Poplar is a widespread hardwood species in France, occupying just over 194,000 hectares, putting France well ahead of the rest of Western Europe.
Although poplars account for over 1% of France’s forest area, they contribute 28% of the hardwood harvest, coming in second place (1.4 Mm3) after oak (2.2 Mm3) in 2021 (total harvest of 5 Mm3).
Poplars are a fast-growing species with a high biological production of 12 m3/ha/year (between spruce and Douglas fir). Over a period of 18 years, a hectare of poplar can produce up to 200 m3 of timber, 2/3 of which is used for peeling and 1/3 for sawing (in addition to timber, smaller-diameter roundwood can be used, in addition to softwood, to make paper pulp).
Cultivars and conservation of poplar genetic resources
In total, France has a register of around sixty poplar cultivars, listed in the “tested and considered interesting for the country’s conditions” category.
But when we talk about “the poplar”, we should be talking about “the poplars”. In fact, the “Populus” genus contains 37 species worldwide (Europe, Asia, North America, etc.). Among the native species in France, the best known are the White Poplar (Populus alba), the Aspen (Populus tremula), and the Black Poplar (Populus nigra), which is sometimes used as an emblem. These spontaneous species are most often found in alluvial areas, including riparian zones, in a procession with willows, for example. As we shall see below, cultivated poplars are mainly made up of hybrids of different species of poplar found around the world.
Black Poplar occurs naturally along French rivers, either as scattered individuals or as relict stands. It is also marketed in the form of six multi-clonal varieties.
Italian Poplar (Populus nigra italica) is a selected variety of Black Poplar. It has been widely distributed and used as an ornamental or windbreak in France since 1750. It has been shown that the Italian poplar, which is a male poplar with a high pollination density, has been widely hybridised with natural black poplars for over two centuries. The wood from these poplars does not normally have the qualities required for peeling.
Why are we talking about cultivars?
Some varieties can be grown for ornamental purposes. This is particularly true of the Italian poplar. But when we talk about cultivated varieties, we are thinking mainly of poplars used for timber production. These are known as “cultivars” for “cultivated varieties”. Cultivars are mainly hybrids between different species, selected for their health tolerance, growth and wood quality. When one of these hybrid trees has good characteristics, it is reproduced by vegetative propagation (cuttings).
Each cultivar has a name under which it is marketed. For example, the cultivars Koster, Diva, Tucano, I45-51, Vesten… are among the most widely planted cultivars today.
Obtaining new cultivars is a long process, taking an average of twenty years, which requires strict monitoring, rigorous attention, and a great deal of know-how. Generally speaking, the selection is based mainly on growth, straightness, shape and disease tolerance. In recent years, criteria such as drought tolerance and wood quality have also come into play.
Breeders initially select parent trees (males and females) according to potentially heritable criteria such as disease tolerance, growth capacity and shape (straightness and branching). Once the resulting seeds have been crossed and germinated, the long selection process begins on the young plants, until only a few dozen that look promising are retained. These will in turn be selected according to the qualities and defects that emerge over the years. The few trees resulting from this initial selection work will then be given a name and multiplied by cuttings, to be tested in groups at different locations in France. Each year, they will be meticulously examined and measured, and their behaviour in the field will be studied. If the results are good, they can then be approved for marketing.
Pests and the effects of climate on poplar cultivation
Like most plant and forest species, poplars have their more or less regular enemies and pests. Whether they be insects or larvae (such as codling moths, poplar borers, etc.), mammals (rabbits, coypu, beavers, deer or squirrels, etc.) or fungi (responsible for leaf diseases, rust, etc.), they all have their place in the plant. Whether they attack leaves and buds, bark, trunk wood or twigs, these pest attacks can cause considerable damage: from slowing growth to damaging the mechanical integrity of the tree and increasing the risk of breakage, these attacks can lead to the tree dying out altogether.
Preventive or control measures that are compatible with environmental restrictions may be available but are technically difficult to implement and not always economically viable. This is one of the major challenges in selecting new cultivars that are more resistant and better adapted to different regions and risk exposure.
Poplars, like many other trees, are sensitive to ‘extreme’ climatic conditions such as prolonged drought, intense frosts, or climatic incidents such as storms, even more so if they are weakened by pest attacks. Work on cultivars is part of the answer, as it will enable poplar plantations to be adapted to climate change.
It is essential to stress that poplar cultivation in France has made significant progress in recent years, thanks to the Merci le Peuplier initiative, which has enabled 800,000 seedlings to be planted in less than 10 years. As elsewhere in the world, France is also experiencing a sharp increase in demand for poplar wood, particularly from the veneer industry (plywood and light packaging). The planting dynamic must be maintained to satisfy the growing interest in poplar wood, with its incomparable mechanical/weight ratio.
Programmes to conserve genetic resources and select appropriate cultivars are essential to adapting the poplar plantation of the future and ensuring the long-term sustainability and health of this precious forest resource in France. It is equally important to ensure appropriate protection of today’s poplar stand to limit the consequences of climate change and maintain a successful and resilient poplar crop in the country.