Elsevier

Biomass and Bioenergy

Volume 33, Issue 9, September 2009, Pages 1108-1115
Biomass and Bioenergy

Review
Ground vegetation exceeds tree seedlings in early biomass production and carbon stock on an ash-fertilized cut-away peatland

https://doi.org/10.1016/j.biombioe.2009.05.009Get rights and content

Abstract

Afforestation is one of the most popular after-use options of cut-away peatlands in Scandinavia since it has both economic and aesthetic values and therefore the interest concerning the carbon stock often focuses on tree stands. Consequently, ground vegetation is readily disregarded in the present calculations of the climatic impact of afforestation. However, at the early stages of afforestation vigorous ground vegetation may have a major role in carbon sequestration. The biomass and C stock of ground vegetation and young tree seedlings were examined on an ash-fertilized and afforested cut-away peatland. Six treatments of different mixtures and quantities of wood-ash, peat-ash, biotite or Forest PK-fertilizer were replicated in three plots. Betula pubescens Ehrh. seeds were sown on randomized halves of split plots while the other halves were left unsown. The plant biomass was harvested four growing seasons after the treatments. The live above-ground biomass of ground vegetation on a cut-away peatland was up to two times that of tree seedlings. Furthermore, the below-ground biomass of ground vegetation and tree seedlings was equal to the above-ground biomass, or even greater. In particular, the biomass of mosses multiplied on ash-based fertilized areas compared to the Forest PK-fertilized areas. Our study proved that at the early stages of afforestation ground vegetation was even more important in biomass production and C stock than tree seedlings. Consequently, our results suggest that ground vegetation biomass should also be considered when the climatic impact of afforestation of cut-away peatlands is being calculated.

Introduction

Peatlands cover about one-third of the land area in Finland and are thus the dominant features of the Finnish landscape. About 35%, i.e., 3.3 million hectares of the peatlands are still in natural condition and one-third of this area is protected. Over half of the original peatland area has been drained for forestry or agricultural purposes and less than one percent is utilized in the peat industry [1].

In their natural stage peatland ecosystems act as long-term sinks for atmospheric carbon which is stocked as peat in anaerobic conditions [2]. Drainage, vegetation removal and harvesting have, however, a crucial effect on carbon cycling in peatlands by disturbing severely the structure and function of the original bog ecosystem [3]. Peat harvesting is usually accomplished in about 20–50 years, but since the post-harvesting conditions are very harsh for plant establishment, cut-away areas may remain non-vegetated even for decades [4], [5], [6]. Coincidently, aerobic decomposition of the residual peat layer causes emissions of CO2 to seep into the atmosphere [7], [8]. Therefore, cut-away peatlands with no after-use act as a long-term source of atmospheric carbon [9], [10].

The after-use of cut-away peatlands is comprised of several alternatives, such as afforestation, agriculture, reed canary grass, Phalaris arundinacea L., cultivation, bird sanctuaries, berry production and habitat restoration [11]. Afforestation is one of the most popular options in Finland, Ireland and Sweden since it has both economical and aesthetic values [12], [13]. In addition to a high tree-growth rate, an important object of afforestation is to get the bare cut-away area covered by vigorous vegetation that restricts erosion and leaching of nutrients and fixes carbon into the ecosystem as soon as possible. Ash-fertilization, in particular, is found to increase significantly both the coverage and variety of ground vegetation and the quantities of birch seedlings on bare cut-away peatlands [6]. Although many studies have focused on the factors affecting the biomass production of tree stands on cut-away peatlands [14], [15], [16], [17], knowledge about the biomass dynamics of the ground vegetation in these areas is very scanty. In fact, changes in the ground vegetation biomass or CO2 fluxes have been studied only in restored cut-away areas [7], [18], [19], [20]. However, the high water-table level and anaerobic conditions of these rewetted cut-away peatlands make them differ completely from the afforested cut-away peatlands.

On afforested cut-away peatlands the decomposition rate of the residual peat layer and the productivity of the tree stands are regarded as the most important factors affecting the carbon balance [21]. Ground vegetation is readily disregarded in the present calculations of the climate impact of afforested cut-away peatlands [22] mainly due to the absence of exact studies. Still, the whole life cycle of the peatland including all the carbon emissions and sinks should be considered [23]. In fact, at the early stages of the afforestation vigorous ground vegetation may have a major role in carbon sequestration, quite comparable to that of tree seedlings. In this study we examined the biomass and carbon stock of ground vegetation and young tree seedlings on an ash-fertilized and afforested cut-away peatland.

Section snippets

Experimental field

The experimental field was established in 2000 in Finland (64° 44′ N, 25° 16′ E, 45 m a.s.l.) on a cut-away peatland, abandoned by the peat industry since 1996. The thickness of the residual peat layer, Sphagnum-Carex (H5), varied between 20 and 50 cm, and the soil below the peat was sandy till. Ditches of the trial area were cleaned in August 2000 but the surface of the area was left uncultivated prior to the establishment of the trial plots.

Three blocks were established on the experimental

Above-ground biomass and carbon stock

Fertilization increased the biomass of tree seedlings significantly (ANOVA F = 3.874, df = 5, p = 0.003) whereas the differences in the tree seedling biomass between the two afforestation methods (F = 1.938, df = 1, p = 0.167) or interaction between the afforestation and fertilization methods (F = 0.501, df = 5, p = 0.774) were statistically non-significant. Consequently, the data of the two afforestation treatments were combined and the results presented hereafter concern only the effects of fertilization. The

Discussion

The biomass of ground vegetation is frequently omitted when estimates are made of the biomass production and carbon balance on cut-away peatlands after afforestation. Our results, however, proved that the combined above-ground biomass of mosses and herbaceous plants and thus also the amount of C sequestered into the ecosystem was as much as two times the amount of the biomass of tree seedlings four growing seasons after fertilization. Surprisingly, the amount of mere ground vegetation biomass

Acknowledgements

Mr. J. Issakainen organized the field trials, Dr. A. Kauppi made competent comments on the manuscript and Mr. R. Gear revised the English text. The plant samples were prepared and analyzed in the laboratories of the Finnish Forest Research Institute (Metla) in Muhos and Vantaa. This study was a part of Metsähiisi – project, which was carried out in co-operation of Vapo Company and Metla. The National Technology Agency of Finland (Tekes) supported this work financially. We thank sincerely all

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