An Assessment of Contemporary and Historic Nitrogen Availability in Contrasting Coastal Douglas-Fir Forests Through δ15N of Tree Rings

Forest inventory tree core archive reveals changes in boreal wood traits over seven decades

Boreal forests play an important role in the global carbon (C) cycle, and there is great interest in understanding how they respond to environmental change, including nitrogen (N) and water limitation, which could impact future forest growth and C storage. Utilizing tree cores archived by the Swedish National Forest Inventory, we measured stemwood traits, including stable N and C isotope composition which provides valuable information related to N availability and water stress, respectively, as well as N and C content, and C/N ratio over 1950-2017 in two central Swedish counties covering an area of ca. 55,000 sq. km (n = 1038). We tested the hypothesis that wood traits are changing over time, and that temporal patterns would differ depending on alternative dendrochronological reconstruction methods, i.e. the commonly applied "single tree method" (STM) or a conceptually stronger "multiple tree method" (MTM). Averaged across all MTMs, our data showed that all five wood traits for Picea abies and Pinus sylvestris changed over time. Wood δ15N strongly declined, indicating progressive nitrogen limitation. The decline in δ13C tracked the known atmospheric δ13CO2 signal, suggesting no change in water stress occurred. Additionally, wood N significantly increased, while C and C/N ratios declined over time. Furthermore, wood trait patterns sometimes differed between dendrochronological methods. The most notable difference was for δ15N, where the slope was much shallower for the STM compared to MTMs for both species, indicating that mobility of contemporary N is problematic when using the STM, resulting in substantially less sensitivity to detect historical signals. Our study indicates strong temporal changes in boreal wood traits and also indicates that the field of dendroecology should adopt new methods and archiving practices for studying highly mobile element cycles, such as nitrogen, which are critical for understanding environmental change in high latitude ecosystems.

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.

Changes in forest nitrogen cycling across deposition gradient revealed by δ15N in tree rings

Tree rings provide valuable insight into past environmental changes. This study aimed to evaluate perturbations in tree ring width (TRW) and δ¹⁵N alongside soil acidity and nutrient availability gradients caused by the contrasting legacy of air pollution (nitrogen [N] and sulphur [S] deposition) and tree species (European beech, Silver fir and Norway spruce). We found consistent declines of tree ring δ¹⁵N, which were temporarily unrelated to the changes in the TRW. The rate of δ¹⁵N change in tree rings was related to the contemporary foliar carbon (C) to phosphorus (P) ratio. This observation suggested that the long-term accumulation of ¹⁵N depleted N in tree rings, likely mediated by retained N from deposition, was restricted primarily to stands with currently higher P availability. The shifts observed in tree-ring δ¹⁵N and TRW suggest that acidic air pollution rather than changes in stand productivity determined alteration of N and C cycles. Stable N isotopes in tree rings provided helpful information on the trajectory of the N cycle over the last century with direct consequences for a better understanding of future interactions among N, P and C cycles in terrestrial ecosystems.

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.

Assessing tree ring δ15N of four temperate deciduous species as an indicator of N availability using independent long-term records at the Fernow Experimental Forest, WV

Nitrogen deposition in the northeastern US changed N availability in the latter part of the twentieth century, with potential legacy effects. However, long-term N cycle measurements are scarce. N isotopes in tree rings have been used as an indicator of N availability through time, but there is little verification of whether species differ in the strength of this signal. Using long-term records at the Fernow Experimental Forest in West Virginia, we examined the relationship between soil conditions, including net nitrification rates, and wood δ¹⁵N in 2014, and tested the strength of correlation between tree ring δ¹⁵N of four species and stream water NO₃^- loss from 1971 to 2000. Higher soil NO₃^- was weakly associated with higher wood δ¹⁵N across species, and higher soil net nitrification rates were associated with higher δ¹⁵N for Quercus rubra only. The δ¹⁵N of Liriodendron tulipifera and Q. rubra, but neither Fagus grandifolia nor Prunus serotina, was correlated with stream water NO₃^-. L. tulipifera tree ring δ¹⁵N had a stronger association with stream water NO₃^- than Q. rubra. Overall, we found only limited evidence of a relationship between soil N cycling and tree ring δ¹⁵N, with a strong correlation between the wood δ¹⁵N and NO₃^- leaching loss through time for one of four species. Tree species differ in their ability to preserve legacies of N cycling in tree ring δ¹⁵N, and given the weak relationships between contemporary wood δ¹⁵N and soil N cycle measurements, caution is warranted when using wood δ¹⁵N to infer changes in the N cycle.

Ectomycorrhizal fungal maladaptation and growth reductions associated with assisted migration of Douglas-fir

Climatic adaptations are the foundation of conifer genecology, but populations also display variation in traits for nitrogen (N) utilization, along with some heritable specificity for ectomycorrhizal fungi (EMF). We examined soil and EMF influences on assisted migration of Douglas-fir (Pseudotsuga menziesii var. menziesii) by comparing two contrasting maritime populations planted up to 400 km northward in southwestern British Columbia. Soil N availability and host N status (via δ(15) N) were assessed across 12 maritime test sites, whereas EMF on local and introduced hosts were quantified by morphotyping with molecular analysis. Climatic transfer effects were only significant with soil N concentrations of test sites as a covariate, and illustrated how height growth was compromised for populations originating from relatively dry or cool maritime environments. We also found evidence for EMF maladaptation, where height declined by up to 15% with the extent of dissimilarity in EMF communities of southern populations relative to local hosts. The results demonstrate how geographic structure in belowground environments can contribute to conifer genecology. Differences in the inherent growth potential of conifers may be partly related to nutritional adaptations arising under native soil fertility, and optimization of this growth potential likely requires close affiliation with local EMF communities.