Is tree growth in boreal coniferous stands on mineral soils affected by the addition of wood ash?

Increasing use of forest fuels for energy production is generating greater quantities of wood ash. In Sweden, it is recommended that this ash should be returned to the forest to counter soil acidification and avoid potential future nutrient deficiencies, but the effects on tree growth require clarification. Thus, 10 field experiments were established in Sweden in 1990–2006 to study the effects of adding wood ash of various origins, doses and combinations of both ash and nitrogen on stem growth in Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L. Karst.) stands on mineral soil. Observations after 5–15 years show that growth responses were strongest when N was added, either alone or with wood ash. Growth responses to additions of wood ash without N were small and variable and statistically insignificant at all study sites. However, there were indications that adding wood ash may increase stem-wood growth at fertile sites and decrease it at less fertile sites. Hence, at fertile sites, it may compensate for the growth reductions that normally follow whole-tree harvests, at least temporarily, presumably due to its effects on soil N turnover. At less fertile sites, adding N is probably essential to counteract these growth reductions.     

The impact of climate,drainage and fertilization on the survival and growth of pinus sylvestris L. in afforestation of open,low‐production peatlands

The objective of this study was to analyse the conditions for forest production on open, low‐production peatlands in Sweden with respect to climate, and water and nutrient regimes. The study focused on survival and growth of Scots pine (Pinus sylvestris L.) seedlings, planted 18 yrs ago in five experimental areas evenly distributed between south and north Sweden. Different ditch spacing and NPK fertilizer treatments were combined systematically in all experiments. Survival was positively correlated with temperature sum, fertilization and drainage intensity. Tree growth was not influenced by variations in temperature sum after merely draining, but in combination with fertilization, growth was strongly correlated with climate. In the southern experiments, fertilization increased stand growth eight to nine times, whereas stands on the northern sites did not respond to fertilization. The most important fertilizer element was P. The application of N had no effect on growth. More intensive drainage increased stand growth by 60%. In the southern areas, height development in the most intensive drainage and fertilizer treatments indicates a mean annual increment of 6–7 m³ ha^?1, and no sign of decline in growth was seen. Turf‐planting had positive effects on both survival and growth, especially in less intensively drained plots. A large proportion of damaged trees was observed in the experiment. The frequency of damaged trees was positively correlated with treatment intensity but negatively correlated with temperature sum. The results show clearly that merely draining is not sufficient for successful afforestation of low‐production peatland sites. Fertilization by P and K is a necessary prerequisite, but the effect of fertilization varies with climatic conditions, probably owing to the amount of plant‐available N.     

Changes of soil chemistry,stand nutrition,and stand growth at two Scots pine (<Emphasis Type="Italic">Pinus sylvestris</Emphasis> L.) sites in Central Europe during 40 years after fertilization,liming, and lupine introduction

Long-term (40 years) effects of two soil amelioration techniques [NPKMgCa fertilization + liming; combination of PKMgCa fertilization, liming, tillage, and introduction of lupine (Lupinus polyphyllus L.)] on chemical topsoil properties, stand nutrition, and stand growth at two sites in Germany (Pfaffenwinkel, Pustert) with mature Scots pine (Pinus sylvestris L.) forest were investigated. Both sites are characterized by base-poor parent material, historic N and P depletion by intense litter-raking, and recent high atmospheric N input. Such sites contribute significantly to the forested area in Central Europe. Amelioration resulted in a long-term increase of pH, base saturation, and exchangeable Ca and Mg stocks in the topsoil. Moreover, significant losses of the forest floor in organic carbon (OC) and nitrogen stocks, and a decrease of the C/N ratio in the topsoil were noticed. The concentrations and stocks of OC and N in the mineral topsoil increased; however, the increases compensated only the N, but not the OC losses of the forest floor. During the recent 40 years, the N nutrition of the stands at the control plots improved considerably, whereas the foliar P, K, and Ca concentrations decreased. The 100-fascicle weights and foliar concentrations of N, P, Mg, and Ca were increased after both amelioration procedures throughout the entire 40-year period of investigation. For both stands, considerable growth acceleration during the recent 40 years was noticed on the control plots; the amelioration resulted in an additional significant long-term growth enhancement, with the NPKMgCa fertilization liming + being more effective than the combination of PKMgCa fertilization, liming, tillage, and introduction of lupine. The comprehensive evaluation of soil, foliage, and growth data revealed a key relevance of the N and P nutrition of the stands for their growth, and a change from initial N limitation to a limitation of other growth factors (P, Mg, Ca, and water).     

