Western redcedar (Thuja plicata Donn ex D. Don; redcedar), an indeterminate conifer in the Cupressaceae family, is vulnerable to maladaptation in the face of climate change. Assisted gene flow is one mitigation strategy and involves human-mediated migration of populations, where the projected climate of the area of deployment matches the source climate of the population. Despite the overall projections of warmer temperatures globally, in British Columbia (B.C.), the risk of seasonal frost events will remain and therefore the potential for cold damage and mortality of redcedar exists if the newly migrated populations cannot withstand these freezing events. Knowledge of redcedar's ability to withstand freezing temperatures (cold hardiness) is therefore crucial. Redcedar, like many Cupressaceae species, produces and accumulates the purple-coloured carotenoid rhodoxanthin during the winter. This was hypothesized to be correlated with cold hardiness. Assessment of variation in overall, fall and spring cold hardiness and associated rhodoxanthin concentrations were done through repeated, seasonal freeze testing of clonal grafts originating from across the range of redcedar, and seedling progeny from a subset of these clones. Cold damage was quantified using electrolyte leakage and rhodoxanthin concentrations were quantified using high performance liquid chromatography. Cold hardiness and rhodoxanthin were individually modelled using univariate and bivariate mixed effect models with clone/family as a random effect. Model outputs were compared to climatic variables associated with clonal origin to test for climatic relationships. This study found genetic variation in cold hardiness of redcedar with weak climatic clines. This indicates that assisted gene flow of redcedar should be done on a case-by-case basis, with no need for a climatic threshold. Overall heritability of cold hardiness was 0.17 ± 0.03. Novel findings included the positive genetic correlation between fall and spring cold hardiness (0.55 ± 0.33); lack of reciprocal or parental effect for overall cold hardiness; and weak climatic relationships between cold hardiness and predominantly temperature, with the strongest correlation between number of frost-free days in January (0.38, p < 0.01) in the location of origin and cold hardiness. All findings related to rhodoxanthin were novel. Rhodoxanthin varied with family/provenance and season with heritabilities of 0.30 ± 0.09 in fall, 0.42 ± 0.09 in winter and 0.28 ± 0.09 in spring. Winter and spring rhodoxanthin concentrations were phenotypically correlated (0.50, p < 0.01) and genetically correlated (0.76 ± 0.14). Surprisingly, rhodoxanthin was not detected in clonal grafts of redcedar in any season. Results also indicate that rhodoxanthin cannot be used to estimate cold hardiness. The absence of rhodoxanthin in the clonal grafts compared to the seedlings suggests that plant age impacts rhodoxanthin accumulation. / Graduate / 2021-12-14