Five tree species comprise nearly 80% of the forest standing stock volume in Western Europe, often in monoculture. These forests are increasingly suffering from climate change and bio-aggressors, confirming the need for forest diversification. In this context, birch colonising forest gaps, e.g., after clear-cuts or windfalls, may be an interesting tree species. The aim of the thesis is to analyse the potential of silver (Betula pendula Roth) and downy (B. pubescens Ehrh.) birch for forestry and the forest-based industry sector, and to provide silvicultural recommendations.
Firstly, we described birch resource (Chapter 2) in terms of a) distribution and abundance, using available forest inventories; b) stand structure and composition, through the example of Wallonia, using data from the Permanent Inventory of Forest Resources in Wallonia (IPRFW); c) evolution, based on literature and on the IPRFW data. Birch has a very wide distribution across Eurasia, dominated by downy birch in the north, where birch is the main hardwood species, and dominated by silver birch in temperate climates. In Western Europe, birch comprises 0.5-6% of the forest standing stock volume, depending on the country. In Wallonia, only 30% of the birch volume concerns pure birch stands. Most of the birch resource is distributed in all kinds of stand composed by other species, mostly with European oaks, and in natural regenerations and in coppice and coppice-with-standards forests. Birch stems with diameter at breast height (dbh) larger than 38 cm and with no visible defect (e.g., branches and knots) are rare (less than 0.3% of the standing stock volume). On the other hand, birch standing stock volume appears to rapidly increase over time.
Secondly, we analysed the potential of birch for forestry and the forest-based industry sector by compiling and discussing literature, and we organised the results according to its strengths, weaknesses, opportunities and threats (SWOT) (Chapter 3). Among the strengths, birch is suitable for a wide variety of climatic conditions; it performs well on a wide range of soils; it has the capacity to adapt physically to changes in its environment and it has a high genetic diversity; to date, it has not been seriously affected by pests or diseases; it promotes biodiversity; it optimises soil functioning; it can compose a variety of mixed stands, and as a nurse species, it facilitates the development of other tree species; wood properties allow versatile timber uses, as illustrated by the market in Northern Europe, in the Baltic States and in Russia. Foresters should manage its weaknesses, such as high sensitivity to crown competition and to wood decay, and its abundance in tree species mixtures. Socio-economic and climatic context offers opportunities to birch for colonising forest gaps that have not been planted or in which birch is one of the rare species to resist to game overpopulation. It also benefits from policies promoting forest diversification and from favourable public perception for its ecological roles. The main obstacles to birch development in Western Europe are the lack of information on its strengths and uses, the absence of silvicultural guidelines adapted to its weaknesses, and the lack of standards for wood use in construction. Based on birch resource and on industry’s organisation, foresters should aim at the rapid production of high quality (large and free from defect) birch logs. Crop tree silviculture is a means to optimise dbh growth by crown release and quality by pruning.
Thirdly, we analysed dbh growth and crown development of 704 crown released target birches in 38 dense birch natural regenerations in southern Belgium and north-eastern France (Chapter 4). We found that birch can reach a dbh of 50 cm in less than 60 years. Crown diameter evolution model indicates that target birches with a dbh of 40, 50 or 60 cm have a crown of about 8, 10 or 12 m wide, respectively. We also compared dbh growth of target birches in response to different crown release earliness in three subsamples. Without intervention, dbh growth declines due to competition as soon as the stand reaches the age of 4-5 years. With crown release at this age, dbh growth is doubled and could remain higher than that of birches later crown released, at least for 20 years. When crowns are released later than 9-12 years of age, birches appear to have reduced their dbh growth potential, and they may never reach the needed dimension for best timber uses.
The discussion (Chapter 5) begins with a reflection about the link between science and empirical knowledge in this research. Secondly, the chapter articulates findings of the three research through successive questions that the forester may ask when choosing which tree species to favour. Birch seems to be an interesting forest tree species, from an autoecological point of view, for its ecological and silvicultural functions at the service of other species, and to produce valuable wood. Industrial use of birch wood is realistic in the Western European context. We therefore proposed silvicultural guidelines for producing high-quality logs, and general management guidelines for common stands composed with birch. Finally, perspectives are proposed. From a practical point of view, there is a need to inform foresters and timber industry about the potential of birch and to develop guidelines and technical training about birch silviculture, and standards for the use of birch wood. Concerning research, we should study the link between birch productivity and site, add experimental plots for dbh growth measuring, and create working groups including scientists, foresters and timber industry.