Effect of wood ash fertilization on soil chemical properties and stand nutrient status and growth of some coniferous stands in Finland

The effects of wood ash or wood ash plus nitrogen (N) fertilization on soil chemical properties, needle nutrient concentrations and tree growth were studied in five coniferous stands, aged 31–75 yrs, after 5 and 10 yrs. In each experiment 3 t ha^?1 of loose wood ash was applied to three replicated plots (30×30 m). In three of the experiments 120–150 kg N ha^?1 was applied together with the same wood ash (WAN). These three experiments also included a stand-specific fertilization (SSF) treatment, which consisted of 120, 150 or 180 kg N ha^?1. Five years after wood ash or WAN application the pH increase in the humus layer was 1–1.7 pH-units and in the 0–5 cm mineral soil layer 0.3–0.4 pH-units. The increase was approximately the same 10 yrs after application, and was also associated with an increase in pH in the 5–10 cm mineral soil layer. Wood ash or WAN significantly increased both the total and extractable calcium and magnesium concentrations in the humus layer on all the sites. Wood ash or WAN had an increasing effect on the boron concentrations, but a decreasing effect on the manganese concentrations in the needles. Wood ash had no significant effect on the volume growth. The trees on the WAN plots grew as well as or slightly better than those on the SSF plots.     

Recovery of N-urea 10 years after application to a Douglas-fir pole stand in coastal British Columbia

The long-term fate of fertilizer N in forest ecosystems is poorly understood even though such information is critical for designing better forest fertilization practices. We studied the distribution and recovery of ¹⁵N (4.934 atom% excess)-labelled fertilizer (applied as urea at 200 kg N ha⁻¹) 10 years after application to a 38–39-year-old Douglas-fir (Pseudotsuga menzeisii (Mirb.) Franco) stand in coastal British Columbia. The urea was applied in the spring (May 1982) or fall (November 1982). Sampling was conducted in October 1992, and we found that after 10 years, there were few differences between the fall and spring fertilizer applications in total N and ¹⁵N distribution within the tree and forest ecosystem. On average total fertilizer-N recovery was 59.4%; about 14.5% of the applied-N was recovered in the trees including coarse roots, with foliage containing 41% of the labelled-N recovered in the aboveground tree biomass. Tissue ¹⁵N remained mobile and could be transferred to new growth. Soil recovery was 39.8%, which had decreased from 57.0% at a previous 1-year sampling, with an average loss of 3.0% per year from the mineral soil and 3.7% from the litter layers. However, it appears that there was little continuing tree uptake. While short-term effects of fall vs. spring urea application were previously reported, there were no long-term effects on either stand productivity or fertilizer use efficiency, suggesting that if fertilization is properly done, timing of fertilization is not a critical issue in terms of maximizing fertilizer use efficiency for the coastal Douglas-fir forest we studied. Our results also highlight the high capacity of this ecosystem to retain externally applied inorganic N over the long-term, the importance of maximizing nitrogen uptake in the first year, and also of the continuing need to develop new approaches to overcome the generally low efficiency of forest N fertilization.     

Nitrogen cycling in Pinus sylvestris stands exposed to different nitrogen inputs

The objective of this study was to quantify the effects of high nitrogen (N) inputs on N cycling in a 35–45-yr-old Scots pine (Pinus sylvestris L.) forest. Nitrogen was added annually (single doses) as NH₄NO₃ in doses of 0 (N0), 30 (N1) and 90 (N2) kg N ha^?1 yr^?1. The only N input to the N0 plots was atmospheric deposition of 10 kg N ha^?1 yr^?1. The N cycle in these plots was tight, with almost complete retention of the incoming N. In the N1 plots the N retention was 83% after 9 yrs of N addition. The trees were the major sink, but the soil also contributed to the N retention. In the N2 plots the N retention was 63%, being mainly accounted for by accumulation in the soil. The leaching of N from the N2 stands was as high as 35 kg N ha^?1 yr^?1. The N2 system was N saturated.     

