Abstract
Relative to mammals and birds, little is known about the mortality trajectories of perennial plants, as there are few long-term demographic studies following multiple yearly cohorts from birth to death. This is particularly important because if reproductively mature individuals show actuarial senescence, current estimations of life spans assuming constant survival would be incorrect. There is also a lack of studies documenting how life history trade-offs and disturbance influence the mortality trajectories of plants. We conducted Bayesian survival trajectory analyses (BaSTA) of a 33-year individual-based dataset of Pulsatilla vulgaris ssp. gotlandica. Mortality trajectories corresponded to “Type III” survivorship patterns, with rapidly decreasing annual mortality rates for young plants, but with constant mortality for reproductively mature individuals. We found trade-off effects resulting in a cost of growth for non-reproductive plants but no apparent cost of reproduction. Contrarily to our expectation, young plants that had previously shrunk in size had a lower mortality. However, accounting for trade-offs and disturbance only had minor effects on the mortality trajectories. We conclude that BaSTA is a useful tool for assessing mortality patterns in plants if only partial age information is available. Furthermore, if constant mortality is a general pattern in polycarpic plants, long-term studies may not be necessary to assess their age-dependent demography.
Highlights
Understanding the mortality trajectories of plants is important for developing life history theory and to predict the dynamics of populations of special interest, such as threatened or economically important populations
Life expectancy at germination varied between 3–11 years among the three populations, but at age ten, it had increased to 27–50 years
Similar patterns were found when including covariates, suggesting that individual variation in growth, reproductive output or environmental conditions resulting in shrinkage did not mask any effects of age on survival in mature plants, even though we did detect possible growth-survival trade-off effects and negative correlations of mortality with reproduction and shrinkage
Summary
Understanding the mortality trajectories of plants is important for developing life history theory and to predict the dynamics of populations of special interest, such as threatened or economically important populations. Increasing mortality with age in reproductively mature individuals, i.e., actuarial senescence, seems to occur in some perennial plants [1,2,3,4], while other studies show no evidence of this [5,6,7,8]. There are numerous studies about the life spans of trees, where growth rings have been used to estimate ages, little is still known about the life spans of long-lived perennials herbs Herbaceous perennials typically display Type III survivorship curves, with a very low seedling survival and a low initial life expectancy, but with high survival and life expectancy for reproductively mature plants [13,14,15]. Little is known about the mortality trajectories at older ages
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