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When plants establish outside their native range, their ability to adapt to the new environment is influenced by both demography and dispersal. However, the relative importance of these two factors is poorly understood. To quantify the influence of demography and dispersal on patterns of genetic diversity underlying adaptation, we used data from a globally distributed demographic research network comprising 35 native and 18 nonnative populations of Plantago lanceolata. Species-specific simulation experiments showed that dispersal would dilute demographic influences on genetic diversity at local scales. Populations in the native European range had strong spatial genetic structure associated with geographic distance and precipitation seasonality. In contrast, nonnative populations had weaker spatial genetic structure that was not associated with environmental gradients but with higher within-population genetic diversity. Our findings show that dispersal caused by repeated, long-distance, human-mediated introductions has allowed invasive plant populations to overcome environmental constraints on genetic diversity, even without strong demographic changes. The impact of invasive plants may, therefore, increase with repeated introductions, highlighting the need to constrain future introductions of species even if they already exist in an area.

Abstract. We know very little about aging (senescence) in natural populations, and even less about plant aging. Demographic aging is identified by an increasing rate of mortality following reproductive maturity. In natural populations, quantifying aging is often confounded because changes in mortality may be influenced by both short-and long-term environmental fluctuations as well as age-dependent changes in performance. Plants can be easily marked and monitored longitudinally in natural populations yet the age-dependent dynamics of mortality are not known. This study was designed to determine whether a plant species, Plantago lanceolata, shows demographic aging in its natural environment. A large, multiple-cohort design was used to separate age-independent and age-dependent processes. Seven years of results show environmental influences on mortality as evidenced by synchronous changes in mortality across four cohorts over time. Age-dependent mortality was found through an age-by-environment interaction when the oldest cohorts had significantly higher mortality relative to the younger cohorts during times of stress. Neither size nor quantity of reproduction could explain this variation in mortality across cohorts. These results demonstrate demographic senescence in a natural population of plants.

The major starting point to life history analysis is the schedule of reproduction and mortality; hence, knowledge of age‐specific demographic dynamics is needed. The key ingredients to studies on age‐specific demography must include large cohorts of individuals of known age, an accurate accounting of all individuals, and an experimental design to facilitate a separation of age‐dependent and age‐independent dynamics. In this study with Plantago lanceolata, multiple, large cohorts were planted over four successive years, and the individuals were censused monthly for nearly five years. Longitudinal analysis showed seasonal variation in demography that was correlated with maximum temperature and cumulative precipitation. Cross‐sectional analysis of the different cohorts showed variation across cohorts in age‐specific demography. The cohort with the lowest juvenile mortality had the highest adult mortality and the lowest fecundity, suggesting that there is an interdependence of demographic patterns across life stages and that the history of mortality within a cohort may be critical to late‐age demographic patterns. Corresponding Editor: L. F. Delph