3. Doktoritööd
Selle kollektsiooni püsiv URIhttp://hdl.handle.net/10492/7081
Sirvi
Sirvi 3. Doktoritööd Autor "Aosaar, Jürgen" järgi
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Kirje The Development and Biomass Production of Grey Alder Stand on Abandoned Agricultural Land in Relation to Nitrogen and Carbon Dynamics(Eesti Maaülikool, 2012) Aosaar, Jürgen; Uri, VeikoDuring the last decades, forest coverage of Estonia has increased, mostly due to the natural afforestation of the abandoned agricultural areas. In Estonian conditions grey alder is one of the main pioneer tree species to occupy such areas. Due to the changes in the fossil fuels sector and the need to mitigate the potential anthropogenic factor in global climate changes, the need for the utilization of renewable energy sources has increased. In Estonia, due to high forest coverage, forest, including grey alder stands, can be considered as a potential green energy source. Today, the growing stock of and the area of grey alder stands in Estonia is approximately 30 million m3 and 180,000 ha, respectively. During the last years, several coproduction power plants which use wood as a fuel have been established. The demand for woody fuel will probably increase in the coming years and the pressure of more extensive management of grey alder stands will increase. Hence, in order to make sustainable and ecologically smart decisions for managing grey alder stands, new knowledge about the functioning of grey alder ecosystems is needed. In the current thesis, the results of a long-term study, carried out in a grey alder plantation growing on a former arable land, are presented. In the thesis, the growth dynamics of the stand, the stand’s impact on the soil and the carbon accumulation in the stand are discussed. Based on results, grey alder is a very fast growing and vigorous tree species for Estonian conditions. The mean annual increment of Estonian grey alder stands is 7.6 m3 ha-1; in Estonian forests, the respective value is 5.2 m3 ha-1. In the studied stand the mean annual increment was 15.7 m3 ha-1. At the age of 16 years, the current annual increment of stem mass peaked, reaching as high as 36 m3 ha-1 y-1. The result can be considered extremely high as it exceeds all the respective values from the scientific literature. Grey alder is a species with a rather short life-span – the common maximum age of grey alder stands is 40-50 years. As a result of the study, the bulk maturity, i.e. the age of the stand when the annual increment of timber has overcome its maximum, is around 15-20 years for grey alder stands. This posture is also supported by the researchers from other countries. Today, approximately 70% of all grey alder stands in Estonia are older than 20 years, i.e. their bulk maturity has been achieved and it would be rational to utilize them more extensively than so far. The growing stock of the stand at the age of 17 years was 265 m3 ha-1, which exceeds the growing predictions of the yield-tables of neighbouring countries. The issue of grey alder stemwood density is understudied so far in Estonia; the published data from other countries are highly variable. In the studied stand, the stemwood density was 396 kg m3. The appropriate density value, inherent in the local region, is needed for practical concerns; it allows us the accurate converting of mass units to volume units, or conversely. The knowledge of the soil’s improving effect of growing alders is widespread; the study affirmed this – the increase of the concentration of the most important nutrient, nitrogen, in the upper 0-10 cm soil layer was significant. During the growth period, it increased 0.6 t ha-1, reaching 2 t ha-1 in a 17-year-old stand. The possible environmental risks, such as nitrogen leaching and the emission of very strong greenhouse gas N2O (laughing gas), linked to managing grey alder stands were also studied. According to the results, some leaching occurred (15 kg ha-1 y-1), however, it mostly accumulated in the deeper soil layers and the amount of nitrogen leached to the groundwater was insignificant. Furthermore, the N2O emissions were very small (0.5 kg ha-1 y-1) and it can not be considered as a possible environmental hazard. Grey alder stands growing on abandoned agricultural land act as a carbon sink. In the studied stand there was approximately 65 t ha-1 of carbon accumulated into the woody biomass (stems, branches, roots). Moreover, 6.5 t ha-1 of carbon was accumulated in the stand’s soil during the 17year long period of stand growth. In conclusion, grey alder as an indigenous tree species is highly productive and resistant, which makes it a suitable tree species for renewable energy for Estonian conditions. Furthermore, growing grey alder improves the soil qualities of the site and the stand acts as a carbon sink.