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The carbon isotope composition (δ13C) of plant tissues is an information-rich signal providing useful insights into different plant functions (Adams & Grierson, 2001). δ13C of plant tissues depends on δ13C of the ambient atmospheric CO2 and on the isotope fractionation within the plant (Farquhar et al., 1989). In C3 plants, the slower diffusion of the heavier 13C isotope, compared with 12C, from the atmosphere to the site of carboxylation, and the strong discrimination of Rubisco against 13C, are largely responsible for the depletion of plant material in 13C relative to the atmosphere. The δ13C composition of plant tissues is described by Farquhar et al. (1989) as follows:
- δ13Cplant = δ13Catm − α − (b−α)Ci/Cα
where δ13C is expressed in ‰, α is the discrimination during diffusion (c. 4.4‰), b is the discrimination during carboxylation by Rubisco (c. 29‰), Ci is the CO2 concentration inside the stomatal cavities, and Cα is the atmospheric CO2 concentration. Due to their effects on Ci, intercepted radiation as well as atmospheric and soil water deficits modify δ13C in plant carbon (Leavitt & Long, 1986; Livingston & Spittlehouse, 1996; Korol et al., 1999). Stomatal closure as a consequence of water deficits reduces Ci, leading to an increase in δ13C (Lauteri et al., 1997). As a consequence, plant water potential can be related to δ13C in leaves and wood and in turn to the availability of water (Damesin et al., 1998; Warren et al., 2001). Alternatively, under light-limiting (but not water-limiting) conditions, photosynthesis and, consequently, Ci depends on radiation. Therefore, organic carbon in leaves from the shaded part of the crown of trees is more depleted in 13C compared with the sun-exposed crown (Leavitt & Long, 1986).
In addition, assessment of δ13C in different plant tissues provides information on the reaction of plants to environmental parameters within various scales. For example, δ13C of wood is considered to be a spatially and temporally integrated measure of leaf internal CO2 concentration of the growing season the wood was produced in and/or of one or more growing season(s) before wood formation (Schleser et al., 1999; Geßler et al., 2001). Furthermore, foliar δ13C is regarded as an indicator of water-use efficiency during longer periods of time (Farquhar et al., 1989) depending upon δ13C of first carbon storage pools remobilised during bud break, second carbon assimilated during leaf expansion and third assimilates produced later in the growing season (Damesin et al., 1998). Carbon isotope composition of phloem sap sampled during the growing season is assumed to be a strong guide to short-time changes in Ci : Cα during the actual growing season (Pate & Arthur, 1998; Adams & Grierson, 2001; Geßler et al., 2001; Keitel et al., 2003).
Owing to its dependence on water availability δ13C in plant organic matter can be a useful tool for assessing plant responses to climate change. In the near future summer droughts are expected to increase both in frequency and duration in Central Europe (Linder et al., 1996; Peñuelas, 1996; Lawlor, 1998; IPCC, 2001; Saxe et al., 2001). Natural regeneration of the drought sensitive European beech (Fagus sylvatica L.) – one of the most important deciduous tree species in central Europe – may be affected intensively by such climate alterations, especially since the area of distribution includes sites with shallow limestone-derived soils with low water storage capacity (e.g. Schwäbische Alb, Fränkische Alb, Schweizer Jura and French Jura).
In a changing climate, the application of forest management practices may have effects on the physiology of beech regeneration different from the ones observed today (Fotelli et al., 2002). At present, silvicultural techniques such as selective thinning of the mature canopy are routinely applied in beech forests (Tarp et al., 2000) in order to support natural regeneration (Dertz, 1996, Ministerium für Ländlichen Raum, Ernährung, Landwirtschaft und Forsten in Baden-Württemberg (ed.), 1997). Thinning is known to improve abiotic conditions (light intensity, nutrient availability, temperature; e.g. Aussenac, 2000; Mizunaga, 2000; Thibodeau et al., 2000) for beech seedlings, but due to its effects on interspecific relations within the forest understorey (Madsen, 1995; Lof, 2000) it may favour drought-tolerant species under altered climatic conditions. Since δ13C is a useful indicator for comparing the ecophysiology of functional groups in complex forest ecosystems (Huc et al., 1994; Brooks et al., 1997; Guehl et al., 1998; Bonal et al., 2000), it may also be used for identifying differences in water balance between beech regeneration and other – competing – functional groups of the understorey vegetation, induced by varying climate and thinning.
