Root Hydraulic Conductance in Laurel Sumac Experiencing Severe Dieback in the Santa Monica Mountains

Malosma laurina (laurel sumac) is a dominant species in coastal chaparral ecosystems, extending its roots down to an average of 40 feet in rocky, sandy soils. The deep root systems of M. laurina provide structural support for the steep Santa Monica Mountains. The roots also allow M. laurina to draw from the deep water table, increasing its drought tolerance. M. laurina dieback is prevalent in coastal exposures of the Santa Monica Mountains presumably due to historic drought predisposing plant tissues to infection by the fungal pathogen Botryosphaeria dothidea. In ecosystems that experience annual dry periods, a high hydraulic conductance in roots is necessary to provide plants with sufficient water to sustain them until the next rainfall. A recent study has examined the effect of Botryosphaeria on the hydraulic conductance of M. laurina stems, but no research has been performed on roots. This study looks at the effect of the fungus Botryosphaeria on the root system of infected M. laurina plants compared to uninfected, well-irrigated control plants. Our focus was on the roots’ ability to transport water to the shoot. We collected root samples of both infected plants experiencing severe dieback as well as healthy plants and tested the hydraulic conductivity (Kh), water movement through each root. We then calculated the specific conductivity per unit xylem area (Ks). We found significantly lower Ks native in infected plants (mean of 1.752 mg • mm • s-1 • kPa-1) than in the healthy, control M. laurina (4.007 mg • mm • s-1 • kPa-1). The data supports the hypothesis that Botryosphaeria greatly reduces water transport function in the roots of Malosma laurina. Furthermore, the greater hydraulic conductance in well-irrigated controls indicates that water stress increases proliferation of Botryosphaeria infection in Malosma laurina