canopy height, but must produce their entire support structure each year, whereas
woody plants merely add small increments to their existing structure. The greater mechanical
efficiency of woody plants taller than 0.5 m is probably an important reason why
there are so few species of forest herbs much greater than that height, together with the
inability of taller herbs to exceed their ecological compensation points and achieve
positive carbon balance in deep shade (see Givnish 1984; Raven 1986; and analysis
below).