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 ) and resistance to cavitation (P ) 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 . 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.

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The xylem tissue of woody plants performs three principal functions: transport of water, mechanical support of the plant body, and storage of nutrients (Pratt and Jacobsen, 2017). Much previous research has focused on the two functions of water transport and mechanical support in the context of xylem tradeoffs, and there has been less research into the function of storage (however, see Plavcová et al., 2016; Chen et al., 2020). In the present study, we focused on how storage relates to other xylem functions. It is likely that xylem functions are interdependent, such that wood proficient at one function is necessarily poor at another, giving rise to tradeoffs. Tradeoffs can arise due to the structural demands required to support a particular function that is at odds with another function (Janssen et al., 2020). Another possibility is that evolutionary forces lead to wood functions that are associated because they are part of an adaptive suite of traits that determine rates of resource acquisition, use, and turnover (Reich, 2014). One force that strongly influences xylem function is water limitation in seasonal environments. Plants widely differ in dehydration tolerance during drought, which we define as how negative their water potentials are during dry periods. One trait that is associated with dehydration tolerance is cavitation resistance (Pockman and Sperry, 2000; Kursar et al., 2009; Parker et al., 2016). Cavitation occurs when the tension of the xylem sap reaches a point where gas is pulled into water-filled conduits, filling them with gas emboli.