Radial growth of Norway spruce and Scots pine: effects of nitrogen deposition experiments

The growth patterns of annually resolved tree rings are good indicators of local environmental changes, making dendrochronology a valuable tool in air pollution research. In the present study, tree-ring analysis was used to assess the effects of 16 years (1991–2007) of chronic nitrogen (N) deposition, and 10 years (1991–2001) of reduced nitrogen input, on the radial growth of Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) growing in the experimental area of Lake Gårdsjön, southwest Sweden. In addition to the ambient input of c. 15 kg N ha^?1 year^?1, dissolved NH₄NO₃ was experimentally added to a 0.52-ha watershed at a rate of c. 40 kg ha^?1 year^?1. Atmospheric N depositions were reduced by means of a below-canopy plastic roof, which covered a 0.63-ha catchment adjacent to the fertilized site. The paired design of the experiment allowed tree growth in the N-treated sites to be compared with the growth at a reference plot receiving ambient N deposition. Nitrogen fertilization had a negative impact on pine growth, while no changes were observed in spruce. Similarly, the reduction in N and other acidifying compounds resulted in a tendency towards improved radial growth of pine, but it did not significantly affect the spruce growth. These results suggest that spruce is less susceptible to changes in the acidification and N status of the forest ecosystem than pine, at least in the Gårdsjön area.     

Phosphorus form and bioavailability in the pine rotation following fertilization: P fertilization influences P form and potential bioavailability to pine in the subsequent rotation

Soils in some parts of the world are known to be severely phosphorus (P) deficient. As little as 20 kg P ha⁻¹ have produced large growth responses in forest stands. If increased tree growth and augmented P cycling are caused by a one-time P application, a valid question is whether the effect of the initial fertilization will be evident in the regenerating forest, thereby reducing the need for P fertilization early in the second rotation. The objective of this study was to evaluate the P status of the forest floor and inorganic P status of mineral surface soil of two long-term fertilizer trials in order to determine if there were increases in soil P amount and bioavailability that benefits the next rotation. Long-term fertilizer trials in Georgia, USA and New Zealand (29 and 22 years after fertilization) were the study sites. In Georgia, forest fertilization increased the mass of the F horizon by 239% and its P content by 318%. Anion exchange membranes measured up to a 47% increase in bioavailable P in the mineral soil in a fertilized treatment. While bioassays did not show significant residual P in the mineral soil under the fertilized plots, soil from an adjacent operationally fertilized stand, which had the forest floor bedded into its planting rows, showed a 100% increase in seedling P content. At the New Zealand site, the mineral soil contained 142% more P in the anion exchange membrane form than in the unfertilized soils. Bioassays indicated a 224% increase in seedling P content when grown in fertilized soil at the highest fertilization rate. Operational levels of fertilization did not yield a significant bioassay result. The data show that there can be a residual effect of fertilizer P in the rotation following fertilization. The bioassays, anion membrane exchangeable P and forest floor P from Georgia suggest that there is enough P in the soil system to support the needs of a regenerating stand of loblolly pine, while residual bioavailable P in the mineral soil from operational levels of fertilization at New Zealand site would require residual P in the forest floor or immediate fertilization to meet the demands of newly planted seedlings. Conservation of the forest floor is an important part of P management in these P deficient sites.     

Modelling the effects of forest management intensification on base cation concentrations in soil water and on tree growth in spruce forests in Sweden

The study investigated the effects of forest residue extraction on tree growth and base cations concentrations in soil water under different climatic conditions in Sweden. For this purpose, the dynamic model ForSAFE was used to compare the effects of whole-tree harvesting and stem harvesting on tree biomass and the soil solution over time at 6 different forest sites. The study confirmed the results from experimental sites showing a temporary reduction of base cation concentration in the soil solution for a period of 20–30 years after whole-tree harvesting. The model showed that this was mainly caused by the reduced inputs of organic material after residue extraction and thereby reduced nutrient mineralisation in the soil. The model results also showed that whole-tree harvesting can affect tree growth at nitrogen-poor forest sites, such as the ones in northern Sweden, due to the decrease of nitrogen availability after residue removal. Possible ways of reducing this impact could be to compensate the losses with fertilisation or extract residue without foliage in areas of Sweden with low nitrogen deposition. The study highlighted the need to better understand the medium- and long-term effects of whole-tree harvesting on tree growth, since the results suggested that reduced tree growth after whole-tree harvesting could be only temporary. However, these results do not account for prolonged extraction of forest residues that could progressively deplete nutrient pools and lead to permanent effects on tree growth.

