Propagating ice front induces gas bursts and ultrasonic acoustic emissions from freezing xylem

Abstract

Ice formation and propagation in the xylem of plants is a complex process. During freezing of xylem sap, gases dissolved in liquid sap are forced out of the ice lattice due to their low solubility in ice, and supersaturation of xylem sap as well as low water potential (Ѱ) are induced at the ice–liquid interface. Supersaturation of gases near the ice front may lead to bubble formation and potentially to cavitation and/or to burst of gases driven out from the branch. In this study, we investigated the origin and dynamics of freezing-related gas bursts and ultrasonic acoustic emissions (AEs), which are suggested to indicate cavitation. Picea abies (L.) H. Karst. and Salix caprea L. branch segments were exposed to frost cycles in a temperature test chamber, and CO2 efflux (indicating gas bursts) and AEs were recorded. On freezing, two-thirds of the observed gas bursts originated from the xylem and only one-third from the bark. Simultaneously with gas bursts, AEs were detected. Branch Ѱ affected both gas bursts and AEs, with high gas burst in saturated and dry samples but relevant AEs only in the latter. Repeated frost cycles led to decreasing gas burst volumes and AE activity. Experiments revealed that the expanding ice front in freezing xylem was responsible for observed gas bursts and AEs, and that branch Ѱ influenced both processes. Results also indicated that gas bursts and cavitation are independently induced by ice formation, though both may be relevant for bubble dynamics during freezing.

Inspection of gas bubbles in frozen Betula pendula xylem with micro‐CT: Conduit size, water status and bark permeability affect bubble characteristics

“…The bubbles also tended to be larger and less spherical, although this is only evident in fibers due to the lack of bubbles in the vessels of the saturated samples (Figure 5). These observations are consistent with previous studies that show significant gas bursts coming out from xylem during ice propagation (Lintunen et al, 2014, 2020). Together these results suggest that bark that is permeable to gases can decrease the risk of embolism formation in trees.…”

“…Based on these results and previous literature, we hypothesize that more, larger and less spherical bubbles are grown due to the low water potential in the remaining sap at the ice front, which locally increases tension in the sap, inducing the formation of large gas voids and acoustic emissions (Charrier et al, 2014, 2015). Previous studies have shown that the amount of acoustic emissions detected from freezing xylem is typically small for saturated samples and increase with increasing dehydration until the majority of the xylem gets embolized due to the prefreezing dehydration (Charrier et al, 2015; Lintunen et al, 2020; Mayr et al, 2007).…”

“…A second factor potentially affecting the gas bubble size is the escape of gases from the xylem during the freezing process. It has been generally assumed that all the gas dissolved in the xylem sap is trapped within the conduits and forms bubbles during freezing, but we have previously shown that this is not the case as substantial freezing-related bursts of CO 2 are released from the stem during ice propagation (Lintunen et al, 2014(Lintunen et al, , 2020. The size and/or number of gas bubbles in the ice are likely to change with the amount of gas diffusing or being pushed out by the high pressures that develop during the freezing process (Robson & Petty, 1987).…”

“…The fate of gas bubbles during thawing, that is whether they collapse or expand to embolize xylem conduits, depends on the water potential of the surrounding xylem sap and the bubble size (Pittermann & Sperry, 2006). Although the gas bubble size is crucial in winter embolism formation, the critical factors affecting bubble formation and subsequent bubble size during freezing are unclear (Charrier et al, 2015; Lintunen et al, 2020).…”

Bark Transpiration Rates Can Reach Needle Transpiration Rates Under Dry Conditions in a Semi-arid Forest

“…The air stream concentration was measured from both cuvettes every three minutes. See details of the measurement system in Lintunen et al (2020) . Surface area of the bark was calculated based on branch diameter measurement assuming a cylinder branch shape inside the cuvette with 5.5 cm inner length.…”