Biomass production and nutrient accumulation in short-rotation grey alder (Alnus incana (L.) Moench) plantation on abandoned agricultural land
Introduction
During the last decade the economic situation has changed drastically in Estonia: agricultural production has declined, as a result of which 223,000 ha of abandoned agricultural land have come into existence (Meiner, 1999). The prices of fossil fuel have risen considerably, which has urged researchers to seek possibilities for a far more extensive use of wood and peat in energetics and heat management. Issues related to the establishment and use of energy forests have been placed on the agenda. In Finland and Sweden, most studies have addressed the use of willows, birches and grey alders as energy forests both on mineral and organic soil (Granhall, 1982, Elowsson and Rytter, 1988, Slapokas, 1991, Saarsalmi, 1995, Hytonen et al., 1995, Rytter, 1996), where willows are generally the most rapidly growing species (Saarsalmi, 1995, Hytonen, 1996). In Estonia, the first energy willow plantations were established during 1993–1995 in co-operation with the Swedish University of Agricultural Sciences (Koppel et al., 1996). Although grey alder has commonly not so high production capacity as some willow species and clones, it has some essential advantages which make it a promising species for short-rotation forestry. It grows rapidly, is symbiotically N2-fixing by the actinomycete Frankia, and has only a few pests and diseases. Decomposition of alder litter improves soil properties. After cutting, a new alder generation emerges by coppicing from root system, and artificial reforestation of clearcuts is not needed. Grey alder seedlings withstand direct sunlight and frost. In younger age, grey alder grows faster than other local tree species, the current annual increment being 7.57 m3 ha−1. However, growth slows down earlier; maturity is attained at the age of 40–50 years. Harvesting can be performed with ordinary equipment without special harvesting machinery.
Natural grey alder stands are widespread in Estonia, forming 4.2% of the whole forested area, while in private forests they account for 11.1%.
Alder forests are also typical riparian ecosystems in Europe, which can retain and transform nutrient fluxes from adjacent intensively exploited territories. Therefore, riparian alder stands are commonly evaluated as buffer zones for protecting waterbodies against pollution (Mander et al., 1995, Mander et al., 1997, Lohmus et al., 1996).
Up to the present, grey alder has not been cultivated in Estonia. The first experimental plantation on former agricultural land was established in 1995 with the aim to study the possibilities of cultivating this tree species as short-rotation forest as well as its impact on soil fertility on abandoned agricultural land.
The main aims of the present study were:
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to analyse the dynamics of the above-ground biomass and production of short-rotation grey alder plantation after establishment;
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to estimate nutrient (NPK) allocation, demand and accumulation in the above-ground part of grey alder plantation;
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to estimate biomass and nutrient (NPK) content in the below-ground part of grey alder plantation;
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to find out the impact of grey alder plantation on soil nutrient status on abandoned agricultural land.
Section snippets
Plantation
The study based on an experimental area located in the southeastern part of Estonia, Põlva county, 58°3′N and 27°12′E. According to the data of the Võru metereological station, which is the closest to the experimental area, mean annual temperature is 6 °C, mean amount of precipitation is 653 mm and the mean length of the vegetation period is 191 days. The experimental plantation was established on abandoned farm land in spring 1995. The soil was classified as Planosol (according to FAO
Above-ground biomass and production
Despite the competition of herbaceous plants, the survival rate of grey alders after the first growing season was high (94.1%), which favoured their further growth and development. Naturally regenerated seedlings displayed the highest growth percentage (Uri and Tullus, 1999).
The dry matter ratio for biomass fractions varied in different years; the content of dry matter in the stemwood varied from 51 to 61%, in the leaves from 39 to 50% and in the primary growth of branches from 41 to 47% in the
Discussion
Although literature data indicate that grey alder suffers from competition with herbaceous plants during the first growing years on abandoned agricultural land (Saarsalmi, 1995), germination and increment of plants in our experimental plantation in the first year after establishment were good in spite of the lack of weed control. Such young plantations are rarely described in the literature. According to Rytter (1996), in the first year after harvesting, the mass of shoots sprouting from root
Conclusions
The fact that the survival and growth of plants in the experimental plantation were good, shows that grey alder is a perspective tree species for afforestation of abandoned agricultural lands, both in the environmental and economic aspects. Soil improvement with respect to available phosphorus was noted already after the second year of the plantation. As the highest current annual increment of above-ground biomass was not attained during the five study years, the investigation will be continued
Acknowledgements
This study was supported by the Estonian Science Foundation Grant no. 3529. We thank Mrs. Ester Jaigma for revising the English text.
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