Physiological responses of a young Picea Sitchensis stand to long-term nitrogen and sulphur deposition: a lesson from d13C, d18O and d15N in tree rings (original) (raw)
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Forests
Increasing atmospheric CO2 concentration and nitrogen deposition are, among the global change related drivers, those playing a major role on forests carbon sequestration potential, affecting both their productivity and water-use efficiency. Up to now, results are however contrasting, showing that the processes underlying them are far from being fully comprehended. In this study, we adopted an innovative approach to simulate the increase of N deposition in a sessile oak forest in North-Eastern Italy, by fertilizing both from above and below the canopy. We observed the dynamics of basal area increment, intrinsic water-use efficiency and of several leaf functional traits over 4 years, to evaluate how the added nitrogen and the two different fertilization system could affect them. We were not able, however, to detect any shift, besides a common yearly variability related to a prevailing background environmental forcing. To this end, we considered as relevant factors both the short time-...
The contribution of nitrogen deposition to the photosynthetic capacity of forests
Global Biogeochemical Cycles, 2013
1] Global terrestrial carbon (C) sequestration has increased over the last few decades. The drivers of carbon sequestration, the geographical spread and magnitude of this sink are however hotly debated. Photosynthesis determines the total C uptake of terrestrial ecosystems and is a major flux of the global C balance. We contribute to the discussion on enhanced C sequestration by analyzing the influence of nitrogen (N) deposition on photosynthetic capacity (A max ) of forest canopies. Eddy covariance measurements of net exchange of carbon provide estimates of gross primary production, from which A max is derived with a novel approach. Canopy A max is combined with modeled N deposition, environmental variables and stand characteristics to study the relative effects on A max for a unique global data set of 80 forest FLUXNET sites. Canopy A max relates positively to N deposition for evergreen needleleaf forests below an observed critical load of~8 kg N ha -1 yr -1 , with a slope of 2.0 AE 0.4 (S.E.) mmol CO 2 m -2 s -1 per 1 kg N ha -1 yr -1 . Above this threshold canopy A max levels off, exhibiting a saturating response in line with the N saturation hypothesis. Climate effects on canopy A max cannot be separated from the effect of N deposition due to considerable covariation. For deciduous broadleaf forests and forests in the temperate (-continental) climate zones, the analysis shows the N deposition effect to be either small or absent. Leaf area index and foliar N concentration are positively but weakly related to A max . We conclude that flux tower measurements of C fluxes provide valuable data to study physiological processes at the canopy scale. Future efforts need to be directed toward standardizing measures N cycling and pools within C monitoring networks to gain a better understanding of C and N interactions, and to disentangle the role of climate and N deposition in forest ecosystems.
Ecological Applications, 2015
As increasing levels of nitrogen (N) deposition impact many terrestrial ecosystems, understanding the potential effects of higher N availability is critical for forecasting tree carbon allocation patterns and thus future forest productivity. Most regional estimates of forest biomass apply allometric equations, with parameters estimated from a limited number of studies, to forest inventory data (i.e., tree diameter). However, most of these allometric equations cannot account for potential effects of increased N availability on biomass allocation patterns. Using 18 yr of tree diameter, height, and mortality data collected for a dominant tree species (Acer saccharum) in an atmospheric N deposition experiment, we evaluated how greater N availability affects allometric relationships in this species. After taking into account site and individual variability, our results reveal significant differences in allometric parameters between ambient and experimental N deposition treatments. Large trees under experimental N deposition reached greater heights at a given diameter; moreover, their estimated maximum height (mean ± standard deviation: 33.7 ± 0.38 m) was significantly higher than that estimated under the ambient condition (31.3 ± 0.31 m). Within small tree sizes (5-10 cm diameter) there was greater mortality under experimental N deposition, whereas the relative growth rates of small trees were greater under experimental N deposition. Calculations of stemwood biomass using our parameter estimates for the diameter-height relationship indicated the potential for significant biases in these estimates (~2.5%), with under predictions of stemwood biomass averaging 4 Mg/ha lower if ambient parameters were to be used to estimate stem biomass of trees in the experimental N deposition treatment. As atmospheric N deposition continues to increase into the future, ignoring changes in tree allometry will contribute to the uncertainty associated with aboveground carbon storage estimates across a forest with a large geographic distribution in eastern North America.
