Age distributions of Greenlandic dwarf shrubs support concept of negligible actuarial senescence

Many plants and sessile animals may not show actuarial senescence, the increase in mortality with age predicted to be ubiquitous by classic evolutionary theories of aging. Age-structured demographic information is, however, limited for most organisms. We assessed the age distributions of nine dwarf shrub species from 863 taproot samples collected in coastal east Greenland. Penalized composite link models (pclm) were used to fill gaps in the observed age ranges, caused by low species-specific sample sizes in relation to life span. Resulting distributions indicate that mortality patterns are independent of age. Actuarial senescence is thus negligible in these dwarf shrub populations. We suggest that smoothing techniques such as pclm enable consideration of noisy age data for determining age distributions. These distributions may, in turn, reveal age effects on demographic rates. Moreover, age determination from the root collars of small plants constitutes a powerful technique to further investigate age dependency of the demography of many plant species, including eudicot herbs. Using these methods for long-lived plants where long-term monitoring is unrealistic, we show that age is unlikely to be an important variable for making population projections and determining extinction risks.