This study aimed at assessing the relationship of δ13C in different tissues of different functional groups of a beech forest understorey to environmental parameters, and at comparing these relationships between the functional groups. Moreover, we intended to validate δ13C signature as a time-integrating indicator of changes in environmental factors, as modulated by climate and thinning. The study was performed with naturally regenerated beech seedlings and other understorey species growing in beech forests on two opposite-exposed aspects (SW compared with NE), differing in local climate, and within each site, in a thinned and an unthinned stand. Our initial hypothesis was that δ13C would largely reflect variations in water availability, which is higher at the NE-facing site (Geßler et al., 2001) and probably also in the thinned stands (Breda et al., 1995). Moreover, we expected to identify differences in δ13C patterns among beech seedlings and neighbouring understorey plants, attributable to differences in their water status (Fotelli et al., 2001).
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The present study aimed to assess the effects of different meso-climatic conditions and of thinning on the C-isotope composition in different tissues of young beech seedlings and two additional functional plant groups in the understorey of a beech forest in Southern Germany. Our initial hypothesis was that δ13C of the studied plants reflects the general aspect- (NE compared with SW) and treatment-specific differences in water and light availability (Geßler et al., 2001). Furthemore, we expected to identify differences in the factors that mainly affected δ13C, among the plant groups studied.
Above-ground tissues and roots of beech seedlings and other understorey species were constantly 13C-enriched on the SW, compared with the NE aspect (Figs 2, 3 and 4). Regression analyses revealed that soil water potential – as a measure of soil water availability – at 0.20 m depth was the main single environmental factor responsible for the significant 13C-enrichment in leaves (R2 = 0.87, P = 0.005; Table 4) and roots (R2 = 0.97; P < 0.001) of beech and in shoots of woody plants (R2 = 0.47; P = 0.048) in the control stand of the SW, compared with the NE aspect. It is a well-established finding that water shortage causes stomatal closure, thus, resulting in lower discrimination against 13C (Farquhar et al., 1989; Lauteri et al., 1997; Arndt et al., 2000; Fotelli et al., 2001; Warren et al., 2001). Since there were also significant lower transpiration rates in beech seedlings grown in the NE compared with the SW control stand (Fig. 5), it is concluded that differences in the water status of understorey species between aspects are mainly responsible for the observed patterns in δ13C of different tissues. This hypothesis is supported by the observation that decreasing plant water potential from the NE to the SW aspect accounted for 56% of the concurrent increase in δ13C in beech foliage (Fig. 7).
Soil water potential and, hence, water availability in deeper soil layers (40 cm) had no significant effect on δ13C, since the rooting zone of beech seedlings and other understorey species was not below 0.20 m.
Within a comparable range of transpiration rates leaves of beech seedlings were significantly 13C enriched on the SW aspect (Fig. 7). The observed differences in δ13C patterns among aspects could not be attributed to differences in stomatal conductance at comparable transpiration rates: regression analysis between stomatal conductance and foliar δ13C showed a relationship comparable with transpiration. As a consequence, δ13C could not be used as a general indicator for instantaneous water use or stomatal reactions for all aspects and treatments. Bearing in mind that a mixture of different carbon pools with different turn-over times contributes to δ13C of bulk leaf material (Adams & Grierson, 2001) it is obvious that its dependency on parameters measured immediately before sampling may be confounded by a whole set of other physiological and environmental variables and their influence over different integrals of time.
Warren et al. (2001) found isotopically heavier tissues in conifers grown in thinned stands, compared with plants in closed stands, and attributed this response to increased radiation interception after thinning which mediates higher photosynthesis and lower discrimination against 13C (Farquhar et al., 1989). Also in the present study, radiation modulated the effect of water availability on the δ13C. Foliar δ13C signatures of beech seedlings on the control plots reflected, to a great extent, the combined effects of soil water availability and light intensity integrated over a 1-month period (R2 = 0.95, P = 0.01; Table 4). PAR alone accounted for 61% of the variation in foliar δ13C in beech and was the main determinant of δ13C in shoots and roots of herbs grown in the control stands (Table 4) where apparently light intensity was a limiting factor (Fig. 1). This supports the hypothesis that light intensity modifies the effect of water availability on δ13C at the site studied when the former is a limiting factor for CO2 assimilation (Geßler et al., 2001). The low foliar δ13C signatures in the NE-control stand (Fig. 2) may, thus, be a result of the restricted light intensities (Fig. 1, NEC), according to the well-documented effect of PAR on Ci and, thus, δ13C (Leavitt & Long, 1986; Farquhar et al., 1989; Broadmeadow & Griffiths, 1993; Livingston & Spittlehouse, 1996). On the other hand the possibility can not be excluded that light intensity increases leaf temperature and, as a consequence, leaf-to-air water vapour pressure difference causing stomatal closure. A decrease in δ13C with increasing radiation could, thus, also be a result of stomatal reaction. The additional assessment of δ18O in plant organic matter and a correlation between δ13C and δ18O as proposed by Scheidegger et al. (2000) and Keitel et al. (2003) allows a differentiation between effects of stomatal reaction and/or photosynthesis on δ13C, and should be applied in future studies.
