Observed compression of in situ tree stems during freezing

Freezing temperatures can influence the material properties of trees. Understanding how forest biometeorological interactions respond to freezing temperatures is important for forest science and management, as its effects can cascade through coupled hydrological and ecological processes including limitations on tree growth, changes to canopy interception, and influences on atmospheric turbulence. This short communication details the effect of sub-freezing temperatures on the mechanical displacement of three in situ Pinus contorta Douglas (lodgepole pine) stems during a winter season (2013–2014) at Niwot Ridge Mountain Research Station (CO, USA). Although previous research on harvested trees suggests longitudinal stem expansion should occur as temperatures decrease below freezing, we observed longitudinal compression in live stems below −3 °C that linearl y correlated with sub-zero air temperatures ranging down to −22 °C. Freeze-related compression of stems frequently achieved displacement magnitudes comparable to applying 55 kN of compressive force (800 μm). Hypotheses are proposed to explain the significant mechanical displacement observed due to freezing: (1) internal gas release as freezing fronts propagate through stems; (2) short-term relocation of water via sap exudation or preferential ice crystal growth. As the observed stem compression response to freezing is substantial, future work on underlying processes and consequences is merited. The measurement technique used here may prove useful to others interested in the dynamics of stem freezing.