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Are mixed-tree plantations including a nitrogen-fixing species more productive than monocultures?

Marron, Nicolas, Epron, Daniel
Forest ecology and management 2019 v.441 pp. 242-252
Eucalyptus, Fabaceae, biomass production, case studies, latitude, legumes, meta-analysis, nitrogen, nitrogen fixation, plantations, soil, temperate zones, trees
The inclusion of N2-fixing tree species in tree plantations has the potential to increase biomass production compared to monocultures. Both successes and failures have been described in the literature; however, it is still difficult to distinguish a general pattern and to disentangle the factors influencing the mixture effect. The first objective of this study was to provide an overview of the published data on the effect of the introduction of N2-fixing trees in tree plantations through a meta-analysis approach and to calculate a mean effect of mixed-tree plantations on biomass production compared to monocultures of the non N2-fixing species in stands 2–20 years of age. The second objective was to evaluate the effects of (1) climate zone (temperate vs. tropical), (2) the species used (eucalypts vs. other non N2-fixing species, and leguminous tree species vs. other N2-fixing species), (3) the proportion of N2-fixing species compared to the non-fixing species, and (4) plant developmental stage. A total of 148 case studies from 34 experimental plantations under tropical (68 case studies) and temperate (80 case studies) conditions were identified from the literature. The global mixture effect was significantly positive, mixed-tree plantations being 18% more productive than the non N2-fixing monocultures, and this effect was significantly different from zero under temperate conditions (24% more productive) but not under tropical conditions (12% more productive). Indeed, the sites where the positive mixture effect was significantly different from zero were mostly located in a temperate climate, where soil nitrogen is generally considered less available than in tropical latitudes. Intermediate and high proportions of N2-fixing species gave similar positive results (27% more productive), while low proportions had no significant impact. Neither plantation age nor type of N2-fixing species (legume trees vs. other N2-fixing species) had any significant effect. In conclusion, it appears that climate is the main factor influencing the success of the mixture; however, it also seems that the degree of mixture success is more marked on sites with low biomass production where the monoculture is the least productive.