Tree seedling growth: effects of shaking.

ence in the productivity of the vegetation on the control plots. Odum et al. (2) observed comparable differences in productivity during old field succession, but these were attributable to previous fertilization and changes in the growth forms of the dominants, factors not adequately discussed by Hurd et al. We have also observed significant differences in the primary production of herbaceous communities among fields of different quality, but not among fields differing principally in age (3). These data suggest that site quality rather than age may have been the more important of the confounded variables in the experiment of Hurd et al. BUFORD HoI.T Biology Department, Brookhaven National Laboratory, Upton, New York 11973