It seems plantations like these are all-around good—besides the efficient reuse of the abandoned land, they provide a wonderful way to save the environment. More woody biomass results in less carbon dioxide released into the atmosphere, providing possible alleviation to climate changes.
During the last decades, production of woody biomass has received more attention in northern Europe and also in Estonia. In Estonia, an SRF pilot program of growing hybrid aspen on former arable land was initiated more than 15 years ago. However, less attention is paid to the impact of SRF plantations to the soil but the researchers of the Estonian University of Life Sciences and University of Tartu have made positive discoveries. Although the first generation of hybrid aspen plantations has altered soil composition, the soil is still highly fertile.
While producing woody biomass from SRF plantations for energy and pulp industries, it is crucial that a high productivity of trees is sustained. Therefore, sustainable soil fertility must be maintained
Researchers compared the data of soil monitoring carried out in hybrid aspen plantations at the ages of 5 (young) and 15 years (mid-term). Arvo Tullus, Senior Research Fellow of Applied Ecology in the University of Tartu, said that the decisive factor is whether the trees have excessively absorbed some nutrient from the soil, resulting in insufficiency that restrains rapid growth.
Survey revealed that the content of macronutrients—nitrogen (N), phosphorous (P) and potassium (K)—in the soil of hybrid aspen plantations is still high. According to Reimo Lutter, a PhD student in the Estonian University of Life Sciences, this provides a continuously good production and carbon sequestration capacity to above-ground biomass.
Arvo Tullus added that, curiously, the average ratio of N had slightly increased. “This might have been partly caused by species binding atmospheric nitrogen extant in the undergrowth, such as the Leguminosae. However, it cannot be ruled out that some of the bacteria living in symbiosis with the aspen have contributed to this,” he said.
At the same time, it appeared that the content of micronutrient boron (B), which is vital for tree growth, decreased significantly. As this was strongly associated with the growth rate of the trees, it is necessary to do a follow-up on the binding of B in the leaves and wood and its role in the growth process of trees.
Peace for the residents
Arvo Tullus said that the research, above all, gave an inkling of how hybrid aspen, proven to be generally fast-growing and cold resistant in northern Europe, grows in Estonia and whether the establishing of plantations here is economically worthwhile. “If we decide to afforest the arable land even temporarily, it is good to know which arable soils are better for hybrid aspen to grow,” said Tullus.
“New knowledge can also be used in practical silviculture,” said Reimo Lutter. “As the nutrient ratios in the soil are still high, to gain more biomass, the nutrient-rich branches can be removed during forest thinning. Also, there is no need for additional fertilization,” explained Lutter.
Furthermore, Lutter noted that if wood production from the hybrid aspen plantations gives good results, it creates the prospect of deforesting fewer natural forests which, in turn, let the aspen forest and its diverse population live in peace. “In the context of Estonia, it is also important that the plantations could create new jobs in rural areas,” said Lutter.
· Parental species are European aspen (P. tremula L.) and North American trembling aspen (P. tremuloides Michx.).
· Hybrid aspen is cold resistant, pathogen resistant and fast-growing (even without fertilization).
· In addition to woodchips, hybrid aspen wood could be used for more valuable pulpwood and logs.
This article was supported by the European Regional Development Fund through Estonian Research Council.