Repeated freeze?thaw cycles induce embolism in drought stressed conifers (Norway spruce, stone pine)

Abstract

Freezing and thawing lead to xylem embolism when gas bubbles caused by ice formation expand during the thaw process. However, previous experimental studies indicated that conifers are resistant to freezing-induced embolism, unless xylem pressure becomes very negative during the freezing. In this study, we show that conifers experienced freezing-induced embolism when exposed to repeated freeze-thaw cycles and simultaneously to drought. Simulating conditions at the alpine timberline (128 days with freeze-thaw events and thawing rates of up to 9.5 K h(-1) in the xylem of exposed twigs during winter), young trees of Norway spruce [Picea abies (L.) Karst.] and stone pine (Pinus cembra L.) were exposed to 50 and 100 freeze-thaw cycles. This treatment caused a significant increase in embolism rates in drought-stressed samples. Upon 100 freeze-thaw cycles, vulnerability thresholds (50% loss of conductivity) were shifted 1.8 MPa (Norway spruce) and 0.8 MPa (stone pine) towards less negative water potentials. The results demonstrate that freeze-thaw cycles are a possible reason for winter-embolism in conifers observed in several field studies. Freezing-induced embolism may contribute to the altitudinal limits of conifers.

Uptake of Water via Branches Helps Timberline Conifers Refill Embolized Xylem in Late Winter

“…These trees experienced frost drought, because frozen soil blocked water uptake for months. Moreover, the crown is exposed to numerous freeze-thaw events, with more than 100 frost cycles per winter, causing embolism even in conifers with small and resilient tracheids (Mayr et al, 2003a).…”

Xylem Sap Surface Tension May Be Crucial for Hydraulic Safety

“…Mean estimated C 50 varied by 0.77 MPa in P. abies and by 0.9 MPa in P. mugo. This might help explain why native PLC in timberline trees often was found to be much higher than PLC based on measured C and the respective standard vulnerability curve (Mayr et al, 2003a(Mayr et al, , 2003cMayr and Charra-Vaskou, 2007). In the studied winter seasons, C in P. abies and P. mugo reached only 23.1 6 0.15 and 22.97 6 0.14 MPa (data not shown); thus, PLC should be 22.6% 6 8.2% and 12.4% 6 2.7% (according to Pammenter and Vander Willigen [1998]) at a maximum for P. abies and P. mugo, respectively.…”

“…As shown in Figure 1, it instead reached up to 70% to 80% in both species. One reason for this unexpected high embolism is the combination of freezethaw events (Mayr et al, 2003a;Pittermann and Sperry 2006) and frost drought at the timberline, but shifts in the in situ vulnerability now appear to be a second important aspect: as g plays a crucial role defining the stability of bubbles and air-water menisci during frost drought, freezing, and thawing (see introduction; Mayr and Sperry, 2010;Charrier et al, 2014Charrier et al, , 2017Mayr and Ameglio, 2016), low g increases the risk of xylem embolism during winter while it decreases it during spring. One might also speculate that the observed changes during spring play a role in refilling processes reported in a previous study .…”

Investigations Concerning Cavitation and Frost Fatigue in Clonal 84K Poplar Using High-Resolution Cavitron Measurements

“…The two main factors causing cavitation and embolism are drought and frost (Mayr et al, 2003;ChristensenDalsgaard and Tyree, 2013). Drought-induced cavitation is caused by the high xylem tension attributed to water stress.…”

“…Hence, the continuity of water flow is disrupted due to cavitation. Frost-induced cavitation, on the other hand, occurs when dissolved gases in the sap freeze out and create bubbles during ice formation because air is not soluble in ice (Mayr et al, 2003;Tyree, 2013, 2014) but remains entrapped between ice crystals. Once the sap melts and tension is regenerated, the entrapped bubbles may expand to embolize the conduits instead of dissolving (Pittermann and Sperry, 2006).…”