Abstract
Mixed cropping in short rotation coppice can be an alternative to monocultures. To design optimized mixtures, field trials are needed. Poplar, as an economically important and fast-growing species, and black locust, as a nitrogen-fixing species, are promising candidates for such studies. RNA sequencing (RNA-seq) was used to monitor effects of mixed and pure cultivations on the gene expression of poplar along with growth measurements during 2017 and 2018. Both biomass production and leaf transcriptomes revealed a strong competition pressure of black locust and the abiotic environment on poplar trees. Gene expression differed between the two study sites and pure and mixed stands. Shading effects from black locust caused the downregulation of photosynthesis and upregulation of shade avoidance genes in mixed stands in 2017. As a result of higher light availability after cutting black locust, plant organ development genes were upregulated in mixed stands in 2018. Drought conditions during the summer of 2018 and competition for water between the two species caused the upregulation of drought stress response genes in mixed stands and at the unfavorable growing site. Further investigations are required to discover the mechanisms of interspecific competition and to develop stand designs, which could increase the success and productivity of mixed plantations.
Highlights
Modern forestry is challenged to meet different and growing demands of society
Poplar cuttings and black locust seedlings were planted at two ecologically different study sites near Goettingen, Germany, in April 2014 to test for differences in the growth performance of eight commercially used poplar clones in monoculture and in mixture with black locust, representing three provenances (Table 1)
DWD weather data show that April 2014, when trees were planted, was unusually warm (11.4 ◦C) and dry (22.9 mm)
Summary
Modern forestry is challenged to meet different and growing demands of society. On the one hand, it has to be highly productive and efficient, and on the other hand, it should take care to protect environment and biodiversity. Mixing different species and growing them together to achieve a positive effect in productivity has been practiced by humans for centuries on a small scale. These practices need to be tested before implementing them into industrial forestry. Many studies were conducted to identify conditions for high productivity of mixed stands. This includes testing species mixtures, planting schemes, interspecific competition effects, soil, climate, etc. Observed effects on stand productivity of such mixtures varied broadly in different studies from 50% less to three times higher productivity than in pure stands [8,9,20,21,22,23,24]. We tested how combinations of poplar genotypes (mostly poplar hybrids) growing together with black locust (a nitrogen-fixing species) in mixed forest experimental stands influenced gene expression
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