Yield table for selected black locust (Robinia pseudoacacia L.) cultivars

Nowadays, black locust (Robinia pseudoacacia L.), the tree species native to North-America, is naturalized and widely planted throughout the world from temperate to subtropical areas (Nicolescu et al., 2018). It was the first North-American tree species imported to Europe at the beginning of the 17 century (Demené and Merzeau, 2007). Since (1710s) its introduction into Hungary black locust has been closely associated with agriculture, and its wood could be utilized for many agricultural and domestic purposes. Recently, black locust has played a role of great importance in the Hungarian forest management, covering approximately 25% of the forested area. Almost half of the approximately 1.6 million m of black locust harvested annually is of industrial wood quality, the rest is only suitable as firewood (Rédei et al., 2017; Nicolescu et al., 2018) In Hungary, after World War II, significance of black locust changed, because large-scale farms had less demand for wood and the timber industry was not willing to buy black locust wood. It was necessary to improve the quality of final products of black locust. Selection breeding was a good option for it. The main target of the selection breeding was to improve the quality of stem, to increase the output of industrial wood. Besides the varieties recommended for timber production as their primary function, there are others that have been improved for honey production or energy production. New cultivars had to be produced by improvement techniques introducing them into the practical forestry use (Keresztesi, 1988; Rédei et al., 2011; 2017). Monoand multiclonal cultivars were developed and a seed orchard was established from the selections. Some cultivars are suitable for both forestry and honey production. Such double-use cultivars are ’Zalai’, ’Kiskunsági’, ’Császártöltési’, ’Egylevelű’, and ’Váti46’ (Keresztesi, 1988). Black locust cultivars that can be suitable for sawlogs production are of decisive importance for primary wood production. Based on this, we selected ‘Nyírség’, ‘Üllői’ and ‘Jászkiséri’ cultivars for systematic yield studies. Based on the evaluations of long-term yield experimental plots (Bujtás, 1984; Keresztesi, 1988; Rédei, 1984, 1994, 2006, 2008, 2013; Hegede, 2018; Rédei et al., 2020) it has also become possible to compile their yield table, which is the first to be published in English in this publication. The Hungarian primary wood production practice uses yield tables with 6 yield classes (Sopp and Kolozs, 2000). However, the cultivation technological objectives and economic aspects of plantation forestry are mainly take into account only the stands of I–IV. yield classes. Consequently, our table presented in this paper contains only 5 yield classes.


INTRODUCTION
Nowadays, black locust (Robinia pseudoacacia L.), the tree species native to North-America, is naturalized and widely planted throughout the world from temperate to subtropical areas (Nicolescu et al., 2018). It was the first North-American tree species imported to Europe at the beginning of the 17 th century (Demené and Merzeau, 2007).
Since (1710s) its introduction into Hungary black locust has been closely associated with agriculture, and its wood could be utilized for many agricultural and domestic purposes. Recently, black locust has played a role of great importance in the Hungarian forest management, covering approximately 25% of the forested area. Almost half of the approximately 1.6 million m 3 of black locust harvested annually is of industrial wood quality, the rest is only suitable as firewood (Rédei et al., 2017;Nicolescu et al., 2018) In Hungary, after World War II, significance of black locust changed, because large-scale farms had less demand for wood and the timber industry was not willing to buy black locust wood. It was necessary to improve the quality of final products of black locust. Selection breeding was a good option for it. The main target of the selection breeding was to improve the quality of stem, to increase the output of industrial wood. Besides the varieties recommended for timber production as their primary function, there are others that have been improved for honey production or energy production. New cultivars had to be produced by improvement techniques introducing them into the practical forestry use (Keresztesi, 1988;Rédei et al., 2011;2017).
Black locust cultivars that can be suitable for sawlogs production are of decisive importance for primary wood production. Based on this, we selected 'Nyírség', 'Üllői' and 'Jászkiséri' cultivars for systematic yield studies. Based on the evaluations of long-term yield experimental plots (Bujtás, 1984;Keresztesi, 1988;Rédei, 1984Rédei, , 1994Rédei, , 2006Rédei, , 2008Rédei, , 2013Hegede, 2018;Rédei et al., 2020) it has also become possible to compile their yield table, which is the first to be published in English in this publication.
The Hungarian primary wood production practice uses yield tables with 6 yield classes (Sopp and Kolozs, 2000). However, the cultivation technological objectives and economic aspects of plantation forestry are mainly take into account only the stands of I-IV. yield classes. Consequently, our table presented in this paper contains only 5 yield classes.

MATERIALS AND METHODS
The yield table was constructed from data gathered on 29 forest subcompartments (56 sampling plots), located in different parts of Hungary ( Figure 1). The age of the examined stands varied between 5 and 35 years. Their sites are typically forest-steppe and Turkey oak-sessile oak forest climate, free-draining site, humic sand and brown forest soil. The initial stem number of afforestations varied between 3000-5000 stems/ha. In stands up to the age of 10 years, one, and in the case of stands over the age of 18-20 years, two stem number reductions were carried out. The sampling plots were square shaped, their areas approximately 1000 m 2 , the boundaries were marked. In the course of the stand surveys the key stand characteristics were measured, and then, on the basis of data collected were calculated the average height, diameter (DBH), volume, basal area and stem number given separately for the main (remaining), secondary (removal) and total stands per hectare (Husch et al., 1982;Avery and Buckhart, 1994;Röhle, 1999;Laar and Akca, 2007). We classified each tree according to their functional role as follows, class 1: future croptree, class 2: subdominant trees, and class 3: trees to extract.
Stem volume was estimated by the following volume function (Sopp and Kolozs, 2000),

RESULTS AND DISCUSSION
Yield class is called the intensity of height growth of a given stand compared to the all same tree species of the country, from best to worst from I to V marked by Roman numerals (stands are classified into the yield class I-V). The normative yield table prepared for the above mentioned black locust selected cultivars contains the most important stand structure and yield factors of the main, removal and total stand (height, diameter at breast height, basal area, volume, age and number of stems), divided into five yield classes with the same relative height growth.
When using the yield table for determining the actual volume per ha (Vact) of a stand, a basal area ratio is to be recommended: The standard deviation of the average height of the stands from cultivars is shown in Figure 2, and the numerical yield table with five yield classes is shown in Table 1.

CONCLUSIONS
The yield table demonstrated in this paper supposedly applies to average rather than full stocking. In other words, an empirical yield table applies only to the average density levels found on the sample plots used. In the last decades, growth models focused on stand level data have gradually been replaced by stand growth models that predict stem number frequencies and individual-tree growth models. In spite of this fact yield tables will remain very useful tools for forest management and forest inventory in the future.
The published yield table can be widely utilized in the following fields of the Hungarian black locust cultivars' management,  appraisal of statistical nature of the black locust cultivars' plantations and stands,  harvest scheduling of black locust cultivars' stands, implementing the volume estimations,  elaborating or further developing silvicultural (tending operation) models for black locust cultivars' stands.
Due to the characteristics of the experiments, the above presented results in connection with the research on the yield, stand structure and tending operations of improved black locust cultivars are preliminary investigations. Thus, the reported yield model and tending guidelines need further improving and care should be taken in applying them.