Pure Norway Spruce Stands Research Articles

The Impact of Different Thinning Regimes on the Growth Dynamics of Pure Norway Spruce Stands in Lithuania

Forest growth under changing environmental conditions has been a major concern worldwide. The estimations of forests’ growth trends may provide significant insight on changing the value of Norway spruce (Picea abies (L.) H. Karst.) forests. The purpose of this study was to estimate the growth dynamics of thinned and un-thinned spruce stands in Lithuania by assessing their standing volume, gross volume yield, and stand density. For this purpose, two pure Norway spruce sites, established in 1990 and 1992, were selected. The thinning treatment scheme for each trial, comprised of five distinct stand density variants and eight different yield parameters, were analysed for each plot. Control plots exhibited the smallest growth in diameter, whereas stands that had been thinned once at a young age displayed the largest diameter increment (peak—1.4 cm/year). An increasing trend in tree stem volume was registered in all thinning treatment plots. The greatest competition index was found in the control plots, indicating an exceptionally high competition (peak—4.6–5.1) between trees for growth space. The competition index was four times smaller (2.0) in the moderately thinned stand. Intensifying natural mortality rate and signs of growth stabilization in the control plots (un-thinned) cannot guarantee them the positions of the highest productivity and accumulated stand volume in the future compared with thinned spruce stands.

The Mechanical Stability of Pure Norway Spruce Stands along an Altitudinal Gradient in the Czech Republic

Norway spruce stands are established and managed along various site conditions in central Europe. Currently, spruce often grows at locations outside of its ecological optimum, resulting in extensive damage elicited by harmful abiotic and biotic factors, which relatively shortens the time to change this adverse status in the adaptation frame by foresters. Except for the rapid change in species composition through clear-cuts, another way is possible, i.e., stabilising current (especially young) spruce stands to extend the time required to implement adaptation measures. The assumption that different site conditions will have to be respected as part of this adaptation was confirmed by our study based on NFI data of the Czech Republic. A semiparametric generalized linear model (GAM) was used to model the relationship between the height-to-diameter ratio and forest stand age, differentially considering particular forest vegetation zones. Spruce stands with lower elevations attain a lower stability (expressed by their height-to-diameter ratio; HDR) than those in the mountains. The HDR culminated in lower and middle altitudes in the first half of the rotation period, representing the most critical timing and effectivity of silvicultural measures. Contrary to previous findings, we found higher HDR values at nutrient-rich sites than those at acid ones, especially up to 50–60 years old. Therefore, more research should be devoted to the issue concerning the same thinning regime under different site conditions.

Effects of Norway spruce shelter density and soil scarification method on mortality and height growth of underplanted European beech, European silver fir and Douglas fir

AbstractIn Europe, 6–7 million ha even-aged, pure stands of Norway spruce grown outside its natural range are prone to butt rot, windthrow, and bark beetle infestation. Climate change may well accelerate these problems and there is an urgent need to develop methods for converting Norway spruce plantations into more heterogeneous forest with larger resistance and resilience towards natural disturbances. Based on a field experiment located on former heathland on an outwash plain in western Denmark, we examined the mortality and height growth of European beech (Fagus sylvatica L.), European silver fir (Abies alba Mill.), and Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) planted under a shelterwood of 17–19 m tall Norway spruce (Picea abies (L.) Karst.) at shelter densities initially ranging by basal area from 10 to 36 m2 ha−1 (271–1099 stems ha−1). Underplanting included plots of beech (100 per cent) and silver fir (100 per cent) as well as plots with two-species mixtures of silver fir/beech (66 per cent/33 per cent and 33 per cent/66 per cent) and Douglas fir/beech (66 per cent/33 per cent). The experiment also included two soil scarification methods (patch and strip scarification). The experiment was laid out in an unbalanced factorial split-plot design, replicated in time and space, and totaling 144 plots in four blocks. The experiment was followed for 19 years during which a gradual reduction in shelter density took place although in some plots the shelterwood was removed abruptly by windthrow and bark beetle infestation. Shelter densities larger than 10 m2 ha−1 had only a minor effect on the mortality of beech and silver fir, while shelter densities larger than 20 m2 ha−1 resulted in increased mortality of Douglas fir. Subsequent abrupt reduction in shelter density resulted in increased mortality, especially of silver fir. Height growth decreased with increasing shelter density, more for Douglas fir than for beech, and marginally more for beech than for silver fir. For shelter densities <15 m2 ha−1, height growth of silver fir was unaffected. Beech grew faster when mixed with Douglas fir. The soil scarification method did not influence mortality, but strip scarification resulted in marginally larger height growth than patch scarification. We hypothesize that the shelter effect was related mainly to competition for light, while late frost and browsing were important factors in open shelters (<10 m2 ha−1). Based on these results, recommended initial shelter densities are at or <10 m2 ha−1 for the establishment of Douglas fir, at ~15–20 m2 ha−1 for silver fir, and around 15 m2 ha−1 for beech.

