Nitrogen fixation rates associated with the feather mosses Pleurozium schreberi and Hylocomium splendens during forest stand development following clear-cutting

• Study on N2-fixation rates along a 123 year chronosequence following clear-cutting.
• N2-fixation increased after cutting, peaked at 16 years, and thereafter declined.
• Total N input was at its peak 1.0 kg N ha−1 year−1 and on average 0.4 kg ha−1 year−1.
• Fixation rates correlated with nutrient pools, NH4+, moss and dwarf shrub biomass.
• N2-fixation contributed on average ∼9% of total N accrued.

Pleurocarpous feather mosses host di-nitrogen (N2) fixing cyanobacteria, and this association serves as an important source of N input to late-successional natural boreal forests. However, little is known about how forest management affects feather mosses and their associated N2-fixation rates, or how these rates change during post-logging stand development. We established a chronosequence of 32 forest stands used for commercial wood production to better understand how stand development after clear-cutting drives changes in biomass and N2-fixation rates of the two dominant feather mosses, Pleurozium schreberi and Hylocomium splendens. These stands included eight replicate stands of each of four stand types: (1) recently clear-cut and newly planted stands (CC, 4 years); (2) pre-commercial thinning stands (PCT, 16 years); (3) first thinning stands (T1, 34 years); and (4) mature uncut forest (MF, 123 years), all dominated by Pinus sylvestris. We found that clear-cutting did not reduce moss biomass relative to the uncut forest. Further, biomass of P. schreberi (but not of H. splendens) increased twofold from CC stands to PCT stands, and remained high throughout the T1 stands. Di-nitrogen fixation capacity, determined as the amount of N fixed per unit moss mass, was ca. six and three times larger in PCT stands compared to the other stand types for P. schreberi and H. splendens respectively. Correlation analyses showed that N2-fixation capacity associated with both moss species increased with increasing Empetrum hermaphroditum biomass, and that N2-fixation capacity of P. schreberi declined with increasing NH4+ availability. Further, correlation analysis showed that N2-fixation capacity of H. splendens declined with increasing tree biomass and decreasing light transmission. The total amount of N fixed at the stand level was highest in the PCT stands (1.0 kg ha−1 year−1 of N), and was associated with both high moss biomass and high N2-fixation capacity. The contribution of N2-fixation to total N accrual per hectare during stand development was ca. 9%, and across the chronosequence N2 was fixed on average at rates of 0.4 kg ha−1 year−1. Our results show that N2-fixation rates in feather moss communities were promoted by the conditions at the PCT stands approximately 16 years after clear-cutting, while N2-fixation rates were lowest under conditions at the newly clear-cut and mature stands. Further, it suggests that mosses and associated N2-fixation can be important in maintaining a long-term N balance, and that this source of N input should be accounted for when modeling N balance in N-limited managed boreal forests.

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