Sabine Braun

Does nitrogen deposition increase forest production? The role of phosphorus.

Effects of elevated N deposition on forest aboveground biomass were evaluated using long-term data from N addition experiments and from forest observation plots in Switzerland. N addition experiments with saplings were established both on calcareous and on acidic soils, in 3 plots with Fagus sylvatica and in 4 plots with Picea abies. The treatments were conducted during 15 years and consisted of additions of dry NH4NO3 at rates of 0, 10, 20, 40, 80, and 160 kg N ha(-1) yr(-1). The same tree species were observed in permanent forest observation plots covering the time span between 1984 and 2007, at modeled N deposition rates of 12-46 kg N ha(-1) yr(-1). Experimental N addition resulted in either no change or in a decreased shoot growth and in a reduced phosphorus concentration in the foliage in all experimental plots. In the forest, a decrease of foliar P concentration was observed between 1984 and 2007, resulting in insufficient concentrations in 71% and 67% of the Fagus and Picea plots, respectively, and in an increasing N:P ratio in Fagus. Stem increment decreased during the observation period even if corrected for age. Forest observations suggest an increasing P limitation in Swiss forests especially in Fagus which is accompanied by a growth decrease whereas the N addition experiments support the hypothesis that elevated N deposition is an important cause for this development.

Growth of Mature Beech in Relation to Ozone and Nitrogen Deposition: an Epidemiological Approach

Stem increment of mature Fagus sylvatica L. from 57 plots in Switzerland was analysed with respect to environmental factors using multilinear regression. Nitrogen (N) deposition was positively correlated with stem growth, with an increase by 41.8% (confidence interval 38.9–44.2) per 10 kg N ha−1 a−1. There was a negative relationship between ozone (03) dose and diameter increment. The fit was better for maximum rather than average exposure values. Coefficients indicated growth changes of −22.5% (14.3–28.6 confidence interval) and −35.1% (17.8–47.3) respectively per 10 ppm hours. These changes exceed the −6.1% found in experiments with beech seedlings. This difference is mainly attributed to the fact that the seedling data are calculated for a single season’s exposure whereas the stem increment data extend over four years, representing the product of four annual reductions. Increased sensitivity of mature trees compared to seedlings, however, is also suggested.

Nitrogen and Its Effect on Growth, Nutrient Status and Parasite Attacks in Beech and Norway Spruce

In permanent observation plots across Switzerland, nitrogen (N) concentration in the foliage of mature beech has increased by 15% and phosphorus (P) concentrations in beech and mature Norway spruce decreased by 12 and 13% respectively between 1984 and 1995, leading to increased N:P ratios. Modelled N deposition was correlated with stem increment in both beech and spruce, with indications of P limitation in some beech plots. Experimental application of 0-160 kg N ha-1> yr-1> over four to five years caused nutrient imbalances in various afforestation plots comparable to those observed in the permanent observation plots. The changes in the trees caused by N treatment led to increased attacks by parasites such as Apiognomonia errabunda, Phomopsis sp., Phyllaphis fagi in beech and Botrytis cinerea, Sacchiphantes abietis and Cinara pilicornis in Norway spruce. The results suggest current N deposition in Switzerland induces significant changes in the forest ecosystem.

Effect of ambient ozone and acid mist on Aphid development

The effect of ambient air with increased ozone concentrations and artificial acid mist on the population growth of two different aphid species was studied: Aphis fabae on Phaseolus vulgaris and Phyllaphis fagi on seedings of Fagus sylvatica. Whereas the ambient air inhibited growth of Aphis fabae, it stimulated population growth of Phyllaphis fagi. In Fagus, analysis of a phloem exudate revealed that the amino acid/sugar ratio was significantly increased by the ambient air compared to filtered air. In Phaseolus, no significant differences in amino acid or sugar content of the phloem exudate could be found. Acid mist inhibited population development in both aphid species; the strongest effect was observed in the first weeks after artificial infestation.

Forest Damages by the Storm ‘Lothar’ in Permanent Observation Plots in Switzerland: The Significance of Soil Acidification and Nitrogen Deposition

In a network of permanent observation plots in Switzerland,the storm ‘Lothar’ uprooted 18.7% of Fagussylvaticaand 14.8% of Picea abies. The percentage of uprootingwas significantly inversely correlated with actual soil basesaturation (NH4Cl-extract). At a base saturation of≤40% (calculated as an average over 0–40cm depth) uprootingwas increased by a factor of 4.8 in beech and by a factor of3.6 in Norway spruce compared to less acidic soils. In beech,the percentage of uprooted trees was also significantlycorrelated with nitrogen concentration in the leaves(positively) and with coarse pore volume in the soil(negatively). There was neither a relation with seasonalozone dose nor a relation with crown transparency, stemdiameter, crown size, slenderness, social position andposition within the stand. The results suggest thatanthropogenic stress factors play an important role in theextent of the ‘Lothar’ damages in Switzerland.

