Wide-spread vulnerability of black ash (Fraxinus nigra Marsh.) wetlands in Minnesota USA to loss of tree dominance from invasive emerald ash borer

Abstract

The emerald ash borer (EAB) has killed ash species (Fraxinus L.) in much of eastern North America, but it has yet to reach the vast ash wetlands in northern Minnesota, USA. In these wetlands, a single species, black ash (Fraxinus nigra Marsh.) comprises a majority of trees and has a foundational role in controlling ecosystem function. Given the likelihood of wide-spread mortality of black ash from EAB and the likelihood of severe ecosystem impacts, we examined the potential for co-occurring tree species to replace black ash, either through gap filling in the overstory or release from the understory. We addressed this objective by examining woody plant communities in 32 mature black ash sites located across a large geographic region and inclusive of two distinct wetland types as defined by hydrologic regime. Our results indicate a region-wide lack of species capable of replacing black ash in both wetland hydrologic types; thus there is very low existing potential for replacement of black ash from expansion in the overstory or through release in the understory. These results point to an urgent need for silvicultural intervention to identify and establish future-adapted non-ash tree species so as to promote resilience in the face of EAB by maintaining aspects of the foundational role of black ash in controlling ecosystem functions.