Tree-ring δ15N Research Articles

Watershed-scale changes in terrestrial nitrogen cycling during a period of decreased atmospheric nitrate and sulfur deposition

Recent reports suggest that decreases in atmospheric nitrogen (N) deposition throughout Europe and North America may have resulted in declining nitrate export in surface waters in recent decades, yet it is unknown if and how terrestrial N cycling was affected. During a period of decreased atmospheric N deposition, we assessed changes in forest N cycling by evaluating trends in tree-ring δ15N values (between 1980 and 2010; n = 20 trees per watershed), stream nitrate yields (between 2000 and 2011), and retention of atmospherically-deposited N (between 2000 and 2011) in the North and South Tributaries (North and South, respectively) of Buck Creek in the Adirondack Mountains, USA. We hypothesized that tree-ring δ15N values would decline following decreases in atmospheric N deposition (after approximately 1995), and that trends in stream nitrate export and retention of atmospherically deposited N would mirror changes in tree-ring δ15N values. Three of the six sampled tree species and the majority of individual trees showed declining linear trends in δ15N for the period 1980–2010; only two individual trees showed increasing trends in δ15N values. From 1980 to 2010, trees in the watersheds of both tributaries displayed long-term declines in tree-ring δ15N values at the watershed scale (R = −0.35 and p = 0.001 in the North and R = −0.37 and p <0.001 in the South). The decreasing δ15N trend in the North was associated with declining stream nitrate concentrations (−0.009 mg N L−1 yr−1, p = 0.02), but no change in the retention of atmospherically deposited N was observed. In contrast, nitrate yields in the South did not exhibit a trend, and the watershed became less retentive of atmospherically deposited N (−7.3% yr−1, p < 0.001). Our δ15N results indicate a change in terrestrial N availability in both watersheds prior to decreases in atmospheric N deposition, suggesting that decreased atmospheric N deposition was not the sole driver of tree-ring δ15N values at these sites. Other factors, such as decreased sulfur deposition, disturbance, long-term successional trends, and/or increasing atmospheric CO2 concentrations, may also influence trends in tree-ring δ15N values. Furthermore, declines in terrestrial N availability inferred from tree-ring δ15N values do not always correspond with decreased stream nitrate export or increased retention of atmospherically deposited N.

Can tree-ring δ15N be used as a proxy for foliar δ15N in European beech and Norway spruce?

For long-term environmental investigations, tree-ring δ 15 N values are inappropriate proxies for foliar δ 15 N for both Fagus sylvatica and Picea abies under moderate N loads. Currently it is unclear whether stable nitrogen isotope signals of tree-rings are related to those in foliage, and whether they can be used to infer tree responses to environmental changes. We studied foliar and tree-ring nitrogen (δ15N) and carbon (δ13C) isotope ratios in European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L.) from six long-term forest monitoring sites in Switzerland together with data on N deposition and soil N availability, as well as a drought response index over the last two decades. For both species, tree-ring δ15N and δ13C values were less negative compared to foliar δ15N and δ13C values, most likely due to recycling and reallocation of N within the tree and fractionation processes associated with the transport of sucrose and the formation of tree-rings, respectively. Temporal trends recorded in foliar δ15N were not reflected in tree-ring δ15N, with much higher variations in tree-rings compared to foliage. Soil N availability and N deposition were partially able to explain changes in foliar δ13C, while there were no significant correlations between environmental variables and either tree-ring or foliar δ15N. Our results suggest an uncoupling between the N isotopic composition of tree-rings and foliage. Consequently, tree-ring δ15N values are inappropriate proxies of foliar δ15N values under low-to-moderate N deposition loads. Furthermore, at such low levels of deposition, tree-ring δ15N values are not recommended as archives of tree responses to soil C/N or bulk N deposition.

