The bark structure and conductance of two co-occurring Mediterranean pine species with contrasting ecological optima

Climate change is intensifying drought conditions in the Eastern Mediterranean, posing a significant threat to its unique forest ecosystems. While residual water loss from leaves (i.e., minimal leaf conductivity) after stomatal closure has been identified to play an important role in drought susceptibility across different tree species worldwide, the role of bark as an additional source of residual transpiration (i.e., bark conductivity-gbark) still remains largely underexplored. This study investigates gbark and bark structural traits in two co-occurring Mediterranean pine species, Pinus brutia Ten. and P. nigra Arnold, in Cyprus. Since P. brutia typically occurs in hotter and drier areas, we expected a lower gbark associated with thicker outer bark. Contrary to our initial hypothesis, P. brutia exhibited significantly higher gbark and thinner outer bark than P. nigra on branches of similar diameter (~1 cm). Along with its higher gbark, P. brutia also showed traits associated with an acquisitive growth strategy, including thicker inner bark and potentially greater bark photosynthetic capacity. Contrary to species-specific relationships, gbark showed a negative relationship with outer bark thickness across the species level. These findings suggest that bark structure and function are intricately linked to species-specific growth strategies.