Understanding how trees prioritize carbon gain at the cost of drought vulnerability under severe drought conditions is crucial for predicting which genetic groups and individuals will be resilient to future climate conditions. In this study, we investigated variations in growth, tree-ring anatomy as well as carbon and oxygen isotope ratios to assess the sensitivity and the xylem formation process in response to an episode of severe drought in 29 mature white spruce (Picea glauca [Moench] Voss) families grown in a common garden trial. During the drought episode, the majority of families displayed decreased growth and exhibited either sustained or increased intrinsic water-use efficiency (iWUE), which was largely influenced by reduced stomatal conductance as revealed by the dual carbon‑oxygen isotope approach. Different water-use strategies were detected within white spruce populations in response to drought conditions. Our results revealed intraspecific variation in the prevailing physiological mechanisms underlying drought response within and among populations of Picea glauca. The presence of different genetic groups reflecting diverse water-use strategies within this largely-distributed conifer is likely to lessen the negative effects of drought and decrease the overall forest ecosystems' sensitivity to it.

Understanding how trees prioritize carbon gain at the cost of drought vulnerability under extreme drought conditions is crucial for predicting which genetic groups and individuals will be resilient to future climate conditions. In this study, we investigated variations in growth, tree-ring anatomy as well as carbon and oxygen isotope ratios to assess the sensitivity and the xylem formation process in response to an episode of extreme drought in 29 white spruce (Picea glauca [Moench] Voss) families. The dual carbon-oxygen isotope approach revealed that reductions in stomatal conductance were the main driver of the observed increase in intrinsic water use efficiency (iWUE) for most families. Different water use strategies were detected within white spruce populations in response to drought conditions. Our results provide new insights into the genetic nature and the relative importance of the physiological mechanisms underlying drought response in conifers. The presence of different genetic groups reflecting diverse water use strategies within tree species is likely to modulate the impacts of drought and the sensitivity of forest ecosystems.

As anthropogenic nitrogen (N) emissions have been rising for decades, it is critical to develop natural archives that help understand how natural processes were modified in the past. Tree-ring δ15N values may represent such an indicator but its validity as faithful record of N cycling changes is still debated. Here we produce long-tree-ring δ15N series for five white spruce stands from two boreal regions submitted to moderate industrial N inputs. The obtained δ15N series show sharp differences among stands, even from the same region, despite the fact that they show similar increases in intrinsic water use efficiency (iWUE), a proxy for foliar strategies derived from δ13C values. The statistical modeling of these series and the basal area increment (BAI) of the trees allow to suggest that the mechanisms controlling the isotopic fractionation of N assimilated by tree rings are decoupled from the foliar strategies under the anthropogenic N emissions. The iWUE trends mainly reflect rise of pCO2 and changes in air quality. The long-term δ15N trends echo different biogeochemical processes responding to N deposition due to distinct original soil pH at the various sites. We contend that tree-ring δ15N series can record changes in the forest N cycle, but their rigorous interpretation requires laborious work, particularly an understanding of the biogeochemistry in the soil immediately around the investigated trees. "Seek simplicity and distrust it", Alfred North Whitehead.

The stable isotopic ratios of carbon and oxygen in tree rings are remarkable paleoclimatic data. In northeastern Canada, where climatic reconstructions longer than 500 years are very rare and where none is based on tree-ring isotopes, subfossil stems from boreal lakes can be used to produce long isotopic series. The most important steps in producing millennial isotopic series consist in selecting the material to study (lakes, living trees, subfossil stems) and the method to sample tree rings in order to obtain a climate reconstruction of high temporal resolution. The climatic significance of the isotopic ratios was determined by correlations with climatic parameters, and validated by our understanding of the physiological and pedogeochemical response mechanisms. The significant correlation obtained for the delta-18O series with summer maximal temperature has allowed reconstructing the mean of June-July maximum temperature over the last millennium by using subfossil stems recovered from a boreal lake. This new millennial series, the first isotopic series in northeastern Canada, is one of the first in the world to use delta-18O values as a proxy for climate. It brings new knowledge of the past climate for northeastern Quebec by highlighting that the medieval warm anomaly (1000-1250 AD) was as warm as the last three decades and the warming observed since 1970 is one of the most important of the last millennium.