A dynamic modelling approach for estimating critical loads of nitrogen based on plant community changes under a changing climate

A dynamic model of forest ecosystems was used to investigate the effects of climate change, atmospheric deposition and harvest intensity on 48 forest sites in Sweden (n = 16) and Switzerland (n = 32). The model was used to investigate the feasibility of deriving critical loads for nitrogen (N) deposition based on changes in plant community composition. The simulations show that climate and atmospheric deposition have comparably important effects on N mobilization in the soil, as climate triggers the release of organically bound nitrogen stored in the soil during the elevated deposition period. Climate has the most important effect on plant community composition, underlining the fact that this cannot be ignored in future simulations of vegetation dynamics. Harvest intensity has comparatively little effect on the plant community in the long term, while it may be detrimental in the short term following cutting. This study shows: that critical loads of N deposition can be estimated using the plant community as an indicator; that future climatic changes must be taken into account; and that the definition of the reference deposition is critical for the outcome of this estimate.

Growth losses in Swiss forests caused by ozone : epidemiological data analysis of stem increment of Fagus sylvatica L. and Picea abies Karst

The estimate of growth losses by ozone exposure of forest trees is a significant part in current C sequestration calculations and will also be important in future modeling. It is therefore important to know if the relationship between ozone flux and growth reduction of young trees, used to derive a Critical Level for ozone, is also valid for mature trees. Epidemiological analysis of stem increment data from Fagus sylvatica L. and Picea abies Karst. observed in Swiss forest plots was used to test this hypothesis. The results confirm the validity of the flux-response relationship at least for beech and therefore enable estimating forest growth losses by ozone on a country-wide scale. For Switzerland, these estimates amount to 19.5% growth reduction for deciduous forests, 6.6% for coniferous forests and 11.0% for all forested areas based on annual ozone stomatal uptake during the time period 1991-2011.

Revitalization of an alpine protective forest by fertilization

In a highly damaged, 120–150 year old Norway spruce (Picea abies L. Karst.) stand, still used for grazing, with a protective function against avalanches and rockfall, a fertilization experiment for revitalization was carried out. A slow release fertilizer containing macro-and micronutrients was applied over three years, from 1986 to 1988. Four years after the last treatment, the proportion of trees with a needle loss of >25% decreased from initially 60–80% to 24% in the treated plots, while in the control plots it remained unchanged. A significant increase of radial increment (+100%), shoot growth (+40%), needle length (+20%) and starch content in the fine roots (+40%) was established as well. The supply with P which had been low initially was significantly increased. No changes could be observed in fine root biomass or mycorrhization. In the soil, an alpine rendzina on dolomite, a significant reservoir of P and tendentially of K was still detectable three years after the last treatment. Cellulose decomposition was slightly enhanced in the uppermost soil layer of the treated plots but no changes in the C/N ratio or in pH were found.

Use of sap flow measurements to validate stomatal functions for mature beech (Fagus sylvatica) in view of ozone uptake calculations

For a quantitative estimate of the ozone effect on vegetation reliable models for ozone uptake through the stomata are needed. Because of the analogy of ozone uptake and transpiration it is possible to utilize measurements of water loss such as sap flow for quantification of ozone uptake. This technique was applied in three beech (Fagus sylvatica) stands in Switzerland. A canopy conductance was calculated from sap flow velocity and normalized to values between 0 and 1. It represents mainly stomatal conductance as the boundary layer resistance in forests is usually small. Based on this relative conductance, stomatal functions to describe the dependence on light, temperature, vapour pressure deficit and soil moisture were derived using multivariate nonlinear regression. These functions were validated by comparison with conductance values directly estimated from sap flow. The results corroborate the current flux parameterization for beech used in the DO3SE model.

Growth trends of beech and Norway spruce in Switzerland: The role of nitrogen deposition, ozone, mineral nutrition and climate

Understanding the effects of nitrogen deposition, ozone and climate on tree growth is important for planning sustainable forest management also in the future. The complex interplay of all these factors cannot be covered by experiments. Here we use observational data of mature forests for studying associations of various biotic and abiotic factors with tree growth. A 30year time series on basal area increment of Fagus sylvatica L. and Picea abies Karst. in Switzerland was analyzed to evaluate the development in relation to a variety of predictors. Basal area increment of Fagus sylvatica has clearly decreased during the observation period. For Picea abies no trend was observed. N deposition of more than 26 (beech) or 20-22kgNha-1year-1 (Norway spruce) was negatively related with basal area increment, in beech stronger than in Norway spruce. High N deposition loads and low foliar K concentrations in Fagus were correlated with increased drought sensitivity. High air temperatures in winter were negatively related with basal area increment in Norway spruce in general and in beech at high N:Mg ratio or high N deposition while on an average the relation was positive in beech. Fructification in beech was negatively related to basal area increment. The increase of fructification observed during the last decades contributed thus to the growth decrease. Ozone flux was significantly and negatively correlated with basal area increment both in beech and Norway spruce. The results show clear non-linear effects of N deposition on stem increment of European beech and Norway spruce as well as strong interactions with climate which have contributed to the growth decrease in beech and may get more important in future. The results not only give suggestions for ecological processes but also show the potential of an integral evaluation of observational data.