FINE-ROOT GROWTH AND DEMOGRAPHIC RESPONSES TO NUTRIENT PATCHES IN 4 OLD-FIELD PLANT-SPECIES

Proliferation of roots in a nutrient patch can occur either as a resul t of an increase in root length (morphological response) or by a chang e in root birth or death rates (demographic responses). In this study we attempted to distinguish between these two mechanisms of response t o nutrient patches and to compare the responses of four old-field plan t species (two annuals, two perennials). For all four species combined , there were significant increases in root numbers and root length in fertilized patches. Root proliferation in fertilized patches was large ly due to increased birth ( = branching) rates of new roots. However, there was also a significant increase in root death rates in the ferti lized patches which reduced the magnitude of the increase in net root numbers. Plots for individual species suggested they differed in the m agnitude and timing of root proliferation in fertilized patches due to differences in root birth and death rates. However, because of the li mited sample size in this study, there was only a marginally significa nt difference among species in root birth rates, and no difference in death rates. Further studies are currently underway to better quantify species differences in the demographic mechanism, as well as magnitud e, of response to nutrient patches and if this would affect the abilit y to exploit small-scale heterogeneity in soil resources.