Analysis of patterns and processes underlying the development of plant invasion patches may contribute to our general understanding of how invasive plants spread. The objective of this study was to reevaluate population development in formerly studied invasion patches of perennial forbs. Herb-chronology (analysis of annual rings in the roots of forbs) was used in this and in the former study to analyze the spatial age structure in stands of spotted knapweed and yellow foxglove in Michigan. Although invasion patches of the two species showed a pronounced spatial age structure in 1999, suggesting frontlike invasion, this pattern was largely or completely lost in 2003. These findings demonstrate how population growth trajectories in invasion patches may take a relatively sudden change because of altered growth conditions or consolidation of the population structure. In addition, they indicate the importance of the timing of a posteriori analyses into the development of plant invasion patches relative to successional stage.
Nomenclature: Spotted knapweed, Centaurea maculosa Lam. #3 CENMA; yellow foxglove, Digitalis grandiflora Mill.
Additional index words: Age determination, age distribution, annual rings, herbaceous perennials, herb-chronology, spatiotemporal invasion pattern.