Sex-specific hydraulic and anatomical strategies underpin differential drought and shade tolerance in Populus cathayana

Global climate change is intensifying the frequency of drought events and altering forest light environments, yet the interactive effects of drought and shade on dioecious tree species remain poorly understood. In this study, we selected female and male Populus cathayana cuttings as research objects to explore their growth, photosynthesis, anatomical and hydraulic characteristics responses to drought, shade and their interactive stress. The results revealed that the growth and photosynthetic capacity of both sexes were reduced under drought and shade stress, and the total biomass and net photosynthesis rates of both sexes were lower under interactive stress than single drought stress, indicating that shade aggravates the negative effects of drought stress. Compared with females, P. cathayana males presented greater biomass, net photosynthesis rate and xylem hydraulic conductivity, and a lower percentage loss of conductivity under drought, shade and their interactive stress. In addition, under drought and shade stress, males presented greater leaf thickness, palisade tissue thickness, palisade tissue thickness to spongy tissue thickness ratio, number of vessels and total vessel area and had a smaller average vessel area, which was favorable for males to maintain water transport and utilization in stressful environments. Collectively, these coordinated anatomical and hydraulic advantages enabled males to sustain superior physiological performance under drought, shade and their interactive stress. Our findings highlight pronounced sexual dimorphism in stress tolerance and suggest that increasing drought and shade under future climates may bias natural populations toward males, with potential consequences for population productivity and ecosystem function.