Three carbon and oxygen dendroisotopic series representative of the upstream sector of the La Grande River watershed and covering the period between 1800 and 2005 were produced under the ARCHIVES project. In order to use tree-ring isotopic ratios to reconstruct climatic conditions of the last centuries, we first assessed their climatic significance by conducting a statistical study of their relationship with hydrometeorological series available for the region. The results show that the two types of isotopic ratios in wood cellulose (delta-13C and delta-18O) are generally sensitive to the same climatic parameters, but at varying degrees, and that this sensitivity increases when the two isotopic indicators are combined (mean delta-13C and delta-18O values). They respond to the maximum temperature and total precipitation of the summer season (June to August), and also to some parameters integrating several aspects of the regional climate such as a climate index combining temperatures and precipitations, the vapor pressure deficit (VPD) and regional River discharge. VPD is the parameter that most strongly correlates with the isotopic values, due to its direct influence on isotope fractionation processes related to stomatal functioning. The results presented in this chapter, clearly show that isotopic ratios of black spruce trees can be considered as excellent climate indicators for the boreal sector of northeastern America.

A tree is as a natural archive of valuable environmental information that allows going back in time if its growth rings are precisely dated, and the indirect indicators or proxies they contain are well understood. Dendroisotopy, a clever combination of dendrochronology and isotope geochemistry, is part of this process because it can provide a very informative look at the biogeochemical conditions of the past, and eventually help reconstructing climate. This possibility exists because the mechanisms that govern the distribution of isotopes in trees are increasingly better known. This section briefly summarizes how dendroisotopy of carbon and oxygen may reflect environmental changes such as the impact of atmospheric pollution, ecological changes, and particularly climatic conditions.

The close relationship between stable carbon and oxygen isotopes in growth rings of black spruce trees and regional climatic conditions allows us to reconstruct over the last two centuries, two hydroclimatic parameters providing the best statistical correlation with isotopic values, namely the maximum summer temperature (June-August Tmax) and the summer discharge (July to November) of major regional rivers. The reconstructed 200 years series shows contrasting conditions marked by alternating climatic ambiances typically associated with Quebec boreal summers, where warm and dry conditions alternate with cold and wet periods. The series also documents specific climate events such as the cooling that followed the Tambora eruption in 1815. The reconstructed hydroclimatic series were then validated by comparing them to long datasets measured at larger scale, and to other series reconstructed from independent proxies. Results from this analysis suggest that mid and long-term hydroclimatic variations of the central part of the Quebec-Labrador peninsula, as reconstructed from dendroisotopes, were not only controlled by northern Québec climate systems, but also by major weather systems controlling the level of rivers of northeastern North America.

Like other dendrochronological networks in the world, the ARCHIVES group has established a collection of sites based on a stratified sampling plan taking into account average hydric conditions (mesic environments), in terms of exposure, and seeking to represent the major regional types of forest formations (spruce-lichen and spruce-moss forests). The distribution of sites also took into account the altitude, continentality and latitude to achieve at best the geographic coverage over the area stretching from James Bay to Labrador and between the 53rd and 56th parallels. The homogeneity and density of the stands, the age of the trees and their physiognomy were selection criteria used to eliminate certain possible effects such as competition during plant succession and other factors related to the heterogeneity of the sites or of the life history of trees. The area covered by the ARCHIVES network is 360,000 km2, a territory large enough to allow the combined analysis of climate and hydrological model data and dendrochonological data. For the millennial series, subfossil trees were sampled in a set of lakes selected on the basis of catchment physiography, spatial distribution over the vast designated area and fire history. The laminated sedimentary series of a few lakes were also used to complete the multi-proxy approach put forward in the ARCHIVES project. Finally, ARCHIVES used a complex assemblage of grid data obtained by kriging of instrumental registers, reanalyses and modeled series.

