The validity of using morphological characteristics as predictors of age in the kelp, Pterygophora californica (Laminariales, Phaeophyta),
Description
Previous studies have used stipe ring counts to estimate the age of several species in the
Laminariales. Although this method is widely accepted, it has rarely been validated. To test the validity
of aging Pterygophora californica by counting rings formed in the stipe, we sampled plants of known
age between 1.0 and 4.4 yr old and plants with a calculated age of 12 yr. For a given cohort, the number
of complete rings closely approximated the known age in years. indicating ring counts are a reliable
method for estimating the age structure of P califomica populations. However, ring counts from
individual plants of known age can vary by ± 2 yr (95 % CI), and among readers counting the same
plants by ± 1 yr (95 % CI). Single plants, there fore, could not be reliably aged by ring counts Linear
relationships between 2 morphometric measures (stipe length and stipe diameter) and number of rings
varied both within and between stands and sites, suggesting these measures are not reliable for
estimating the age of this alga in the areas sampled. Stand density may be an important factor
contributing to the variation in these morphome tric measures. Short-term (22 wk) field manipulations of
stand density showed that plants at high density (30 plants m^-2) grew faster in stipe length and slower in
stipe diameter than those at medium (6 plants m^-2) and low (2 plants m^-2) densities. Plants collected in a multi-site survey, however, showed no relationship between stand density and stipe morphology,
indicating factors other than density also influence stipe growth Short-term (22 wk) field manipulations,
which reduced ambient light levels by up to 58 %. had no significant effects on stipe morphology., , ,
