Occurrence, pattern of change, and factors associated with American beech-dominance in stands of the northeastern USA forest

American beech (Fagus grandifolia Ehrh) in the understory of tolerant hardwood stands is a major obstacle to improving the quality of forest regeneration in the northeastern USA and southeastern Canada. To better understand patterns of beech occurrence and stand conditions when it is present, we used US Forest Service Forest Inventory and Analysis (USFS-FIA) data from four northeastern USA states (Maine, New Hampshire, New York, and Vermont) across a 16-year period (1999-2015) to: (i) classify stand conditions (i.e., FIA plots) where American beech was present, (ii) identify beech-dominated stand types, (iii) assess the trend of occurrence (percentage of total forest area) of each identified stand type across the study area during the past 16 years, and (iv) evaluate the relationship of each identified stand type with key biotic and abiotic factors. Based on understory, midstory, and overstory characteristics, we identified four stand types where beech was present across the region including: (i) beech-dominated, (ii) commercial hardwood-dominated, (iii) other hardwood-dominated, and (iv) softwood-dominated. Among these four stand types, the beech-dominated stand type currently occupies 27.4% of total northeastern USA forest area and has had a ∼5% increase over the past 16 years with an even greater increase (>5%) across the forests of New Hampshire, New York, and Vermont. Our results showed increasing mean annual precipitation and greater overstory basal area were positively correlated with the occurrence probability of the beech-dominated stand type, but negatively correlated with the occurrence probabilities of the other identified stand types. Beech-dominated stands were generally associated with higher elevations, greater mean annual precipitation, warmer temperatures, and larger overstory basal areas. Beech dominance in the understory was associated with overstories dominated by beech, as well as overstories dominated by commercial hardwood species. Therefore, overstory harvesting in stands with beech dominant in the understory (irrespective of overstory composition) will tend to promote beech into the overstory. Our results emphasize the need for greater attention to improving regeneration composition in stands where beech is dominant to increase the long-term productivity and commercial value of northeastern forests of the USA.