The effects of age on the demography of a perennial plant depend on interactions with size and environment

Age-dependence of the demographic rates survival, fecundity and individual
growth is a fundamental aspect of population biological theory. Knowledge
about plant ageing can also be important for conservation and agriculture
as it will improve the accuracy of population viability assessments and
long-term performance assessments in perennial crops. Recent studies show
age effects on demographic rates for several plant species, yet much
remains to be learned about the patterns and mechanisms of plant ageing,
particularly about how age effects interact with the environment and with
plant size. We collected age-and-size-based demographic data, as well as
individual-based environmental data, for the perennial herb Plantago
lanceolata in Denmark over three annual transitions (four years). We
combined frequent field monitoring of carefully mapped individuals with
the underused technique of root histology to determine age of herbaceous
plants. We used generalized linear mixed effects models to assess how age,
soil properties, and year influenced survival, growth, and reproduction.
Our results show no strong evidence of consistent age declines, rather, we
found mostly positive effects of age on vital rates. For all vital rates,
i.e. survival, growth, flowering, and reproductive output, age effects
also differed significantly among years. Additionally, we detected an
interactive effect of age and size in the growth model. Size, and temporal
and spatial environmental variation also affected vital rates
independently of age. Synthesis: Our study shows that age-dependence of
demographic rates can depend both on individual size and environmental
variation. These results suggest that a consideration of potential
age-interactions may improve the accuracy of comparative studies of ageing
and population projections. Moreover, this study shows that much is still
unknown about how plant ageing can be affected by the environment, and
illustrates how age determination using herb chronology provides an
opportunity to increase our knowledge about ageing in herbaceous plants.