Pre-Commercial Thinning Increases the Profitability of Norway Spruce Monoculture and Supports Norway Spruce–Birch Mixture over Full Rotations

Pre-commercial thinning (PCT) is a common measure in Norway spruce (Picea abies L. Karst.) stands but the profitability of doing PCT and timing of PCT has not been fully investigated over a full rotation. Further, limited knowledge is available for mixed forest management compared to monocultures. In this study, different PCT strategies were tested to investigate the effect of PCT and timing of PCT on the production and profitability of Norway spruce monocultures and mixed Norway spruce–birch stands. A forest decision support system was used to simulate stand development during the whole rotation. Our study findings show that there is a positive effect of PCT on Norway spruce plantations’ long-term profitability but that the timing of PCT has little effect on profitability. However, site variation might influence the effect of PCT timing on the profitability of Norway spruce stands. Moreover, we also found that retaining 1000 Norway spruce ha−1 and 1000 birch ha−1 after PCT supports a mixture of Norway spruce and birch over a full rotation, with little or no economic loss compared to pure Norway spruce stands. Therefore, such a mixture can lead to profitable production while still providing other ecosystem services.

Differential Effects of Tree Species on Soil Microbiota 45 Years after Afforestation of Former Pastures

Several decades ago, many former pastures in Central Europe were afforested or colonized by trees after being abandoned. Knowledge of the effects of tree species on soil properties is important for planning of the composition of future forests. In this regard, a research location in Vrchdobroč (Central Slovakia), which is former agricultural land used as pasture, enables the exploration of ecosystem processes and properties in stands of different tree species after afforestation. The goal of our study was to find out whether changes in soil properties, including soil microbial activity and diversity among different stands, were already observable 45 years after the afforestation, and how the effects differed among stands of different tree species. The study was conducted in the pure stands of Norway spruce (Picea abies L. Karst.), Douglas fir (Pseudotsuga menziesi (Mirb.) Franco), European beech (Fagus sylvatica L.) and sycamore maple (Acer pseudoplatanus L.). Multivariate analyses of physico-chemical soil properties indicated an overlap between the soils under the Douglas fir and the spruce, but a clear separation of beech from sycamore. In general, both microbial activity and diversity were, surprisingly, highest under the Douglas fir, followed by the sycamore, with the beech and the spruce showing mostly lower values.

Heterotrophic and rhizospheric respiration in coniferous forest soils along a latitudinal gradient