Although the environmental impacts of metal atmospheric emissions from point sources such as smelter have been extensively studied, very few studies have attempted to understand the influence of those emissions on nutrient cycles in the surrounding forests. This study investigates nutrient variations in space and time along with trace metals by statistical analysis of tree-ring series combined with the characterization of element concentrations in soil horizons. The research focuses on the Horne smelter (Rouyn Noranda, Québec, Canada), because it released high atmospheric emissions of metals and gases between 1928 and 1990s. Tree-ring Sr/Mn ratios, and Mn and Sr z-score series reveal that surface soil pH recovered progressively within the 45 km footprint of the smelter since the end of acidic deposition in the late 1970s. The influence of acidic deposition on the current soil pH has become negligible. In other words, element bioavailability and root assimilation have changed through time due to soil acidification at proximal sites. The detrended tree-ring elemental series during the last century also suggest that summer temperatures partly control the elemental bioavailability to trees in soils. Moreover, tree-ring Zn and Mg series appear as key environmental indicators of metal deposition from the smelter. This research confirms previous findings indicating that elemental concentrations in black and white spruce trees may be used to evaluate the potential influence of smelter emissions on nutrient cycles. For a future informed and adaptive management of forests, understanding the potential modifications of nutrient regimes caused by anthropogenic contaminations is critical, especially in the context of global warming.

Anthropogenic N emissions represent a potential threat for forest ecosystems, and environmental indicators that provide insight into the changing forest N cycle are needed. Tree ring N isotopic ratios (delta N-15) appear as a contentious choice for this role as the exact mechanisms behind tree-ring delta N-15 changes seldom benefit from a scrutiny of the soil-to-tree N continuum. This study integrates the results from the analysis of soil chemistry, soil microbiome genomics, and delta N-15 values of soil N compounds, roots, ectomycorrhizal (EcM) fungi and recent tree rings of thirteen white spruce trees sampled in five stands, from two regions exposed to moderate anthropogenic N emissions (3.9 to 8.1 kg/ha/y) with distinctive delta N-15 signals. Our results reveal that airborne anthropogenic N with distinct delta N-15 signals may directly modify the NO3- delta N-15 values in surface soils, but not the ones of NH4+ , the preferred N form of the studied trees. Hence, the tree-ring delta N-15 values reflect specific soil N conditions and assimilation modes by trees. Along with a wide tree-ring delta N-15 range, we report differences in: soil nutrient content and N transformation rates; delta N-15 values of NH4+ , total dissolved N (TDN) and EcM mantle enveloping the root tips; and bacterial and fungal community structures. We combine EcM mantle and root delta N-15 values with fungal identification to infer that hydrophobic EcM fungi transfer N from the dissolved organic N (DON) pool to roots under acidic conditions, and hydrophilic EcM fungi transfer various N forms to roots, which also assimilate N directly under less acidic conditions. Despite the complexities of & nbsp;soil biogeochemical properties and processes identified in the studied sites, in the end, the tree-ring delta N-15 averages inversely correlate with soil pH and anthropogenic N inputs, confirming white spruce tree-ring delta N-15 values as a suitable indicator for environmental research on forest N cycling. Crown Copyright (C) 2021 Published by Elsevier B.V.

Although requiring laborious analytical treatment, tree-ring series of nitrogen isotopes (delta N-15) have gained popularity amongst researchers for their potential as environmental indicators as anthropogenic emissions increase globally with potential effects on forest N cycles. Previous studies suggested that tree-ring series correlate with climatic and air quality parameters. However, none discussed the level of replication required for expressing the population signals of specific species of trees. In this investigation, we studied 27 white spruce trees from two sites under distinct environmental conditions to evaluate the appropriate protocol for preparing consistent treering delta N-15 series. The produced series indicate that high frequency (short-term,< 7 years) delta N-15 changes at a replication as high as 10 trees cannot serve environmental purposes. Conversely, the low frequency (middle-, 7-15 years, to long-term, > 15 years) delta N-15 trends show coherence between arithmetic means of individual series at replication levels as low as three trees, whereas middle-term pooled trends do not perform as coherently. The low frequency mean trends of individual series obtained for the two sites suggest that local biogeochemical soil conditions modified by anthropogenic emissions modulate the delta N-15 responses in trees. Hence, we propose that long-term tree-ring delta N-15 series constitute reliable environmental indicators.

