Root freezing injury as a possible contributor to yellow-cedar decline

Yellow-cedar is an ecologically and economically important tree species that has experienced a dramatic increase in mortality in southeastern Alaska for about 100 years. It has been hypothesized that this decline may result from increased root freezing injury following climate change-induced reductions in protective snow cover.  To test this hypothesis, we measured the freezing tolerance and injury of yellow-cedar seedlings in three treatments that differed in the insulative protection they provided soils: 1) full exposure to ambient temperatures (exposed treatment), 2) continuous protection from ambient temperatures via addition of perlite over pots (full protection), and 3) perlite protection during winter and then exposure to ambient temperatures during spring (partial protection). Foliage from all treatments was cold tolerant enough to prevent direct freezing injury throughout the study period.  However, on all sample dates, roots of seedlings from all treatments were only tolerant to about -5°C – a temperature considerably above the reported maximum cold tolerance for the species and well above the actual soil temperatures recorded in exposed treatment soils.  As a result, roots of seedlings from the exposed treatment had significantly higher relative electrolyte leakage (REL) and visible injury during winter and early spring than seedlings in either soil protection treatment. Foliage from the exposed treatment also had significantly higher REL and visual injury than foliage from the other treatments in the spring.  All seedlings in the exposed treatment eventually had 100% fine root damage and died.  Damage began with freezing injury to roots and foliar browning became evident after spring temperatures increased.  This is the first experimentally induced injury expression consistent with reports of decline symptoms in the field.