Biomass production of grey alder, hybrid alder and silver birch stands on abandoned agricultural land

Halli lepa, hübriidlepa ja arukase biomassi produktsioon endistel põllumaadel The present study is based on four experimental sites, located in Southern-Estonia: hybrid alder and grey alder plantations located in Põlva county, and two sample plots of silver birch, located in Tartu county. The stand characteristics, above-ground biomass and current annual production (CAP) were estimated in order to evaluate production capacity of different tree species growing on abandoned agricultural lands. Due to fast growth and high biomass production capacity the most promising tree species for short-rotation forestry in Estonia is grey alder. The stem mass in the 13-years-old grey alder and hybrid alder stand was 63.4 t ha-1 and 40.0 t ha-1, respectively. However, the different biomass production is mainly affected by stand densities, 6170 trees per ha and 4080 trees per ha, respectively. During ageing, the differences between the alder stands diminish. At the age of 14, mean height and diameter at breast height were practically equal. Also the mean stem mass in the older, 13-year old stand, is almost equal: 10.3 kg in grey alder stand and 9.8 kg in hybrid alder stand. At a younger age, the mean stem mass was higher in grey alder stand, but later, at the age of 13, the mean stem mass has become almost the same (10.3 kg in grey alder stand and 9.8 kg in hybrid alder stand). The rotation period for hybrid alder is longer than for grey alder and bulk maturity will occur later. Silver birch is also a highly productive tree species and has a prospect for short-rotation forestry. The mean stem mass and annual current increment of 8-year-old silver birch stand was in same the magnitude as in the grey alder stand. Although the average stand diameter and height were lower in the silver birch stand than in the grey alder stand, it is compensated by the higher wood density of birch wood. The number of trees has affected silver birch stand production, the above-ground biomass in the very high density birch stand (35 600 trees per ha) was significantly lower than in the sparse stand (11 600 trees per ha), 22.8 t ha-1 and 31.2 t ha-1, respectively. Biomass production of grey alder, hybrid alder and silver birch stands on abandoned agricultural land Due to the high prices and lack of fossil fuels in the world, it is necessary to look for new, alternative fuel sources. In Estonia one of the possibilities for renewable energy would be growing deciduous tree species on abandoned agricultural land. Usually silver birch (Betula pendula) and grey alder (Alnus incana) are the most common tree species to occupy agricultural lands which are left out of use. Also the hybrid alder (Alnus hybrida) may be considered a fast-growing species in our conditions. The main aim was to investigate the above-ground biomass production and the dynamics of young stands of fast-growing deciduous tree species on former agricultural land. This study is based on four experimental sites, located in South-Estonia: grey alder and hybrid alder plantations in Põlvamaa, established in 1995 and 1996 respectively, and two silver birch natural stands in Tartumaa, Haaslava and Kambja, established in 2001 and 2004 respectively. The basal density of experimental sites was very different. In Haaslava it was the highest (36 200 trees per ha) and in hybrid alder stand it was the lowest (6700 trees per ha). In Kambja it was 11 600 and in grey alder stand 15750 trees per hectare. The soil conditions were very similar in all sites. Mean diameter at breast height and mean height of the stands were measured annually. The above-ground biomass of the stands was estimated during 2003-2007, by the method of model trees. It was not done in every stand every year due to its labor-intensity. 8-year old stands were compared, because the above-ground biomass of all four experimental sites was determined at that age. Mean height and diameter growth of grey alder at young age were higher and faster than in hybrid alder stand, but at the age of 14 these parameters were practically equal. When comparing the above-ground biomasses of the alder stands, it can be noted, that at the age of 8 grey alder overcomes the hybrid alder stand (biomasses respectively 26.8 t ha-1 and 9.76 t ha-1), but later, at the age of 13 the difference between the two sites diminishes (76.1 t ha-1 and 51.5 t ha-1). The average dry mass of grey alder stems in 8-year old stand was bigger than that of hybrid alders (2.87 and 2.01 kg). The annual increment of above ground-biomass was almost at the same level in Kambja birch stand and in grey alder stand (11.9 and 14.0 t ha -1 y respectively). Although the dimensions of trees are smaller in Kambja stand, the large number of trees and the wood density makes the biomass accumulation high per hectare. Mean height, diameter, average tree stem mass, and above-ground biomass was bigger in Kambja than in Haaslava stand. In 8-year old stands, the average dry mass of stem of alders is bigger than that of birches, the highest is grey alders (2.87 kg) and the lowest is in very dense Haaslava stand (0.44 kg).