Little is known about how forests adjust their gas-exchange mode while atmospheric CO2 rises globally and air quality changes regionally. The present study aims at addressing this research gap for boreal spruce trees growing in three different regions of Canada, submitted to distinct levels of atmospheric emissions, by examining the amount of carbon gained per unit of water lost in trees, i.e., the intrinsic water use efficiency (iWUE). Under pristine air quality conditions, middle-to long-term trends passed from no-reaction mode to passive strategies due to atmospheric CO2, and short-term iWUE variations mostly ensue from year-to-year climatic conditions. In contrast, in trees exposed to pollutants from a copper smelter and an oil-sands mining region, air quality deterioration generated swift, long-term iWUE rises immediately at the onset of operations. In this case, the very active foliar strategy sharply reduced the intra-foliar CO2 (Ci) pressure. Statistical modeling allowed identifying emissions as the main trigger for the iWUE swift shifts; subsequent combined effects of emissions and rising CO2 led to passive foliar modes in the recent decades, and short-term variations due to climatic conditions appeared all along the series. Overall, boreal trees under different regional conditions modified their foliar strategies mostly without changing their stem growth. These findings underline the potential of acidifying emissions for prompting major iWUE increases due to lowering the stomatal apertures in leaves, and the combined influence of rising CO2 in modulating other foliar responses. A fallout of this research is that degrading air quality may create true divergences in the relationship between tree-ring isotopes and climatic conditions, an impact to consider prior to using isotopic series for paleo-climatic modeling.

Globally increasing anthropogenic airborne emissions of reactive nitrogen (N) generate several environmental issues that require investigating how N accumulation modifies the N cycle. Tree-ring δ15N series may help understanding past and current perturbations in the forest N cycle. Although several studies have addressed this issue, most of them were of local scale or based on short δ15N series. The development of this environmental indicator however would benefit from examining, at the regional scale, the relationships of long tree-ring series with soil N biogeochemical processes. Here we explore these links for tree stands of the oil-sands region in northern Alberta, and the coal-fired power plants region in central Alberta, Canada. We characterize the tree-ring δ15N trends, the N modification rates and bacterial and fungal communities of soil samples collected in the immediate surrounding of the characterized trees. The dataset suggests that specific soil pH, and N-cycling bacterial and fungal communities influence tree-ring δ15N responses to anthropogenic emissions, correlating either directly or inversely. Overall, tree-ring δ15N series may record changes in the forest-N cycle, but their interpretation requires understanding key soil biogeochemical processes. «In nature nothing exists alone», Rachel Carson.

In northeastern Canada (Labrador), instrumental climatic data cover less than 70 years and long reconstructions from natural archives are non-existent. This study specifically aims at helping filling this gap of knowledge by testing the possibility of reconstructing the regional 1800–2009 discharge of the lower Churchill River from black spruce tree-ring δ 13 C and δ 18 O series. The results illustrate direct relationships of summer climatic variables/derived parameter (maximum temperature, total precipitation and vapor pressure deficit) with tree-ring isotope values. Importantly, they show an inverse correlation between combined tree-ring isotope series and regional river discharge due to common climate forcing. To a lesser extent, transpiration also affects the river discharge and tree-ring isotopic compositions. The reconstructed river discharge series agrees with an independent reconstruction based on the ANATEM method (1880–2009 period). The agreement between the two reconstructions validates the two approaches for reconstructing regional hydroclimatic conditions at high latitudes. Moreover, the reconstructions suggest that summer discharge has decreased over the past 200 years in eastern Labrador and more broadly at the Québec-Labrador peninsula scale. This trend correlates with the long-term summer Arctic Oscillation (AO) that influences summer regional climatic conditions. This research contributes with other studies to build up observations linking summer AO and eastern Canada climatic conditions, and calls for research on mechanisms explaining these relationships during summer.

Studies in dendroisotope chemistry suggested that latewood cellulose contains better climatic records than whole-ring cellulose. However, this approach has never been tested on northeastern Canadian spruce trees. This study compares dendroisotopic series of cellulose from late and whole ring, and analyses their statistical relationships with hydro-climatic variables with the aim of selecting the best suited protocol for future hydro-climatic reconstruction in the downstream sector of Churchill River basin of Labrador, Canada. To this end, δ13C and δ18O series from latewood (LW) and whole ring (WR) α-cellulose of black spruce trees (Picea mariana [Mill.] B.S.P.) were produced for the 1940–2010 period. The results show strong correlations between LW and WR isotopic series suggesting that there are no important variation in the isotopic ratios during the growing year and that black spruce trees use photosynthates of the current growing season to form their earlywood. Moreover, LW and WR δ13C and δ18O show similar relationships with both maximum temperature (Tmax) and Churchill River discharge. Correlations are higher when combining δ13C and δ18O for LW and WR. Overall, those correlations support the indirect relationship between tree-ring isotopic series and river discharge, as they are integrators of several climatic variables and derived parameters (Tmax, relative humidity, evapotranspiration, etc.). The LW and WR isotopic series give similar statistical relationships with hydro-climatic variables, and the WR treatment is faster (separation easier compared to LW). Thus, for black spruce the use of combined isotopic series in WR can be favored over LW for hydro-climatic reconstruction in the study region.