Northern forest soils are a major carbon (C) reservoir of global importance. To estimate how the C balance in these soils will change, the roles of tree roots and soil microbes in C balance should first be decoupled. This study determined how the activity of heterotrophs and tree roots together with root-associated microbes in the rhizosphere varies in coniferous forest soils in boreal, hemiboreal, and temperate climates along a latitudinal gradient using a trenching approach. We created experimental plots without living tree roots, measured soil respiration (CO2 efflux) from these and from unmanipulated plots using the chamber technique, and partitioned the efflux into root-rhizosphere (RR) and heterotrophic (RH) respiration. The share of RR in ecosystem gross primary production (GPP) decreased from north to south in the Scots pine (Pinus sylvestris L.) and the Norway spruce (Picea abies (L.) Karst.) forests, with the exception of a mixed site, where the share of RR in GPP varied strongly between the years. RR per ground area and per root biomass were mainly independent of climate within the gradient. RH per ground area increased from north to south with temperature, while RH per soil C did not change with temperature. Soil moisture did not significantly affect the respiration components in the northernmost site, whereas soil moisture was positively connected with RH and negatively with RR in other Scots pine sites and positively connected with RR in pure Norway spruce stands. The dynamic ecosystem model LPJ-GUESS was able to capture the seasonal dynamics of RH and RR at the sites, but overall accuracy varied markedly between the sites, as the model underestimated RH in the southern site and RR elsewhere. Our study provides knowledge about the nature of soil respiration components. The valuable insights can be used in more accurate land-ecosystem modelling of forest ecosystems.

Assessing transformation scenarios from pure Norway spruce to mixed uneven-aged forests in mountain areas

Mixed mountain forests, primarily made up of Norway spruce (Picea abies (L.) Karst.), silver fir (Abies alba Mill.) and European beech (Fagus sylvatica L.), cover about 10 × 106 ha of submontane–subalpine altitudes in Europe. They provide invaluable ecosystem services, e.g. protection against avalanches, landslides or rockfall. However, pure Norway spruce stands have, since mediaeval times, been heavily promoted as productive stand types for salt works at sites naturally supporting mixed mountain forests. Damage to these secondary pure spruce stands has been steadily increasing in recent decades. Furthermore, due to their previous limitation due to low temperatures and a short growing season, forest ecosystems in higher elevations are expected to be strongly affected by climate warming. To address these problems, alternative management concepts are being intensively discussed. A possible option to improve the stability and resilience of the stand is the transformation from pure Norway spruce stands into site-appropriate, sustainable and stable mixed mountain forests. In this study, we have tested seven different transformation scenarios (e.g. slit, shelterwood and gap-coupes, strip clear-cutting, do-nothing) and their impact on five evaluation criteria (forest growth, economics, carbon sequestration, (stand) stability and biodiversity). As there are hardly any practical examples for some of the transformation scenarios available, we have used the forest growth simulator SILVA to assess whether the tested transformation scenarios differ in transformation success and to observe trade-offs between the criteria of evaluation. Of the investigated scenarios, we consider the ones with gap or slit-coupes with the most beneficial overall utility values for the portfolio of the five evaluation criteria. However, we showed with our results that it is possible, by means of several trajectories, to return destabilised forests to sustainable and stable systems. We showed that a transformation is realistic, even if sophisticated silvicultural concepts are not strictly pursued.

THE INTENSIVE MANAGEMENT OF NORWAY SPRUCE: A COMPROMISE BETWEEN FINANCIAL GAIN AND GENETIC DIVERSITY?

