Walnut tree plantations whose final product was lumber traditionally existed throughout the entire Mediterranean basin. The walnut tree, in addition to producing fruit for many years, was cut down and used for its wood. Before the 1950s, Spain was a country with a great many Juglans regia L. trees and was considered an exporter of walnut wood, a native/naturalized species very present in fields throughout the Iberian Peninsula and well adapted to very different conditions.
Starting in the first third of the 20th century, indiscriminate lumbering was carried out without any accompanying replantation of new trees, nor was any support given to the systems of regeneration. As living standards rose, so too did the consumption and demand for quality wood, which could only be satisfied by turning to imported wood.
Good walnut tree groves exist in lowland areas and the bottoms of valleys with good soils.
In recent years, broad-leaved plantations for quality wood production have become increasingly attractive due to the scarcity of this type of natural wood, agricultural policies of reforestation, their environmental benefits and, most importantly, the search for more profitable productive alternatives.
The quality of walnut wood has been widely recognized since the 14th century, when the use of furniture began to be widespread in Europe. Nowadays, it is still used to make furniture, especially quality veneers and overlays, although it is also used for other purposes, most notably for the manufacture of shotgun butts.
An economic and environmental option for many former agricultural lands is to repopulate them with tree species that produce quality wood. These species can also be used to repopulate the borders, banks and edges of streams and rivers and the zones less suited for agriculture of numerous estates whose primary function is agricultural. These species can also be used for the reforestation of forest lands, albeit limiting their plantation to small zones with appropriate edaphic-climatic characteristics, which would lead to the formation of small, dispersed woods.
Plantations of species that produce quality wood are considered to be an alternative to the traditional uses of numerous estates. The primary attractions of this alternative are its economic profitability (revaluation of the estate, given that the value of the expected production has a direct repercussion on the value of the estate), and its capacity to create environmental wealth and biological and landscape diversity. The subsidy currently offered by the European Union for changes in crops must also be taken into consideration, given that it makes these plantations competitive against most widespread traditional crops.
CHARACTERISTICS AND USES OF WALNUT WOOD:
The sapwood is perfectly differentiated and varies from grey to clear brown, while the heartwood ranges from greyish brown to greyish and has a very dark grain. The radii are very hard to perceive and the rings are notable but only lightly marked. The fiber is straight although at times wavy and the grain is thick. When boiled, it acquires a reddish tone and the grain becomes more evident; it should be dried slowly or else deformations and internal splitting may occur.
Sawing walnut wood is difficult due to its hardness and the defects of some pieces used. Nor is it apt for rotary cutting, but yes for obtaining flat veneer. For years almost all walnut wood has been flat cut and old trunks with lumps and knots are highly valued for use as visible veneer or overlay for furniture.
It is also suitable for curving, gluing and finishing, and can be easily nailed or screwed. For this reason it is used in indoor carpentry to cover doors, floors, friezes, moldings and stairs. It is also used as a decorative covering, in high-quality cabinetmaking, the crafting of small objects and above all for the butts of firearms.
EXISTING VEGETAL MATERIAL FOR THESE PLANTATIONS:
Among the possible Juglans used for forestry production, there are two differentiated groups, the pure speciess and the interspecific hybrid species. There are two pure species used for producing wood: one is J. regia and the other is J. nigra.
This is a species native to Central Asia although naturalized in Europe, with an excellent ability to adapt to the climate and soil conditions of the Mediterranean area. Its habitat is that of a solitary tree, which means it needs ample living space. It resists winter temperatures of up to -20ºC, although some varieties have adapted to continental conditions (-30ºC). It doesn’t put up with flooding/waterlogging and always grows in places where it has access to some type of hydric source. It needs deep soil to grow well. In plantations, it grows well in terms of height but the thickening of the trunk is often slow in relation to branching, which can be slight and vigorous. Its traditional period of production is 50 years, but new plantations that receive appropriate care have expected periods of 30 to 35 years. In many traditional zones, wood and fruit production often coexist.
The existence of a vegetal material offering guarantees for wood plantations of J. regia was necessary.
Esta especie es originaria de América del Norte y es conocida como nogal negro, por el color oscuro de su madera. Es una especie de hábitat forestal que aguanta muy bien el frío invernal, hasta –45ºC. Para tener un buen crecimiento la especie precisa de suelos profundos, y destaca por su resistencia al encharcamiento temporal. Presenta una marcada dominancia apical y la ramificación es abundante. En plantación tradicional, entorno tipo bosque, su turno se establece entre 40 y 50 años; en las nuevas plantaciones en las que se aplica técnicas selvícolas y de cultivo más controladas se rebaja este turno a 30 años.
