Aboveground biomass and nutrient accumulation dynamics in young black alder, silver birch and Scots pine plantations on reclaimed oil shale mining areas in Estonia
Research highlights
▶ Soil properties of post-mining area improved with stand age. ▶ The better survival, growth and largest biomass occurred in black alder stands. ▶ The largest amounts of nutrients accumulated in the aboveground biomass were in black alder stands. ▶ Deciduous trees needed more nutrients for biomass production; Scots pine used nutrients most efficiently. ▶ Black alder is a promising tree species for the reclamation of oil shale post-mining areas.
Introduction
The opencast mining of oil shale in Northeast Estonia creates significant areas of degraded land that need restoration. By the year 2006 the area damaged by oil shale opencast mining was 13,098 ha, of which 10,347 ha was afforested (Kaar and Tomberg, 2006). The relief of the alkaline (pH ∼ 8) wasteland is rugged. Oil shale mining spoil is heterogeneous and extremely stony with low nitrogen and organic content (Kaar, 2002, Lõhmus et al., 2006a). Since the use of the post-mining landscapes for agriculture is made difficult by heterogeneity of the soil and by poor moisture conditions, their afforestation is more suitable (Kaar, 2002). Afforestation is a sustainable reclamation option for post-mining landscapes (Singh et al., 2002, Lõhmus et al., 2006a, Pietrzykowski and Krzaklewski, 2007) to reduce CO2 emission and to create renewable energy sources. Results of previous research (Kaar, 2002) of restoration of post-mining areas in Estonia showed that new stands are of better quality than the stands that grew there before mining. A total of 52 indigenous and introduced species have been used for restoration. Various introduced larch species (Larix europaea, L. sibirica, L. kurilensis) have shown the best growth among coniferous trees, and the native deciduous species of silver birch and black alder have been the most productive deciduous trees (Kaar, 2002, Kuznetsova and Pärn, 2004, Korjus et al., 2007). Until recent years a disproportionately large proportion of conifers (>90%), mainly Scots pine (86% of the area) has been planted (Lõhmus et al., 2006a). Moreover, the opencast mining area in Northeast Estonia has been moving toward areas with deeper excavation (Kaar, 2002, Korjus et al., 2007) and the quality of the post-mining substrate has declined and its stoniness has increased due to the thickening overburden in new mining areas. These harsh growing conditions might limit growth and survival of different tree species at different rates, and hence the reasonable choice of tree species for the reclamation of levelled opencast mines will be even more important in the future.
Deciduous species have several advantages (faster growth, higher resistance to pests, diseases and fires) over conifers on post-mining sites, and their use in reclamation should rise (Kaar, 2002, Vares et al., 2004, Lõhmus et al., 2007). Black alder can be considered to be a “biological fertiliser”, which improves soil nitrogen status, fixing N2 in symbiosis with Frankia in its root nodules and increasing phosphorus availability in soil by the activity of its roots and associated microbial communities (Giardina et al., 1995, Uri et al., 2002). However, the potential of different tree species for the recultivation of exhausted opencast oil shale mines in relation to biomass production, nutrient accumulation and nutrient use efficiency is still poorly investigated. Analysis of morphological and chemical characteristics of trees has often been used to evaluate the nutritional status (Brække, 1996) as well as biomass increment and growth conditions of trees (Niinemets et al., 2002). The success of the restoration of damaged areas has been assessed by evaluating the growth and biomass production of various plants (Kumar et al., 1995, Singh and Singh, 1999). Previously it has been found that the higher growth rate of black alder growing on oil shale post-mining areas corresponds to the higher activity of microbial communities in its rhizosphere, and the higher specific root area of its short roots than in the case of conifers (Vares et al., 2004, Lõhmus et al., 2006b, Ostonen et al., 2006). Also, Vares et al. (2004) found the aboveground productivity in a 21-year-old black alder plantation on reclaimed oil shale mining area to be comparable with that of the stands of the same age growing on fertile mineral soils.
The present study deals with survival, growth, biomass production and nutrient accumulation of young black alder, silver birch and Scots pine stands growing on reclaimed oil shale mining areas. The main aim of the study was to assess the suitability of the studied species for the reclamation of oil shale post-mining areas. The working hypothesis of the present study was that black alder can be the preferred species for the recultivation of the exhausted oil shale mining area due to its faster growth and ability to restore soil fertility. Our findings are important for a better understanding of the development of stands and allow giving recommendations for restoration of exhausted oil shale mining areas in ecologically and economically changing conditions. The approaches of this study can be used for other regions where wastelands require reclamation.
The specific objectives of this study were (i) to analyse the early dynamics of the survival, growth, aboveground biomass allocation and production in relation to the tree species in black alder, silver birch and Scots pine plantations on reclaimed oil shale mining area; and (ii) to analyse the nutrient concentration, allocation, accumulation and nutrient use efficiency in relation to the tree species and stand age in the studied plantations.
Section snippets
Study areas
The study was carried out in experimental plantations established on Narva reclaimed oil shale mining area, Northeast Estonia (59°15′N, 27°42′E). According to the data of the Narva meteorological station, which is closest to the experimental area, the mean annual temperature in the region during the studied period was 5.8 °C, and mean annual precipitation was 747 mm. The plantations had been established directly on levelled quarry spoil. No soil preparation was done before planting. The soil type
Soil parameters
The changes in soil pH, N and P concentrations depended on age of plantation (F[1,30] = 67.5; 38.4; 17.2; p < 0.001 for soil pH, N and P concentrations, respectively), but not on species (F[2,30] = 0.27; 0.49; 1.04; p > 0.05 for respective values) and interaction between species and age (F[2,30] = 0.28; 0.52; 1.03; p > 0.05 for respective values). During 7 years of trees growth the decrease of soil pH and P concentration and increase the soil N concentration with stand age for all studied species appeared (
Discussion
Considering that the levelled dumps of oil shale mining spoil are heterogeneous, stony and alkaline with a low initial N and organic content (Kaar, 2002, Lõhmus et al., 2006a, Lõhmus et al., 2007), the first years of stand development in these harsh conditions are most critical for tree survival and growth. However, our results showed that soil improvement was remarkable in the oil shale mining area during the first 7 years. The initially high pH decreased and the soil N concentration increased
Conclusion
Afforestation of alkaline mining spoil with fast-growing trees is the best means to accelerate the development of new forest ecosystems. Deciduous trees should be favoured because they increase soil fertility more and have higher productivity than conifers. The present study revealed changes in soil properties with increasing stand age. Soil pH and P concentration decreased and soil N concentration increased with stand age. The survival and productivity of black alder stands on a reclaimed oil
Acknowledgements
This study was supported by Target Funding Projects Nos. 0170021s08 and 0182732s06 of the Estonian Ministry of Education and Research.
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