INTRODUCTION
 Norway spruce (Picea abies (L.) Karst.) is a high-yielding commercial tree species grown in the Baltic sea region. It not only ensures timber production, but also provides a notable amount of crown branch biomass and a substantial amount of technically accessible stump biomass for energy. Thus, it has a high potential as a source of renewable materials and energy in the bioeconomy. Recent studies in Latvia have shown no long-term negative consequences to forest ecosystems from whole-tree harvesting. Hence, this management method can be considered sustainable for Norway spruce stands in fertile mineral soils. Not only yield, but also risks need to be considered to ensure financial sustainability, mainly the impact of wind-storms, drought, and pests. A combination of silviculture and genetics (tree breeding) can be used to reduce the probability of damage to Norway spruce stands. The aim of our study was to assess the potential of simultaneously ensuring both genetic gain and diversity in Norway spruce plantations.
 MATERIALS AND METHODS
 Data characterizing/showing tree growth – current height, diameter at breast height, survival, as well as radial increment (increment cores) – were obtained from a 50-year-old Norway spruce plantation. Data characterizing genetic diversity were collected from a gene reserve stand (48 trees), Norway spruce seed orchard progenies consisting of 20 clones, as well as 12 pure Norway spruce stands. DNA was extracted and analysed with 6 to 14 nuclear SSR markers.
 RESULTS
 No significant differences were observed between the seed orchard progenies, the trees from the gene reserve stand and other Norway spruce stands using the assessed parameters – allelic richness, observed heterozygosity, genetic diversity and relatedness. This indicates that the use of a seed orchard containing a relatively low number of clones as a seed source for plant production and forest regeneration would not have a negative impact on genetic diversity. However, notable gains in productivity can be achieved using selected plant material. At the age of 50 years, phenotypically selected clones in the low-density (5×5m) plantation had a mean yield of 327±42 m3ha-1, significantly exceeding the mean yield (277±56 m3ha-1) observed in Norway spruce stands of the same age and the same site conditions (forest type), while no significant differences were observed compared to the average stand yield at the age of 80 years (347±47 m3ha-1). The target diameter of 31 cm was reached at the age of 42±0.9 years on average, but this varied significantly among clones.
 CONCLUSION
 The results demonstrate a notable potential to reduce the rotation period, thereby: a) increasing the availability of raw material for further processing and energy production, and b) reducing financial risks due to lowering the probability that the stand will sustain substantial damage (i.e. wind storm). This gain can be achieved without significantly compromising genetic diversity. Further studies shall address potential changes in genetic diversity at the landscape level over a longer period when using a very limited set of clones. Stands in nature reserves could serve as a basis for comparison in such studies.

Economics of harvesting boreal uneven-aged mixed-species forests

The subject of this study is the economics of harvesting boreal uneven-aged mixed-species forests consisting of Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), birch (Betula pendula Roth and B. pubescens Ehrh.), and other broadleaves. The analysis is based on an economic description of uneven-aged forestry, applying a size-structured model. The optimization problem is solved in its general dynamic form using gradient-based interior point methods. When volume yield is maximized, the optimal steady state is a nearly pure Norway spruce stand at all site types, producing slightly higher yields than single-species stands. After including sawlog and pulpwood prices, the net present value of stumpage revenues is maximized using 1%, 3%, and 5% interest rates and a 15-year harvesting interval. At less productive sites, the stands are nearly pure Norway spruce stands, regardless of the interest rate. At more productive sites, increasing the interest rate increases the species diversity, with optimal steady states consisting of both Norway spruce and birch. In some cases, rather small changes in relative prices change the optimal steady state into a birch-dominated stand. Optimal solutions converge to the same steady-state solutions, independent of the initial stand state. If other broadleaves without commercial value are not harvested, they will eventually dominate the stand.

Climate effects on productivity and resource-use efficiency of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica [L.]) in stands with different spatial mixing patterns

Species mixing is widely held to stabilize productivity, increase resilience and contribute to risk minimization in forest stands in need of special as a result of longevity. However, research on the effects of mixing on productivity and resource consumption so far yielded fairly incoherent results rather than general findings. We focused on the effects of the spatial mixing pattern and the annual climate conditions on the mixing effect, which to date have seldom been considered as modifiers of mixing effects. Nine years of intensive survey of four pure stands of Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica [L.]) and two mixed plots with different mixing pattern showed: (1) mixing hardly changed annual net primary productivity at stand level when Norway spruce and European beech are cultivated group-wise but increased by 37 % on account of a higher efficiency of water and light use in individual tree-wise mixture. (2) Favourable climatic conditions increased the superiority of mixed versus pure stands productivity, while, in particular, water stress cancelled the benefit of mixing considerably. (3) An interaction between the spatial pattern and variable climatic conditions was revealed. Both improved light and water use were found in favourable years in close inter-specific intermingling. However, in unfavourable years the spatial pattern played a less pronounced role in terms of productivity.

