Population-level responses to nutrient heterogeneity and density by Abutilon theophrasti (Malvaceae): An experimental neighborhood approach

An experimental approach was used to examine the effects of spatial nutrien t heterogeneity and planting density on the sizes of plants within populati ons of Abutilon theophrasti. Planting locations were generated using random numbers and replicated among populations growing on two different scales o f heterogeneity and homogeneous soils. The same quantity of nutrients (dehy drated cow manure) was added to each population, regardless of the spatial nutrient distribution. The higher density was achieved by adding additional planting locations to those present at the lower density. Plant biomass wa s compared among ten planting locations present in all populations. Plants in seven locations were smaller at the higher density, but the spatial dist ribution of nutrients affected plant size in only two locations. At the pop ulation level, the higher density reduced mean plant biomass and increased both total biomass and the coefficient of variation in biomass, a measure o f size inequality. Only when populations on both scales of heterogeneity we re together compared with those on homogeneous soils were population-level measurements found to be significantly affected by soil treatment; heteroge neity resulted in decreased total biomass and an increase in the coefficien t of variation, apparently due to an increase in the number of small plants in the population. These results, together with the finding that fine root biomass increased in nutrient-enriched patches, suggest that on heterogene ous soils most plants were able to access nutrient patches.