This paper chooses two news-genre dependency treebanks, one in Chinese and one in English and examines the synergetics or the interrelations among dependency tree widths, heights and sentence lengths in the framework of dependency grammar. When sentences grow longer, the dependency trees grow both taller and higher. The growths of heights and widths are competing with each other, resulting in a minimization of dependency distance. The product of the widest layer and its width corresponds to the sentence length. These correlations are integrated into a tentative synergetic syntactic model based on the framework of dependency grammar.