Contrasting physiological strategies explain heterogeneous responses to severe drought conditions within local populations of a widespread conifer.

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 carbon­oxygen 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.

On the significance of long-term trends in tree-ring N isotopes - The interplay of soil conditions and regional NOx emissions.

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.

Nitrogen isotopes in the soil-to-tree continuum - Tree rings express the soil biogeochemistry of boreal forests exposed to moderate airborne emissions.

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 (δ15N) appear as a contentious choice for this role as the exact mechanisms behind tree-ring δ15N 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 δ15N 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 δ15N signals. Our results reveal that airborne anthropogenic N with distinct δ15N signals may directly modify the NO3- δ15N values in surface soils, but not the ones of NH4+, the preferred N form of the studied trees. Hence, the tree-ring δ15N values reflect specific soil N conditions and assimilation modes by trees. Along with a wide tree-ring δ15N range, we report differences in: soil nutrient content and N transformation rates; δ15N 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 δ15N 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 soil biogeochemical properties and processes identified in the studied sites, in the end, the tree-ring δ15N averages inversely correlate with soil pH and anthropogenic N inputs, confirming white spruce tree-ring δ15N values as a suitable indicator for environmental research on forest N cycling.

Response strategies of boreal spruce trees to anthropogenic changes in air quality and rising pCO2.

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.

Tree-ring nitrogen isotopes reflect anthropogenic NOx emissions and climatic effects.

Anthropogenic emissions of atmospheric nitrogen have increased over the last century, but the monitoring of nitrous oxide concentrations is only recent. Can trees from temperate regions be used to infer past changes in nitrogen cycles? To considerthis question, we investigate nitrogen isotope (delta15N) ring series from pine and beech trees near Montréal, and beech specimens of Georgian Bay Islands. The delta15N values show coherent intertree and interspecies trends, independent of the sapwood-heartwood transition zones, implying that these results reflect local environmental conditions. At both sites, short-term isotopic fluctuations correlate directly with precipitation and inversely with temperature. Long-term isotope decreases of 1.5 to 2 per thousand suggest progressive changes in soil nitrogen after 1951. In Georgian Bay, an additional important change is inferred on the basis of a 1.5 per thousand increase initiated after 1971. At both sites, long-term series correlate with a proxy for NOx emissions. We propose that the contrasted long-term delta15N changes of Montreal and Georgian Bay reflect deposition of NOx emissions from cars and coal-power plants, with higher proportions from coal burning in Georgian Bay. This research suggests that tree-ring delta15N series may record both, regional climatic conditions and anthropogenic perturbations of N cycles.

Effects of smelter sulfur dioxide emissions: a spatiotemporal perspective using carbon isotopes in tree rings.

We wanted to test the hypothesis that forest exposure to phytotoxic gases indirectly affects their carbon uptake. We estimated that the reduction of photosynthesis may have reached 20 to 30% at a site located 9 km (test site) from the Horne copper smelter in Rouyn-Noranda, which is a point source of SO2. Twenty-one spruce trees older than 100 yr were selected from seven sites at various distances from the smelter to evaluate conditions prior to and during the periods of smelter operation. The carbon isotope results obtained from spruce tree rings at our test site reveal an unprecedented and abrupt shift of +4/1000 after the onset of smelter operations. This large and permanent shift exceeds natural variations in regional pre-smelter series or in the series at a remote control site. All trees up to 116 km downwind from the smelter show delta13C positive shifts following the onset of operations. There is also a clear inverse relationship between the amplitude of the first-order trends and distance from the smelter. Those delta13C trends indicate that trees exposed to high levels of SO2 decrease their level of CO2 uptake through activation of stomatal closure. This is strongly supported by the significant departure of the Rouyn-Noranda trends from those measured for trees from non-industrialized areas of the Northern Hemisphere, or calculated using global atmospheric conditions. Considering the large number of SO2 point sources in North America, our results imply that CO2 uptake by the boreal forest in the vicinity of these sources may be lower than previously thought.