Compositional convergence and forest architecture following windstorm in two Minnesota forests

Background/Question/Methods

In forests, instances of compositional convergence following wind disturbances are an important phenomenon, but one that should be examined in the context of the forests’ structural architecture. Here we examine the diameter distributions of two compositionally converging forest communities and their constituent species to identify patterns of architectural change through time since a windstorm. We performed this study at the Cedar Creek Natural History Area (CCNHA) LTER Site, Anoka and Isanti Counties, Minnesota, USA. Forests within the CCNHA were struck by a severe windstorm in 1983 that caused widespread tree mortality. Permanent plots were established in two forest communities, one dominated by oak, the other by pine, where all tree species were recorded, stems measured, and damage assessed. Plots were resampled 4 times in the 21 years following the windstorm in order to monitor forest recovery. We used ordination and diameter distributions to describe the compositional and structural characteristics of the two forest communities.
Results/Conclusions

We found that compositional convergence between the forests has been persistent through 21 years of recovery. Structurally, both forest communities transitioned from weakly bimodal diameter distributions to reverse-J distributions. This change seems to be happening at close to the same rate, with the pine forest lagging somewhat behind the oak forest. In contrast, however, the structural architecture of individual species varied between sites through time. In the case of the dominant species (Quercus ellipsoidalis and Pinus strobus), their size hierarchies may prevent total replacement by later successional species. We conclude that at the community level, compositionally converging forests may also have convergent architectural structure through time. However, the architecture of individual species within that overall pattern of convergence may be different. This suggests that convergent forest communities may remain ecologically distinct if the architecture of their constituent species continues to be different through time.