Age matters: Demographic senescence in the moss Polytrichastrum formosum

1. It is known that many animal species senesce demographically, showing a decrease in survival and/or fertility with age. Although there is mounting evidence for and against senescence in various flowering plant species, the question of whether senescence also occurs in other plant taxa, such as mosses and other bryophytes, remains unanswered. 2. We used GAMMs (generalised additive mixed models) and GLMMs to assess the effects of age on survival, size and the monocarpic female ramets' sexual reproduction in Polytrichastrum formosum using demographic, ramet-level data collected over 25 years in Solhomfjell nature reserve, Norway. 3. Survival was found to be negatively correlated with ramet age, with 1-year survival decreasing from 83% in 1-year-old to 43% in 10-year-old plants. Stem growth was positively correlated with survival but accounting for it in our models did not change the relationship between age and survival qualitatively. Annual growth decreased during the first years of the ramets' life but then stabilised. Sexual reproduction in the monocarpic females peaked at ramet age 2-4 years but was observed at ages up to 13 years. The longest-living (non-reproducing) ramet died at age 14. 4. Synthesis. Our findings indicate that Polytrichastrum formosum ramets experience constant actuarial senescence, at least in terms of survival. This finding improves our understanding of age-dependent demography in mosses, as well as in plants in general. This new knowledge about moss demography may also aid in management decisions for threatened species, as it can improve the precision of predictions of population longevity.