Contrasting controls on tree ring isotope variation for Amazon floodplain and terra firme trees

Climate variability of the southern Amazon inferred by a multi-proxy tree-ring approach using Cedrela fissilis Vell

The analysis of climate variability and development of reconstructions based on tree-ring records in tropical forests have been increasing in recent decades. In the Amazon region, ring width and stable isotope long-term chronologies have been used for climatic studies, however little is known about the potential of wood traits such as density and chemical concentrations. In this study, we used well-dated rings of Cedrela fissilis Vell. from the drought-prone southern Amazon basin to assess the potential of using inter-annual variations of annually-resolved ring width, wood density, stable oxygen isotope (δ¹⁸O_TR) measured in tree-ring cellulose and concentration of Sulfur (S_TR) and Calcium (Ca_TR) in xylem cells to study climate variability. During wet years, Cedrela fissilis produced wider and denser rings with higher Ca_TR and lower S_TR, as well as depleted δ¹⁸O_TR values. During dry years, a wider range of responses was observed in growth, density and S_TR, while lower Ca_TR and enriched δ¹⁸O_TR values were found. The annual centennial chronologies spanning from 1835 to 2018 showed good calibration skills for reconstructing local precipitation, evapotranspiration (P-PET), Amazon-wide rainfall, as well as climate modes related to sea surface temperature (SST) anomalies such as El Niño South Oscillation (ENSO), Tropical Northern Atlantic (TNA), and the Western Hemisphere Warm Pool (WHWP) oscillations. Ca_TR explained 42 % of the variance of local precipitation (1975-2018), RW explained 30 % of the P-PET variance (1975-2018), while δ¹⁸O_TR explained 60 % and 57 % of the variance of Amazon rainfall (1960-2018) and El Niño 3.4 (1920-2018), respectively. Our results show that a multi-proxy tropical tree-ring approach can be used for high-reliable reconstructions of climate variability over Amazon basin at inter-annual and multidecadal time scales.

Intra-annual tree-ring δ18O and δ13C reveal a trade-off between isotopic source and humidity in moist environments

Tree-ring intra-annual stable isotopes (δ13C and δ18O) are powerful tools for revealing plant ecophysiological responses to climatic extremes. We analyzed interannual and fine-scale intra-annual variability of tree-ring δ13C and δ18O in Chinese red pine (Pinus massoniana) from southeastern China to explore environmental drivers and potential trade-offs between the main physiological controls. We show that wet season relative humidity (May-October RH) drove interannual variability of δ18O and intra-annual variability of tree-ring δ18O. It also drove intra-annual variability of tree-ring δ13C, whereas interannual variability was mainly controlled by February-May temperature and September-October RH. Furthermore, intra-annual tree-ring δ18O variability was larger during wet years compared with dry years, whereas δ13C variability was lower during wet years compared with dry years. As a result of these differences in intra-annual variability amplitude, process-based models (we used the Roden model for δ18O and the Farquhar model for δ13C) captured the intra-annual δ18O pattern better in wet years compared with dry years, whereas intra-annual δ13C pattern was better simulated in dry years compared with wet years. This result suggests a potential asymmetric bias in process-based models in capturing the interplay of the different mechanistic processes (i.e., isotopic source and leaf-level enrichment) operating in dry versus wet years. We therefore propose an intra-annual conceptual model considering a dynamic trade-off between the isotopic source and leaf-level enrichment in different tree-ring parts to understand how climate and ecophysiological processes drive intra-annual tree-ring stable isotopic variability under humid climate conditions.

Tree-ring oxygen isotopes record a decrease in Amazon dry season rainfall over the past 40 years

Extant climate observations suggest the dry season over large parts of the Amazon Basin has become longer and drier over recent decades. However, such possible intensification of the Amazon dry season and its underlying causes are still a matter of debate. Here we used oxygen isotope ratios in tree rings (δ¹⁸O_TR) from six floodplain trees from the western Amazon to assess changes in past climate. Our analysis shows that δ¹⁸O_TR of these trees is negatively related to inter-annual variability of precipitation during the dry season over large parts of the Amazon Basin, consistent with a Rayleigh rainout model. Furthermore δ¹⁸O_TR increases by approximately 2‰ over the last four decades (~ 1970-2014) providing evidence of an Amazon drying trend independent from satellite and in situ rainfall observations. Using a Rayleigh rainout framework, we estimate basin-wide dry season rainfall to have decreased by up to 30%. The δ¹⁸O_TR record further suggests such drying trend may not be unprecedented over the past 80 years. Analysis of δ¹⁸O_TR with sea surface temperatures indicates a strong role of a warming Tropical North Atlantic Ocean in driving this long-term increase in δ¹⁸O_TR and decrease in dry season rainfall.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00382-021-06046-7.

