Recovery of post-fire forest sites with alder admixture—biodynamic principles and perspective

Introducing N-fixing species into non-N-fixing tree stands may accelerate the recovery of soil function and restoration of forest ecosystems after fire through a higher rate of soil organic carbon (SOC) and nitrogen (N) accumulation. This assumption derives from a biodynamic approach to restoring degraded ecosystem functions. Here, we examine the effects of admixing alder ( Alnus glutinosa (L.) Gaertn.), a nitrogen-fixing species, into even-aged common birch ( Betula pendula Roth) stands approximately 30 years after fire, focusing on soil properties, tree biometric parameters, and recovery of tree nutrient supply at wildfire-affected sites. Our results indicate that the admixture of alder into even-aged birch stands improved soil physicochemical properties. This effect manifested primarily as higher SOC (34.23 g kg −1 ) and N (2.39 g kg −1 ) content in the soils under mixed birch–alder stands compared to pure (25.38 g kg −1 and 1.71 g kg −1 for SOC and N, respectively) birch stands. Birches growing in mixed stands were also characterized by higher biometric parameters, particularly height (13.0 m), compared with birches in pure stands (height = 10.2 m). Therefore, in degraded sites after large-scale fires, we recommend applying the biodynamic approach of admixing N-fixing alder into birch monocultures.