50 RESEARCH FOR RURAL DEVELOPMENT 2015, VOLUME 2 GREY ALDER ALNUS INCANA (L.) MOENCH ADDITIONAL GROWTH CHANGES AFTER THINNING IN AEGOPODIOSA SITE TYPE Olga Miezīte, Imants Liepa, Arnita Šereiko, Solveiga Luguza Latvia University of Agriculture olga.miezite@llu.lv Abstract Grey alder stands Alnus incana (L.) Moench have a number of distinctive features. In fertile soils it successfully forms productive forest stands without any human intervention. Therefore, relatively few publications can be found on the thinning effects on stand reaction after thinning. It is possible that due to the highly intensive circulation of substances in the ecosystems of grey alder (high photosynthesis and canopy thinning, withering and breaking off of the lower branches, litter decomposition within a few years, thereby ensuring a continuous and stable plant mineral nutrition substance complementarity in the soil) response reaction of the remaining trees and management of grey alder forest stands could be different comparing to other tree species. The study analyses stock volume additional increment dynamics during 10-year period after the thinning in 24-year-old grey alder pure stands in Aegopodiosa site type. Thinning of grey alder forest stands have caused a moderate positive reference reaction – during 10 years, in addition to the total increase, 3.17 m 3 ha -1 have been added. During the valuation interval response reaction differs among the years. In the frst four years it is relatively small as accumulation of the growing potential is taking place. From the ffth to seventh year after felling an intensive growing takes place, which results in repeatedly additional annual increment. Starting from the eighth year, the trees show tendency to return to a steady state as it was before the thinning. Key words: grey alder, effect of thinning, additional volume increment, Aegopodiosa site type. Introduction Thinning is always such a radical intervention in any forest stand that its effect persists for years after the thinning (Liepa and Zaļkalns, 2014). The nature and intensity of the impact depends on a wide variety of conditions. So during the long stand thinning history numerous studies have been done to fnd an environmentally substantiated and economically advantageous way of thinning (Zviedris et al., 1961). The criterion for comparing of different study results is the state of health and productivity changes of thinned stands (Eberts, 1996; Zālītis, 2010). There is no doubt that the thinning should not become a risk factor of explosion of tree diseases, such as root rot or enhanced proliferation of pests, for example a massive invasion of needles consumers. No less important is to increase or at least keep the pre-thinning timber productivity. This is being substantially dependent on the tending intensity (Aнтанайтис и др., 1986). Experience has shown that the selection of inappropriate thinning effciency criteria, such as the annual tree ring width change after the thinning can lead to, although scientifcally based, over-rarefying of the stands. In this case, the width of tree ring annual increase cannot compensate for the increase of the thinned trees, thereby resulting in the decrease of the productivity of the remaining forest stand. For the most tree species this problem has been identifed and partially solved by the development of thinning regulations (Donis et al., 2012). The situation is different with the grey alder Alnus incana (L.) Moench stands management research. As a reason two considerations can be mentioned – wood of grey alder is with relatively low assessment comparing to other tree species, as well as grey alder biological characteristics. The use of wood depends on the physical and mechanical properties, chemical composition, dimensions of the derived wood products, wood defects and other factors. According to these conditions, grey alder wood is mainly used in containers, wood fbre and board industry, lathing, charcoal, frewood, meat and fsh smoking (Daugavietis, 2006). New opportunities for the use of grey alder can be given for renewable energy production. This tree species is quite suitable for use in heat energy industry because of its outstanding fast growth, high biomass productivity, easy afforestation and foster regeneration as well as wide occurrence. Latvia has a considerable amount of grey alder resources. This refers to both the forest and in particular abandoned agricultural lands occupied area as well as to the stock volume and the amount of the biomass (Miezīte, 2008; Daugavietis et al., 2009; Miezīte et al., 2011). According to the State Forest Service data (Meža statistikas..., 2014), the total area of grey alder stands in Latvia is 210,704.5 hectares with a total volume stock of 30,305,201 m 3 , which is an important alternative for energy production. Especially important is the fact that grey alder mostly regenerates naturally in fertile forest site types and in uncultivated agricultural land forms healthy young stands. Reasonable use of these properties of grey alder allows the forest owner to avoid the expenditure for the planting and reproductive material, focusing more on further forest pre-commercial thinning. In dry and sandy soils feasible is planting of the grey alder too (Rokjānis, 1957). Economic signifcance of the grey alder is not yet suffciently identifed. Over FORESTRY AND WOOD PROCESSING