Initial Conditions, History and Successional Pathways in Ten Contrasting Old Fields

Abstract

-We examined the PCA ordination scores of 10 old fields through 22 yr of succession. Clonal species Dactylis glomerata and Hieracium pratense had the largest correlation with the first principal component in the 1st 18 yr of succession. After 18 yr, Rosa multiflora and Toxicodendron radicans had the largest correlation with the first principal component. Successional pathways of the 10 old fields show great variation in early succession but variation of pathway decreases later. Comparison with a null model reveals that the trend of increasing similarity of old fields through time is real and convergence is occurring. Because mechanisms causing convergence might include the effect of initial abandonment conditions, we divided the 10 old fields into groups defined by the last crop, season of abandonment, year of abandonment, and mode of abandonment (plowed or not). Analysis of the minimum spanning tree needed to connect the old fields of the same age in ordination space showed that the greatest difference between groups was in the 1st 8 yr between old fields that had Dactylis glomerata (orchard grass) as a last crop and those that did not. Of common abandonment conditions, last crop greatly influences successional pathways and convergence in these old fields.