Effects of different tree species on microclimate and microbial processes on a natural regeneration site

Introduction

Investigations on water balance, soil fertility and microbial decomposition processes were conducted on a 20 year old wind-throw site. The former spruce monoculture is now replaced by a mixed stand of birch (Betula pendula), trembling aspen (Populus tremula), spruce (Picea abies) and oak (Quercus robur). The site is located in a warm area at low altitude on heavy soils, where the unfavourable environ-mental conditions for spruce trees are prevailing. The aim of this project part was to determine whether birch could be considered as beneficial to forest regrowth in this area. In addition possible ecological effects of different trees concerning litter decomposition, carbon sequestration and the emission of greenhouse gases could be detected.

Material and methods

The experimental plot is located at about 320 m a.s.l. in the south-east of Styria in Austria. Annual mean air temperature is 8.8 °C with an average precipitation of 765 mm. The soil is characterised by a high content of clay in the mineral soil horizons ("Stagnogley"). Six sampling sites including 3 groups of birch and 3 groups of spruce were selected. Measurements were made at 13 sampling periods, every two weeks from May to October. Climate and soil water tension were monitored continuously. In addition nitrogen mineralisation and microbial biomass were determined in the litter layer and in the mineral soil (Schinner et al. 1996). Microbial respiration and the emission of the greenhouse gas nitrous oxide (N2O) were measured in the field by static chambers (Hahn et al. 2000).

Results and discussion

A strong seasonal trend of microbial nutrient dynamics was detected. This trend was dependent on tree species as birch and spruce groups showed different microclimate, root distribution, shading, litter quality and transpiration rates. It turned out that the mineral soil under birch was dryer due to enhanced transpiration. However, the humic layer under birch was more moist and less susceptible to drying during summer. Accordingly, the microbial biomass in birch humus showed a different seasonal dynamic compared to spruce humus. Birch plots had a higher soil respiration and less acidic pH. In soils under birch microbial biomass correlated with NH4+ availability (r=0.6; p<0.05), which often exceeded spruce plots. Respiration rates were dependent on air temperature. We conclude that 20 year old birch trees already exerted a positive influence on microbial mineralisation processes and on soil chemistry. In contrast microbial biomass and respiration rates in spruce plots relied on precipitation and on soil moisture. They declined under dry periods, which means that in this climate microbial decomposition processes and nutrient release may be inhibited in summer under spruce. There was no significant difference of N2O emissions between spruce and birch plots. Highest emissions occurred during warm and rainy weather. The availability of NO3- and NH4+ did not seem limiting for the emission of N2O as there was no correlation between these parameters during the vegetation period. We cannot suggest preference of either investigated tree species in terms of green-house gas mitigation. The results of our study emphasise the possible value of birch as a pioneer tree species. Due to the better litter quality and the stimulation of microbial mineralisation, birch can help in the restoration of secondary coniferous forests and in the conversion into more stable mixed deciduous forests in this area. According to our measurements there is no danger of enhanced nitrogen losses, such as NO3--leaching or emission of the greenhouse gas N2O.

Figure 1: Seasonal course of soil respiration in spruce and birch plots.

Figure 1: Seasonal course of soil respiration in spruce and birch plots.

References
Schinner F., Öhlinger R., Kandeler E., Margesin R. (Eds.) Methods in Soil Biology. Springer Verlag, Berlin
Hahn M., Gartner K., Zechmeister-Boltenstern S. (2000) Greenhouse gas emissions (N2O, CO2 and CH4) from beech forests near Vienna with different water and nitrogen regimes. Die Bodenkultur - Austrian Journal of Agricultural Research 51, 115-125.

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Effects of different tree species on microclimate and microbial processes on a natural regeneration site
Leitgeb E., Zechmeister-Boltenstern S., Gartner K.
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