Disentangling the effects of root foraging and inherent growth rate on plant biomass accumulation in heterogeneous environments : a modelling study

Empirical evidence indicates that fast-growing species generally display a higher degree of selective root placement in heterogeneous environments than slow-growing species. Such root foraging is accomplished by root morphological responses, but since some morphological responses are simply the result of enhanced growth of the roots in the enriched patch it is difficult to separate the effects of root foraging and growth rate on the biomass accumulation of species in heterogeneous environments. Here a simple model is presented to disentangle these effects. Root foraging is incorporated as the selective allocation of root biomass per unit time to the nitrogen-rich patch. Growth rate differences among the model plants result from differences in nitrogen utilization efficiency. In the model, the degree of selective root placement can be varied independently of growth rate. The model shows that when plants are compared at a common point in time, selective root placement and growth rate interact positively with respect to the enhancement of plant biomass accumulation in heterogeneous compared to homogeneous environments. However, by evaluating the model at a common plant biomass, the main and interactive effects of growth rate are eliminated. These results suggest that growth rate by itself does not confer an advantage in terms of resource acquisition and biomass accumulation in heterogeneous environments. Only the selective placement of resource acquiring structures (such as roots) leads to such benefits. The essential differences between foraging and growth, as well as the consequences of differences in foraging ability and growth rate between species on competition for a limited resource, are discussed.