Deborah A. Roach

Abstracts of recently published papers

Roach, D.A. 2003. Age-Specific Demography in Plantago: Variation Among Cohorts in a Natural Plant Population. Ecology 84(3):749-756.

Abstract: 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.

Lacey, E.P., D.A. Roach, D.Herr, S. Kincaid, and Rachel Perrott. 2003. Multigenerational effects of flowering and fruiting phenology in Plantago lanceolata. Ecology (in press).

Abstract: Phenological patterns of flowering and fruiting can be influenced by the effects of flowering/fruiting time on seed set. We propose here that these patterns are influenced also by phenological effects on offspring quality. Furthermore, we hypothesize that there are cross-generational tradeoffs between parental and offspring components of parental fitness influencing the evolution of flowering/fruiting phenology.

To test our hypothesis we examined the multigenerational effects of flowering and fruiting phenology in Plantago lanceolata. We transplanted into field plots offspring of 30 families to measure the effects of onsets of flowering and fruiting, duration of fruiting, and percent fungal infection and damage by grasshoppers on seed set, our measure of the parental component of parental fitness. We also weighed seeds and measured the germination of seeds maturing at two times in 12 families.

Families significantly differed in flowering and fruiting onsets after accounting for the effect of plant size. Larger plants began flowering earlier and earlier flowering plants began fruiting earlier, and produced fruits for a longer time. Earlier fruiting significantly reduced seed predation by grasshoppers and increased seed set. In contrast, later-maturing seeds were significantly heavier and germinated more rapidly. The directions of the multigenerational effects support the hypothesis that there are cross-generational tradeoffs between parental and offspring components of parental fitness. The experiments indicate that multigenerational fitness effects should be considered in future studies addressing the evolution of flowering and fruiting phenology. Results also suggest that parental environmental effects influence the direction of evolutionary change without being adaptive.

Dudycha, J. and D.A. Roach. 2003. Pathogen frequency in an age-structured population of Plantago lanceolata. Oecologia 136:141-147.

Abstract: Life history traits can play important roles in determining the course of ecological species interactions. We explored the consequences of host age on a host-pathogen interaction by quantifying pathogen frequency in an age-structured host population. Our project was motivated by an interest in whether the demographic structure of a host population has consequences for species interactions. In two successive years, we planted large cohorts of the perennial herb Plantago lanceolata into its natural environment and observed infection by Fusarium moniliforme, a non-lethal floral fungal pathogen, over three years. We documented substantial variation of pathogen frequency across years and between cohorts. Logistic regression revealed that pathogen frequency increased with the number of inflorescences produced and with evidence of prior pathogen presence, whereas it decreased with increasing plant longevity. In addition, interannual variation and an age-year interaction contributed to the observed pathogen frequencies. There was a significant positive effect of age on pathogen frequency overall, but this was not consistent over all ages. Pathogen frequency was higher in two-year-old plants than in one-year-olds, suggesting that age-structure can influence the host-pathogen interaction. This pattern did not continue into three-year-old plants. A possible explanation for this is that selective mortality allows only generally robust plants, and consequently the most resistant plants, to survive to the oldest ages.

Roach, D.A.2003. Evolutionary approaches to the study of whole plant senescence. In: Plant Cell Death Processes (L.D. Nooden, Ed.). Academic Press (in press).

Abstract: There have been two distinct definitions of ‘plant senescence’ which have developed within the literature. First, physiologists and cell biologists use the term senescence to describe the continual turnover of cells and plant parts that occurs within an individual as part of an internally controlled program of development. In cases of monocarpy (semelparity), this program can be responsible for the death of the whole organism. The details of this program of ‘physiological senescence’ within individuals are addressed in the other chapters of this book. The second, alternative approach to senescence is termed ‘evolutionary senescence’ and it addresses theories and experimental evidence explaining variation in mortality patterns among individuals within populations and between species. Senescence, as viewed by most animal and evolutionary biologists, is a decline in age-specific survival and reproduction with advancing age. The evolutionary theories of senescence are designed to explain why this senescence occurs in most species, and to explain the variation in the rates of evolutionary senescence between different species. The objective of this chapter is to present the study of whole plant senescence within an evolutionary and demographic context. In the first part of this chapter the theories which have been proposed to explain the evolution and persistence of senescence will be discussed, and experimental tests of the theories will be evaluated. To study senescence at the level of the whole plant, demographic evidence for a decline in mortality and reproduction with age is essential. In the second part of this chapter, demographic evidence for senescence in plants will be evaluated and the techniques and problems that are unique to demographic studies of whole plant senescence will be discussed.