Nitrogen dynamics in managed boreal forests: Recent advances and future research directions

Abstract

Nitrogen (N) availability plays multiple roles in the boreal landscape, as a limiting nutrient to forest growth, determinant of terrestrial biodiversity, and agent of eutrophication in aquatic ecosystems. We review existing research on forest N dynamics in northern landscapes and address the effects of management and environmental change on internal cycling and export. Current research foci include resolving the nutritional importance of different N forms to trees and establishing how tree-mycorrhizal relationships influence N limitation. In addition, understanding how forest responses to external N inputs are mediated by above- and belowground ecosystem compartments remains an important challenge. Finally, forestry generates a mosaic of successional patches in managed forest landscapes, with differing levels of N input, biological demand, and hydrological loss. The balance among these processes influences the temporal patterns of stream water chemistry and the long-term viability of forest growth. Ultimately, managing forests to keep pace with increasing demands for biomass production, while minimizing environmental degradation, will require multi-scale and interdisciplinary perspectives on landscape N dynamics.

Keywords: Boreal forests; Clear cutting; Forest management; Nitrogen fixation; Plant–soil interactions; Watersheds.

Fig. 1

Total tree volume and annual growth increment for Västerbotten and Norrbotten counties in northern Sweden. Data are based on measurements from approximately 7000 national forest inventory (NFI) plots (Swedish NFI 1953–2012). Annual growth increment is based on a five-year moving average

Fig. 2

Summary of key pools and fluxes characteristic of a northern boreal watershed. Ranges for different N fluxes (kg ha⁻¹ year⁻¹) are based on literature values for northern Sweden and Finland wherever possible, and linked to the following references: 1 Gundale et al. ; 2 Lindo et al. ; 3 Kortelainen et al. ; 4 Korhonen et al. ; 5 Futter et al. , Palviainen et al. ; 6 Mustajärvi et al. ; 7 Binkley and Högberg ; 8 Ukonmaanaho et al. ; 9 Gundale et al. ; 10 Olsson et al. ; 11 Nohrstedt et al. ; 12 Kortelainen et al. , . N₂O fluxes (ref 3) are based on soils with C/N > 20. Blue and yellow circles represent rates of inorganic and organic N (DIN and DON) leaching from undisturbed and recently clear-cut forests, respectively. Rates of internal N cycling and export by forest stands can be highly variable in space and time, and the estimates for net nitrification and denitrification are particularly uncertain, but are very likely to be low in northern boreal forests. The forest inset provides ranges for N stocks in overstory, surface soils, and mineral soil compartments (see Egnell et al. ; Stuiver et al. ; Maaroufi et al. 2015)

Fig. 3

Long-term trends in a river inorganic nitrogen (DIN) concentration, b forest volume, c mean annual precipitation, and d mean annual temperature for the Vindeln River watershed of northern Sweden (10 000 km²). DIN represents the sum of nitrate and ammonium for samples collected monthly by the Swedish national monitoring program (Fölster et al. 2014). Forest volume was determined from national forest inventory (NFI) plots located within watershed boundaries (Lucas et al., 2016). Precipitation and temperature were estimated from the Hydrologiska Byråns Vattenbalansavdelning (PTHBV) database (from the Swedish Meteorological and Hydrological Institute, SMHI; Sponseller et al. 2014)

Fig. 4

Stommel diagram summarizing key elements of N dynamics in boreal landscapes and their potential interrelationships across spatial and temporal scales. Stand-scale biomass production (green) represents the primary target of forest agencies and private landowners. This yield reflects ecological processes (blue with green arrows) related to tree N nutrition and a variety of plant–soil interactions that constrain N availability at small scales, as well as the constraints on growth imposed by broad-scale variation in climate and atmospheric N inputs. In addition, management-related decisions (yellow with red arrows) at the national scale drive local forestry operations that additionally influence ecosystem N dynamics, N losses to aquatic ecosystems, and tree growth. Boxes summarize key questions related to forest N cycling at each of these scales, as highlighted in the text