A simple method to observe water distribution in tracheid-bearing wood of subalpine conifer

Key message: Distributions of water- and air-filled conduits can be distinguished by observing a xylem cross-section of frozen conifer stem. This method is applicable to field observation in cold winter. Abstract: Xylem embolism resulting from summer drought and frost drought is one of the critical stresses responsible for the dieback of stems and individuals of subalpine evergreen conifers. The occurrence of xylem embolism in conifers depends not only on the species but also on the plant microhabitats, causing difficulties in understanding possible adaptive strategies against xylem embolism. This study examines a simple method of using a digital camera to photograph the xylem water distribution (CXW method) in cross-sections of frozen stems. Light is transmitted through the water-filled tracheid lumen but reflects and scatters at the surface of embolized tracheids, resulting in contrast in wood color between darker (water-filled tracheids) and lighter (air-filled tracheids) colored regions. The CXW method was effective in detecting water distribution in conifers, although the colored xylem in latewood and reaction wood decreased the color contrast between air- and water-filled regions. By cutting the frozen stem with a cryostat, sequential changes in the water distribution of stem xylem were easily monitored. In the cold winter of the subalpine region, the spatial distribution of embolized conduits can be detected when a branch is collected. If a cryostat is available, this method is applicable to other tracheid-bearing wood collected in any season and does not require additional instruments or time-consuming intensive labor in the field. Information about the hydraulics of conifers growing in extreme environments contributes to the understanding of their adaptive strategy and facilitates accurate prediction of forest dynamics under future climatic conditions.