Structural determinants of increased susceptibility to dehydration-induced cavitation in post-fire resprouting chaparral shrubs

It is well established that transpiration and photosynthetic rates generally increase in resprouting shoots after fire in chaparral shrublands. By contrast, little is known about how plant hydraulic function varies during this same recovery period. We hypothesized that vascular traits, both functional and structural, would also shift in order to support this heightened level of gas exchange and growth. We examined stem xylem-specific hydraulic conductivity (K-s) and resistance to cavitation (P-50) for eight chaparral shrub species as well as several potential xylem structural determinants of hydraulic function and compared established unburned plants and co-occurring post-fire resprouting plants. Unburned plants were generally more resistant to cavitation than resprouting plants, but the two groups did not differ in K-s. Resprouting plants had altered vessel structure compared with unburned plants, with resprouting plants having both wider diameter vessels and higher inter-vessel pit density. For biomechanics, unburned plants had both stronger and denser stem xylem tissue than resprouting plants. Shifts in hydraulic structure and function resulted in resprouting plants being more vulnerable to dehydration. The interaction between time since disturbance (i.e. resprouting versus established stands) and drought may complicate attempts to predict mortality risk of resprouting plants. It is well established that transpiration and photosynthetic rates generally increase in resprouting plants after fire in chaparral shrublands, but little is known about how plant hydraulic function varies during this same recovery period. The ecophysiology of eight species of chaparral shrubs was compared between resprouting plants and adjacent unburned plants. Resprouting plants varied from unburned plants in their hydraulics, xylem structure and biomechanics. The observed shifts resulted in resprouting plants being more vulnerable to dehydration than unburned plants and demonstrate potential variability of plants in their hydraulic physiology depending on disturbance recovery state.