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 tha n slow-growing species. Such root foraging is accomplished by root morpholo gical responses, but since some morphological responses are simply the resu lt 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 accum ulation of species in heterogeneous environments. Here a simple model is pr esented to disentangle these effects. Root foraging is incorporated as the selective allocation of root biomass per unit time to the nitrogen-rich pat ch. Growth rate differences among the model plants result from differences in nitrogen utilization efficiency. In the model, the degree of selective r oot placement can be varied independently of growth rate. The model shows t hat when plants are compared at a common point in time, selective root plac ement and growth rate interact positively with respect to the enhancement o f plant biomass accumulation in heterogeneous compared to homogeneous envir onments. However, by evaluating the model at a common plant biomass, the ma in and interactive effects of growth rate are eliminated. These results sug gest that growth rate by itself does not confer an advantage in terms of re source acquisition and biomass accumulation in heterogeneous environments. Only the selective placement of resource acquiring structures (such as root s) leads to such benefits. The essential differences between foraging and g rowth, as well as the consequences of differences in foraging ability and g rowth rate between species on competition for a limited resource, are discu ssed. (C) 1999 Annals of Botany Company.