Comparison of dendroclimatic relationships using multiple tree-ring indicators (tree-ring width and δ 13C) from Masson pine

This study addressed the effects of climate drivers on the tree-ring width (TRW) parameters (total ring width (TR), earlywood width (EW) and latewood width (LW)) and the total ring δ 13C series of different wood components (whole wood, α-cellulose and holocelluose) from Masson pine in subtropical China. Pairwise correlation coefficients between three ring width parameters were statistically significant. EW and LW did not reveal much stronger climate sensitivity rather than TR. This indicated that the use of intra-annual ring width has little benefit in extracting more climate information. The mean δ 13C series of the three components of the total ring had the strongest climate response to the July-September relative humidity (r = -0.792 (whole wood), -0.758 (holocellulose) and -0.769 (α-cellulose)). There are no significant differences in the dendroclimatic relationships of the δ 13C series of different wood components. Through both stationary temporal and spatial-statistical perspectives, the moisture drivers (summer/autumn) had a significant impact on three ring width parameters and three components of Masson pine. Overall, the radial growth and the δ 13C series showed different responses to the same climate drivers during the same period. Moreover, the R-squared values of the strongest climate-proxy correlation coefficients were smaller than 50% for TRW. Consequently, the δ 13C series of Masson pine may be a more representative climate proxy than TRW parameters for dendroclimatology in subtropical China.

Intra-annual oxygen isotopes in the tree rings record precipitation extremes and water reservoir levels in the Metropolitan Area of São Paulo, Brazil

The impacts of climate change on precipitation and the growing demand for water have increased the water risks worldwide. Water scarcity is one of the main challenges of the 21st century, and the assessment of water risks is only possible from spatially distributed records of historical climate and levels of water reservoirs. One potential method to assess water supply is the reconstruction of oxygen isotopes in rainfall. We here investigated the use of tree-ring stable isotopes in urban trees to assess spatial/temporal variation in precipitation and level of water reservoirs. We analyzed the intra-annual variation of δ¹³C and δ¹⁸O in the tree rings of Tipuana tipu trees from northern and southern Metropolitan Area of São Paulo (MASP), Brazil. While variation in δ¹³C indicates low leaf-level enrichments from evapotranspiration, δ¹⁸O variation clearly reflects precipitation extremes. Tree-ring δ¹⁸O was highest during the 2014 drought, associated with the lowest historical reservoir levels in the city. The δ¹⁸O values from the middle of the tree rings have a strong association with the mid-summer precipitation (r = -0.71), similar to the association between the volume of precipitation and its δ¹⁸O signature (r = -0.76). These consistent results allowed us to test the association between tree-ring δ¹⁸O and water-level of the main reservoirs that supply the MASP. We observed a strong association between intra-annual tree-ring δ¹⁸O and the water-level of reservoirs in the northern and southern MASP (r = -0.94, r = -0.90, respectively). These results point to the potential use of high-resolution tree-ring stable isotopes to put precipitation extremes, and water supply, in a historical perspective assisting public policies related to water risks and climate change. The ability to record precipitation extremes, and previously reported capacity to record air pollution, place Tipuana tipu in a prominent position as a reliable environmental monitor for urban locations.

Disentangling the effects of atmospheric CO2 and climate on intrinsic water-use efficiency in South Asian tropical moist forest trees

Due to the increase in atmospheric CO2 concentrations, the ratio of carbon fixed by assimilation to water lost by transpiration through stomatal conductance (intrinsic water-use efficiency, iWUE) shows a long-term increasing trend globally. However, the drivers of short-term (inter-annual) variability in iWUE of tropical trees are poorly understood. We studied the inter-annual variability in iWUE of three South Asian tropical moist forest tree species (Chukrasia tabularis A.Juss., Toona ciliata M. Roem. and Lagerstroemia speciosa L.) derived from tree-ring stable carbon isotope ratio (δ13C) in response to variations of environmental conditions. We found a significantly decreasing trend in carbon discrimination (Δ13C) and an increasing trend in iWUE in all the three species, with a species-specific long-term trend in intercellular CO2 concentration (Ci). Growing season temperatures were the main driver of inter-annual variability of iWUE in C. tabularis and L. speciosa, whereas previous year temperatures determined the iWUE variability in T. ciliata. Vapor pressure deficit was linked with iWUE only in C. tabularis. Differences in shade tolerance, tree stature and canopy position might have caused this species-specific variation in iWUE response to climate. Linear mixed effect modeling successfully simulated iWUE variability, explaining 41-51% of the total variance varying with species. Commonality analysis revealed that temperatures had a dominant influence on the inter-annual iWUE variability (64-77%) over precipitation (7-22%) and atmospheric CO2 concentration (3-6%). However, the long-term variations in iWUE were explicitly determined by the atmospheric CO2 increase (83-94%). Our results suggest that the elevated CO2 and concomitant global warming might have detrimental effects on gas exchange and other physiological processes in South Asian tropical moist forest trees.

Tropical Trees as Time Capsules of Anthropogenic Activity

After the ice caps, tropical forests are globally the most threatened terrestrial environments. Modern trees are not just witnesses to growing contemporary threats but also legacies of past human activity. Here, we review the use of dendrochronology, radiocarbon analysis, stable isotope analysis, and DNA analysis to examine ancient tree management. These methods exploit the fact that living trees record information on environmental and anthropogenic selective forces during their own and past generations of growth, making trees living archaeological 'sites'. The applicability of these methods across prehistoric, historic, and industrial periods means they have the potential to detect evolving anthropogenic threats and can be used to set conservation priorities in rapidly vanishing environments.