Conservation of functional traits leads to shrub expansion across a chronosequence of shrub thicket development

Abstract

Robust physiology of Myrica cerifera across a chronosequence (i.e., space for time substitution) of shrub thicket age classes contributes to rapid cover expansion observed in the last 50 years. Many studies have documented the causes of woody expansion into grasslands, but few address unique morphological and physiological traits that facilitate expansion. Myrica cerifera, an evergreen N-fixer, is the dominant shrub on many barrier islands of the southeastern United States. Cover of Myrica cerifera has expanded by ~400 % on Hog Island, Virginia, in the past 50 years. Accretion of the northern end of the island has resulted in a chronosequence (i.e., space for time substitution) of both soil age and shrub thicket development. We investigated functional traits and physiological parameters related to light capture, processing and water balance of M. cerifera across shrub thickets of four age classes from ~10 to ~50 years. We hypothesized that light processing capabilities and hydraulic capacity would be reduced with thicket age. Spatial variation in morphology (i.e., leaf thickness, leaf area) and structure (i.e., leaf angle) related to light capture was observed. Yet, little or no differences were detected in stomatal density, photosynthetic pigments, electron transport rate (ETR) and hydraulic conductivity across sites. Previous research has shown declines in leaf N content, productivity and leaf litter production across the chronosequence. In contrast, we observed that physiology remains consistent despite considerable differences in thicket age and development. Myrica cerifera maintains high photosynthetic and hydraulic efficiency, factors which enable expansion and maintenance of shrub thickets in mesic coastal environments.