0501-B3

Early growth of fast-growing broadleaved tree species on abandoned agricultural lands in Estonia

Aivo Vares ¹ , Veiko Uri ², Hardi Tullus ³ and Arno Kanal ⁴

Abstract

The present paper is based on the study of 21 plantations of broadleaved trees growing on abandoned agricultural land in Estonia. For the establishment of the plantations, four fast-growing broadleaved tree species (Betula pendula Roth., Alnus incana L. Moench., Alnus hybrida A. Br., Populus x wettsteinii Hämet-Ahti) were used as the planting material. Silver birch as the most important tree species in Estonia in the economic aspect grew well on Glossic Podzoluvisol/Mollic Glossagualf, Calcaric Luvisol/Oxyaquic Eutrudept and Dystric Gleysol/Typic Endoaquent. Grey alder and hybrid alder appeared to grow faster on Glossic Podzoluvisol/Mollic Glossagualf compared with silver birch and hybrid aspen. All studied fast-growing broadleaved tree species are suitable for afforestation of abandoned agricultural lands in Estonia.

Introduction

During the last decade the economic situation changed drastically in Estonia as well as in other post-socialist Eastern and Central European countries (Mander and Jongman 2000). The total area of Estonia is 4.5 million ha, including 2.2 million ha of forest land and 1.1 million ha of agricultural land. Due to socio-economic reasons the intensity of the use of agricultural land decreased significantly in Estonia, as a result of which at least 228 000 ha of abandoned agricultural land have come into existence (Meiner 1999). In the last decade part of this area was already regenerated naturally with broadleaved pioneer tree species: alders, birches, aspens and willows. Unfortunately, the structure of the natural regeneration is highly variable and the economic value of naturally regenerated stands is low. Thus, afforestation of abandoned agricultural lands enables to increase the stands' economic and ecological value. The attention attracted by broadleaved trees is the result of the increased need for energy and pulp, as well as of the possibility to afforest abandoned agricultural areas. Because of their low disease resistance, coniferous species are not recommended for afforestation of abandoned agricultural areas in Eastern and Northern Europe. Besides, broadleaved species have also become valued in sustainable forestry.

Very few studies have dealt with afforestation of abandoned agricultural land with broadleaved tree species in the Estonian conditions. Therefore, experimental plantations of grey and hybrid alder (Alnus incana L. Moench., Alnus hybrida A. Br), silver birch (Betula pendula Roth.) and hybrid aspen (Populus x wettsteinii Hämet-Ahti) were established and monitored in 1995-2002.

The aims of the present study were (i) to investigate the mean height and annual height growth of four fast-growing broadleaved tree species on different soil types after the fourth growing season, (ii) to find out the suitability of the studied fast-growing broadleaved tree species for afforestation of abandoned agricultural land in Estonia and (iii) to give recommendations for practical applications. Owing to comparable climatic conditions, our results may also present interest in the Baltic Sea region.

Material and methods

The present paper is based on the study of 21 plantations of broadleaved trees on private agricultural land, which are located in different parts of Estonia (58-59ºN; 22-28ºE)(Figure 1).

Figure 1. Location of the plantations (1- silver birch; 2 - grey alder; 3 - hybrid alder; 4 - hybrid aspen) in Estonia.

The plantations of silver birch and hybrid aspen were established in spring of 1999 and the plantations of grey alder and hybrid alder, already in 1995 and 1996, respectively. In the establishing the plantations, mainly one-two-year-old seedlings from the nursery were used. Fixed planting density was used for the tree species in each plantation. Soil preparation (ploughing) was done in most plantations to suppress weed competition and to improve soil properties (Table 1).

Following the principle of contingency, 0.1 ha sample plots were established in each plantation. The location of the sample plots in the plantations was marked to facilitate finding and re-measuring of model trees in the following years. In each sample plot, the height and the annual height growth of all trees (minimum 100 trees) were measured. Observation of the plantations was based on the method of single-tree, in which all measured trees were marked with numbered metal labels for further long-term observation. In the case of silver birch and alders, the trees were measured during four growing seasons after the establisment of the plantations, while the plantations of hybrid aspen were monitored only after the third and the fourth growing seasons.

