Hydraulic capacitance: Biophysics and functional significance of internal water sources in relation to tree size

Abstract

Components of the tree water transport pathway; roots, trunks, branches and leaves; can also serve as water storage compartments and therefore act transiently as intermediate sources of water for transpiring leaves. However, most previous work has focused on gradual depletion and recharge of tree internal water reserves as soil water availability varies over seasonal cycles. This chapter focuses on the underappreciated role that internal water storage plays in stabilizing the physiological function of trees under the dynamic conditions that prevail over the course of a day. Capacitive discharge of water into the transpiration stream can buffer daily fluctuations in xylem tension, thereby diminishing the risk of xylem embolism and hydraulic failure under dynamic conditions. Intrinsic sapwood capaci­t­ance and reliance on stored water increase with tree size. An inverse relationship between sapwood capacitance and resistance to embolism across diverse woody species suggests that above a minimum threshold value of capacitance, the tree survives by using capacitance to provide hydraulic safety by buffering fluctuations in tension, rather by relying on xylem structural features that directly reduce vulnerability to embolism. Progress in understanding the physiological role of capacitance in trees is impeded by non-uniformity in the way capacitance is measured and expressed, preventing much of the available information from being synthesized. To remedy this, standard protocols are described for defining and expressing capacitance and water storage capacity.