The aim of the field study was to quantify the below-ground competitiveness of 50 to 60-year-old beech (Fagus sylvatica) and spruce (Picea abies) trees by means of space-related cost/benefit relationships. The study was conducted at the experimental site “Kranzberger Forst” within the framework of the interdisciplinary research program Sonderforschungsbereich 607 (SFB 607; Project B4) “Growth and Parasite Defence – Competition for Resources in Economical Plants from Agronomy and Forestry”. It was postulated that costs (resource investments) and returns (resource gains) as based on soil volume have the potential to quantify the plant competitive ability below-ground. This idea relates to the definition of Begon et al. (1996) that the competitive success of plants depends on their ability to acquire efficiently resources shared with neighbours from external pools located in a given space. The below-ground competitive ability (i.e. competitiveness) of beech and spruce was quantified by using three types of space-related cost/benefit relationships (i.e. “efficiencies”, cf. Grams et al., 2002): (1) Efficiency of below-ground space occupation (occupied soil volume per unit C investment into standing fine-root biomass), (2) Efficiency of below-ground space exploitation (resource uptake as related to the occupied soil volume), and (3) Efficiency of below-ground “running costs” (occupied soil volume per unit of C costs of root respiration to sustain occupation and exploitation of the occupied soil volume). Here, tree response to free-air ozone fumigation at the crown level (the O3 regime was experimentally raised by a factor of 2 relative to the ambient O3 regime; i.e. 1xO3 and 2xO3, respectively) was related to different water availability across the three experimental years (including the extraordinary summer drought of 2003) to analyse the belowground responsiveness of both competitors to disturbance under forest stand conditions.
The following hypotheses were to be evaluated: (i) Limited carbon gain by the foliage, as caused by O3 stress or water limitation, results in stimulation of compensatory fine-root production; (ii) Spruce being more conservative in resource turnover is less affected below-ground by O3 stress than beech; conversely, relative to beech, the root system of spruce is more affected by drought; (iii) O3 exposure enhances the CO2 efflux from the soil, whereas drought reduces CO2 efflux; (iv) “Autotrophic” soil respiration (i.e. CO2 release from roots + mycorrhizosphere) and “heterotrophic” soil respiration (i.e. respiration of free-living soil micro-organisms) differ in their response to changing soil temperature and soil water content, with higher sensitivity reflected in “autotrophic” soil respiration ; (v) The responses of beech and spruce plants to O3 stress and water limitation depend on the type of competition (i.e. intra or interspecific); (vi) Below-ground competitiveness of adult beech and spruce trees is determined by similar efficiency ratios as above-ground competitiveness.
Sampling and analysis were conducted within the uppermost 20 cm of the soil where most intense below-ground competition between the neighbouring trees was expected. In addition, within this soil depth two soil sub-samples were distinguished according to the existing gradient in nutrient and water supply: organic sub-sample (consisting of the humus layer, and the humic topsoil (A-horizon)); and mineral sub-sample (consisting of the loamy B-horizon). At mono-specific subplots, the space occupied by roots was derived from the volume of the organic or mineral soil sub-sample which was extracted with a soil coring cylinder. At mixed subplots, a factor kc (i.e. defined at the monospecific sub-plots under control conditions as the ratio between the soil volume and the volume of contained root biomass) was used to split the soil volume into parts occupied by beech or spruce roots, respectively. Bio- and necromass of the fine roots was monitored using soil coring and in-growth coring methods. In addition, the natural variations of δ13C signature were assessed in newly formed fine-roots as obtained by in-growth technique. For assessment of the physiological, biochemical and morphological parameters, the fine-root fraction was differentiated into three functional categories according to root position and anatomy. Water uptake was studied in situ on intact fine rootlets by using the non-destructive approach of Göttlein et al. (2001) adapted to field conditions. Root respiration was measured with a portable, integrated infrared gas analyzer (IRGA) and gas exchange assessment system throughout the annual course and in response to temperature. Total soil respiration was assessed accordingly throughout the annual courses of 2002 and 2003. In addition, the root exclusion method was employed to differentiate between the components in total soil respiration, i.e. “autotrophic” and “heterotrophic” respiration. Available soil water content (calculated by S. Raspe, LWF, through LWF-BROOK90; Hammel & Kennel, 2001) and soil temperature data were used to parameterize a modelling approach for estimation of the annual “autotrophic” and “heterotrophic” soil respiration.
At monospecific sub-plots, beech responded to both disturbing factors (drought or ozone) with enhanced fine-root production and turnover. Spruce, in contrast, showed inhibited root growth under dry conditions, and had no response to enhanced O3. Hence, hypothesis (i) was corroborated for beech and rejected for spruce in that resource limitation stimulated fine-root growth. The reduced fine-root production in spruce shows that this species is more affected below-ground by drought than beech, thus confirming hypothesis (ii). The contrasting physiological root responses in these tree species to drought may relate with the different extent of water limitation underneath mono-specific spruce and beech groups during 2003. In contrast to beech, spruce which has not responded below-ground to O3 showed aboveground decrease in stem diameter increment of fumigated trees (Wipfler et al., 2005). Possibly, the capacity for ensuring sufficient nutrient supply for growth and/or repair processes via enhanced fine-root turnover is the “strategy of success” in beech to cope with O3 stress. In beech, the combination of factors “O3+drought” influenced fine-root dynamics in a way similar to that by drought or O3 impact alone, whereas in spruce such scenario induced responses similar as that caused by drought. At mixed sub-plots, only spruce showed different response to the studied scenarios remaining non-affected by drought in any of the studied root parameters (i.e. hypothesis (v) was confirmed only for spruce). Apparently the shallow fine-root system of spruce has profited from a more effective re-charge and use of soil water in beech/spruce mixed sub-plots relative to monospecific sub-plots.
