The Use of Size as an Estimator of Age in the Subantarctic Cushion Plant, Azorella selago (Apiaceae)

Abstract

Plant species with morphological features that enable the age of individuals to be estimated are potentially useful for understanding past environmental conditions. Here, the size and growth rate of the cushion plant, Azorella selago Hook. (Apiaceae), are examined to determine if an accurate and reliable age can be assigned to individual plants using the phytometric model detailed by Frenot et al. (1993). Plant size, growth rate, epiphyte load, nearest-neighbor characteristics, and spatial position (used as a surrogate, to encompass a range of abiotic conditions to which plants were exposed) were measured at three sites. Additionally, variation in some of these variables was quantified across three altitudinal transects. Relationships were examined using regression, trend surface and partial regression approaches. Growth rate was independent of plant size, differed between sites, and was related to abiotic as well as other biotic factors. As a result, the phytometric model’s age estimates may be biased by environmental variables. The results of the phytometric model, albeit in the absence of support for one assumption, estimated mean plant age at 30 yr, with some plants estimated as older than 80 yr. Using a simulation model, the accuracy of age estimates was shown to vary with temporal variation in plant growth rate and plant size. Nonetheless, even a conservative approach suggested these estimates to be accurate to within 2 to 15 yr. While further development of the phytometric model would improve its reliability, the model remains a valuable tool for estimating plant ages in an environment where many related techniques can not be applied.