This species is native to North America and is known as the black walnut tree because of the dark color of its wood. It is a species of a forest habitat that resists winter temperatures very well, up to -45º C. Deep soil is needed for the species to grow well, and its notable for its resistance to seasonal flooding. It has a marked apical dominance and abundant branching. In traditional plantations, in a wood-like setting, its period of production is between 40 and 50 years; in new plantations that apply forestry techniques and swhere growth is more controlled, this period can be reduced to 30 years.
CYCLES OF LUMBERING AND PRODUCTION:
Straight trees are sought, with a diameter greater than 45 cm at chest height, without branches or knots until at least 3 or 4 meters of height and with a regular growth, whose final purpose will be to produce quality veneer or board. For this purpose, during the first 5 years the tree is subjected to training pruning, about which we shall talk later. With a plantation and appropriate care, these goals can be reached in 20-30 years after planting. The period can be lengthened in order to obtain exceptional sizes and darker colors that multiply the wood’s value. The maximum age for felling shouldn’t be greater than 80 – 90 years.
According to Luna (1990), the walnut tree can have an average annual growth of 2.5 to 3 cm in circumference and a production of 1 to 3.5 m3/ha/year. Walnut trees for wood at the age of lumbering (25-30 years) approximately can have a wood volume of 1 to 1.3 m3. The price paid for walnut wood oscillates between 600€ and 1,500€ per m3, depending on the quality and the end use. In recent decades, hardwood prices have risen consistently and consumer trends indicate that demand for this wood will continue in the future.
WALNUT TREE ECOLOGY:
The walnut tree is a species with important spatial requirements and reacts very poorly to competition. It is typically found on border areas or in isolation, related with human settlements given that humans have mostly been responsible for distributing it. This species is allelopathic, in that it generates substances that impede or make difficult the development of other species in its vicinity.
The climate requisites of the walnut tree are minor, given that it is found in zones ranging from hot and dry to others that are cool and humid. It is, however, a species that demands heat during the leafy period. To produce good quantities of wood, the average annual temperature should be above 8º C, or for at least five months of the year to be warmer than 10º C. High temperatures, above 40º C, cause damage to the fruit and the precocious falling off of leaves. The tree is resilient to low winter temperatures, resisting up to -20º C. That is to say, it isn’t the cold that excludes growing walnut trees in the mountains, but rather the low heat in the summer.
The tree requires quite a bit of water. In general, it isn’t recommendable to grow the tree without artificial irrigation if annual rainfall is less than 700 mm. Although rainfall in many Spanish counties doesn’t reach this minimum (< 500 mm), it’s still common to find walnut trees there. They require at least 100-150 mm during the leafy period, although their rainfall demands are very much related to the characteristics of the soil.
The physical characteristics of the soil are more important than its chemical qualities for the walnut tree to adapt well. An appropriate soil for its plantation is one that drains rapidly and is capable of retaining water. For an appropriate retention of water, a content of organic material between 1.5% and 2% is required, and loamy soils (< 25% clay, 30-50% lime and 30-50% sand). These characteristics are found in alluvial, siliceous-clay and clay-lime soils, limestone mesetas, siliceous and rocky soils, etc. The pH of the soil should be about neutral (6.5-7.5). Due to its strongly pivoting root, a minimum soil depth of 0.8 to 1m should be kept in mind. Soils with the phreatic layer above these levels or which are waterlogged for part of the year are not appropriate.
PLANTATION AND CARE:
Plantation density and frame:
Seedlings of 1-2 years of age should be used for the plantation, although those aging 1 year are preferable. The walnut tree is planted in a definitive frame, that is to say, without clearings and all of the trees are cut down at the end of the cycle. The frame of the plantation is very ample given the trees’ need for significant living space for their development; this can range from 7×7 m and 12×12 m depending on the quality of the season, which means a variable density of between 100-250 trees per hectare. In case of mediocre seasons or of seedlings of unknown origin, smaller spacing may be adequate, but never less than 5×5 m, to permit the selection of the best seedlings in the future. Whatever the case, creating clearings has produced poor results, given that the roots remaining in the soil are very sensitive to the dye and easily transmit it to the trees that remain standing through root connections.
