Concentrations of nutritional and trace elements in needles of Norway spruce (Picea abies [L.] Karst.) as functions of the needle age class

The endogenous concentrations of the essential elements Ca, Cl, Cu, Fe, K, Mg, Mn, N, P and Zn, and of the nonessential elements Al, As, Ba, Br, Co, Cr, Cs, Hg, I, La, Na, Rb, Sb, Sc, Si and Sr were determined in 5 successive needle age classes. 40 mature spruce trees from 6 different sites were investigated individually. A given element usually shows smooth changes with the needle age class t. Trees on a given site usually have a similar dynamic behaviour. The same holds for the different site means. The concentrations can be approximated by functions c=f(t). Three different types of functions are required to describe the dynamic behaviour of 3 groups of elements that increase with t, and one for the elements that decrease with t. A given element usually can be described by the same type of function at all sites, even if its concentration differs widely. Exceptions are Mn, Co and Zn, which change from a decreasing function at low concentrations to an increasing function at high concentrations. Further irregulatities are found at some sites with Ca, Sr and Ba. These findings are corroborated by a multivariate statistical analysis.     

Multivariate interpretation of the foliar chemical composition of Norway spruce (Picea abies)

Twenty-four chemical elements were analysed by INAA, ICP-AES and CN in needles of Norway spruce (Picea abies (L.) Karst.). Branches were sampled from 54 trees on eight sites in Switzerland and South Germany. From each tree, twigs were sorted into the most recent four or five age classes and their needles analysed separately. All measured concentrations could be considered as log-normally distributed and statistical analyses were, therefore, performed on logarithms. Variance components were estimated by maximum likelihood and compared between elements. Non-essential elements varied more than essential nutrients (Mn was an exception). The sites and the age of the needles were the most important sources of variance. The interaction between site and age, the individual tree, the sampled branch and the residual variance were usually much smaller sources of variance. The effects of the most significant factors – site and age – were further described by principal components and cluster analyses. Mineral elements either increased or decreased with the age of the needles according to their mobility in the phloem. Two different components were identified in the effect of the sites: a geochemical component linked to soil pH and a climate component linked to altitude, temperature and precipitation. Multivariate statistics are discussed as a tool for the interpretation of complex interaction patterns between element concentrations in plants.     

Increased nitrate availability in the soil of a mixed mature temperate forest subjected to elevated CO2 concentration (canopy FACE)

In a mature temperate forest in Hofstetten, Switzerland, deciduous tree canopies were subjected to a free‐air CO₂ enrichment (FACE) for a period of 8 years. The effect of this treatment on the availability of nitrogen (N) in the soil was assessed along three transects across the experimental area, one under Fagus sylvatica, one under Quercus robur and Q. petraea and one under Carpinus betulus. Nitrate, ammonium and dissolved organic N (DON) were analysed in soil solution obtained with suction cups. Nitrate and ammonium were also captured in buried ion‐exchange resin bags. These parameters were related to the local intensity of the FACE treatment as measured from the ¹³C depletion of dissolved inorganic carbon in the soil solution. Over the 8 years of experiment, the CO₂ enrichment reduced DON concentrations, did not affect ammonium, but induced higher nitrate concentrations, both in soil solution and resin bags. In the nitrate captured in the resin bags, the natural abundance of the isotope ¹⁵N increased strongly. This indicates that the CO₂ enrichment accelerated net nitrification, probably as an effect of the higher soil moisture resulting from the reduced transpiration of the CO₂‐enriched trees. It is also possible that N mineralization was enhanced by root exudates (priming effect) or that the uptake of inorganic N by these trees decreased slightly as the result of a reduced N demand for fine‐root growth. In this mature deciduous forest, we did not observe any progressive N limitation due to elevated atmospheric CO₂ concentrations; on the contrary, we observed an enhanced N availability over the 8 years of our measurements. This may, together with the global warming projected, exacerbate problems related to N saturation and nitrate leaching, although it is uncertain how long the observed trends will last in the future.     