Human footprints in urban forests: implication of nitrogen deposition for nitrogen and carbon storage

Rising levels of nitrogen (N) deposition are influencing urban forest carbon (C) and N dynamics due to greater human disturbance compared to those in rural areas. N deposition in combination with increased atmospheric carbon dioxide (CO2) and water limitation may alter C and N storage in urban forests. This review aimed to provide a better understanding of N and C storage under N deposition scenarios in urban forests. Globally, fuel combustion and biomass burning contribute in approximately 70 and 16 % of the NOx emission respectively. It is also estimated that NHy and NOx are two to four times higher in urban forests compared to rural areas. However, higher N deposition may not always result in increased N and C storage in urban forests. In fact, urban forests may even show early symptoms of N and C losses under climate change. For example, urban forests in fire-prone areas require higher frequency of burning to reduce the threat of wildfires, leading to an acceleration of C and N loss. Additionally, chronic N deposition may result in an early N loss in urban forests due to faster N saturation and soil acidification in urban forests compared to rural forests. Studies of N deposition on urban forests using N isotope composition (δ15N) also showed that N loss from urban forests can occur through the direct leaching of the deposited NO3 −-N. We also noted that using different 15N signal of soil and plant in combination of tree ring δ15N may provide a better understanding of N movement in urban forests. Although urban forests may become a source of C and N faster than rural forests, N-limited urban forests may benefit from N deposition to retain both N and C stocks longer than non-N-limited urban forests. Appropriate management practices may also help to delay such symptoms; however, the main source of emission still needs to be managed to reduce both N deposition and rising atmospheric CO2 in urban forests. Otherwise, the N and C stocks in urban forests may further decline when prolonged drought conditions under global climate change increase the frequency of fires and reduce plant photosynthesis.

(15)N in tree rings as a bio-indicator of changing nitrogen cycling in tropical forests: an evaluation at three sites using two sampling methods.

Anthropogenic nitrogen deposition is currently causing a more than twofold increase of reactive nitrogen input over large areas in the tropics. Elevated 15N abundance (δ15N) in the growth rings of some tropical trees has been hypothesized to reflect an increased leaching of 15N-depleted nitrate from the soil, following anthropogenic nitrogen deposition over the last decades. To find further evidence for altered nitrogen cycling in tropical forests, we measured long-term δ15N values in trees from Bolivia, Cameroon, and Thailand. We used two different sampling methods. In the first, wood samples were taken in a conventional way: from the pith to the bark across the stem of 28 large trees (the “radial” method). In the second, δ15N values were compared across a fixed diameter (the “fixed-diameter” method). We sampled 400 trees that differed widely in size, but measured δ15N in the stem around the same diameter (20 cm dbh) in all trees. As a result, the growth rings formed around this diameter differed in age and allowed a comparison of δ15N values over time with an explicit control for potential size-effects on δ15N values. We found a significant increase of tree-ring δ15N across the stem radius of large trees from Bolivia and Cameroon, but no change in tree-ring δ15N values over time was found in any of the study sites when controlling for tree size. This suggests that radial trends of δ15N values within trees reflect tree ontogeny (size development). However, for the trees from Cameroon and Thailand, a low statistical power in the fixed-diameter method prevents to conclude this with high certainty. For the trees from Bolivia, statistical power in the fixed-diameter method was high, showing that the temporal trend in tree-ring δ15N values in the radial method is primarily caused by tree ontogeny and unlikely by a change in nitrogen cycling. We therefore stress to account for tree size before tree-ring δ15N values can be properly interpreted.

Temporal changes in nitrogen acquisition of Japanese black pine (Pinus thunbergii) associated with black locust (Robinia pseudoacacia)