Does nitrogen and sulfur deposition affect forest productivity
2010
We studied the effects of atmospheric nitrogen and sulfur deposition on forest productivity in a 10year-old, aggrading forest stand at the Fernow Experimental Forest in Tucker County, WV. Forest productivity was expressed as total aboveground wood biomass, which included stem and branch weight of standing live trees. Ten years after stand regeneration and treatment initiation, total aboveground wood biomass was compared among three treatments: whole tree harvest (WT), whole tree harvest plus annual nitrogen (N) and sulfur (S) additions at two times ambient deposition rates (WT+NS), and whole tree harvest plus N, S (two times ambient), and dolomitic lime (WT+NS+CA) additions. Furthermore, future stand productivity was estimated for a subsequent 70 years using growth projection simulator SILVAH. Total aboveground wood biomass at 10 years was not significantly different among treatments (ANOVA: F = 1.20, p = 0.33, n = 9). Mean total aboveground wood biomass values for the WT, WT+NS, and WT+NS+CA treatments were 47.5 (± 15.3) Mg ha-1 , 53.0 (± 14.3) Mg ha-1 , and 51.0 (± 15.5) Mg ha-1 , respectively. The dominant tree species was pin cherry (average aboveground dry weight of the three treatments was 38.2 Mg ha-1). Lack of significant differences in the aboveground wood component at the stand level suggests that 10 years of three times ambient rates of nitrogen and sulfur deposition and mitigation did not impact the ability of this site to produce woody biomass. At the species level, however, yellow-poplar had significantly higher diameter at breast height and aboveground wood biomass in the WT+NS+CA stands, indicating the potential for N, S, and Ca additions to impact individual species' growth over forest succession. Projected aboveground wood biomass at stand age 80 was 230.3 Mg ha-1 , 229.9 Mg ha-1 , and 349.7 Mg ha-1 for the respective treatments WT, WT+NS, and WT+NS+CA. Our results suggest that although N and S additions alone do not increase stand growth, N, S, and dolomite additions increase growth of individual species within the first 10 years of stand development. Based on this early biomass increase, long-term growth in the WT+NS+CA treatment may increase 34 percent by forest age 80.
No evidence that chronic nitrogen additions increase photosynthesis in mature sugar maple forests
Ecological Applications, 2011
Atmospheric nitrogen (N) deposition can increase forest growth. Because N deposition commonly increases foliar N concentrations, it is thought that this increase in forest growth is a consequence of enhanced leaf-level photosynthesis. However, tests of this mechanism have been infrequent, and increases in photosynthesis have not been consistently observed in mature forests subject to chronic N deposition. In four mature northern hardwood forests in the north-central United States, chronic N additions (30 kg NÁha À1 Áyr À1 as NaNO 3 for 14 years) have increased aboveground growth but have not affected canopy leaf biomass or leaf area index. In order to understand the mechanism behind the increases in growth, we hypothesized that the NO 3 À additions increased foliar N concentrations and leaf-level photosynthesis in the dominant species in these forests (sugar maple, Acer saccharum). The NO 3 À additions significantly increased foliar N. However, there was no significant difference between the ambient and þNO 3 À treatments in two seasons (2006)(2007) of instantaneous measurements of photosynthesis from either canopy towers or excised branches. In measurements on excised branches, photosynthetic nitrogen use efficiency (lmol CO 2 Ás À1 Ág À1 N) was significantly decreased (À13%) by NO 3 À additions. Furthermore, we found no consistent NO 3 À effect across all sites in either current foliage or leaf litter collected annually throughout the study (1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007) and analyzed for d 13 C and d 18 O, isotopes that can be used together to integrate changes in photosynthesis over time. We observed a small but significant NO 3 À effect on the average area and mass of individual leaves from the excised branches, but these differences varied by site and were countered by changes in leaf number. These photosynthesis and leaf area data together suggest that NO 3 À additions have not stimulated photosynthesis. There is no evidence that nutrient deficiencies have developed at these sites, so unlike other studies of photosynthesis in N-saturated forests, we cannot attribute the lack of a stimulation of photosynthesis to nutrient limitations. Rather than increases in C assimilation, the observed increases in aboveground growth at our study sites are more likely due to shifts in C allocation.
Chronic N deposition alters root respiration-tissue N relationship in northern hardwood forests
Global Change Biology, 2012
Specific root respiration rates typically increase with increasing tissue N concentration. As a result, it is often assumed that external factors inducing greater root N concentration, such as chronic N deposition, will lead to increased respiration rates. However, enhanced N availability also alters root biomass, making the ecosystem-level consequences on whole-root-system respiration uncertain. The objective of this study was to determine the effects of chronic experimental N deposition on root N concentrations, specific respiration rates, and biomass for four northern hardwood forests in Michigan. Three of the six measurement plots at each location have received experimental N deposition (3 g NO À 3 -N m À2 yr À1 ) since 1994. We measured specific root respiration rates and N concentrations of roots from four size classes (<0.5, 0.5-1, 1-2, and 2-10 mm) at three soil depths (0-10, 10-30, and 30-50 cm). Root biomass data for the same size classes and soil depths was used in combination with specific respiration rates to assess the response of whole-root-system respiration. Root N and respiration rate were greater for smaller diameter roots and roots at shallow depths. In addition, root N concentrations were significantly greater under chronic N deposition, particularly for larger diameter roots. Specific respiration rates and root biomass were unchanged for all depths and size classes, thus whole-root-system respiration was not altered by chronic N deposition. Higher root N concentrations in combination with equivalent specific respiration rates under experimental N deposition resulted in a lower ratio of respiration to tissue N. These results indicate that relationships between root respiration rate and N concentration do not hold if N availability is altered significantly. For these forests, use of the ambient respiration to N relationship would over-predict actual root system respiration for the chronic N deposition treatment by 50%.
