Stable carbon and oxygen isotopes in tree rings show physiological responses of Pericopsis elata to precipitation in the Congo Basin

Earlier onset and slower heartwood investment in faster-growing trees of African tropical species

BACKGROUND AND AIMS

Heartwood plays an important role in maintaining the structural integrity of trees. Although its formation has long been thought to be driven solely by internal ageing processes, more recent hypotheses suggest that heartwood formation acts as a regulator of the tree water balance by modulating the quantity of sapwood. Testing both hypotheses would shed light on the potential ecophysiological nature of heartwood formation, a very common process in trees.

METHODS

We measured quantities of heartwood and sapwood, xylem conduits and the width and number of growth rings on 406 stems of Pericopsis elata with ages ranging from 2 to 237 years. A subset of 17 trees with similar ages but varying growth rate were sampled in a shaded (slower-growth) site and a sun-exposed (faster-growth) site. We used regression analysis and structural equation modelling to investigate the dynamics and drivers of heartwood formation.

KEY RESULTS

We found a positive effect of growth rate on the probability of heartwood occurrence, suggesting an earlier heartwood onset in faster-growing stems. After this onset age, heartwood area increased with stem diameter and age. Despite the similar heartwood production per unit stem diameter increment, shaded trees produced heartwood faster than sun-exposed trees. Tree age and hydraulics showed similar direct effects on heartwood and sapwood area of sun-exposed trees, suggesting their mutual role in driving the heartwood dynamics of sun-exposed trees. However, for shaded trees, only tree hydraulics showed a direct effect, suggesting its prominent role over age in driving the heartwood dynamics in limited growing conditions. The positive relationship between growth rate and maximum stomatal conductance supported this conclusion.

CONCLUSIONS

Heartwood area increases as the tree ages, but at a slower rate in trees where water demand is balanced by a sufficient water supply. Our findings suggest that heartwood formation is not only a structural process but also functional.

Wood anatomy and tree-ring stable isotopes indicate a recent decline in water-use efficiency in the desert tree Moringa peregrina

The ability of desert plants to adapt to future climate changes and maximize their water-use efficiency will determine their survival. This study uses wood anatomy and δ¹³C and δ¹⁸O isotope analyses to investigate how Moringa peregrina trees in the Egyptian desert have responded to the environment over the last 10 years. Our results show that M. peregrina tree-ring widths (TRWs) have generally declined over the last decade, although individual series are characterized by high variability and low Rbars. Vessel lumen area percentages (VLA%) are low in wet years but increase significantly in dry years, such as the period 2017-2020. Stable δ¹³C isotope values decrease between 2010 (- 23.4‰) and 2020 (- 24.9‰), reflecting an unexpected response to an increase in drought conditions. The mean δ¹⁸O value (± standard error, SE) for the first ten rings of each tree from bark to pith (2020-2010) is 33.0 ‰ ± 0.85 with a range of 29.2-36.3‰, which indicates a common drought signal. The intrinsic water-use efficiency (iWUE) declines gradually with time, from 130.0 µmol mol^-1 in 2010 to 119.4 µmol mol^-1 in 2020. The intercellular carbon concentration (C_i) and C_i/C_a ratio increase over the same period, likely as a result of decreasing iWUE. The results show that M. peregrina trees seem to cool their leaves and the boundary air at the cost of saving water.

Tree-ring δ18O climate signals vary among tree functional types in South Asian tropical moist forests

We present the first annually resolved and statistically reliable tree-ring δ¹⁸O (δ¹⁸O_T) chronologies for the three South Asian tropical moist forest tree species (Chukrasia tabularis A. Juss., Toona ciliata M. Roem., and Lagerstroemia speciosa Roxb.) which differ in their shade tolerance and resistance to water stress. We found significantly higher mean δ¹⁸O_T values in light-demanding T. ciliata than in intermediate shade tolerant C. tabularis and shade tolerant L. speciosa (p < 0.001). δ¹⁸O_T in C. tabularis was mainly influenced by pre-monsoon vapor pressure deficit (VPD; r = -0.54, p < 0.01) and post monsoon maximum temperature (T_max) (r = 0.52, p < 0.01). δ¹⁸O_T in T. ciliata was strongly negatively correlated with a dry season drought index PDSI (r = -0.65, p < 0.001) and VPD (r = -0.58, p < 0.001). Pre-monsoon T_max was strongly positively linked with δ¹⁸O_T in L. speciosa (r = 0.65, p < 0.001), indicating that climatic influences on δ¹⁸O_T are species-specific and vary among tree functional types. Although there was a week correlation between local precipitation and δ¹⁸O_T in our studied species, we found a strong correlation between δ¹⁸O_T and precipitation at a larger spatial scale. Linear mixed effect models revealed that multiple factors improved model performance only in C. tabularis, yielding the best model, which combined VPD and T_max. The top models in T. ciliata and L. speciosa included only the single factors PDSI and T_max, highlighting that the way C. tabularis interacts with climate is more complex when compared with other two species. Our analyses suggest that stable oxygen isotope composition in tree rings of South Asian tropical moist forest trees are a suitable proxy of local and regional climate variability and are an important tool for understanding the physiological mechanisms associated with the global hydrological cycle.