     

Results of long-term K and Mg fertilizer experiments in afforestation

Results from long-term fertilizer experiments on Scots pine (Pinus sylvestris) planted on old arable land in the north-eastern German lowlands demonstrate the necessity for fertilization with KMg on poorly buffered sandy soils.Applying KMg to young Scots pine increased pole production over a 60 year period by a total of 129 m³ ha⁻¹ (26%). In other experiments the improvement was as much as 93%.KMg manuring on light soils ensures optimal K and Mg nutrition and creates the conditions for uninterrupted growth and healthy development of the plantation. On degraded soils, KMg treatment leads to improved utilisation of the low soil nitrogen supply.With increasing eutrophication due to atmospheric nitrogen, KMg fertilization overcomes the depletion of soil cations and resulting imbalance in tree metabolism.Adequate application of KMg is a prerequisite for success in underplanting with beech (Fagus silvatica) on soils with a land use classification below 30, increasing the growth in height of beech by about 30%.KMg fertilizer favours biomass production and thus the fixation of N deposited through the atmosphere by the vegetation. Thereby, KMg fertilizer use and other silvicultural measures reduce the negative effects of atmospheric N-input in forest ecosystems to a certain extent, especially the N-pollution of groundwater. However, they cannot replace the technologies to reduce the N emissions.     

Long-term nutritional trends of conifer stands in Europe: results from the RECOGNITION project

Since 1999–2002 the European Commission has funded and the European Forest Institute has coordinated the interdisciplinary RECOGNITION project aiming at elucidating the causes of the growth acceleration which has been observed in some forest tree species in several parts of Europe. Within this project, it was our task to identify and quantify long-term changes in the nutritional status of representative forest stands that potentially could explain this growth increase, using available long-term series of foliar analyses. An inquiry among 25 forest research institutions in Europe resulted in 28 Scots pine (Pinus sylvestris L.) and 21 Norway spruce (Picea abies [L.] Karst.) stands for this historical development investigation (HDI). The stands generally are control plots of fertilization experiments and are located mainly in Central Europe and in Scandinavia. The monitoring periods vary from 15–40 years. The foliar data were given to us by our partner organisations, subjected to rigorous tests for plausibility and comprehensively evaluated using mainly single linear regression approaches. Most Scots pine stands under study in Central Europe, which grow predominantly on naturally poor or devastated soils in regions with relatively high atmospheric N deposition, suffered from N deficiency at the start of the monitoring period, but exhibited a considerable improvement in N nutrition over the past decades. The increase in N levels was usually associated with an increase in the ratios of N and P, K, Ca, and Mg, although critical values of these ratios are far from being reached. For the majority of the investigated Scots pine stands in Central Europe, growth acceleration due to a better N supply is highly probable. In contrast, N seems to be far less important as a potential driving factor for long-term growth changes in Scots pine in Scandinavia. Norway spruce stands examined in Central Europe were, in general, initially well supplied with N and characterised by a decreasing trend in foliar N levels although they received considerable N deposition from the atmosphere. This decrease in concentrations of N and other macronutrients is believed to be mainly a dilution effect as indicated by a simultaneous increase in needle weights. Spruce in Finland also turned out to be adequately provided with N at most sample sites, and no general trend of improving N nutrition was detected.     