The Tambora eruption (1815 AD) was one of the major eruptions of the last two millennia and has no equivalents over the last two centuries. Here, we collected an extensive network of early meteorological time series, climate simulation data and numerous, well-replicated proxy records from Eastern Canada to analyze the strength and the persistence of the Tambora impact on the regional climate and forest processes. Our results show that the Tambora impacts on the terrestrial biosphere were stronger than previously thought, and not only affected tree growth and carbon uptake for a longer period than registered in the regional climate, but also determined forest demography and structure. Increased tree mortality, four times higher than the background level, indicates that the Tambora climatic impact propagated to influence the structure of the North American taiga for several decades. We also show that the Tambora signal is more persistent in observed data (temperature, river ice dynamics, forest growth, tree mortality) than in simulated ones (climate and forest-growth simulations), indicating that our understanding of the mechanisms amplifying volcanic perturbations on climates and ecosystems is still limited, notably in the North American taiga.

Northeastern North America has very few millennium-long, high-resolution climate proxy records. However, very recently, a new tree-ring dataset suitable for temperature reconstructions over the last millennium was developed in the northern Quebec taiga. This dataset is composed of one δ18O and six ring width chronologies. Until now, these chronologies have only been used in independent temperature reconstructions (from δ18O or ring width) showing some differences. Here, we added to the dataset a δ13C chronology and developed a significantly improved millennium-long multiproxy reconstruction (997–2006 CE) accounting for uncertainties with a Bayesian approach that evaluates the likelihood of each proxy model. We also undertook a methodological sensitivity analysis to assess the different responses of each proxy to abrupt forcings such as strong volcanic eruptions. Ring width showed a larger response to single eruptions and a larger cumulative impact of multiple eruptions during active volcanic periods, δ18O showed intermediate responses, and δ13C was mostly insensitive to volcanic eruptions. We conclude that all reconstructions based on a single proxy can be misleading because of the possible reduced or amplified responses to specific forcing agents.

Oxygen isotopes in tree rings (δ18OTR) are widely used to reconstruct past climates. However, the complexity of climatic and biological processes controlling isotopic fractionation is not yet fully understood. Here, we use the MAIDENiso model to decipher the variability in δ18OTR of two temperature-sensitive species of relevant palaeoclimatological interest (Picea mariana and Nothofagus pumilio) and growing at cold high latitudes in North and South America. In this first modelling study on δ18OTR values in both northeastern Canada (53.86° N) and western Argentina (41.10° S), we specifically aim at (1) evaluating the predictive skill of MAIDENiso to simulate δ18OTR values, (2) identifying the physical processes controlling δ18OTR by mechanistic modelling and (3) defining the origin of the temperature signal recorded in the two species. Although the linear regression models used here to predict daily δ18O of precipitation (δ18OP) may need to be improved in the future, the resulting daily δ18OP values adequately reproduce observed (from weather stations) and simulated (by global circulation model) δ18OP series. The δ18OTR values of the two species are correctly simulated using the δ18OP estimation as MAIDENiso input, although some offset in mean δ18OTR levels is observed for the South American site. For both species, the variability in δ18OTR series is primarily linked to the effect of temperature on isotopic enrichment of the leaf water. We show that MAIDENiso is a powerful tool for investigating isotopic fractionation processes but that the lack of a denser isotope-enabled monitoring network recording oxygen fractionation in the soil–vegetation–atmosphere compartments limits our capacity to decipher the processes at play. This study proves that the eco-physiological modelling of δ18OTR values is necessary to interpret the recorded climate signal more reliably.