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Research article2021Peer reviewed

Belowground resource utilization in monocultures and mixtures of Scots pine and Norway spruce

Lutter, Reimo; Henriksson, Nils; Lim, Hyungwoo; Blasko, Robert; Magh, Ruth-Kristina; Nasholm, Torgny; Nordin, Annika; Lundmark, Tomas; Marshall, John D.


Mixed forests have been recommended to replace monocultures, often being more productive and resilient ecosystems. Those benefits of species mixing have been suggested to be attributed to lower competition (aboveand belowground) due to potential separation of resource acquisition strategies, yet a mechanistic understanding of belowground processes has largely been missing. We applied an isotopic-labelling technique using N-15 and H-2 at similar to 5 cm soil depth to study acquisition of two important resources, nitrogen (N) and water, in a replicated field experiment including mature Scots pine and Norway spruce monocultures and their mixture in boreal Sweden. Based on the isotopic data, we modelled distance-dependent N and water uptake and described the horizontal reach of active rooting areas around trees. The active root area for both N and water uptake in monocultures of both species was approximately 6-10 m(2). Substantially wider areas were observed inside the mixture for N acquisition (27 m(2) for pine and 21 m(2) for spruce). Water was mainly acquired from within a 12-15 m(2) area in mixture. The mixture and the pine monocultures exhibited similar recovery of the added N-15 (3.8-4.5%) and its uptake per unit of foliage mass. The recovery of N-15 for the spruce monocultures was generally lower (2.1%) but no difference was noted in the uptake per unit of foliage mass between spruce monoculture and mixture. No differences were found for recovery of H-2 between the stand types. Mixing pine and spruce did not improve the resource uptake in comparison with the estimated theoretical mixture (according to the normalized basal area in monocultures). However, the mixing revealed significant differences between tree species whereas pines inside the mixture recovered 11.3-fold more N-15 than accompanying spruce. Root overlap of 3-11 trees on 1 m(2) was observed in all stand types, far exceeding the degree of canopy overlap. We discuss this overlap in terms of competition between individual trees. Spruces have proportionally larger horizontal rooting area relative to their aboveground size than pines. Scots pine and Norway spruce mixing in this type of boreal forest does not significantly enhance resource acquisition but leads to compensatory partitioning of growth, suggesting fierce interspecific competition, rather than niche separation.


Mixed forests; Stable Isotopes; Root competition; Nitrogen; Water

Published in

Forest Ecology and Management
2021, Volume: 500, article number: 119647
Publisher: ELSEVIER