The alien woody legume, black locust (Robinia pseudoacacia), has invaded Japanese black pine (Pinus thunbergii) forests located in Japan’s coastal plain and hill regions where gaps are formed in pine forests after nematode infestation. Nitrogen fixation by legumes accelerates N cycling in forest ecosystems. We studied temporal change in the annual tree-ring resolution N stable isotope composition (δ15N, a per mil deviation of δ15N/14N ratio, relative to atmospheric N2δ15N=0‰) at two natural locations of Japanese black pine forest with black locust that differed in the time since black locust establishment (Shohnai in northeast and Kita-Kyushu in southwest Japan). Analyzed tree-rings covered the period from 1990/1992 to 2009. N acquisition by Japanese black pine from black locust N input to the soil was evidenced by temporal shifting of N stable isotope composition on the annual pine tree rings. With progressive development of the forest stand, δ15N values of earlier tree-rings δ15N of −5‰) from black pine associated with black locust shifted towards values similar to those of black locust δ15N values nearly to −1‰), which suggests acquisition of N by N2 fixation (Shohnai site). In contrast, in a forest where black locust had settled for two or three generations, in a black pine stand (Kita-Kyushu site), longer periods of N enrichment in the soil were reflected in the elevated tree-ring δ15N values of newly established black pine trees. Based on tree-ring δ15N data from the Shohnai site, we determined that about 10 years after black locust establishment, soil N had already been enriched by black locust N, this, in turn, contributed to N fertilization of surrounding trees in mixed stands.

Climate imprints on tree-ring δ15N signatures of sessile oak (Quercus petraea Liebl.) on soils with contrasting water availability

In the present study we investigated long-term climate imprints (160 year period) on tree-ring δ15N signatures of sessile oak (Quercus petraea) at sites with contrasting water availability (i.e. Cambisols vs. Regosols in Luxembourg, Central Europe, with 175mm and 42mm available water capacity, respectively). We hypothesized that tree-ring δ15N signatures constitute a sensitive indicator to long-term shifts in climatic conditions. Our findings revealed a close positive correlation between winter and spring temperatures and tree-ring δ15N signatures. These relationships were stronger for Cambisol than for Regosol sites. If entire chronologies were considered, peaks in annual mean temperatures closely corresponded with peaks in tree-ring δ15N signatures, with both annual mean temperatures and δ15N signatures reaching their maxima within the last two decades. In addition, we found a weak but significant impact of February precipitation on δ15N signatures, but only for Cambisols. We hypothesize that these findings are attributable to climate- (particularly temperature-) mediated nitrification rates in forest soils. As nitrification is a strongly fractionating process that produces 15N-depleted nitrate and higher isotopic ratios for ammonium in the topsoil, increased nitrification leads to 15N-enriched pools of Ninorg in the upper soil horizons and therefore higher δ15N signatures in plant tissues. Weaker correlations at Regosol sites were likely related to dryer and more acidic site conditions, both of which may reduce nitrification rates. Comparisons of oak and beech trees in the study area point to species-specific trajectories of wood nitrogen isotopes, likely related to the partitioning of ammonium and nitrate among species. In conclusion, tree-ring δ15N signatures may serve as an integrator of terrestrial N cycling and as such constitute a valuable tool in the identification of spatial and temporal patterns of N cycling in relation to environmental changes. Due to the mediating effects of the isotopic composition of the respective N sources, analyses of the isotopic composition of airborne N loads would support the interpretation of wood δ15N patterns, particularly in areas that are subject to high N pollution.

Tree-ring growth recovers, but δ13C and δ15N do not change, after the removal of point-source air pollution: a case study for poplar (Populus cathayana) in northwestern China

Pollution from urban centers and fossil fuel combustion can decrease forest growth and interfere with physiological processes. To evaluate whether tree growth and the carbon isotope ratio (δ13C) and nitrogen isotope ratio (δ15N) in tree rings can serve as proxies for air pollution, this study compared these indices for poplar (Populus cathayana) growing at urban and suburban locations in Lanzhou, in northwestern China. Basal area increment values were much lower at the urban site than in the suburbs from 1985 to 2009, were negatively correlated with NO2 (r = −0.56, p < 0.01) and SO2 (r = −0.52, p < 0.05) emissions from 1990 to 2009, and increased abruptly after the Lanzhou urban steel factory closed. Urban tree-ring δ13C values were not significantly correlated with NO2 and SO2 concentrations, and did not differ significantly between the two sites, indicating that other environmental effects (such as precipitation) masked the pollution effects. Tree-ring δ15N values in the urban samples were much higher than the suburban values. Such differences may be attributable to uptake of 15N-enriched compounds caused by a higher urban N deposition rate. Tree growth is a promising tool for detecting ecophysiological responses of trees to both diffuse and point-source air pollution, but δ13C and δ15N in poplar were not sensitive to point-source air pollution in a heavily polluted environment.

