Biomass production and nitrogen balance of naturally afforested silver birch (Betula pendula Roth.) stand in Estonia

Silver birch (Betula pendula Roth.) is one of the main pioneer tree species occupying large areas of abandoned agricultural lands under natural succession in Estonia. We estimated aboveground biomass (AGB) dynamics during 17 growing seasons, and analysed soil nitrogen (N) and carbon (C) dynamics for 10 year period in a silver birch stand growing on former arable land. Main N fluxes were estimated and nitrogen budget for 10-year-old stand was compiled. The leafless AGB and stem mass of the stand at the age of 17-years were 94 and 76 Mg ha(-1) respectively. The current annual increment (CAI) of stemwood fluctuated, peaking at 10 Mg ha(-1) yr(-1) at the age of 15 years; the mean annual increment (MAI) fluctuated at around 4-5 Mg ha(-1). The annual leaf mass of the stand stabilised at around 3 Mg ha(-1) yr(-1). The stand density decreased from 11600 to 2700 trees ha(-1) in the 8- and 17-year-old stand, respectively. The largest fluxes in N budget were net nitrogen mineralization and gaseous N-2-N emission. The estimated fluxes of N2O and N-2 were 0.12 and 83 kg ha(-1) yr(-1), respectively; N leaching was negligible. Nitrogen retranslocation from senescing leaves was approximately 45 kg ha(-1), N was mainly retranslocated into stembark. The N content in the upper 0-10 cm soil layer increased significantly (145 kg ha(-1)) from 2004 to 2014; soil C content remained stable. Both the woody biomass dynamics and the N cycling of the stand witness the potential for bioenergetics of such ecosystems.