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Ageing in an herbaceous plant: Increases in mortality and decreases in physiology and seed mass

Little is known about plant age‐dependent trait expression and how environmental conditions might affect ageing in the wild. This study evaluates age variation in multiple traits of a short‐lived perennial herb using a manipulative field experimental design. Two different‐aged cohorts were followed in a field plot for over a year to evaluate trait expression in response to a competition treatment and seasonal stress. Traits measured included size, mortality, reproduction, and physiology, including photosynthetic efficiency and chlorophyll content (SPAD). We hypothesized that the stress of competition and seasonal changes would accentuate age‐dependent trait declines in older plants. The results highlight consistent age differences in plant size, mortality, and seed size with older plants being smaller, more likely to die, and producing smaller seeds. Some of the ageing declines were sensitive to environmental conditions such that it was only during certain seasons when older plants had higher mortality, lower photosynthetic efficiency, and lower chlorophyll content than young plants. Age‐dependent trait expression also varied in response to competition such that age differences in size were only present in the “no competition treatment,” and old individuals in the competition treatment had a higher mortality than all other age‐environment combinations. Synthesis. These findings show that ageing in plants is a complex phenotype where declines in traits are uncoordinated and can be, but are not always, sensitive to environmental conditions. This study shows age‐dependent maternal effects on offspring quality which, together with the decline in performance of older individuals, may have impacts on an individual’s fitness and on natural population demography.