Tree Height-Related Hydraulic Strategy to Cope with Freeze-Thaw Stress in Six Common Urban Tree Species in North China

Urban trees are sensitive to extreme weather events under climate change.Freeze-thaw induced hydraulic failure could induce urban tree dieback and nullify the services they provide.Plant height is a simple but significant trait for plant ecological strategies.Understanding how urban trees with different heights adapt to freeze-thaw stress is increasingly important under climate change.We investigated the relationship between tree height and stem hydraulic functional traits of six common urban tree species in North China to explore tree height-related hydraulic strategies to cope with freeze-thaw stress.Results showed that tall trees had wider vessels,higher hydraulic conductivity,more winter embolism,but lower vessel and wood densities.Positive relationships were found between tree height and vessel diameter,hydraulic conductivity,and freeze-thaw induced embolism,and negative relationships were found between tree height and vessel and wood densities,which implied that short trees employ more conservative ecological strategies than tall trees.Tall and short tree species were well separated by multiple stem hydraulic functional traits;this is consistent with the fact that tall and short trees occupy different niches and indicates that different hydraulic strategies for freeze-thaw stress exist between them.Tall trees might face more pressure to survive under extreme cold weather caused by climate change in the future.Therefore,more attention should be paid to tall urban tree management in North China to cope with extreme cold weather.