Tree growth patterns and soil nutrient variation in a maple-basswood forest restoration after sixteen years

Growth patterns and survival of trees in restored maple-basswood forest in southeastern MN, USA, were examined over a sixteen-year period. Heights and diameters of 1,001 individually tagged trees were measured in four adjacent old-field sites. Mean tree heights and mortality varied by species and with soil moisture and nutrient levels. The tallest species, Quercus rubra, Fraxinus americana, and Tilia americana, were 6-7 m while the shorter species, Juglans nigra and Acer saccharum, were 3-4 m. Tree mortality rates ranged from 17.6% in Tilia americana and 26.3% in Quercus rubra to 51.3 % in Acer saccharum. Soil extraction and ion exchange resins showed lower phosphate levels and ash free dry mass in the restored forest sites than in nearby mature forests. These data suggest that nutrient levels change slowly in agricultural soils. Tree growth patterns of Fraxinus, Juglans and Quercus fit a linear growth model; while Acer and all trees combined closely fit an exponential growth model. Variability in mean height increased over time within species as some individuals became much larger than others. Recent growth rates slowed in moderately shade tolerant species like Fraxinus americana, while shade tolerant species, such as Acer saccharum, still showed rapid growth. Acer saccharum is expected to become a more dominant component of the forest as growth rates decrease, more trees begin reproducing, and the canopy closes over.