In all plantations one characteristic soil pit (down to a depth of 1.0 m) was prepared and soil type was determined according to the FAO-UNESCO and USDA classifications (FAO-UNESCO 1994; Keys to soil taxonomy 1998). Bulk density from each genetic horizon was estimated with a 50 cm³ open-ended steel cylinder. Soil samples were taken from each genetic horizon and were analysed for pH_KCl, total Kjeldahl nitrogen (Tecator ASN 3313), available (ammonium lactate extractable) phosphorus (Tecator ASTN 9/84) and potassium by flame photometric method (Table 2). Organic carbon was analysed by wetchemical combustion with CrO₃+H₂SO₄ according to Tinsley (1950).

Table 1. Planted broadleaved trees, survival of the plants after the first growing season, initial height of the planting stock, planting density and soil preparation in the plantations

One-way analysis of variance (ANNOVA) and T-test were used in statistical analysis (p<0.05). Throughout the study, the means are presented together with standard error (_ SE).

Table 2. Main soil characteristics of topsoils in the studied plantations

Results and discussion

The survival of silver birches during the four growing seasons was high in all 11 plantations under investigation. By the end of the fourth growing season the tallest silver birches were measured in the plantation of Veneküla (2.7±0.2 m) and the shortest ones, in the plantation of Reigi (1.4±0.1 m). Mean annual height growth for the fourth growing season, too, was the most intensive in the plantation of Veneküla (0.85±0.02 m) and the least in the plantation of Reigi (0.26±0.02 m) (Figure 2). The growth trend of silver birch in different plantations in the fourth year was comparable with the results from the second year (Vares et al. 2001), which indicates the stability of the height growth of the tree species in different years. It can be supposed that the growth of silver birch in the first years after planting is significantly more affected by soil conditions and competition than by climatic conditions, which is generally characteristic of all pioner tree species.

According to edaphic conditions in the plantations, the height growth of silver birch was found to be more intensive on Glossic Podzoluvisol/Mollic Glossagualf, Calcaric Luvisol/Oxyaquic Eutrudept and Dystric Gleysol/Typic Endoaquent (Table 2). Thus, soils with aquic conditions, i.e. periodic saturation with water, are preferable for young silver birches. In general, silver birch showed sufficient growth and survival rate also on other soil types occurring in the plantations. Based on our measurements, we conclude that silver birch is able to grow in different edaphic conditions and is a suitable tree species for afforestation of abandoned agricultural lands in Estonia.

Figure 2. Mean height (± SE) of silver birches during the four growing seasons after the establishment of the plantations.

Silver birch has also proved a fast growing tree species on agricultural land in Finland (Hynönen and Saksa 1997; Hynönen 2000) where by 1994 it had been planted on an area of 100 000 ha (Ferm et al. 1994). In Finland, silver birch has been found to be more profitable for afforestation of abandoned agricultural lands compared with conifers (Niskanen 1999).

Alder plantations were established with higher initial density compared with the other plantations of broadleaved species, with the purpose to produce bioenergy, which is the most widespread area of using alder wood in Estonia. The survival rate of alders was high after the first growing season (87-94%) despite the intensive growth of herbaceous plants. The mean height of the studied grey alder plantation after the fourth growing season was 4.7±0.2 m. When comparing the growth of alders in the Holvandi areas, the grey alder plantation was significantly higher than the hybrid alder plantation (3.5±0.1 m). The mean height of the studied plantations of hybrid alder (Holvandi II and Kambja) also revealed significant differences. Mean annual height growth in the fourth growing season varied in alder plantations from 0.35±0.01 m to 0.98±0.02 m, being the highest in the grey alder plantation. When in the hybrid alder plantation in Holvandi the mean height of trees in autumn of the fourth growing season was 3.5±0.1 m, then in Kambja the respective parameter was 1.7±0.1m. Since the plants originated from the same batch and were planted at the same time, and since the plantation with a lower growth rate (Kambja) was even tended, the significant difference in the growth rate of the trees was evidently related to the site, primarily to edaphic conditions. When in 1999 P content in topsoil in Holvandi was estimated at 62 mg kg^-1 and K content was estimated at 118 mg kg^-1, then in Kambja P content was 5.5 times and K content two times lower than in Holvandi (Table 2). Agreeing with Ingestad (1987) we concluded that limited P and K supply of the soil limits the growth nitrophilous alders, especially hybrid alder, because their demand for all macronutrients is high in comparision with the other tree species. In fertilization experiments with alders, tree growth has been affected mostly by addition of phosphorus fertilizers (Hytonen et al. 1995).

