Utility of Herbaceous Annual Rings as Markers of Plant Response to Disturbance: A Case Study Using Roots of a Common Milkweed Species of the US Tallgrass Prairie

Abstract

Herb-chronology, the study of annual growth rings in the root to shoot transitional zone of perennial forbs, involves efforts mostly devoted to finding correlations between growth increment and annual climate. The potential of using growth rings as markers of plant growth response to more ecological phenomena such as periodic disturbance still remains mostly untapped. By monitoring the 2016 growth increment of 64 individuals of a common milkweed species (Asclepias viridis) from the US tallgrass prairie system we investigate plant response to factorial treatments of early season shoot removal and neighbor thinning. These treatments simulate bouts of herbivory, grazing, and dormant-season fire, each of which should have differential effects on plant growth. Neighbor thinning had the strongest effects of the study, moderately increasing ring widths. Conversely, ring widths were moderately decreased by shoot removal. Individual age had negative effects on ring width. These results are the first evidence of herbaceous annual ring sensitivity to sudden amelioration of resources as well as a significant loss of aboveground biomass. Herb-chronology could be useful in future studies monitoring the effects of disturbance on plant growth, increasing our understanding of these phenomena and their overall effect on grassland composition.