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NordGen Forest Conference 2016 - selected topics

Establishment of high-productive mixtures

Mixed forest plantations can be designed and managed to meet a variety of social, economic, and environmental objectives, and can provide key ecosystem services, help preserve remaining primary forests, and sequester an important proportion of the atmospheric carbon released by human activities.

Mixed_forests_FrancescoPelleri

Photo: Francesco Pelleri
November 2016

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.

Mixed_Forests_PhilComeau

White spruce (Picea glauca) under aspen (Populus tremoluides) is a very common natural mixture in boreal Canada. Its management and productivity have been studied in detail. Here the aspen is a pioneer regenerated after large disturbances such as fire and the white spruce are naturally regenerated afterwards. Photo: Phil Comeau

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

References:

Bergqvist G. (1999). Wood volume yield and stand structure in Norway spruce understorey depend¬ing on birch shelterwood density. Forest Ecology and Management 122: 221–229.

Comeau P. (2014). Effects of aerial strip spraying on mixedwood stand structure and tree growth. Forestry Chronicle 90: 479–485.

Gardiner E.S., Stanturf J.A., Schweitzer C.J. (2004). An afforestation system for restoring bottom¬land hardwood forests: biomass accumulation of nuttall oak seedlings interplanted beneath east¬ern cottonwood. Restoration Ecology 12: 525–532.

Hulvey K.B., Hobbs R.J., Standish R.J., Lindenmayer D.B., Lach L., Perring P. (2013). Benefits of tree mixtures in carbon plantings. Nature Climate Change 3: 869–874.

Kabzems R., Nemec A.L., Farnden C. (2007) Growing trembling aspen and white spruce intimate mixtures: early results (13–17 years) and future projections. BC Journal and Ecosystems and Management 8: 1–14.

Löf M., Bolte A., Jacobs D.F., Jensen A.M. (2014). Nurse trees as a forest restoration tool for mixed plantations: effects on competing vegetation and performance in target tree species. Restoration Ecology 22: 758–765.

Mård H. (1996) The influence of a birch shelter (Betula spp) on the growth of young stands of Picea abies. Scandinavian Journal of Forest Research 11: 343–350.

Nord-Larsen T., Meilby H. (2016). Effects of nurse trees, spacing, and tree species on biomass production in mixed forest plantations. Scandinavian Journal of Forest Research 31: 592–601.

Paquette A., Messier C. (2010). The role of plantations in managing the world’s forests in the Anthropocene. Frontiers in Ecology and the Environment 8(1): 27–34.

Stark H., Nothdurft A., Block J., Bauhus J. (2015) Forest restoration with Betula ssp. And Populus ssp. Nurse crops increases productivity and soil fertility. Forest Ecology and Management 339: 57–70. Tham Å. (1994). Crop plans and yield predictions for Norway spruce (Picea abies (L.) Karst.) and 33 birch (Betula pendula Roth & Betula pubescens Ehrh.) mixtures. Studia Forestalia Suecica 195. 21 p.