Different from the observations of Warren et al. (2001), no consistent trend towards an increase in δ13C in the thinned stands could be observed in the present study, although radiation increased intensively as a consequence of thinning. On the SW aspect there was almost no difference in δ13C between treatments. The effect of thinning on δ13C of plant tissues from the NE aspect was not constant among sampling dates and species with both, increase and decrease of δ13C as a consequence of treatment. As described by Buchmann et al. (2002), differences in plant δ13C in the forest understorey may be due to differences in δ13C of CO2 assimilated. As a result of microbial and root respiration δ13C of CO2 near the forest floor is typically more negative than values of above-canopy CO2 (Buchmann et al., 1998). The variable effects of thinning on δ13C of understorey species growing on the NE aspect may, thus, be a result of variable influence of the silvicultural treatment on the intensity of soil respiration during the growing season and, in addition, of generally high spatial and seasonal variation of CO2 efflux from the soil (Buchmann, 2000). Still, we lack data on δ13C of the CO2 assimilated for verifying this hypothesis.
The finding that foliar δ13C of beech seedlings is an indicator of environmental conditions within a month integral before sampling contradicts the results of other studies with adult trees. In deciduous trees such as European beech, foliage that develops in spring is formed mainly from stored carbon and nutrients (Kozlowski & Pallardy, 1997) – newly assimilated carbon is mixed with the previously stored carbon to form the new leaves. It has been reported that foliar δ13C largely reflects conditions of the previous growing season(s) (Damesin et al., 1998; Geßler et al., 2001; Le Roux-Swarthout et al., 2001). Still, the close relationship between foliar δ13C and water potential of beech seedlings in a greenhouse-experiment (Fotelli et al., 2001), as well the results of the present study, support that foliar δ13C signature of young beech seedlings reflects – to a great extent – the isotopic composition of recently produced assimilates.
Not only δ13C of beech foliage but also of roots proved to be measure for soil water availability: averaged over a 2-month period soil water potential accounted for c. 97% of δ13C variation in the control stands (Table 4). This finding supports the assumption that δ13C in the roots is – with a considerable time lag – mainly defined by the carbon isotope composition of carbon exported from the leaves. This observation is further strengthened by the results of Högberg et al. (2002) who demonstrated that there is a significant flux of recently fixed carbohydrates from the leaves to the roots, which greatly affects root respiration, and thus carbon balance of roots. Furthermore, Ekblad & Högberg (2001) showed that newly produced photo-assimilates significantly accounted for the variation in root respiration and in its δ13C composition.
In contrast to leaves and roots, soil water availability had no direct effect on the δ13C signature of beech wood. The carbon isotope composition of wood is an integrator of environmental conditions over longer periods of time (Livingston & Spittlehouse, 1996; Pate & Arthur, 1998; Geßler et al., 2001; Porté & Loustau, 2001). Therefore, the positive relationship between δ13C signature of wood and soil temperature (Fig. 6d Table 4) most probably reflects the differences in the light environment of aspects and treatments, which induce long-term differences in soil temperature (Table 1). In agreement with our results, Dupouey et al. (1993) found an increase in δ13C of wood cellulose of European beech with increasing temperature.
The observed relationships between δ13C of herbs or woody plants, and environmental factors were weaker, compared with beech seedlings. This may be due to the diversity of species forming these functional groups, and/or the complexity of the conditions occurring close to the forest ground. Since species within both groups differ in life form and phenology, the time when the major part of carbon-containing material is laid down may vary significantly among members of each functional group. Thus, effects of environmental parameters on δ13C may be concealed.
Tissues of above-ground parts from herbs and woody plants had a tendency to 13C-enrichment – compared with beech seedlings – in May and, especially, in July (Table 3). This difference may be attributed to the higher root biomass and, hence, probably better water access of beech seedlings compared with plants of the other groups. However, the conditions in the field seem to be more complex, as herbs were the plants with highest 13C depletion in autumn. Since most of the herbaceous species assessed are vernal (cp. Oberdorfer, 1983) they show signs of senescence in mid-September before visible yellowing of leaves occurs in beech (Kirchgäßner, 2001). Decreasing rates of photosynthesis (Proietti, 1998), and, thus, increasing Ci in senescing leaves might be the reason for the observed increase in 13C depletion in shoots of herbaceous species at the end of the growing season (Fig. 4).