COS 280-2 - Evaluating the functional lifespan of ring-porous earlywood vessels in Quercus lobata under non-freezing mediterranean-type climate conditions

Background/Question/Methods

Wood porosity (ring-porous vs. diffuse-porous structure) and associated anatomical traits influence tree hydraulic function and resistance to environmental stress, affecting tree phenology and carbon use. Ring-porous trees have large earlywood vessels, which may become dysfunctional within one year after formation due to their high vulnerability to freezing and drought-induced cavitation. However, in areas where winter frost is absent and water-stress limited, earlywood vessels may be able to transport water for longer. In this study, we evaluated earlywood vessel functionality in Quercus lobata Née (a native California oak species with deciduous leaf habit and ring-porous growth rings) trees growing in southern California. We combined three different approaches: observation of branch micro-sections after conducting active xylem staining, in vivo observation of branches with microcomputed tomography (microCT), and observation of tyloses in branch and stem micro-sections. Surveys were conducted in 2021 on 11 adult trees located in an irrigated plot to mitigate summer drought. Very long (2 m) distal branches were regularly collected during leaf development in spring (March 1 to April 27) and fall (October 12-18). Additional wood cores were collected in December for tyloses observation.

Results/Conclusions

Stained micro-sections and microCT images from branches showed that earlywood vessels up to three years old were still functional in all the sampled trees, even though there were abundant embolized vessels in previous rings, and also in current rings in the fall. Such a long functional lifespan was in line with the rare occurrence of tyloses in branch earlywood vessels. However, this contrasted with micro-sections taken from the bole at the end of the year, with up to six trees showing at least one earlywood vessel occluded by tylosis in the last two rings. Our results suggest that, in the absence of winter frost and severe drought stress, the functional lifespan of earlywood vessels in ring-porous oaks may largely depend on the tree organ, with intrinsically longer functional lifespans in branches than at the bole. The high transport capacity of earlywood vessels compared to latewood vessels and tracheids suggests that old functional earlywood vessels in branches may not only represent an important source of water for developing leaves and shoots at the time when new vessels are not mature yet, but also for mature leaves during high evaporative demand.