The planting is done in holes, dug a few days before, of a size that allows a good placement of the roots of the seedling inside the hole (0.60 x 0.60 x 0.40 meters). Care must be taken when planting the tree to not bury the neck and much less the graft point. The holes are then covered with fine, loose soil. The seedlings must be watered immediately after being planted, with 40 to 50 liters of water for a hole of 60 to 70 cm in diameter poured around the tree’s trunk. The planting can be done throughout the period of vegetative repose, with the best moment being in autumn after the leaves have fallen off, given that in this period the seedlings produce a large mass of roots thanks to the reserves accumulated in the trunk, ensuring a good sprouting in the spring.
Preparation of the terrain and base fertilization:
Before planting, the terrain should be painstakingly prepared, performing different tasks depending on the nature the terrain. If the soil is deep, with a subsoil of the same nature, ripping it is enough, going as deep as possible, with a minimum depth of 0.60 meters. If the soil is shallow and the subsoil isn’t favorable for the development of the roots, it will be necessary to subsoil or rip it two or more times in crossed ways. This operation must be done several months before the planting. One or two shallow tillings should also be done before planting. If the irrigation system is localized, it isn’t necessary to move any earth, an operation that isn’t always beneficial given that it entails modifying the soil’s layers.
Before fertilizing, an analysis of the soil should be made, in order to know what nutritive elements it contains and to be able to correct any insufficiencies regarding those elements with the appropriate fertilizer. The walnut tree demands considerable nitrogen and lesser amounts of phosphorus and potassium. For a soil of medium phosphorous content, it is advisable to add 200 to 250 units of fertilizer per hectare of P2O5, which will appear in the form of superphosphate in calcareous terrains and as hyper-phosphate in acidic terrains. In terms of potassium fertilizer, for a soil with a medium content of this element, it is advisable to add between 300 and 350 fertilizer units per hectare of K2O, which will show up in the form of sulfate in calcareous or poorly drained soils and in the form of chloride in gravelly, light soils or those that are irrigated. In very acidic soils, it is best to add lime to the base fertilizer, with the amount depending on the pH of the soil and its texture.
A base fertilizer allows for a good development of the trees for 20 to 30 years; it is placed 20 to 25 days prior to the planting, spreading the fertilizer over the entire surface of the terrain and burying it through a shallow tilling of the land (for example, with the cultivator) that precedes the planting. If manure is available and the organic richness of the soil is less than 2%, 40 to 60 metric tons of manure can be added per hectare, burying it by tilling up to a depth of 25 to 30 cm.
Irrigation and fertilization:
As mentioned previously, the walnut tree needs a minimum amount of water to be able to vegetate and produce. The correct practice of irrigation is essential to obtaining a fast and homogenous development of the tree and achieving significant future production. Above all, it is necessary for the walnut tree not to suffer from a lack of water during the period of vegetative and formation of the fruit (May, June and July), given that an important hydric deficit exists in this period due to high evapo-transpiration and scarce rainfall, and the formation of significant vegetal mass (leaves, branches, fruit), which require a large amount of water.
On the contrary, during the months of August and September, when the fruit is lignified, it is best to reduce the availability of water (without eliminating it). The amount of water used in irrigation will vary, given that the needs of the seedlings vary greatly according to the hydric index, the soil’s retention capacity and the presence or not of spontaneous vegetation. Forest trees are not very dependent on fertilizer, although fertilizer may be very useful during the first years to speed up growth. If the soil is sufficiently deep and humid, in addition to an initial base fertilizer (prior to planting), growth is usually satisfactory without fertilizer, because the trees are adapted to live in soil with low fertility.
When the soil has a medium level of fertility, enough for the normal growth of the seedlings but for one reason or another one wishes to accelerate that growth – a common goal in very valuable wood species -, fertilizer may be added to the plantation. The type of fertilizer will depend on the effects one wishes to achieve, the demands of the species and the characteristics of the soil.
The primary threat to successful reforestation of agricultural lands is posed by competition from weeds (weeds that damage the primary crop). Competition for water is the most negative factor of the presence of accessory vegetation in Mediterranean climates and especially affects young seedlings with a shallow root system. To combat weeds that compete with the cultivated species, annual work can be done such as hoeing (overturning the upper layer of the soil) either mechanically or manually (around each seedling), or through brushcutting through mechanical or shoulder-strapped brushcutters to eliminate brush vegetation. This work must be done from the moment before vegetative activity begins up to the start of summer, when the dryness of the climate saps the production of weeds.