Nitrogen budgets for Scots pine and Norway spruce ecosystems 12 and 7 years after the end of long-term fertilisation

The magnitude of nitrogen storage and its temporal change in forest ecosystems are important when analysing global change. For example, the accelerated growth of European forests has been linked to increased nitrogen deposition, but the changes in the N inputs that cause long-term changes in ecosystems have not yet been identified. We used two Swedish forest optimum nutrition experiments with Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) to study the long-term fate of N applied to these forest ecosystems. In the pine experiment, in addition to fertiliser (NPK) application, soil acidity was manipulated by application of lime and dilute sulphuric acid. From the spruce experiment, we selected treatments with similar fertiliser doses as in the pine experiment and with and without lime addition.We quantified various terms in the N budget 12 years (pine) and 7 years (spruce) after the last N addition. In the pine stand the NPK-treatment was the only treatment to produce a significant increase in N in the tree biomass (97% above control), whereas in the spruce stand the N additions increased tree N in all treatment combinations (207% above control). In the pine stand the relative distribution of nitrogen between trees and soil did not vary across treatments, with trees containing around 12% of ecosystem N and humus containing around 44% of soil N. The increases in N stocks in the pine stands were mainly in the soil. In contrast, in the spruce ecosystem trees accumulated most of the added N and the increase in the soil was restricted to the humus layer.In the pine ecosystem, large losses of added N (between 254 and 738 kg ha⁻¹ out of 1040 kg ha⁻¹ added as fertiliser) occurred, whereas in the spruce ecosystem we recovered more N than could be accounted for by inputs (between 250 and 591 kg ha⁻¹). There was no clear pattern in the interaction between acidification/liming and N additions.     

Low- to moderate-severity historical fires promoted high tree growth in a boreal Scots pine forest of Norway

Abstract

Fire is the most important ecological factor governing boreal forest stand dynamics. In low- to moderate-severity fires, the post-fire growth of the surviving trees varies according to fire frequency, intensity and site factors. Little is known about the growth responses of Scots pine (Pinus sylvestris L.) following fires in boreal forests. We quantified changes in tree growth in the years following 61 historical forest fires (between 1210 and 1866) in tree-ring series collected from fire-scarred Scots pine trees, snags and stumps in Trillemarka nature reserve in south-central Norway. Basal area increment 10 years pre-, 5 years post-, and 11–20 years post-fire were calculated for 439 fire scars in 225 wood samples. We found a slight temporary growth reduction 5 years post-fire followed by a marked growth increase 11–20 years post-fire. Beyond 20 years post-fire, the long-term tree growth declined steadily up to approximately 120 years. Our results indicate that recurring fires maintained high tree growth in remnant Scots pines, most probably due to a reduction in tree density and thus decreased competition.     

Deciduous trees affect small-scale floristic diversity and tree regeneration in conifer forests

Abstract

There is a growing interest in the effects of deciduous trees on biodiversity, soil processes and long-term productivity in boreal, conifer-dominated forests. This study investigated whether individual birch trees allowed to grow to maturity in the coniferous forest can have a local effect on floristic richness and regeneration of tree saplings. The ground vegetation was compared in 2?m radius plots around the stem under the canopies of matched conifer–deciduous trees in a mature, conifer-dominated forest, and included in the analysis variables that could potentially mediate the tree effect (soil pH, cover of lichens, bryophytes, leaf and needle litter). The field layer vegetation was more species rich under birch (Betula pendula and B. pubescens) than under conifers (Picea abies and Pinus sylvestris), and several vascular plant species (including saplings of tree species) occurred more often under birch than under conifers. However, when the effect of the number of subordinate trees was taken into account the difference between birch and pine was not significant. The number of tree regenerations (saplings) was lowest under pines, but did not differ between spruce and birch. There were no effects of the canopy species on soil pH or on cover of lichens and bryophytes. The difference in diversity may be caused by the different effects of leaf and needle litter, and it is also likely that canopy structure has an influence via interception and throughfall and by affecting the light and microclimate.     

