Embolism resistance of different aged stems of a California oak species (Quercus douglasii): optical and microCT methods differ from the benchtop-dehydration standard

Vulnerability of xylem to embolism is an important trait related to drought resistance of plants. Methods continue to be developed and debated for measuring embolism. We tested three methods (benchtop dehydration/hydraulic, micro-computed tomography (microCT) and optical) for assessing the vulnerability to embolism of a native California oak species (Quercus douglasii Hook. & Arn.), including an analysis of three different stem ages. All three methods were found to significantly differ in their estimates, with a greater resistance to embolism as follows: microCT > optical > hydraulic. Careful testing was conducted for the hydraulic method to evaluate multiple known potential artifacts, and none was found. One-year-old stems were more resistant than older stems using microCT and optical methods, but not hydraulic methods. Divergence between the microCT and optical methods from the standard hydraulic method was consistent with predictions based on known errors when estimating theoretical losses in hydraulic function in both microCT and optical methods. When the goal of a study is to describe or predict losses in hydraulic conductivity, neither the microCT nor optical methods are reliable for accurately constructing vulnerability curves of stems; nevertheless, these methods may be useful if the goal of a study is to identify embolism events irrespective of hydraulic conductivity or hydraulic function.