Based on our results, hybrid aspen has proved to be a fast-growing tree species in Estonia. The mean height of the studied plantations of the hybrid aspen varied from 1.0±0.1 to 3.0±0.2m (mean annual height growth varied from 0.32±0.01 to 0.84±0.03 m) after the fourth growing season. Larger mean height in these plantations was noted after the fourth growing season on Gleyic Podzoluvisol/Umbric Albaqualf, Buried-gleyic soil/Cumulic Humaquept and Eutric Histisol/Typic Haplosaprist. The fertility of all these soils varied considerably, but their common characteristic was good water supply. However, it is not possible to draw any profound conclusions about the relationship between the growth of hybrid aspen and soil, as in several plantations tree growth was significantly influenced by the activity of game (moose, roe deer) and by the mass occurrence of Galega orientalis Lam.

More hybrid aspen plantations have been established in Finland and Sweden where this species has proved to be among the most fast-growing ones on abandoned agricultural land (Hynönen and Hytönen 1997; Johnsson 1967). Aspen pulp has great economic importance in Northern Europe and it may become one of the main raw materials for paper industry in the 21^st century (Croon 1992). Utilization of aspen for pulp in North America too has increased significantly in the last 20 years (Li 2002).

Comparison of the studied broadleaved tree species was performed on a similar soil (Figure 3), Glossic Podzoluvisol/Mollic Glossagualf, which is the dominating automorphic field soil in Estonia (Kokk et al. 1991). The soil is the second most productive forest soil after Luvisols in Estonia (Kõlli 2002). T-test revealed a statistically significant difference in mean height between the studied tree species. According to analysis alders grew faster on this soil type, followed by birch and hybrid aspen whose growth rate was roughly equal. Nitrogen fixing ability of alders might have been advantageous in this case, because the nitrogen mineralization capacity of Podzoluvisols was relatively low.

⁵ hybrid aspen plantations were not monitored

Figure 3. Mean height (± SE) of the studied broadleaved tree species on Glossic Podzoluvisol/Mollic Glossagualf in Estonia.

Conclusions

Silver birch and hybrid aspen grew well on most studied soil types, however, their cultivation in Estonia can be influenced in the future by moose and roe deer. Alders appeared to grow faster on Glossic Podzoluvisol/Mollic Glossagualf compared with silver birch and hybrid aspen. The growth rates of silver birch and hybrid aspen on this soil are roughly equal. From the ecological point of view, all fast-growing broadleaved tree species under investigation are suitable for afforestation of abandoned agricultural lands in Estonia. From the economic point of view, alder have great importance in meeting the energy need, while silver birch and hybrid aspen are of significance in producing raw material for paper industry.

Acknowledgements

We would like to thank Mrs. Ester Jaigma for the linguistic revision of the English text. This study was supported by the Environmental Investment Centre of Estonia, the Estonian Science Foundation grant No 4821 and the Eastern and Southern Sweden Baltic Sea Co-operation.

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¹ Department of Silviculture, Estonian Agricultural University, Kreutzwaldi 5,
51014 Tartu, Estonia. Tel. +372-7-313-117, E-mail: [email protected]
² Department of Silviculture, Estonian Agricultural University, Kreutzwaldi 5,
51014 Tartu, Estonia. Tel. +372-7-313-111, E-mail: [email protected]
³ Department of Silviculture, Estonian Agricultural University, Kreutzwaldi 5,
51014 Tartu, Estonia. Tel. +372-7-313-157, E-mail: [email protected]
⁴ Institute of Geography, University of Tartu, Vanemuise 46,
51014 Tartu, Estonia. Tel. +372-7-375-824, E-mail: [email protected]