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Individuals in a large experimental field population, of the short-lived perennial species Plantago lanceolata, were followed to determine the sources of variation that influence mortality and life span. The design included multiple age groups with initially similar genetic structure, which made it possible to separate age effects from period effects and to identify the genetic component to variation in life span. During a period of stress, individuals of all ages showed parallel increases in mortality but different cohorts experienced this period of high mortality at different ages. This then influenced the distribution of life spans across cohorts. Age and size-age interactions influenced mortality during the period of stress. Smaller individuals died but only if they were old. Additionally, growth and age interacted with stress such that older individuals had negative growth and high mortality whereas younger individuals had positive growth and relatively lower mortality during stress. The results of this study show that it is not simply the environment that can have a major impact on demography in natural populations; rather, age, size and growth can interact with the environment to influence mortality and life span when the environment is stressful., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Most experimental studies of senescence have been done with short-lived organisms under controlled laboratory conditions and it is not clear whether the insights gained from these studies can be broadly generalized. This study was designed to detect senescence in a natural population and to compare the patterns of mortality for a single species in natural and protected conditions. It was done with Plantago lanceolata, a perennial plant for which the demography of a large population of individuals in their natural environment is relatively straightforward. An initial cohort of 10,000 individuals was established in the natural field environment. In order to separate the effects of environment- and age-dependent factors on mortality, an additional cohort was planted in the field one year later. To study the demography of mortality under protected conditions, a population of 1000 individuals was established in the greenhouse. The results of the comparative analysis of two different-aged cohorts in the field and of the field and greenhouse populations show that senescence patterns can be very plastic. The results show that senescence in the natural environment is caused by an increased vulnerability of older individuals to environmental stress. Under the protected environmental conditions of the greenhouse senescence was negligible.