  Here tree dependency structures are studied from three different
perspectives: their degree variance (hubiness), the mean dependency length and
the number of dependency crossings. Bounds that reveal pairwise dependencies
among these three metrics are derived. Hubiness (the variance of degrees) plays
a central role: the mean dependency length is bounded below by hubiness while
the number of crossings is bounded above by hubiness. Our findings suggest that
the online memory cost of a sentence might be determined not just by the
ordering of words but also by the hubiness of the underlying structure. The 2nd
moment of degree plays a crucial role that is reminiscent of its role in large
complex networks.
