Research Assessment Exercise 2007 University of Oulu
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3a.2 Year 2002Kaarina Hänninen (2002) Tree - cover crop interactions: birch growth, competition and soil properties. Acta Universitatis Ouluensis A 384. University of Oulu, Department of Biology. http://herkules.oulu.fi/isbn9514267184/ The main objective of this thesis was to investigate tree - cover crop interactions and soil response in an intercropping system, in which leguminous and grass cover crops were used with birch (red-leaved Downy birch Betula pubescens L.). The investigation was conducted in two field experiments in a nursery and in a greenhouse experiment. In the latter the effect of the water supply in interspesific competition was also investigated. The cover crops in the field experiments were perennial clovers Trifolium pratense L., T. repens L. and T. hybridum L. and annual clovers T. incarnatum L., T. resupinatum L. and T. subterraneum L. and perennial Festuca rubra L. The height, stem diameter, leaf area and nutrient status of the birch were determined, as well as soil nutrient status and microbial characteristics. The cover crops in the greenhouse experiment were T. repens, T. subterraeum and F. rubra. The biomass, height, leaf area, leaf area index, specific leaf area and N concentration of the birch, the biomass and N concentration of the cover crops were measured, and soil N and microbial characteristics, as well. Bare ground was the control in all the experiments. The perennial clovers and Festuca strongly decreased the birch growth and nutrient status, but the annual clovers sown in midsummer in the field experiments provided about as good growth as bare ground. In the greenhouse experiment all cover crops were effective competitors with the birch. The soil NO3-N was, in general, the highest on bare ground and second highest in the annual clover plots. Though there were, in general, only minor differences in the soil nutrient concentrations between the treatments, there were significant differences in the tree growth and nutrient concentrations. The interspecific competition in this kind of intercropping system is mainly belowground. The growth reduction in the birch was mainly due to competition for nitrogen but water seems to play an important role in regulating the competitive interaction between the birch and cover crops. The competition for these resources seems to be most crucial at the beginning of the growing season. The microbial biomasses and soil respiration were greatest in the Festuca and perennial clover treatments, which may indicate that microorganisms together with these cover crops may seriously compete with birch for nutrients. Intercropping system is complex and comprises both negative and positive influences. In order to minimize negative competition effects, the cover crop should be non-competitive or the ground should be kept free of vegetation at the beginning of the growing season. By improving soil microbial characteristics, the vegetative ground covers make this cropping system one possiblity towards sustainability in the long-term. Keywords: Betula pubescens, Festuca, Trifolium, nutrient status, water supply Jukka Lamppu (2002) Scots pine needle longevity and other shoot characteristics along pollution gradients. Acta Universitatis Ouluensis A 395. University of Oulu, Department of Biology. http://herkules.oulu.fi/isbn9514268822/ Branches of adult Scots pine (Pinus sylvestris L.) trees were sampled from boreal dry pine forests to study needle longevity, its variation and its relation to other shoot characteristics. The stands studied were located along transects from two smelters and one city, e.g. along gradients of pollution impact. Constant needle age structures were assumed and static life-tables generated. Mean needle longevity was calculated as the sum of the proportions of living needle fascicles on the successive annual shoots. It fully incorporated the information of the static life-tables and was preferred to median or maximum ages because of its significantly lower variation. The first half of the shedding span, the duration of the period when the needle survival gradually dropped from 90 to 50 %, proved to validly reflect the changes in the needle age structure. Needle longevity decreased 15-40 % towards the pollution sources along the transects studied. Near the smelters, needle longevity decreased with the increasing needle Fe, or Fe, Ni and Cu concentration that represented the main constituents of the airborne particle emissions. Near the city, needle longevity decreased with the increasing needle N and P concentrations, annual needle mass and needle mass packing and decreasing needle area packing. In the urban forests, needle Mg, P and K concentrations decreased linearly with the decreasing needle survival from the second to the fourth needle age class. Concentrations in the living needles of the fourth age class stayed over 80 % of the average for all the age classes, though needle survival dropped below 50 %. A decreasing needle Mn concentration was detected towards all the emission sources. Leaching, especially from the soil, as a possible cause was discussed. Needle longevity had the lowest variation among the shoot characteristics, which increases its value as a tool in ecological monitoring. Low plasticity in needle longevity could be an acclimation to the ambient environmental conditions and length of the growing season and to maximise the carbon gain per time. Needle longevity decreased and annual needle mass and leaf mass per area increased upwards in the crowns of mature Scots pines, reflecting the acclimation to irradiance. Keywords: acclimation, air pollution, life-span, monitoring, plasticity Sari Stark (2002) Reindeer grazing and soil nutrient cycling in boreal and tundra ecosystems. Acta Universitatis Ouluensis A 382. University of Oulu, Department of Biology. http://herkules.oulu.fi/isbn9514266927/ In northernmost