Shade tolerance of <Emphasis Type="Italic">Ailanthus altissima</Emphasis> revisited: novel insights from southern Switzerland

The tree of heaven (Ailanthus altissima (Mill.) Swingle) is considered to be an early-successional, gap-obligate pioneer species with vigorous height growth, low shade tolerance, early fecundity and large seed production. It is a highly invasive species in many temperate and Mediterranean ecosystems outside its natural range, especially after disturbance. Due to its low shade tolerance, the potential of A. altissima to colonise undisturbed forests is thought to be low. In this study we analysed the potential of juvenile A. altissima to grow and survive in sweet chestnut (Castanea sativa Mill.) forests in southern Switzerland. We used hemispherical photography to assess the light conditions of 204 individuals of A. altissima (31 % generative, 69 % vegetative) aged between 1 and 7 years (median: 3 years) in six sites. Generative (seed-borne) and vegetative (clonal ramet) offspring of A. altissima are able to grow in light conditions well below the requirements of shade-intolerant tree species such as European larch (Larix decidua Mill.) and Scots pine (Pinus sylvestris L.). The relatively low light conditions found to be sufficient for the growth and survival of generative regeneration of A. altissima suggest a higher shade tolerance for this species than previously stated, at least for early regeneration. Consequently, the colonisation frontier of A. altissima should be intensively monitored in both forest openings but also in closed canopy forests in the vicinity of seed-bearing A. altissima.     

Regional Assessment of N Saturation using Foliar and Root \varvec {\delta}^{\bf 15}{\bf N}

N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar may be a robust, regional-scale measure of the onset of N saturation in forest ecosystems, we assembled a large dataset on atmospheric N deposition, foliar and root and N concentration, soil C:N, mineralization and nitrification. The dataset included sites in northeastern North America, Colorado, Alaska, southern Chile and Europe. Local drivers of N cycling (net nitrification and mineralization, and forest floor and soil C:N) were more closely coupled with foliar than the regional driver of N deposition. Foliar increased non-linearly with nitrification:mineralization ratio and decreased with forest floor C:N. Foliar was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root was more tightly coupled to forest floor properties than was foliar . We observed a pattern of decreasing foliar values across the following species: American beech>yellow birch>sugar maple. Other factors that affected foliar included species composition and climate. Relationships between foliar and soil variables were stronger when analyzed on a species by species basis than when many species were lumped. European sites showed distinct patterns of lower foliar , due to the importance of ammonium deposition in this region. Our results suggest that examining values of foliage may improve understanding of how forests respond to the cascading effects of N deposition.     

Long-term tracing of whole catchment 15N additions in a mountain spruce forest: measurements and simulations with the TRACE model

Despite numerous studies on nitrogen (N) cycling in forest ecosystems, many uncertainties remain, particularly regarding long-term N accumulation in the soil. Models validated against tracer isotopic data from field labeling experiments provide a potential tool to better understand and simulate C and N interactions over multiple decades. In this study, we describe the adaptation of the dynamic process-based model TRACE to a new site, Alptal, where long-term N-addition and ¹⁵N-tracer experiments provide unique datasets for testing the model. We describe model parameterization for this spruce forest, and then test the model with 9- and 14-year time series of ¹⁵N-tracer recovery from control and N-amended catchments, respectively. Finally, we use the model to project the fate of ecosystem N accumulation over the next 70?years. Field ¹⁵N recovery data show that the major sink for N deposition is the soil. On the control plot, tracer recovery in the soil increased from 32?% in the second year to 60?% in the ninth year following tracer addition, whereas on the N-saturated plot, soil recovery stayed almost constant from 63?% in the third year to 61?% in the twelfth year. Recovery in tree biomass increased over the decadal time scale in both treatments, to ca. 10?% over 9?years on the control plot and ca. 13?% over 14?years on the N-amended plot. We then used these time series to validate TRACE, showing that the adaptation and calibration procedure for the Alptal site was successful. Model-data comparison identified that the spreading method of ¹⁵N tracers needs to be considered when interpreting recovery results from labeling studies. Furthermore, the ground vegetation layer was recognized to play an important role in controlling the rate at which deposited N enters soil pools. Our 70-year model simulation into the future underpinned by a Monte-Carlo sensitivity analysis, suggests that the soil is able to immobilize a constant fraction of 70 and 77?% of deposited N for the treated and the control plot, respectively. Further, the model showed that the simulated increased N deposition resulted in a relatively small elevated C sequestration in aggrading wood with an N use efficiency of approximately 7?kg C per kg?N added.     

