A Comparison of Leaf Mechanical Strength and Water Relations among Three Life History Types in California Chaparral
Author:Taylor S Stucky
Mentor:Stephen D Davis, Distinguished Professor of Biology, Pepperdine University
The Santa Monica Mountains of southern California are located in a Mediterranean-type climate region which experiences reoccurring wildfires due to summer drought and Santa Ana winds. Chaparral vegetation, which consists mostly of evergreen, sclerophyllous shrubs, dominates the landscape. Species of chaparral shrubs have evolved three different life history types in response to fire – those that sprout after fire but do not germinate seeds (obligate sprouters = OS), those that do not sprout after fire but reestablish by seed germination (non-sprouters = NS), and those that both sprout and germinate seeds after fire (facultative sprouters = FS). The purpose of this study was to determine whether or not life history type in response to wildfire is related to leaf mechanical strength, leaf bulk modulus of elasticity (cell wall rigidity), and hydraulic conductivity. We also hypothesized that there would be a tradeoff between mechanical strength and hydraulic conductivity. The hypotheses were tested using an Instron Mechanical Testing Device to estimate mechanical strength, a Shcolarnder-Hammel pressure chamber to estimate bulk modulus of elasticity using pressure-volume curves, and an evaporative flux method to estimate leaf hydraulic conductivity. Species examined were Rhamnus californica (OS), Rhamnus ilicifolia (OS), Ceanothus crassifolius (NS), Ceanothus oliganthus (FS), and Ceanothus spinosus (FS). Statistical analysis showed that C. crassifolius (NS) had several fold higher hydraulic conduction than the other species, and R. ilicifolia (OS) had twofold higher mechanical strength and bulk modulus of elasticity than the other species. Though significant differences were found across species it could not be concluded that these differences necessarily related to life history type or that there was a tradeoff between mechanical strength and hydraulic conductivity.