Comparison of selected Nordic stand growth models for Norway spruce, Scots pine and birch

Abstract Thirteen Nordic stand growth models have been validated by use of a test data set from long-term research plots in Norway. The evaluated data was from time-series of even-aged, pure stands of Norway spruce, Scots pine and birch (silver birch and downy birch). In selected models from Finland, Norway and Sweden measures of site productivity, mean tree size and various stand characteristics are represented. Different models display both strengths and weaknesses in their predicting ability. Several measures of precision and bias have been calculated and the models are ranked due to their performance. We observed site quality, stand density and average tree size as the three main components in the models. Basal area increment model for spruce from Sweden had the lowest standard deviation with 23%. The mean R2 between residuals and stand characteristics from this model was also low (1.3%), which indicates that independent variables are well included. For Scots pine and birch, Finnish volume increment models showed the best fit to the Norwegian test data, with a R2 between residuals and stand characteristics of 2.8 and 6.7%, respectively. Several of the models from Sweden and Finland predicted the growth as well as stand models frequently in use in Norway. The results indicated that similar forest conditions and traditional even-aged forest management practice in the Nordic countries could be seen as a suitable basis for developing a joint family of growth models. By careful recalibration of existing models, a reasonable accuracy could be achieved and the prediction bias could be reduced.

Silver fir stand productivity is enhanced when mixed with Norway spruce: evidence based on large-scale inventory data and a generic modelling approach

Questions: How to evaluate the mixture effect on basal area increment in two-species forest stands? Is a mixed Norway spruce–silver fir stand more productive than pure adjacent stands of either species? How to develop generic modelling approaches to assess mixture effects in forest stands? Location: In addition to a case study on Norway spruce–silver fir stands in French mountain forests, the generic approach used goes beyond local applications. Methods: We took advantage of National Forest Inventory data to develop a unique stand basal-area-increment model for pure and mixed stands of Norway spruce and silver fir that responds to ecological site conditions. The database was made up of 284 pure Norway spruce stands, 196 pure silver fir stands, and 323 mixed stands of these species. Results: Pure silver fir basal area increment is strongly influenced by spring climatic conditions, whereas pure Norway spruce is more influenced by soil conditions. The mixture of these species has a positive effect on silver fir, which decreases as the proportion of fir increases. In contrast, the mixture has no noticeable effect on Norway spruce. Conclusion: We developed a stand basal-area-increment model evidencing an advantage of the mixture on silver fir basal area increment, but not on Norway spruce. The mathematical formulation of the model developed is generic and can be used in all two-species mixture situations. It also makes it possible to compare different mixture situations with each other.

Impacts of changing climate on the productivity of Norway spruce dominant stands with a mixture of Scots pine and birch in relation to water availability in southern and northern Finland

A process-based ecosystem model was used to assess the impacts of changing climate on net photosynthesis and total stem wood growth in relation to water availability in two unmanaged Norway spruce (Picea abies) dominant stands with a mixture of Scots pine (Pinus sylvestris) and birch (Betula sp.). The mixed stands were grown over a 100-year rotation (2000-99) in southern and northern Finland with initial species shares of 50, 25 and 25% for Norway spruce, Scots pine and birch, respectively. In addition, pure Norway spruce, Scots pine and birch stands were used as a comparison to identify whether species' response is different in mixed and pure stands. Soil type and moisture conditions (moderate drought) were expected to be the same at the beginning of the simulations irrespective of site location. Regardless of tree species, both annual net canopy photosynthesis (P(nc)) and total stem wood growth (V(s)) were, on average, lower on the southern site under the changing climate compared with the current climate (difference increasing toward the end of the rotation); the opposite was the case for the northern site. Regarding the stand water budget, evapotranspiration (E(T)) was higher under the changing climate regardless of site location. Transpiration and evaporation from the canopy affected water depletion the most. Norway spruce and birch accounted for most of the water depletion in mixed stands on both sites regardless of climatic condition. The annual soil water deficit (W(d)) was higher on the southern site under the changing climate. On the northern site, the situation was the opposite. According to our results, the growth of pure Norway spruce stands in southern Finland could be even lower than the growth of Norway spruce in mixed stands under the changing climate. The opposite was found for pure Scots pine and birch stands due to lower water depletion. This indicates that in the future the management should be properly adapted to climate change in order to sustain the productivity of mixed stands dominated by Norway spruce.

