"The trees in question are mountain ash, the tallest flowering trees in the world. They are not quite the tallest trees of any kind: that record belongs to the coast redwoods of the western US. But that might be because things have been skewed against the mountain ash..." - Maitani

"...the tallest trees can suffer from "xylem cavitation", in which gas bubbles form in the cells carrying water up the trunk. These tiny gas embolisms [sic] can prevent water from moving up the tiny conduit cells, much like a pulmonary embolism can stop blood flow to the lungs in humans. To avoid this, the tree regulates how much water is lost through its leaves by closing down the tiny pores all over their surfaces. But these pores are also the pathways for carbon dioxide to come in, so by closing them the trees limit how much sugar they can make." - Maitani