European Journal of Forest Research, 2010
Nitrogen (N) deposition exceeds the critical loads for this element in most parts of Switzerland apart from the Alps. At 17 sites (8 broadleaved stands, 8 coniferous stands, and 1 mixed stand) of the Swiss Long-term Forest Ecosystem Research network, we are investigating whether N deposition is associated with the N status of the forest ecosystems. N deposition, assessed from throughfall measurements, was related to the following indicators: (1) nitrate leaching below the rooting zone (measured on a subset of 9 sites); (2) the N nutrition of the forest stand based on foliar analyses (16 sites); and (3) crown defoliation, a non specific indicator of tree vitality (all 17 sites). Nitrate leaching ranging from about 2 to 16 kg N ha -1 a -1 was observed at sites subjected to moderate to high total N deposition ([10 kg ha -1 a -1 ). The C/N ratio of the soil organic layer, or, when it was not present, of the upper 5 cm of the mineral soil, together with the pool of organic carbon in the soil, played a critical role, as previous studies have also found. In addition, the humus type may need to be considered as well. For instance, little nitrate leaching (\2 kg N ha -1 a -1 ) was recorded at the Novaggio site, which is subjected to high total N deposition ([30 kg ha -1 a -1 ) but characterized by a C/N ratio of 24, large organic C stocks, and a moder humus type. Foliar N concentrations correlated with N deposition in both broadleaved and coniferous stands. In half of the coniferous stands, foliar N concentrations were in the deficiency range. Crown defoliation tended to be negatively correlated with N concentrations in the needles. In the majority of the broadleaved stands, foliar N concentrations were in the optimum nutritional range or, on one beech plot with high total N deposition ([25 kg ha -1 a -1 ), above the optimum values. There was no correlation between the crown defoliation of broadleaved trees and foliar concentrations.
The legacy of enhanced N and S deposition as revealed by combining d13C, d18O and d15N in tree rings
This study aimed to evaluate the effects of long-term repeated aerial nitrogen (N) and sulphur (S) misting over tree canopies of a Sitka spruce plantation in Scotland. We combined d 13 C and d 18 O in tree rings to evaluate the changes in CO 2 assimilation (A) and stomatal conductance (g s ) and to assess their contribution to variations in the intrinsic wateruse efficiency (WUE i , i.e., the A/g s ratio). Measurements of d 15 N enabled shifts in the ecosystem N cycling following misting to be assessed. We found that: (i) N applications, with or without S, increased the ratio between A and g s in favour of A, thus supporting a fertilizer effect of added N. (ii) After the treatments ceased, the trees quickly adjusted to the reductions of N deposition, but not to the reduction in S deposition, which had a negative effect on WUE i by reducing A. This indicates that the beneficial role of N deposition may be negated in forests that previously received a high load of acid rain. (iii) d 15 N in tree rings reflected the N dynamics caused by canopy retention, with the fingerprint also present in the litter, after the experiment stopped. (iv) Both our results (obtained using canopy applications) and a collection of published data (obtained using soil applications) showed that generally WUE i increased in response to an increase of N applications, with the magnitude of the changes related to soil conditions and the availability of other nutrients. The shifts observed in d 15 N in tree rings also suggest that both the quantity of the applied N and its quality, mediated by processes occurring during canopy N retention, are important determinants of the interactions between N and C cycles. Stable isotopes are useful probes to understand these processes and to put the results of short-term experiments into context.
Retention of Dissolved Inorganic Nitrogen by Foliage and Twigs of Four Temperate Tree Species
Ecosystems, 2012
Nitrogen (N) retention by tree canopies is believed to be an important process for tree nutrient uptake, and its quantification is a key issue in determining the impact of atmospheric N deposition on forest ecosystems. Due to dry deposition and retention by other canopy elements, the actual uptake and assimilation by the tree canopy is often obscured in throughfall studies. In this study, 15 N-labeled solutions ( 15 NH þ 4 and 15 NO À 3 ) were used to assess dissolved inorganic N retention by leaves/needles and twigs of European beech, pedunculate oak, silver birch, and Scots pine saplings. The effects of N form, tree species, leaf phenology, and applied NO À 3 to NH þ 4 ratio on the N retention were assessed. Retention patterns were mainly determined by foliar uptake, except for Scots pine. In twigs, a small but significant 15 N enrichment was detected for NH þ 4 , which was found to be mainly due to physicochemical adsorption to the woody plant surface.