Effects of root trenching of overstorey Norway spruce (Picea abies) on growth and biomass of underplanted beech (Fagus sylvatica) and Douglas fir (Pseudotsuga menziesii) saplings

In Central Europe, the conversion of pure Norway spruce stands (Picea abies [L.] Karst.) into mixed stands with beech (Fagus silvatica L.) and other species like e.g. Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) is accomplished mainly by underplanting of seedlings beneath the canopy of overstorey spruce trees after partial cutting treatments what means exposure to shade and below-ground root competition by the overstorey to the seedlings. Particularly about the second factor, our knowledge is limited. Therefore, we carried out a below-ground competition exclusion experiment by root trenching and investigated the effects on soil resources, growth, and biomass partitioning of underplanted beech and Douglas fir saplings under target diameter and strip cutting treatments. The exclusion of overstorey root competition by trenching increased the soil water potential in the second year that had a fairly dry growing season and led to significantly higher foliar concentrations of most nutrients, particularly in Douglas fir, indicating an amended nutrient supply. Both improvements were accompanied by an increase in length and diameter increment of the underplanted saplings, appearing in both species only after having surpassed a species-specific threshold light value (Douglas fir 16% of above canopy radiation, beech 22%). We also found significant interactions between trenching and light for specific fine root length and further biomass and morphological parameters. Judged by the much steeper increase in height and diameter growth with increasing light after release from below-ground competition, Douglas fir saplings appeared to be more sensitive to root competition than beech saplings what conforms to older findings for beech. According to our results, a strip cutting seems to be more appropriate than a target diameter cutting treatment to replace a pure spruce stand by a mixed stand with beech and Douglas fir.

Detection and Classification of Bark Beetle Infestation in Pure Norway Spruce Stands with Multi-temporal RapidEye Imagery and Data Mining Techniques

Detection and Classification of Bark Beetle Infestation in Pure Norway Spruce Stands with Multi-temporal RapidEye Imagery and Data Mining Techniques

Evaluating the Suitability of Management Strategies of Pure Norway Spruce Forests in the Black Forest Area of Southwest Germany for Adaptation to or Mitigation of Climate Change

The study deals with the problem of evaluating management strategies for pure stands of Norway spruce (Picea abies Karst) to balance adaptation to and mitigation of climate change, taking into account multiple objectives of a forest owner. A simulation and optimization approach was used to evaluate the management of a 1000 ha model Age-Class forest, representing the age-class distribution of an area of 66,000 ha of pure Norway spruce forests in the Black Forest region of Southwest Germany. Eight silvicultural scenarios comprising five forest conversion schemes which were interpreted as "adaptation" strategies which aims at increasing the proportion of Beech, that is expected to better cope with climate change than the existing Norway spruce, and three conventional strategies including a "Do-nothing" alternative classified as "mitigation", trying to keep rather higher levels of growing stock of spruce, were simulated using the empirical growth simulator BWINPro-S. A linear programming approach was adapted to simultaneously maximize the net present values of carbon sequestration and timber production subject to the two constraints of wood even flow and partial protection of the oldest (nature protection). The optimized plan, with the global utility of 11,687 <euro>/ha in forty years, allocated a combination of silvicultural scenarios to the entire forest area. Overall, strategies classified as "mitigation" were favored, while strategies falling into the "adaptation"-category were limited to the youngest age-classes in the optimal solution. Carbon sequestration of the "Do-nothing" alternative was between 1.72 and 1.85 million tons higher than the other alternatives for the entire forest area while the differences between the adaptation and mitigation approaches were approximately 133,000 tons. Sensitivity analysis showed that a carbon price of 21 <euro>/t is the threshold at which carbon sequestration is promoted, while an interest rate of above 2% would decrease the amount of carbon.