Pruning is a very important process in walnut tree plantations established for the production of high quality wood. A good wood for veneer must be straight, without branches, with a regular and homogenous growth in diameter, and 3-5 meters of length. The wood must be free of knots, or for these to be located between the first 10-12 cm of the diameter. To obtain wood with these characteristics, pruning is essential in the first years of the plantation. It has been shown that walnut trees pruned during the vegetative period (June and July) generate few or no suckers the following year. Thus, pruning operations and pruning for cleaning can be grouped in a single pass between June 15 and July 15. If this period is respected, the emergence of suckers is very reduced, even when the pruning is relatively drastic.
Detail of a prop and its attachment to the trunk.
It is necessary to insist on the principle of the balance of the crown, that is to say, the conservation of the well-spaced horizontal branches at all levels in such a way that the walnut tree can ascend straight and tall, without losing its stability. Dynamic pruning is currently the most used pruning method and the best for walnut plantations given its effectiveness and easy realization. The goal of this pruning is to form lumber 3 to 4 meters in length with the minimal number possible of pruning scars on the trunk; for this, the lowest branches are removed as soon as possible, while controlling at the same time the development of the crown to avoid having to place props for the trees.
In July of the first two years of the plantation, all of the tree’s branches are eliminated, with the tree having reached about 2 meters in height. In the third and fourth years, the elimination of branches continues on the lower part, leaving some to grow between 2-4 meters in height, and at the same time the length of the branches that have been left must be controlled so that they don’t unbalance the crown. Once the shaft has reached 3-4 meters (5 or 6 years), it is still necessary to cut back or remove some of the remaining horizontal branches. Finally, at 6 or 7 years, the walnut tree is ready. Lastly, it should be mentioned that this type of pruning is not advisable in very windy zones.
The dynamic pruning of the walnut tree.
CURRENT PROJECTS AND RESEARCH:
Since 2003, the company Cultivos Forestales along with the Institut de Recerca i Tecnología Agroalimentaries (I.R.T.A.) have been carrying out a research project called “Selection of material of Juglans for forestry use and production of seedlings in container”. The main goals of the project are: the production of walnut tree seedlings for forestry use in container (the most appropriate container and substrate to be determined); to have available walnut tree seed specially selected for the production of seedlings destined for lumber production; and to establish plots of production of the above-mentioned seed, as a source for the commercial production of seedlings.
One of the actions realized over the course of the project has been to establish seed orchards in different fields to obtain controlled seeds of J. regia from clones previously pre-selected by I.R.T.A. The goal is to find phenotypes with a more vigorous forestry behavior, a mid-to-late sprouting and a greater tolerance to drought..
The seed from a clonal seed orchard brings improvements to the plantation: the characteristics for which the parent plant was selected are known. If, furthermore, the seed-producing trees, both masculine and feminine, are able to contribute defined forestry characteristics, the controlled seed obtained will offer improved, homogenous and repeatable progenies over the years..
Furthermore, another objective of improved progenies or seedlings of J. regia, is that, by applying good growing practices after the plantation, the possibility exists of exploiting the trees after 8-10 years, combining fruit extraction (nuts) and wood production at the end of the cycle.
Another of the actions realized over the course of the project has been to create a seed orchard for the production of hybrid seeds of J. regia x J. nigra. The peculiarity of this hybrid material is that the feminine progenitor is J. regia and the masculine is J. nigra, the opposite of the existing hybrids on the market in which the feminine progenitor is J. nigra and the masculine is J. regia.
As producers of wood, the interspecific hybrids have demonstrated that they are very suitable for forest production in terms of speed of growth and forestry behavior. The feminine parents of the commercial hybrids are all black walnut trees. The maternal contribution has revealed itself to be very important given that the offspring are trees with good forestry conformation, abundant branching with a small branch diameter and marked apical dominance, however, an appropriate hydric presence is necessary for their development. The use of J. regia as the feminine genitor will unquestionably contribute greater rusticity, will more closely align the qualities of the wood to those of common walnut trees and will furthermore avoid one of the principle problems of current hybrid progenies, mainly the trees’ lack of productivity. The nut production of J. nigra is greatly inferior to that of J. regia and enters into production much later. Thus, the wood from a hybrid whose feminine progenitor is J. regia is of greater commercial value, given that in Europe, J. regia is the most valued species for the manufacture of furniture.