The article explores the mechanisms that limit tree height and growth, focusing on the hydraulic limitation hypothesis. Trees grow to different heights in different environments, and their growth slows with age. The hydraulic limitation hypothesis suggests that as trees grow taller, the hydraulic resistance of their xylem increases, leading to reduced water transport and photosynthesis. This results in lower carbon assimilation, which further slows growth. Studies show that stomata close earlier in the day in older trees, reducing photosynthesis. Hydraulic resistance increases with tree height and age, and older trees have lower hydraulic conductance. Evidence supports the hydraulic limitation hypothesis, including reduced photosynthesis and stomatal conductance in old trees. The hypothesis explains why different species and individuals attain different maximum heights and why height growth rates vary with resource availability. The article concludes that hydraulic limitations are a universal constraint on tree growth and height, and new techniques are needed to fully understand these processes.The article explores the mechanisms that limit tree height and growth, focusing on the hydraulic limitation hypothesis. Trees grow to different heights in different environments, and their growth slows with age. The hydraulic limitation hypothesis suggests that as trees grow taller, the hydraulic resistance of their xylem increases, leading to reduced water transport and photosynthesis. This results in lower carbon assimilation, which further slows growth. Studies show that stomata close earlier in the day in older trees, reducing photosynthesis. Hydraulic resistance increases with tree height and age, and older trees have lower hydraulic conductance. Evidence supports the hydraulic limitation hypothesis, including reduced photosynthesis and stomatal conductance in old trees. The hypothesis explains why different species and individuals attain different maximum heights and why height growth rates vary with resource availability. The article concludes that hydraulic limitations are a universal constraint on tree growth and height, and new techniques are needed to fully understand these processes.