Classic evolutionary theory predicts all organisms will face demographic senescence; a gradual decrease in fertility and an increased risk of death with an advancing age after reaching sexual maturity. Natural selection is believed to not have counteracted the physiological deterioration leading to this senescence with repairs, because at older ages, individuals have increasingly smaller portions of their lifetime reproductive output left. This would lead to a decrease in the force of selection over life spans and allow accumulation of deleterious mutations or favor pleiotropic genes conferring fitness advantages to young individuals at the expense of lower late-life fitness. However, it is not clear whether assumptions of these theoretical predictions hold for plants, and in addition, very little detailed age-based demographic data is available for plants. Understanding how plants age is not only interesting to advance the understanding of the evolution of demographic senescence, but will also be useful to improve forecasting efforts that aim to assess population viability. We address this knowledge gap by combining the underutilized technique of age determination of herbs through annual growth increments derived via root histology and polarization microscopy, with detailed demographic monitoring.
Results/Conclusions
We present preliminary results of three years of individual-level age + size demographic data for the herbaceous perennial Plantago lanceolata L. (Plantaginaceae) from Denmark. Age was determined from the annual growth increments caused by production of large xylem vessels early in the season, and we illustrate how the precise height of the cut in the root collar plays a crucial role in determining the actual age of the individual. The results elucidate the impact of size and age on survival, reproduction and individual growth in this population, as well as their impact on population growth rate. We also present age-distributions from four locations with differing resource availabilities and discuss what these distributions suggest regarding age-dependent mortality. Our preliminary results show how age may be important to account for when attempting to understand the demography of plants.