Several species of the foxglove tree family Paulowniaceae, also known as the empress or princess tree, are native to China, Taiwan, Korea and Japan. All species of the genus are relatively fast-growing and have excellent timber properties. Within 20 years, individual trees can produce up to one cubic metre of timber.
Sunny, sheltered positions and light, well-aerated sandy soils are ideal for the Paulownia. In its natural range, average annual precipitation is between 500 mm and 2500 mm. As long as sufficient water is available to it in the vegetation period, it can also thrive on dry sites. Under favourable conditions, the life expectancy of the Paulownia is 60 to 70 years.
The Chinese princess tree (Paulownia tomentosa) was first brought to Europe in 1830. Its occurrence has also been documented in Germany since the late 19th century. Here it has been able to establish itself outside botanic gardens and arboretums, especially in the west (Rhineland Pfalz and Baden Württemberg), and it has proven cultivable.
In comparison with other Paulownia species, P. tomentosa is considered to be relatively hardy, being able to tolerate temperatures down to -20 °C, with young trees being somewhat more susceptible than older ones. The tree species has so far proven highly resistant to disease and infestation with insects.
The timber of the Paulownia can be used for many different purposes. It is easy to work with and has good physical properties. It is light-coloured, light-weight, hard and very durable. Thanks to its stability of form, it is suitable for roof struts, doors, windows and partition walls. It has one of the lowest values for thermal conductivity, and is thus suitable for use as an insulating material. Because of its excellent acoustic qualities it is also used to make musical instruments.
At the moment there is not however enough sawn Paulownia timber on the market in Germany. Even with direct imports from China, there are only limited quantities available, and the sawn Paulownia timber imported from China does not have the required qualities, either. If we were to succeed in cultivating Paulownia in greater quantities in Europe, the situation could be improved.
P. tomentosa is a fast-growing, shade-intolerant tree species that reaches its most vigorous growth rate at around 20 years old, but lives a relatively short time. Under favourable conditions it may achieve an average height growth of up to 1.6 metres and increase its diameter by four centimetres per year. Ten-year-old trees reach an average diameter (BHD) of 35 to 40 centimetres and accumulate around 0.5 cubic metres of timber, with some specimens even producing up to 1.5 cubic metres. Because of its high growth rate, this species is also suitable for biomass production and wood chip production in short rotation plantations.
Cultivation trials over extensive areas with Paulownia have only taken place in Germany in the last few years. A research project of the Technical University of Munich has been set up to show whether Paulownia thrives under current climatic conditions, whether timber with the expected favourable characteristics can be produced, and how long it takes to produce timber of commercially viable dimensions. From the spring of 2009, P. tomentosa was cultivated from seed. It was planted out in the autumn of 2011 in three different places in Bavaria (Unterfranken, Niederbayern, Oberbayern) along a climate gradient.
The research focuses mainly on recording the success of the establishment measures, the survival percentage, growth, intra-specific competition and susceptibility to disease and pests. Special attention is paid to the onset of flowering, as the species is considered to be invasive in some countries because of its abundant and lightweight seeds and its persistence (reproduction from coppice shoots and root shoots).
After the establishment phase, the study investigates whether silvicultural tending measures can successfully improve the growth of individual trees and the stand, and improve the quality of the trunk wood. Research into the timber quality rounds off the programme, being carried out until the end of the project period in the year 2020.
Testing of the germination rate resulted in the following figures for living sprouts per kilogramme of seed:
- P. tomentosa 1.65 million
- P. elongata 0.04 million
- P. catalpifolia 0.19 million
This more or less correlates with the results of another investigation into germination rates (Table 1). Detailed investigations into sprout behaviour at the ecological botanic garden in Bayreuth can be summarised as follows:
The Paulownia species differ in both the weight of their seeds and in the biomass of their sprouts. Seeds of P. tomentosa had the highest germination percentages, followed by those of the hybrids. Seeds of P. elongata and P. catalpifolia proved to be largely unable to germinate. In open countryside, the seeds of none of the species germinated.
The seedlings for the project were cultivated at the experimental forest garden in Grafrath (FVG). Table 2 gives an overview of the number of seedlings produced in the experimental forest garden.
Paulownia displays mainly sympodial branching patterns. As the terminal buds die off during the dormancy period, the development of the main shoot is interrupted, and the lateral shoots take on the main growth in the next vegetation period. The natural development of the trunk can thus result in very different shapes, and shapes undesirable in terms of timber use.
Considerably more tending is thus necessary than is customary with monopodially or monoaxially growing trees in order to correct the growth forms. Often, however, the simple cutting of the young trees allows the stump shoots to re-grow very long in the first year, thus creating a long, straight first branch-free section of trunk.
Because of its fast growth, its excellent timber properties and its adaptability to the prevailing climatic conditions in Bavaria, Paulownia could in fact be a really valuable asset, and a beacon of hope for the forestry industry. However, before the tree is cultivated in practice, we should wait for the results of trials. To avoid unforeseeable risks, we should not cultivate this non-native tree species in large numbers, either. As well as its cultivation on short rotation plantations and high-value timber plantations, individual groups of Paulownia can be established in the forest to reduce the economic risk of forest companies and for aesthetic reasons (attractive blossoms).