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Establishment of high-productive mixtures

Datum: 2016.11.17

With increased awareness of wood production for bioenergy, high-productive mixed stands become more relevant for forestry where high growth rate, high carbon sequestration and resilience are combined with other important goals. For example, two-storied mixed plantations combining late successional and shade tolerant tree species valuable for high quality timber and for environmental purposes in long rotations with fast-growing tree species in short rotations for biomass production goals is an interesting concept to develop further.

Nurse trees – a cost-effective strategy

Fast growing nurse trees have a potential for rapidly building new forest structures and simultaneously increase productivity, which might be a cost-effective strategy for raising new forests. Nurse trees can reduce competing vegetation, protect against late spring frost, facilitate establishment and improve stem form of slow growing and often shade tolerant target tree species.

There are several species combinations described in the literature that may be suit¬able such as poplar and oak, birch, larch or grey alder under-planted with beech, oak or Norway spruce. Other candidate species for this concept include high productive target species such as Douglas fir or grand fir which are known from Danish forestry to benefit from shelter in the regenera¬tion phase. The many species combinations that are relevant for these systems make it possible to adapt to a wide range of site conditions.

Novel silviculture

The overall yield is expected to increase in two-storied plantations, as have been shown for several combinations such as naturally regenerated mixed stands of for example birch and Norway spruce with 10–20% transgressive over-yielding or trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench.) Voss), also with 20 % transgressive over-yielding. Additional gain is expected by combining this approach with e.g. genetically improved material.

The nurse crop system needs further development to identify appropriate thinning regimes or canopy densities of the nurse crops that allow various main species, with different manage-ment objectives, a successful establishment. A transgressive over-yielding of 10%, depending on product and rotation length, can offset increased costs associated with planting and managing mixed-species stands. The role of high productive mixed forests is, however, unclear regarding protection of biodiversity and therefore this aspect needs to be included in the overall development of these novel silvicultural systems.

Text: Magnus Löf. Magnus Löf is a professor in silviculture at the Swedish University of Agricultural Sciences (SLU). His research interests to date have been related to forest restoration and adaptation of forest management regimes to global change. Thus, for example, he has studied afforestation and reforestation and the restoration of mixed broadleaved-conifer stands through natural and artificial regeneration. The topic of this article was presented at the recent NordGen Forest Conference in Växjö in September 2016

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