Tree regeneration in black ash (Fraxinus nigra) stands exhibiting crown dieback in Minnesota

Abstract

Crown dieback and mortality of black ash (Fraxinus nigra) has been noted across the range of the species in North America for several decades. Causes of dieback and mortality are not definitive, but may be related to spring drought or excessive moisture. Where black ash is the dominant tree species in the forest, continued dieback and mortality may result in open, non-forest systems in the future. There is only limited research that has examined tree regeneration in black ash-dominated forests and none in stands experiencing dieback and mortality. Such studies are needed to better understand likely successional dynamics of impacted stands. Our objective was to quantify tree regeneration of black ash and other species in stands with and without crown dieback and mortality, to elucidate potential successional trajectories with decline and loss of black ash. We assessed 54 stands in Minnesota exhibiting a range of black ash crown dieback and mortality. Dieback and mortality in the black ash sapling layer were positively correlated with the same condition in the overstory, suggesting that saplings may not replace dead overstory trees. The next most abundant sapling-sized species was speckled alder, a non-canopy species, indicating that replacement tree species for black ash are not currently abundant. Black ash seedling densities were generally low, suggesting that a pool of advance regeneration is not available to replace black ash. Other canopy potential species were limited in the seedling layer, while abundance of shrubs was generally high. Our results suggest a high potential for loss of a tree layer in these stands and conversion to open, shrub dominated systems.