Shade affects fine-root morphology in range-encroaching eastern redcedars (Juniperus virginiana) more than competition, soil fertility and pH

Abstract

Fine-root morphological variables are often directly correlated with soil nutrient availability and water absorption. Indirectly, these morphological variables could be related to nutrient depletion by other plants. The relationships of fine-root variables with shade, direct or indirect, are unknown. I tested the relationships between a number of soil quality variables (levels of nitrogen and lime), competition with a co-dominant tree, and shade for the range-encroaching eastern redcedar Juniperus virginiana. I measured seven fine-root parameters as indices of potential nutrient-uptake or competitive ability. I also assessed the relationships of these fine-root parameters with foraging return (i.e., tissue nitrogen, non-structural carbohydrates, relative growth rate and biomass). There were no significant (main) effects of soil amendments (added nitrogen and lime) or tree competition on any of the seven root parameters. There was a significant (indirect) effect of shade (affecting five out of seven fine-root morphological variables), and a significant interaction effect between fertilizer and lime amendments, although the last-mentioned interaction affected one fine-root variable only. There were two additional pieces of evidence that support the effect of shade: a significant difference between shaded and unshaded plants in the slope of the negative relationship between root diameter and specific root length, and a significant overall difference among all fine-root morphological variables in a principal components analysis between shaded and unshaded plants. I found that fractal dimension was the only variable for which there were positive correlations with foraging return, probably because a higher fractal dimension leads to more effective use of nutrients. I expected a negative correlation between foraging returns and fine-root characteristics that incurred substantial costs. I only found a significant cost for specific root length, which may be mediated by the unmeasured parameter of root lifespan.