Long-Term Trends in Tree-Ring Width and Isotope Signatures (δ13C, δ15N) of Fagus sylvatica L. on Soils with Contrasting Water Supply

We investigated long-term responses (since 1850) of Fagus sylvatica (Luxembourg; central Europe) to shifts in temperature, precipitation, and nitrogen deposition by analyzing diameter at breast height (DBH) increment, basal area increment (BAI), and tree-ring stable isotopes (δ13C, δ15N). We compared stands on soils with contrasting water supply (Regosols and Cambisols with an available water capacity of ca. 40 and 170 mm, respectively) and of two different age classes (ca. 60 vs. 200 years). All stands showed a peak in DBH increment in the decade 1978–1987, but a decline in increment growth in subsequent decades. In addition, BAI declined in mature stands in the last two decades. Decreasing increment rates were attributable to an increasing drought limitation of stands, mainly induced by increasing temperatures in the last two decades. Contrary to our expectations, stands on Cambisols showed a similar susceptibility to shifts in temperature and precipitation as stands on Regosols, suggesting a strong adaptation of the respective ecotypes grown at dryer sites. This result was in line with long-term trends for tree-ring δ13C signatures, which did not differ significantly between stands on Cambisols and Regosols. Climate impacts on tree-ring δ15N signatures were low. High spring precipitation and temperatures caused increasing and decreasing δ15N values, respectively, but only in mature stands on Cambisols. Stands on Regosols tended to have lower δ15N values than stands on Cambisols. Decreasing δ15N values in recent decades suggest an increasing impact of allochthonous N loads with isotopically lighter N.

Relationships between salmon abundance and tree-ring δ15N: three objective tests

Quantification of a relationship between salmon escapement in rivers and riparian tree-ring δ15N could allow reconstruction of prehistorical salmon abundance. Unfortunately, attempts to quantify this link have met with little success. We examined the feasibility of the approach using natural abundance of δ15N in riparian tree rings formed before and after extirpation of salmon and 15N tracer studies in a river and riparian soils. We concluded that (i) extractable (sap) N must be removed for interpretation of tree-ring δ15N because it contains up to 78% of the N in wood, is mobile, and differs from structural N in isotopic composition, (ii) no significant change in structural tree-ring δ15N was associated with salmon extirpation in a natural system, (iii) 500‰ 15NH4+ added to a stream was detected in riparian tree rings spanning at least 8 years, demonstrating interring movement of N that confounds detection of an annual signal, and (iv) addition of 28 000‰ 15NH4+ to riparian soils at a rate equaling 7.25 kg salmon·50 m–2 resulted in maximum tree-ring δ15N of ∼100‰–600‰. Thus, the calculated maximum signal possible from salmon was 0.08‰–0.43‰, which is within the range of natural variation. Evidence suggested that neither total nor structural tree-ring δ15N was useful for reconstructing salmon abundance.

Nitrogen deposition influences nitrogen isotope composition in soil and needles of Pinus massoniana forests along an urban-rural gradient in the Pearl River Delta of south China

Purpose Atmospheric nitrogen (N) deposition remains globally and regionally a significant N source in forest ecosystems, with intensive industrial activities. Stable N isotope ratio (δ15N) is a useful indicator widely adopted to assess environmental and ecological impacts of anthropogenic N inputs. On the basis of temporal changes in tree ring δ15N established recently, the present study investigated the influence of N deposition on δ15N in needles of Masson pine (Pinus massoniana L.) and forest soil along an urban–rural gradient in the Pearl River Delta of south China.