Fertilization of Danish Forests: A Review of Experiments

Fertilization experiments in Norway spruce stands on nutrient-poor sites have been practised in Denmark since the eighteenth century. Until 1950, the main aim was to find a nitrogen (N) source that improved the survival of plants in the early growth phase. The N supply could be improved by intercropping with N 2 -fixing plants, and fertilization with phosphorus (P) and potassium (K) improved the growth of the N 2 fixers, thereby improving tree growth and survival. Positive responses of P and K in spruce cultures were seen in the 1960s, and in most cases the N response was dependent on the supply of P and K. Fertilization experiments in mature spruce stands began in the 1960s. Despite 20 yrs of experimentation, no trends can be deduced. The results discouraged fertilization aimed at increasing growth in mature stands, and fertilization stopped in 1989. After 1980, fertilizers were used in experiments aimed at clarifying the causes and effects of forest decline. Fertilization and liming counteracted soil acidification and increased needle nutrient concentrations when they were below deficiency levels. However, positive growth responses have only been recorded in some of the experiments where nutrient deficiencies were overcome. The experiments have not resulted in practical revitalization programmes. Future fertilization of Danish forests may be limited to systems where nutrient extraction is in excess of the carrying capacity of the ecosystem, and presumably where spruce plantations are converted to broadleaved forests. Finally, future events of deposition of acidifying compounds may necessitate counteractive measures.     

Variation in Stemwood Nutrient Concentrations in Scots Pine Growing on Peatland

Radial and longitudinal variation in the nutrient concentrations of Scots pine (Pinus sylvestris L.) stemwood and the effect of fertilization (PK and NPK) were studied on three peatland sites in eastern Finland. The results showed distinctive patterns in longitudinal and radial nutrient concentrations in stemwood. The N, P and K concentrations were highest in the youngest tissues both in the radial and longitudinal directions independent of the site type or fertilization. Ca and Mn accumulated in the older parts of stemwood. Increased N, P, K and Ca concentrations were found 9 yrs after fertilization in the annual rings formed during the first-5-yr period after fertilization. Decreased Mn concentrations were found in the annual rings formed during the 5-and-9-yr periods after fertilization. The results indicate that in the intensive utilization of Scots pine stemwood, e.g. whole stem harvesting instead of stem harvesting to a fixed diameter (7-8 cm in Finland), nutrient loss increases proportionally more than the amount of harvested biomass. The results also imply that this wood contains more undesirable elements, such as K, which may cause problems in wood combustion in power plants, and that fertilization raises these concentrations in wood. However, the concentrations in unfertilized trees were generally at about the same level as in Scots pine stemwood grown on infertile mineral soil sites.     

Inorganic and organic soil phosphorus and sulfur pools in an Amazonian multistrata agroforestry system

In the central Amazon basin, the effects of secondary vegetation and primary forest on inorganic and organic P and S pools were compared with those of different fruit and timber tree species in a multistrata agroforestry system. The soils (Xanthic Ferralsols) were low in readily available P and S. Fertilizer applications increased the less accessible nutrient pools more than the plant available pools. For example, dilute-acid extractable P increased substantially (from 2 to 76 mg P kg⁻¹), whereas Mehlich P (plant available) increased less (from 3 to 19 mg P kg⁻¹). In contrast, the recalcitrant soil P pools, such as the residual P, did not increase on the short term, but only after more than six years following application. The proportion of less available ester-sulfate S was significantly higher in fertilized sites than in unfertilized sites, in contrast to soluble inorganic sulfate S or carbon-bonded S. The marked increase of successively available soil P and S pools through fertilization was advantageous with respect to the long-term effect of nutrient applications. Soil nutrient availability was not only related to the amount of nutrients applied but was also influenced by tree species. Nutrient return by litterfall and litter quality played an important role in soil P and S dynamics. Incorporation of applied nutrients into successively available organic nutrient pools will decrease potential P fixation and S losses by leaching and increase long-term nutrient availability. Therefore, tree species with rapid above-ground nutrient cycling and high quality litter (such as annato [Bixa orellana] and peach palm [Bactris gasipaes]) should constitute the majority of crops in multistrata agroforestry systems on infertile soils to ensure adequate medium to long term availability of P and S. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of chronic nitrogen amendment on dissolved organic matter and inorganic nitrogen in soil solution

