Leaf Mechanical Strength Predicts Physiological Traits among Three Life History Types in California Chaparral
Authors:Nicole Rodriguez-Purcell, Taylor Stucky
Mentor:Stephen Davis, Distinguished Professor of Biology, Pepperdine University
The Santa Monica Mountains of southern California represent a Mediterranean-type climate region experiencing frequent wildfires. Chaparral shrubs dominate the landscape and have evolved three life history types in response to wildfire – 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 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 in terms of Young’s modulus, leaf bulk modulus of elasticity (cell wall rigidity), hydraulic conductivity, water storage capacity or capacitance, osmotic potential, and photosynthetic rate. We hypothesized that greater mechanical strength would result in lower hydraulic conductivity. We used an Instron Mechanical Testing Device to estimate Young’s modulus, a Scholander-Hammel pressure chamber to estimate leaf bulk modulus of elasticity, an evaporative flux method to estimate leaf hydraulic conductivity, and a field portable gas-exchange system to estimate maximum photosynthesis and transpiration, in situ. Ten species were examined, seven in the family Rhamnaceae and three in Ericaceae. Opposite our predictions, statistical analyses indicated that leaf mechanical strength tended to be greatest in obligate sprouters and least in non-sprouters. There was no evidence that greater mechanical strength resulted in lower hydraulic conductance. Greater leaf mechanical strength was associated with more negative osmotic potentials and turgor loss points. Bulk modulus of elasticity was associated with lower gas-exchange rates, more negative osmotic potentials, and lower leaf capacitance. Leaf mechanical strength was found to be a predictor of physiological traits associated with water stress tolerance and gas-exchange but not water transport efficiency.