THE ECOLOGICAL SIGNIFICANCE OF PLASTICITY IN ROOT WEIGHT RATIO IN RESPONSE TO NITROGEN - OPINION

We analyzed data on root weight ratio from a range of experimental stu dies documenting plant allocation changes in response to altered nitro gen availability. Our goal was to determine the degree to which plasti city in allocation between roots and shoots exists and to search for p atterns in such plasticity among species. Our survey included 77 studi es representing 206 cases and 129 species. As expected, we found that root weight ratio decreased with increased nitrogen availability in th e majority of cases examined, and this response was most consistent wh en plants were grown individually or in intraspecific competition (ver sus interspecific competition). Surprisingly, however, we found no evi dence to support existing hypotheses that fast-growing species adapted to high soil fertilities exhibit the highest levels of morphological plasticity, or that plasticity is positively associated with competiti ve ability. Rather, we found that average amounts of plasticity in roo t weight ratio in response to nitrogen availability were similar among species grouped by maximum relative growth rate and habitat fertility . Similar results were obtained for species categorized by life form, life history or root weight ratio itself, and plasticity in root weigh t ratio also had no consistent relationship with competitive ability. Numerous difficulties are associated with the attempt to search for pa ttern using independent studies, however our results lead to the concl usion that strong patterns in plasticity of root weight ratio in respo nse to nitrogen availability among species do not exist. We discuss tw o reasons for this: (1) the costs of plasticity relative to its benefi ts are lower than previously predicted and (2) plasticity in traits ot her than root weight ratio is more important to plant foraging ability .