Critical assessment of Thalassia testudinum (turtle grass) aging techniques:implications for demographic inferences

Abstract

Variability in the time intewal between the formation of successive leaves (i.e. the plastochron) directly influences estimates of seagrass shoot age. Age-frequency distributions were constructed for Thalassia testudinurn short shoots from a shallow and a deep site in the Lower Laguna Madre, Texas, during the summers of 1995 and 1996. In each year the ages of 200 to 250 shoots, collected by coring, were estimated from leaf scars and annual leaf production rates. Two years of monthly leaf production measurements were used to calculate annual leaf production. The leaf production rate was site-specific: plants at the shallow site produced about 13 leaves yr-' while plants from the deep site produced about 10 leaves yr-'; the 95% confidence limits around the mean were *20 to 25%. At the shallow site, the mean annual leaf formation rate during 1996 was 15% higher than during 1995, indicating substantial interannual variability. Age-frequency distributions were adjusted to account for misclassification errors as a result of age-specific leaf initiation rates. The oldest shoot was estimated to be between 8 and 12 yr. Long-term leaf marlung indcates that the seagrass T, testudinum in Lower Laguna Madre violates the assumption that the leaf production rate is constant and successive plastochrons are of equal duration. Reduced leaf formation rates during summer were hkely caused by resource allocation to developing fruits and seeds. As a result of site-specific, seasonal, and interannual variabhty in annual leaf formation rates, the application of the plastochron method to estimate T. testudinurn short shoot age is limited. Furthermore, a review of the hterature indicates that age-frequency &stributions cannot be used to predict population growth. Predicting population growth requires tradltional demographic methods (e.g. mapping individuals through time).