Looking for promising silvicultural treatments to regenerate boreal forests

Clearcutting is the dominant harvesting method in boreal forests of North America and Scandinavia, but today there is a growing interest in silvicultural alternatives that offer a more sustainable forest management. A new study of black spruce regeneration under different experimental harvest treatments suggests that shelterwood is an efficient option to regenerate Canadian forests. Harvesting mini-strips (5 meters wide) that are surrounded by equally sized strips of intact forest was one promising method. Combined with soil scarification it enabled adequate natural regeneration in black spruce stands, reaching a high level of stocking and seedling density.

The study was published recently in Frontiers in Plant Science by Miguel Montoro Girona, currently working at SLU in Umeå in the Ecological Restoration Group at the department of Wildlife, Fish and Environmental Sciences, and colleagues from the University of Quebec at Chicoutimi and from the Canadian Forest Service.

Clearcuttings are criticized for creating highly fragmented landscapes, declines in habitat diversity and losses of productivity (Photo 1). Therefore, there is often a desire to maintain a continuity in the tree layer to maintain biological and social values, such as biodiversity and an attractive landscape. In many countries, researchers are now investigating silvicultural alternatives to today’s clearcuttings, or adaptations, that make it possible to conciliate commercial timber harvest with biodiversity conservation.

Photo 1: Forest state after clearcutting treatment.

“One of the biggest challenges in the boreal region today is to make forest ecosystems more resilient to climate change, and there is much work to do in Scandinavia to diversify silvicultural practices, thereby increasing the heterogeneity in forest structures and species composition”, says Miguel Montoro Girona.

Miguel Montoro Girona and his colleagues have focused on black spruce (Picea mariana), a tree species that is widely spread in northern North America (from Quebec to Alaska) and has an economic significance for the forest industry due to its excellent wood properties. The black spruce is a shade-tolerant species that, in the absence of fire, regenerates by layering from mature trees, but also with seeds when germination conditions are favorable (e.g. mineral soil).

The management strategies that the researchers wanted to investigate were partial cuttings surrounded by seed trees or experimental shelterwoods, which are potential options for forestry in eastern Canada for two reasons (Photo 2). One is that they make it possible to combine commercial forestry with the conservation of important ecological values ​​of mature stands. The second is that many environmental certificates do not allow reforestation by planting, but require natural regeneration.


Photo 2: Spatial patterns of trails and residual strips in the study treatments. White areas represent the harvested surface or intervention trails, black areas indicate the intact residual strips.

In a unique experiment, the researchers have now compared black spruce regeneration in clearcuttings with that in harvested strips surrounded by different combinations of seed trees and shelterwood (Photo 3). In order to favour seedlings, scarified squares with exposed mineral soil were created to reduce competition from other vegetation. In this way, they could study natural regeneration of black spruce (age, diameter, height etc.) along gradients of environmental conditions, regarding factors such as solar radiation, soil moisture, microsites, competition, distance to seed trees, etc. More than 30 variables were measured in this experiment at the site, plot and tree level.

Photo 3: Experiment design where A and B correspond to two study blocks. Numbers represent the experimental silvicultural treatments: 1) Control, 2) Mini-strip shelterwood, 3) Distant selection, 4) Close selection 5) Seed-trees and 6) Clear-cut.

“In summary, our results show that shelterwood and seed trees, combined with soil scarification, produce a density of conifer regeneration that is sufficient to maintain forest productivity. They also show that these management methods are viable alternatives to clearcutting, when required by sustainable forest management objectives, says Miguel Montoro Girona.

Photo 4: Regeneration 0 and 10 years after experimental partial cuttings in Canada. 

One treatment that was particularly promising was to harvest mini-strips over a width of five metres, then leaving a strip of equal measure intact, and so on. The secret of this success was that the residual trees provide lateral shadow and scarification exposes the mineral soil: It is the best conditions to promote the natural regeneration of black spruce. Consequently, some microplots (4m2) contained more than 200 seedlings (Photo 4 and 5).

Photo 5: Satisfactory regeneration ten years after experimental partial cutting in Canada. 

The experiments were carried out in forests dominated by even-aged black spruce (established after forest fire) in one of the most productive forest areas in Eastern Canada. Soil scarification turned out to be necessary for a high black spruce seedling density (Photo 6). Otherwise, a species that is less attractive for the industry would be favoured, namely the balsam fir (Abies balsamea), whose larger seeds are better at establishing themselves in undisturbed humus layers.

Photo 6: State of black spruce stands after mini-strip harvest and soil scarification.

Regeneration in black spruce is very complex due to the vegetative and sexual strategies and continue being a subject where there are considerable gaps in knowledge. Thus, the experiment’s layout makes the results of great value, both through the new basic knowledge of the regeneration biology of the black spruce and through the clear link to practical forestry. To reach it, two persons have counted and measured all conifer seedlings inside 1545 microplots during 6 months.

“There are no other long-term studies that have monitored seedling responses along a gradient of different forest management alternatives”, explains Miguel Montoro Girona. “This study provides the regeneration response ten years after treatment, while most other studies only cover a few years”.

Miguel Montoro Girona emphasizes that it is important to take into account the composition of a forest – for example when it comes to tree species and age distribution – when selecting the harvesting method to use.

“Harvests can resemble natural disturbances, such as storms and fires, and must be adapted to local conditions. In some areas, clearcuttings may work, while they do not in others”.

Miguel Montoro Girona is currently working at SLU in Umeå, where he models the effects of natural disturbances in forests, by factors such as moose, storms and insect outbreaks, in a changing climate.

More information

Contact person

Miguel Montoro Girona, Postdoctor
Department of Wildlife, Fish, and Environmental Studies
Swedish University of Agricultural Sciences
E-mail: [email protected]

Personal website: https://miguelmontorogirona.weebly.com/

CV:  https://www.slu.se/en/cv/miguel-montoro-girona/

Twitter: https://twitter.com/Boreal_Spanish?lang=fr

Virtual visit to the experiment: http://visitesvirtuelles.partenariat.qc.ca/monts-valin/

The article

Miguel Montoro Girona, Jean-Martin Lussier, Hubert Morin and Nelson Thiffault. Conifer regeneration after experimental shelterwood and seed-tree treatments in boreal forests: Finding silvicultural alternatives. Front. Plant Sci. 9:1145. doi: 10.3389/fpls.2018.01145

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