- Top of page
- Materials and Methods
Fractionations of stable carbon and oxygen isotopes (δ13C and δ18O) in tree rings of many species in the temperate (Leavitt & Long, 1988; McCaroll & Loader, 2004; Etien et al., 2008; Esper et al., 2010; Kress et al., 2010), boreal (Kagawa et al., 2006a,b; Gagen et al., 2007) and Mediterranean (Battipaglia et al., 2010; De Micco et al., 2012; Szymczak et al., 2012) zones have been analyzed at inter- and intra-annual resolutions. Understanding of fixation, allocation and remobilization processes of δ13C within an annual cycle and various plant compartments is a prerequisite for isotope dendroclimatology (Damesin & Lelarge, 2003; Helle & Schleser, 2004; Augusti et al., 2006; Kagawa et al., 2006a,b). Helle & Schleser (2004) illustrated a ‘triphase seasonal δ13C pattern in broad-leaf deciduous trees’ with: δ13C increase at the beginning of the vegetation period owing to a carryover effect of stored carbohydrates assimilated during the previous vegetation period; δ13C decrease during the main vegetation period; and a slight increase of δ13C at the very end of the vegetation period as a result of the changed carbohydrate metabolism of senescent leaves. Furthermore, Helle & Schleser (2004) assumed that the mid-section of the seasonal δ13C variation is most affected by environmental factors and may be suitable for reconstruction of seasonal environmental changes. Kagawa et al. (2006a,b) investigated carbon translocation, storage, and remobilization in time and different plant compartments after 13CO2 pulse labeling of Larix gmelini growing in the boreal zone. The results show that earlywood of tree rings contains photoassimilates from the previous summer and autumn as well as from the current season. Usually, only latewood represents the photoassimilates of the current summer and autumn, although a contribution of stored material cannot be totally excluded. Hence, ‘carbon storage is a key mechanism behind autocorrelation in (isotope) dendroclimatology’ (Kagawa et al., 2006b). However, these results were obtained from tree saplings and the authors stress the necessity of carrying out respective experiments on adult trees, which might differ in their behavior (Kagawa et al., 2006b). As evident from a recent review article on pulse-labeling studies (Epron et al., 2012), comparable studies for tropical regions have not been assessed.
A number of studies have already successfully applied stable isotope approaches in tropical regions, most of them working on an annual resolution (Verheyden et al., 2004; Robertson et al., 2006; Gebrekirstos et al., 2009; Brienen et al., 2010; Fichtler et al., 2010; Wils et al., 2010; Gebrekirstos et al., 2011a, 2012; Williams et al., 2011). For instance, a significant negative correlation between annual precipitation and δ13C time series of tree rings was found for several broadleaved tree species in various tropical climates (Fichtler et al., 2010). Gebrekirstos et al. (2011b, 2012) found significant negative correlations of annual δ13C and δ18O variations in West African Sahel woodland species with precipitation amount, humidity, drought (Palmer drought severity index, PDSI), and positive correlations with temperature. Despite those encouraging correlations with climate parameters, physiologically based understanding of δ13C variations in different tree species and functional types is still incomplete.
Stable isotopes have also been used to prove the annual nature of tree rings, to verify visually indistinct growth ring boundaries, and to delineate annual growth layers when visible tree rings are not present in tropical wood (Evans & Schrag, 2004; Poussart & Schrag, 2005; Anchukaitis et al., 2008; Roden, 2008; Pons & Helle, 2011). Anatomically indistinct rings of two tree species growing in the tropical rainforest of central Guyana were identified as annual by Pons & Helle (2011). They used minima of intra-annual δ18O and δ13C variations as primary and secondary indicators of annual growth boundaries, verified by simultaneously measured diameter increment rates of the same trees. However, retrospective stable isotope analyses lack a precise time control when the respective part of the xylem was formed. High-resolution electronic dendrometers are a tool to register the dynamics of stem diameter incremental growth with great precision (Volland-Voigt et al., 2011; Krepkowski et al., 2011), thus providing a time control for the wood formed at a certain distance from the initial starting point of the measurements. Nevertheless, stem diameter increment alone cannot unambiguously be considered as an indicator of cambial growth (Krepkowski et al., 2011, 2012). Positive radial change may result from stem swelling as a result of water uptake and saturation of wood tissues after dry periods; cambial activity with formation of new cells; or enlargement of juvenile cells before lignification (Deslauriers et al., 2009; Krepkowski et al., 2011). Hence, wood anatomical indications of cell formation have to be included for determination of periods of active cambium.
Here, we study differences in carbon carryover effects between different tree functional types linked with dendrometer measurements and wood anatomy to expand our understanding of tropical dendrochronology. To this end, we studied two native tree species with contrasting physiological traits in a tropical mountain forest in Ethiopia. Podocarpus falcatus is a late-successional evergreen conifer, while Croton macrostachyus is an early-successional deciduous tree, but seedlings and saplings of both species compete on the same sites after forest disturbance (Tesfaye et al., 2010). In contrast to P. falcatus, C. macrostachyus shows a shorter life span, higher rates of photosynthesis and transpiration, a higher metabolic activity, and a higher transfer rate of phloem sugars to arbuscular mycorrhiza fungi in the soil (Seyoum et al., 2012; Shibistova et al., 2012). We hypothesize that in the case of the early-successional deciduous tree, the allocation of recently assimilated carbon from the tree canopy to wood will be faster than in the late-successional gymnosperm, and that as a result of the more conservative life strategy, the carryover effect will be more pronounced in the gymnosperm tree. In addition, we propose that intra-annual 13C allocation patterns are related to wood anatomical structures that might indicate annual growth boundaries.
The specific objectives of the study were to analyze the intra-annual patterns of recent 13C allocation in wood of tropical trees belonging to different functional types in order to detect differences in carbon carryover effects; to study differences in cambial dynamics and wood formation of the two studied tree species; and to investigate the relationships among intra-annual δ13C variations, wood anatomical structures and moisture availability.