-history traits within phylogenetic groups and environments, but shared traits among related species are also common, making both diverse and similar patterns probable. Here, we explore how mortality changes with advancing age in four closely related species (Dactylorhiza incarnata, D. lapponica, D. maculata and Gymnadenia conopsea) across two sites in Norway: the coastal Nordmarka and inland Sølendet. Using data collected over 34 years, following more than 2500 individual plants, we conduct Bayesian survival trajectory analysis to assess mortality age trajectories. A simple Weibull model, illustrating increasing mortality at a decelerating rate with age, was the best fit for all species at both sites. From these models, we calculate rates of senescence and compare them using Kullback–Leibler divergences, finding no notable differences in rates between species or sites. Synthesis. Our findings suggest that actuarial senescence, an increase in mortality with advancing age, may be common in orchids and show that demographic ageing can proceed similarly in closely related taxa across different environments.