Sodium dynamics in forest ecosystems and the animal starvation hypothesis.

Sodium may be a critical limiting element for certain vertebrate herbivore populations. It has been hypothesized that the relative exclusion of sodium from the tissues of most land plants may help them against grazing by making it difficult for the grazers to obtain as much of this ion as they need. Contrary to this animal starvation hypothesis, we contend that forest trees do not exhibit any aboveground allocation strategy for this element and that sodium concentrations in forest trees are at least one to two orders of magnitude above those levels found in soil percolates. As a null hypothesis, we state that sodium levels in plants do not affect levels of herbivory; however, alternatives include not only the animal starvation hypothesis but also its antithesis, i.e., sodium levels observed in plants stimulate consumption.