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). There are two families of chaparral shrubs that contain all three life history types -- Rhamnaceae and Ericaceae. The purpose of this study was to determine if life history type in response to wildfire is related to leaf mechanical strength, leaf bulk modulus of elasticity (cell wall rigidity), hydraulic conductivity, capacitance, osmotic potential, and photosynthetic rate. 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 Scholander-Hammel pressure chamber to estimate bulk modulus of elasticity using pressure-volume curves, an evaporative flux method to estimate leaf hydraulic conductivity, and a LiCor-6400XT to estimate maximum gas exchange rates, in situ. Ten species were examined, seven in the family Rhamnaceae and three in the family Ericaceae. Statistical analyses indicated that leaf mechanical strength tended to be greatest in the obligate sprouter life history type and least for non-sprouters. There was no evidence of a tradeoff between mechanical strength and leaf hydraulic conductivity. Greater leaf mechanical strength was associated with more negative osmotic potentials at saturation and the turgor loss point and greater bulk modulus of elasticity was associated with lower gas-exchange rates and more negative osmotic potentials and lower capacitance.
Stucky, Taylor S.; Rodriguez Purcell, Nicole M.; and Davis, Stephen D., "Leaf Mechanical Strength Predicts Physiological Traits among Three Life History Types in California Chaparral" (2013). Pepperdine University, Featured Research. Paper 149.