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| Linking Resource Allocation with Space Sequestration - a Mechanistic Basis of Plant Competitiveness and Self-Thinning of Stands? |
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R. Matyssek, H. Pretzsch, H. Schnyder
K. Klumpp, A. Kozovits, I. Reiter, T. Grams, K.-H. Häberle, M. Lötscher, B. Winkler,H. Blaschke, P. Fabian, H.-D. Payer, H. Werner
Ökophysiologie der Pflanzen, Waldwachstumslehre, Grünlandlehre
Bioklimatologie & Immissionsforschung,
Wissenschaftszentrum Weihenstephan / Technische Universität München
GSF-Forschungszentrum/EPOKA
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Two basic criteria of individual plant fitness are the capacities in resource sequestration as an expression of competitiveness relative to neighboring plants and, once being incorporated, the defence of resources against consumers (e.g. pathogens, herbivores). These two capacities need to be adjusted, therefore, to each other internally through processes of resource allocation. The question arises about costs and benefits that may originate from shifts in resource allocation and be of relevance for competitiveness.
In stands of sunflower, high nitrogen (N) availability enabled subordinate individuals to re-adjust carbon (C) allocation under increasing shading towards enhancements in shoot growth and foliage area and, by this, improvement in light interception. Although N limitation did favor root growth, adjustments in light use could apparently not be afforded, and plants became prone to develop negative C balances and to become gradually constrained by the dominant individuals. High N availability prevented the C gain to drop below the C demand of maintenance respiration which was a function of whole-plant biomass and fed from reserve storage. Within the respired C, the proportion originating from current photosynthesis correlated with the extent of C fixation and biomass increment. Hence, this proportion of respired C was a measure of the costs of growth and of competitiveness, as the new increment contributed to the capacity in aboveground space exploration and exploitation. As plants proceed in growth and gain in biomass and size, the competition for resources exacerbates within stands so that a trade-off emerges between individual plant mass and dimension on the one hand, and plant population density that can be sustained at the stand level on the other. Self-thinning is the phenomenon that mediates this trade-off, and examples are presented from herbaceous and forest systems which indicate similar rules in adjusting the balance between plant allometry and population density despite contrasting lifespans and plant dimensions.
Are principles of self-thinning and, hence, population dynamics and structural stand differentiation, pre-determined by strategies in space sequestration and exploitation ? Are such strategies quantifiable as efficiency ratios that relate resource investments (i.e. internal allocation) and gains to space sequestration and its "running costs", the latter being regarded as the respiratory and transpiratory demands for keeping conquered space occupied ? A concept that links competitiveness intimately with both structural allometry and its associated resource turnover is presented and applied to adult and juvenile spruce and beech trees that grow under stand conditions in the field or in phytotrons, respectively. In the adult trees, space-related C gains and "running costs" of sun branches were similar in spruce and beech despite their contrasting foliage type and growth habit, whereas in the shade crowns the C balance of beech branches tended to become negative. Beech concentrated its foliage within the upper crown while strongly shading the lower part. In parallel, beech was most efficient in sequestering crown space per unit of resource investment, leading to a horizontally extending architecture that rendered the crown a proficient means in constraining neighbors.
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Beech apparently afforded poorly productive branches in the lower crown, which did not weaken the whole-tree production but - due to their efficient space sequestration - were significant in keeping crown volume occupied.
In the juvenile stands of the phytotron study, beech was a poor competitor relative to spruce, while the latter profited both from elevated CO2 supply and the O3 sensitivity of beech when growing in mixed culture. Nevertheless, beech was more efficient in space-related C gain and belowground space sequestration through enhanced specific root length, but tended to be less efficient in the "running costs". Most distinct, however, was the declining efficiency in aboveground space sequestration per unit of resource investment so that it is again concluded that the efficiency in space rather than resource sequestration is a significant determinant of competitiveness. Remarkably, CO2/O3 regimes exerted minor impacts only on tree performance relative to those by competitors, and changes in growth pattern were size-dependent rather than the result of regulatory changes in whole-plant resource allocation.
Efficiency in space sequestration - while inherently being an expression of plant-internal resource allocation - appears to play a key role in constraining neighbors and increasing the chance of becoming a survivor during the self-thinning process of stands. Hence, the introduced concept is capable of providing a mechanistic basis of plant competitiveness and population dynamics at the stand level. The concept is currently being extended through analyses on belowground interactions and assessments of effects by pathogens and symbionts on resource allocation within and between competing plants.
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