Soil warming opens the nitrogen cycle at the alpine treeline

Climate warming may alter ecosystem nitrogen (N) cycling by accelerating N transformations in the soil, and changes may be especially pronounced in cold regions characterized by N‐poor ecosystems. We investigated N dynamics across the plant–soil continuum during 6 years of experimental soil warming (2007–2012; +4 °C) at a Swiss high‐elevation treeline site (Stillberg, Davos; 2180 m a.s.l.) featuring Larix decidua and Pinus uncinata. In the soil, we observed considerable increases in the pool size in the first years of warming (by >50%), but this effect declined over time. In contrast, dissolved organic nitrogen (DON) concentrations in soil solutions from the organic layer increased under warming, especially in later years (maximum of +45% in 2012), suggesting enhanced DON leaching from the main rooting zone. Throughout the experimental period, foliar N concentrations showed species‐specific but small warming effects, whereas δ¹⁵N values showed a sustained increase in warmed plots that was consistent for all species analysed. The estimated total plant N pool size at the end of the study was greater (+17%) in warmed plots with Pinus but not in those containing Larix, with responses driven by trees. Irrespective of plot tree species identity, warming led to an enhanced N pool size of Vaccinium dwarf shrubs, no change in that of Empetrum hermaphroditum (dwarf shrub) and forbs, and a reduction in that of grasses, nonvascular plants, and fine roots. In combination, higher foliar δ¹⁵N values and the transient response in soil inorganic N indicate a persistent increase in plant‐available N and greater cumulative plant N uptake in warmer soils. Overall, greater N availability and increased DON concentrations suggest an opening of the N cycle with global warming, which might contribute to growth stimulation of some plant species while simultaneously leading to greater N losses from treeline ecosystems and possibly other cold biomes.     

Estimating below‐canopy light regimes using airborne laser scanning: An application to plant community analysis

Light is a key driver of forest biodiversity and functioning. Light regimes beneath tree canopies are mainly driven by the solar angle, topography, and vegetation structure, whose three‐dimensional complexity creates heterogeneous light conditions that are challenging to quantify, especially across large areas. Remotely sensed canopy structure data from airborne laser scanning (ALS) provide outstanding opportunities for advancement in this respect. We used ALS point clouds and a digital terrain model to produce hemispherical photographs from which we derived indices of nondirectional diffuse skylight and direct sunlight reaching the understory. We validated our approach by comparing the performance of these indices, as well as canopy closure (CCl) and canopy cover (CCo), for explaining the light conditions experienced by forest plant communities, as indicated by the Landolt indicator values for light (L_light) from 43 vegetation surveys along an elevational gradient. We applied variation partitioning to analyze how the independent and joint statistical effects of light, macroclimate, and soil on the spatial variation in plant species composition (i.e., turnover, Simpson dissimilarity, β_SIM) depend on light approximation methodology. Diffuse light explained L_light best, followed by direct light, CCl and CCo (R² = .31, .23, .22, and .22, respectively). The combination of diffuse and direct light improved the model performance for β_SIM compared with CCl and CCo (R² = .30, .27 and .24, respectively). The independent effect of macroclimate on β_SIM dropped from an R² of .15 to .10 when diffuse light and direct light were included. The ALS methods presented here outperform conventional approximations of below‐canopy light conditions, which can now efficiently be quantified along entire horizontal and vertical forest gradients, even in topographically complex environments such as mountains. The effect of macroclimate on forest plant communities is prone to be overestimated if local light regimes and associated microclimates are not accurately accounted for.     

Correction to: Vertical Redistribution of Soil Organic Carbon Pools After Twenty Years of Nitrogen Addition in Two Temperate Coniferous Forests

Ecosystems - In the article by Forstner et al. (2018), the surnames of co-authors Katharina M. Keiblinger and Patrick Schleppi were misspelled. We apologize and ask readers to cite the corrected...     

Regional Assessment of N Saturation using Foliar and Root $\varvec {\delta}^{\bf 15}{\bf N}$\varvec {\delta}^{\bf 15}{\bf N}

N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar may be a robust, regional-scale measure of the onset of N saturation in forest ecosystems, we assembled a large dataset on atmospheric N deposition, foliar and root and N concentration, soil C:N, mineralization and nitrification. The dataset included sites in northeastern North America, Colorado, Alaska, southern Chile and Europe. Local drivers of N cycling (net nitrification and mineralization, and forest floor and soil C:N) were more closely coupled with foliar than the regional driver of N deposition. Foliar increased non-linearly with nitrification:mineralization ratio and decreased with forest floor C:N. Foliar was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root was more tightly coupled to forest floor properties than was foliar . We observed a pattern of decreasing foliar values across the following species: American beech>yellow birch>sugar maple. Other factors that affected foliar included species composition and climate. Relationships between foliar and soil variables were stronger when analyzed on a species by species basis than when many species were lumped. European sites showed distinct patterns of lower foliar , due to the importance of ammonium deposition in this region. Our results suggest that examining values of foliage may improve understanding of how forests respond to the cascading effects of N deposition.