Repeated freezethaw cycles induce embolism in drought stressed conifers (Norway spruce, stone pine)
Planta, (2003)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 freezinginduced 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 freezethaw cycles and simultaneously to drought. Simulating conditions at the alpine timberline (128 days with freezethaw 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 freezethaw cycles. This treatment caused a significant increase in embolism rates in drought-stressed samples. Upon 100 freezethaw 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 freezethaw 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.