Increased atmospheric deposition of N to forests is an issue of global concern, with largely undocumented long-term effects on soil solution chemistry. In contrast to bulk soil properties, which are typically slow to respond to a chronic stress, soil solution chemistry may provide an early indication of the long-term changes in soils associated with a chronic stress. At the Harvard Forest, soil solution was collected beneath the forest floor in zero tension lysimeters for 10 years (1993–2002) as part of an N saturation experiment. The experiment was begun in 1988 with 5 or 15 g N m⁻² per year added to hardwood and pine forest plots, and our samples thus characterize the long-term response to N fertilization. Samples were routinely analyzed for inorganic nitrogen, dissolved organic nitrogen (DON), and dissolved organic carbon (DOC); selected samples were also analyzed to determine qualitative changes in the composition of dissolved organic matter. Fluxes of DOC, DON, and inorganic N were calculated based on modeled water loss from the forest floor and observed concentrations in lysimeter samples. The concentration and flux of inorganic N lost from the forest floor in percolating soil solution are strongly affected by N fertilization and have not shown any consistent trends over time. On average, inorganic N fluxes have reached or exceeded the level of fertilizer application in most plots. Concentrations of DOC were unchanged by N fertilization in both the hardwood and pine stands, with long-term seasonal averages ranging from 31–57 mg l⁻¹ (hardwood) and 36–93 mg l⁻¹ (pine). Annual fluxes of DOC ranged from 30–50 g m⁻² per year. DON concentrations more than doubled, resulting in a shift toward N-rich organic matter in soil solution percolating from the plots, and DON fluxes of 1–3 g m⁻² per year. The DOC:DON ratio of soil solution under high N application (10–20) was about half that of controls. The organic chemistry of soil solution undergoes large qualitative changes in response to N addition. With N saturation, there is proportionally more hydrophilic material in the total DON pool, and a lower C:N ratio in the hydrophobic fraction of the total DOM pool. Overall, our data show that fundamental changes in the chemistry of forest floor solution have occurred in response to N fertilization prior to initiation of our sampling. During the decade of this study (years 5–14 of N application) both inorganic N and dissolved organic matter concentrations have changed little despite the significant biotic changes that have accompanied N saturation.     

Indications that site preparation increases forest ecosystem carbon stocks in the long term

Mechanical site preparation (MSP) causes a mixing disturbance of the soil, which may increase decomposition of soil organic matter and subsequent carbon (C) dioxide emissions to the atmosphere. MSP also promotes the establishment and growth of tree seedlings, and hence ecosystem C fixation. However, there are uncertainties regarding the net effects of MSP on C stocks at the ecosystem scale. To assess decennial effects of MSP on ecosystem C stocks, C stocks in soil, ground vegetation and trees at three experimental forest sites with Pinus contorta, Pinus sylvestris and Picea abies in Sweden were sampled and measured for ca. 25 years in a control and after three MSP treatments: disc trenching, mounding and ploughing. After 25 years, all of the MSP treatments resulted in larger ecosystem C stocks than the control treatment due to positive effects on the tree biomass C stock. The tree C stock was highest after ploughing, intermediate after mounding or disc trenching and lowest in untreated control plots at all experimental sites. The MSP treatments did not affect the soil C stocks down to 30?cm. We recommend mounding or disc trenching to promote C sequestration as they disturb sites’ ecological, aesthetic and recreational values less than ploughing.     

Effects of environmental changes on the vitality of forest stands

Using the physiological single tree growth model BALANCE, vitality of forest stands was simulated in dependence of the site-related factors, climate and stand structure. At six level II plots in southern Germany with the main tree species beech (Fagus sylvatica L.), oak (Quercus robur L.), spruce (Picea abies [L.] Karst.), and pine (Pinus sylvestris L.), simulated results were compared to measured values (soil water content, bud burst and leaf colouring, diameter at breast height, tree height and crown density) in order to validate the model. Sensitivity tests were done to examine the influence and the interactions of the environmental parameters. The validation results show that BALANCE is capable of realistically simulating the growth and vitality of forest stands for central European regions for medium term time spans (several years). The validation of the water balance module produces mean absolute errors based on field capacity between 2.7 and 6.9% in dependence of sites and forest stands. Senescence of foliage as well as crown density is reproduced with a correlation coefficient of 0.70 compared to measurements. Differences between measured and simulated diameter values were smaller than 1% for spruce and smaller than 6.5% for beech after 7 years of simulation, and smaller than 1% for oak after 8 years of simulation. On the other hand, the simulations for pine trees conform less with the measurements (difference: 22.6% after 8 years). The sensitivity of the model on environmental changes and on combinations of these parameters could be demonstrated. The responses of the forest stands were quite different.