Differentiating the effects of pot size and nutrient availability on plant biomass and allocation

L. Hess and H. de Kroon (2007. J. Ecol. 95: 241-251) hypothesize that apparent plant responses to neighbour presence could be artifacts of pot-size manipulations. They posit that root size increases with pot size, regardless of nutrient concentration, while plant size increases with nutrient amount in the pot. Simple manipulations of pot size do not test this hypothesis, because larger pots contain more nutrients when nutrients are applied in solution at constant rate. We tested the effects of pot size on growth and allocation in two life stages of Cakile edentula (Bigel.) Hook. subsp. edentula var. lacustris while manipulating nutrient amount and concentration. Although pot size did affect growth and allocation, the patterns were not those predicted by the Hess and de Kroon hypotheses. In several treatments, total plant mass decreased with pot size. Root mass did not increase with pot volume independently of nutrients. Root mass increased with pot volume only when plants were given soluble fertilizer, and to a much greater extent in older plants. Contrary to predictions, root mass increased with pot size only in the treatment with high water-soluble fertilizer where plant size increased greatly with pot size. Root allocation showed a complex pattern of responses with plant life stage and nutrient treatments that were not predicted by the Hess and de Kroon hypotheses.