Summary
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A two-year study was performed in phytotrons in order to analyse and quantify the competflveness of young beech (Fagus sylvatica) an spnice (Picea abies) plants grawing in mono- and mixed plantations. Efficiency rates are defined that relate resource investments and gains to above and below-ground s~ace sequestration. Plants were exposed to gaseous regimes of ambient er eleveted 002 (ambient + 300 ppm) concentratons in combina~on with embient or twice-ambient 03 Ievels. The C02/03 regimes were used as disturbants for examining the sensitivity of the efflciency ratios. In mixed plantations. the growth of spruce was enhanced and that of beech reduced as compared with the corresponding monocultures. This effect related to a high efficiency of spruce in above-ground space sequestration per resource investment, whereas the efficiency in carton gain (retum per unit cl crown volume) was ef minor importance. In contrast. beech displayed advantages in below-ground space sequestration. Compelitiveness of young beech trees was hardly affected by the applied C0j03 treatments. A compahson of such efflciencies between young and mature trees resulted in conspicuous consistencies across plant age and species. The concept proves to be adequate for quantitatively comparing coinpetitiveness across species age and growth condit ans.
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Introduction
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Plant competitiveness is analysed jn our study in terms of efficiency ratios which are related to above and below-ground space sequestration, resource gain maintenance costs. C0j03 regimes were used as disturbants for examining sensitivity of these rafios. A hyoothesis was tested as to whether the competitiveness of beech rather than spruce is affected by elevated CO2 and ozone regimes. Comparisons were made with findings from an associated study an mature trees at a forest site (Project B4 ).
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Experimental design
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The investigation was conducted et the GSF-National Research Center (Fig.1), In spring 1998, 20 young trees (Fa gus sylvatica and Picea abies, twa and three yeerold, respectively), were planted inta containers (0084 m3) es mono- and mixed cultures. Measurements were concentrated an the six central individuals of each container. In the first year plants were kept in a ciimete-controlled greenhouse et ambient and eleveted (ambient +300 ppm) 002 Ievels. During the following two growing seasons, plants were placed into phytotrons at both C02 concentrations being combined with ambient or elevated twiceambient 03 Levels (150 ppb maximum).
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Results
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Relative growth increment
In general, beech showed higher proportional increments than spruce. The relative growth of spruce was not affected by the gasecus treatmdnts. In conb~st, beech es the more responsive species reflected slight (but not signiflcant) stimulation under elevated CO2 when growing in monocultures. Spruce has a competitive advantage over beech in the mixed cultures. since the relative abovegrcund growth of spruce was enhanced at the expense ofthat in beech
Fig.3: Relative increment (9%) of the total abovegroung non-green biomass in one growing season. A CQ+300/1s03 o CO~+300/2xO3
Efficiency in carbon gain
Under elevated 03 regimes, spruce displayed an increased carben gain efficiency. Beech achieved its highest efficiencies under elevated £02 and ambient 03 exposure treatment. Relative to the growth in monoculture, beech tended to increase the carbon gain efticiency in the mixed plantetons. But in general, differences between mixed and monocultures were rather small and cannot explain the advantage cl' spruce over beech displayed in the Fig. 3. In parallel, beech showed a higher efticiency in belowground space sequestration (when expressed as root length by weight), whereas spruce neither responded to the gaseous regime ncr plantation type (data not shown). Also the letter results cannot explain the linding of Fig. 3.
Fig. 4: Efficency in carbon gain (annual carbon gain per unit of crown volume).
Efflciency in aboveground space sequestration
The crown architecture of beech reflected lowest efficiency in space sequestration under elevated 002 conditions (Fig. 5). In contrast, spruce increased its correponding efliciency ratio in the mixed plantations under ambient 002 and elevated CO2/2x03 conditions, whereas in beech ne significant changes from' mono towards mixed ctjltures were observed. Overall, crown architecture rather than productivity (Fig. 3) per se appears to dominate the competitiveness cl spruce.
Fig. 5: Efficiency in aboveground space sequestratlon (crown volume per biomass investment). Die bars reprement changes relative to control (amb. CO2/1*O monoculture).
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Comparison between trees of different age
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In both species the resource gain efficiency of young trees was similar to that of the sun branches cl mature trees. In general, ipruce displayed a higher carbon gain efficiency than beech. The below-ground space sequestration efficiency was highet in die young as compared with the mature trees regardless of the species, and - irrespective of age - was lower in spruce than in beech.
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Conclusions
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Spruce displayed higher competitiveness in the mixed plantations than beech.
Efficiency of resource gain does not explain the increased competitive performance of spruce.
The crucial factor tor the competitiveness of spruce in the mixed cultures appears to be the higher efficiency of above-ground space occupation.
The ccncept of assessing volume-based resource gains versus investments proved to be a promising real tor quantifying above and below-ground competitveness. and tor scaling mechanisms of competition across plant ontogeny.
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Fig. 1: Phytotron at GSF-National Reserch Center-Munich
Pattern of a mixed plantation
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