Fennoscandia, grazing by reindeer (Rangifer tarandus L.) has a substantial impact on the vegetation of boreal forests and arctic-alpine tundra heaths, which are reflected in below-ground processes, such as nutrient mineralization and soil organic matter decomposition. In the present thesis, the effects of reindeer grazing on soil nutrient cycling were studied by comparing grazed situation with an ungrazed control area in ten boreal forests and six arctic-alpine tundra heaths. In boreal forests, reindeer grazing reduced microbial respiration in both the oligotrophic and mesotrophic study areas, indicating a deficiency of labile substrates for the soil microbes due to reindeer grazing. Simultaneously, there was heterogeneity in the impact on nitrogen mineralization rates as at some sites, mineralization was enhanced by grazing. The fertilization effect of urine and faeces can therefore be strong enough a factor to outweigh a reduction in quality of soil organic matter. In the oligotrophic forests, low soil moisture content in the grazed areas could sometimes limit the mineralization rates even when the potential for mineralization was enhanced by grazing. In the tundra ecosystems, there was spatial variation in the impact of grazing on microbial respiration and nitrogen mineralization. Low grazing intensity occurring outside the growing season had a retarding impact on nutrient cycling in both unfertilized, nutrient-poor and fertilized, nutrient-rich conditions. In contrast, a relatively high grazing intensity enhanced the mineralization rates in two nutrient-poor and two nutrient-rich tundra heaths. When three different grazing intensities were compared in one oceanic, nutrient-rich and one continental, nutrient-poor tundra heath, the strongest positive effect of grazing on soil nutrient cycling occurred in the heavily grazed areas. The data do not support the assumption that soil nutrient availability regulates whether herbivores enhance or retard nutrient cycling in the soil. Instead, the net effect of grazing is determined by the balance between the underlying mechanisms that may work at opposite directions. The most important of these mechanisms are the grazer-mediated impact on the decomposability of the dominant vegetation and fertilization by urine and faeces. The duration, intensity and seasonal timing of the grazing seem to be important factors that regulate whether reindeer grazing enhances or retards soil nutrient cycling in each specific area. Due to the high spatial and temporal variation in the effects of grazing observed in this study, it is not possible to generalize the overall impact of grazing. Further study is required in order to determine the exact conditions under which grazing enhances or it retards soil nutrient cycling. Keywords: carbon, herbivory, litter decomposition, microbial biomass, microbial respiration, mineralization, nitrogen Pirjo Welling (2002) Regeneration by seeds and vegetation structure in alpine plant communities, subarctic Finland. Acta Universitatis Ouluensis A 391. University of Oulu, Department of Biology. http://herkules.oulu.fi/isbn951426861X/ The aims were to examine the importance of regeneration by seeds, the influence of plant traits and disturbances, and the role of seed-seedling conflicts in regeneration and in the determination of vegetation structure. The study was carried out at in a subarctic alpine area (Kilpisjärvi 69°01'N 20°50E', Finland). Seed bank and seedling densities were high in many plant communities (ranges 99 -1109 viable seeds/m2 and 0.2-227 seedlings/m2, respectively). Effective seedling recruitment is reflected in vegetation as a high proportion of plants with poor or no vegetative reproduction ability. This development may take place in meadows and snowbeds where herbs (e.g. Gnaphalium supinum, Sibbaldia procumbens, Veronica alpina and Viola biflora) are abundant. On the other hand, the low proportion of these plants in heath vegetation reflects ineffective seedling recruitment. Floristic similarities between the consecutive phases in the regeneration pathway may be low despite effective seedling recruitment. Clonality, large and small seed sizes and appendaged diaspores limit the movement of species from phase to phase. Generally, disturbances facilitate effective regeneration by seeds. Grazing promotes species with large seed banks and is therefore one reason for high seed bank densities. Freezing and melting processes negate a negative influence of altitude on seed bank densities in the phase of seedlings. However, if disturbances are severe and continuous and the soil is compact, unstable or dry, disturbances are not beneficial. The same is true if there is a shift in the species composition of seedlings from gaps to closed vegetation. This phenomenon occurred in a rich meadow. Seed-seedling conflicts limit regeneration by seeds in low-herb snowbeds and Ranunculus glacialis-Gymnomitrion snowbeds. Vegetative reproduction and infrequent pulses of seedling recruitment negate an influence of short-term seedling recruitment on the spatial structure of vegetation. Extreme conditions, such as low temperatures, instability of the soil and late snowmelt modify the influence of factors that are important in more moderate conditions. To conclude, all transitions limit regeneration by seeds. However, favourable conditions (e.g. moist conditions in a meadow) partly eliminate the obstacles against seedling emergence. Regeneration by seeds therefore has a major impact on the dynamics and structure of vegetation. In heath vegetation, where bare soils are dry and the moss cover is thick, large seed banks and seed rains do not guarantee effective seedling recruitment. The regeneration process is reduced in the early phases, and plants that reproduce primarily by seeds have a minor role in vegetation. The accumulation of seed banks is effective in these circumstances. Keywords: seed bank, seed rain, seedling emergence, seedling recruitment |