Genetic enhancement of tree species is integral to global forest management practices with mass propagation of enhanced plant material being used to reforest whole landscapes. It is unclear, however, how genetic enhancement of basic traits such as tree growth may influence the function of life supporting soil ecosystems. We studied the potential cascading effects of genetic increases in growth of Norway spruce (Picea abies) on a range of soil chemical and biological properties. Because this species is a prime candidate for the genetic enhancement of boreal forest landscapes and it has been introduced around the world, its impacts on soil microbiomes are likely of importance both locally and globally. In a 40-year common garden, we assessed how genetic increases in growth generated through controlled crossing of high-quality "plus" trees from across the central boreal zone of Sweden influenced a range of soil properties beneath the canopies. Properties included pH, carbon, nitrogen, nitrate, ammonium, phosphate, respiration rate, and the composition of microbial communities assessed via phospholipid fatty acids (PLFAs). We found that Norway spruce family significantly affected each of the seven chemical properties assessed, with differences of up to 140% among families, and that three of the seven were significantly correlated with mean family growth rate. We also found that fungal PLFAs varied significantly across Norway spruce families, but these differences were not strongly related to mean family growth rate. This study, representing just one cycle of selective breeding, suggests that genetic increases in tree growth rates may also be inadvertently altering soil communities and ecosystem services. Such alterations across forest landscapes may have unexpected implications for the function of forest ecosystems (i.e., nutrient cycling) as well as processes of global significance (i.e., carbon sequestration)