In both species, beech and spruce, and independent of the type of competition, drought of 2003 reduced and O3 exposure increased the total CO2 efflux from the soil thus corroborating hypothesis (iii) and rejecting hypothesis (v). In the context of “global change” scenarios with increase of O3 levels, the risk of chronic O3 stress to promote CO2 efflux from forests with sufficient water supply cannot be ruled out. In both species, beech and spruce, “autotrophic” rather than “heterotrophic” soil respiration was sensitive to changing soil temperature and soil moisture (i.e. hypothesis (iv) was corroborated in both tree species). Drought limitation of “autotrophic” soil respiration was accompanied, on an annual basis, in both tree species by decrease in radial stem growth (according to P. Wipfler, pers. comm.), with spruce being more sensitive than beech.
Under control conditions, the below-ground competitive advantage of beech within the uppermost 20 cm of the soil was indicated by higher efficiency of space occupation and space exploitation, whereas the efficiency of “running costs” remained similar in both tree species. The competitive success of beech was associated with the capacity to exploit available resources rapidly from soil (i.e. via constructing fine-roots with high specific fine-root length). This finding supports the theory of Grime (1977), that the competitive success is a reflection of the individual capacity to exploit resources rapidly. Additionally, the present study shows that beech which was the superior competitor for light (Reiter et al., 2005) was the superior competitor also for soil resources (i.e. soil water). These results are in agreement with the other hypothesis of Grime (1977), that superior competitors for light would be equally successful in competing for soil resources (cf. section 1.1.2).
When focusing on space occupation within both soil sub-samples, beech was more efficient than spruce within the organic soil sub-sample, whereas spruce was more efficient within the lower mineral soil. Such “habitat partitioning” between both competitors resulted in decreased competition for water: Beech was more efficient in space exploitation within the mineral, whereas spruce was more efficient in space exploitation within the organic soil sub-sample. Such “habitat partitioning” may represent the mechanism allowing beech and spruce to coexist in a short-term scale. On the other hand, such kind of partitioning may result in a shift towards a more superficially distributed fine-root system of spruce in mixture with beech, which suppose spruce to be at higher risk than beech at prolonged drought or regarding hazards like wind-throw.
The competitive below-ground advantage of beech over spruce vanished under chronically enhanced O3 exposure independent of the type of competition: The disturbance through elevated O3 lowered the efficiency of beech in below-ground space occupation and the efficiency of “running costs” relative to the corresponding efficiencies in control sub-plots. Spruce, in contrast, showed similar below-ground space occupation under both O3 regimes and types of competition.
The novel approach of a quantitative, space-related assessment of cost/benefit relationships proved suitable to analyse the below-ground competitive ability of forest trees. Within the studied soil depth of 20 cm and under control conditions, the higher below-ground competitive ability of beech over spruce was reflected by higher efficiency in space occupation and space exploitation. Both “efficiencies” can be explained with higher morphological plasticity (i.e. specific fine-root length, turnover) of the beech fine-root system relative to spruce. In contrast, spruce fine-roots, which appear to be more expensive than beech fine-roots in terms of their structural costs per root length, were less efficient in space occupation and exploitation within the studied soil depth. Independent of the type of disturbance (i.e. drought or O3-fumigation) beech maintained an enhanced fine-root production. Although the enhanced fine-root production is related with additional structural costs (i.e. lower efficiency in space occupation), on a whole-tree level beech was less imperilled to such disturbances than spruce.
Im Mischbestand „Kranzberger Forst“ wurde die Konkurrenzfähigkeit adulter Buchen und Fichten im Wurzelraum raumbezogen quantifiziert. Die erstellten Kosten-Nutzen-Bilanzen bezogen sich auf die Effizienz der Raumbesetzung und der Raumausbeutung sowie die Unterhaltskosten beider Baumarten. Dazu wurden Biomasse, Wasseraufnahme und Atmungsrate der Feinwurzeln aus dem Hauptwurzelhorizont pro Bodenvolumen bestimmt. Der Einfluss experimentell erhöhter Ozonkonzentrationen im Kronenraum und der Trockenheit von 2003 wurde im Vergleich zu Kontrollbedingungen untersucht. Ungestört erwies sich die Buche mit weniger C-Investition pro Bodenvolumen und morphologisch variableren Feinwurzelnstrukturen als die effizientere Baumart. Unter erhöhten Ozonkonzentrationen oder Trockenheit verlor Buche an unterirdischer Effizienz aufgrund kompensatorisch erhöhter Investition in die Feinwurzelbiomasse, erwies sich aber weniger gestört auf Baumebene als die konservative Fichte. «
Im Mischbestand „Kranzberger Forst“ wurde die Konkurrenzfähigkeit adulter Buchen und Fichten im Wurzelraum raumbezogen quantifiziert. Die erstellten Kosten-Nutzen-Bilanzen bezogen sich auf die Effizienz der Raumbesetzung und der Raumausbeutung sowie die Unterhaltskosten beider Baumarten. Dazu wurden Biomasse, Wasseraufnahme und Atmungsrate der Feinwurzeln aus dem Hauptwurzelhorizont pro Bodenvolumen bestimmt. Der Einfluss experimentell erhöhter Ozonkonzentrationen im Kronenraum und der Trockenhe... »