Chemical Fingerprinting of Wood Sampled along a Pith-to-Bark Gradient for Individual Comparison and Provenance Identification

Background and Objectives: The origin of traded timber is one of the main questions in the enforcement of regulations to combat the illegal timber trade. Substantial efforts are still needed to develop techniques that can determine the exact geographical provenance of timber and this is vital to counteract the destructive effects of illegal logging, ranging from economical loss to habitat destruction. The potential of chemical fingerprints from pith-to-bark growth rings for individual comparison and geographical provenance determination is explored. Materials and Methods: A wood sliver was sampled per growth ring from four stem disks from four individuals of Pericopsis elata (Democratic Republic of the Congo) and from 14 stem disks from 14 individuals of Terminalia superba (Côte d’Ivoire and Democratic Republic of the Congo). Chemical fingerprints were obtained by analyzing these wood slivers with Direct Analysis in Real Time Time-Of-Flight Mass Spectrometry (DART TOFMS). Results: Individual distinction for both species was achieved but the accuracy was dependent on the dataset size and number of individuals included. As this is still experimental, we can only speak of individual comparison and not individual distinction at this point. The prediction accuracy for the country of origin increases with increasing sample number and a random sample can be placed in the correct country. When a complete disk is removed from the training dataset, its rings (samples) are correctly attributed to the country with an accuracy ranging from 43% to 100%. Relative abundances of ions appear to contribute more to differentiation compared to frequency differences. Conclusions: DART TOFMS shows potential for geographical provenancing but is still experimental for individual distinction; more research is needed to make this an established method. Sampling campaigns should focus on sampling tree cores from pith-to-bark, paving the way towards a chemical fingerprint database for species provenance.

Carbon Isotopes of Riparian Forests Trees in the Savannas of the Volta Sub-Basin of Ghana Reveal Contrasting Responses to Climatic and Environmental Variations

Stable isotopes of tree rings are frequently used as proxies in climate change studies. However, species-specific relationships between climate and tree-ring stable isotopes have not yet been studied in riparian forests in the savannas of West Africa. Four cross-dated discs, each of Afzelia africana Sm. (evergreen) and Anogeissus leiocarpus (DC.) Guill. & Perr. (deciduous) in the humid (HSZ) and dry (DSZ) savanna zones of the Volta basin in Ghana were selected from a larger tree-ring dataset to assess the relationships between the tree-ring carbon isotope composition (δ13C values) and climatic parameters. The atmospherically corrected δ13C values of both studied species showed that A. africana was enriched in 13C compared to A. leiocarpus. Strong correlations were found between δ13C values of A. africana and A. leiocarpus with temperature, but weak correlations with precipitation. Spatial correlation analysis revealed significant relationships between δ13C values of both tree species and Sea Surface Temperatures in the Gulf of Guinea in the southern Atlantic Ocean. The results suggest that the carbon isotope composition of riparian trees in the Volta river basin has a potential to reconstruct climate variability and to assess tree ecological responses to climate change.

Climate signals in tree-ring δ18 O and δ13 C from southeastern Tibet: insights from observations and forward modelling of intra- to interdecadal variability

Stable isotopes in tree rings are increasingly used as proxies for climatic and ecophysiological changes. However, uncertainties remain about the strength and consistency of their response to environmental variation at different temporal (i.e. seasonal to inter-decadal) scales. We developed 5 yr of intra-seasonal and 62 yr of early- and late-wood δ¹³ C and δ¹⁸ O series of Smith fir (Abies georgei var. smithii) on the southeastern Tibetan Plateau, and used a process-based forward model to examine the relative importance of environmental and physiological controls on the isotopic data. In this temperate high-altitude region, the response, both δ¹⁸ O and δ¹³ C, is primarily to variations in relative humidity, but by different processes. In δ¹⁸ O, the response is via source water δ¹⁸ O but also arises from leaf water ¹⁸ O enrichment. In δ¹³ C, the response is via changes in stomatal conductance but is modified by carry-over effects from prior periods. We conclude that tree-ring δ¹⁸ O may be a more robust climate proxy than δ¹³ C, and δ¹³ C may be more suited to studies of site-related physiological responses to the local environment.