Transgressive overyielding in mixed compared with pure stands of Norway spruce and European beech in Central Europe: evidence on stand level and explanation on individual tree level

The temperate, humid climate and nutrient-rich soils in the pre-alpine areas of southern Bavaria represent conditions where European beech and Norway spruce come out with rather equal above ground biomass production when cultivated in pure stands. In order to reveal the effect of mixture we established 37 experimental plots in even-aged pure and mixed stands of Norway spruce and European beech covering an age span of 37–155 years. The site conditions ranged from warm, dry and base-rich to cool, wet and acidic sites. The ratio of above ground biomass growth of Norway spruce in relation to European beech decreases from 1.14:1 in the monocultures to 1.04:1 in the mixed stands. The mixing of spruce and beech results in a mutual stimulation of biomass production and acceleration of size growth. Together both species produce up to 59% more above ground biomass than the neighboring pure stands. On average the overyielding amounts to 21% in the case of Norway spruce and 37% in the case of European beech. A total of 67% out of the plots indicate overyielding and 57% transgressive overyielding. In mixed stands both species’ tree sizes are significantly ahead of the corresponding pure stands. Facilitation of spruce and competitive reduction of beech yields mutualism with respect to growth on tree and stand level. Consequences for analyzing and modeling interspecific competition and for silvicultural prescriptions are discussed. Ecological implications of the mixing effect on the occurrence and stability of natural and man-made mixed stands of spruce and beech are considered.

Are Self-Thinning Constraints Needed in a Tree-Specific Mortality Model?

Abstract Can a tree-specific mortality model elicit expected forest stand density dynamics without imposing stand-level constraints such as Reineke's maximum stand density index (SDImax) or the −3/2 power law of self-thinning? We examine this emergent properties question using the Austrian stand simulator PROGNAUS. This simulator was chosen specifically because it does not use stand density constraints to determine individual tree mortality rates. In addition, it is based on a probability sample of the population that includes the span of the data being used to test the hypothesis. Initial conditions were obtained from 27 permanent research plots that were established in young pure stands of Norway spruce (Picea abies L. Karst.) and Scots pine (Pinus sylvestris L.) in Austria. A growth period of 250 years was simulated. We conduct our test in two parts. First, we compare our simulated results to Reineke's theory of maximum density and stand density index by examining the self-thinning relationship between stem number per hectare and quadratic mean diameter (log-log scale). Second, we compare our results to Sterba's full competition density rule, which incorporates dominant height along with stem number and quadratic mean diameter. From the results for Norway spruce, we conclude that stand-level density constraints are not necessary to obtain Reineke's maximum size-density relations. Norway spruce results confirm that the maximum size-density relationship reflects reasonable and stable stand dynamics and conforms to that expected by Reineke's theory. Results from simulation of Scots pine also display reasonable and stable stand dynamics, except that they greatly exceed Reineke's maximum stand density index determined empirically from the literature. This Scots pine result argues for stand-level constraints (such as specifying SDImax) to ensure that the appropriate intercept for the maximum density line is used. Our second test revealed that the estimated maximum stand density index according to Sterba's theory was too high for both species, but that the relative rankings across plots were correct. Thus, we are left with ambiguous results. First, that a density-dependent individual-tree mortality model, developed on an adequate data set, is sufficient for the desired stand-level behavior of Reineke to emerge. Second, that stand-level constraints on SDImax need to be imposed if the underlying mortality modeling database is not adequate. FOR. SCI. 50(6):848–858.

Factors controlling soil carbon and nitrogen stores in pure stands of Norway spruce ( Picea abies) and mixed species stands in Austria

Soil data of 18 pairs of secondary pure spruce stands ( Picea abies) and adjacent mixed stands of spruce and broadleaf trees (at least 40% broadleaf trees, mainly beech, Fagus sylvatica) on comparable sites were used to study effects of species composition and other driving forces on carbon and nitrogen stores. It was hypothesized that carbon input by litter and roots to different soil horizons has a marked effect on carbon and nitrogen stores and, therefore, silvicultural methods, e.g., admixture of beech versus spruce, can be used to manage nitrogen retention and release. The study sites of different stand age (pole to mature stage) were chosen on two different bedrock materials (Flysch and Molasse). In general, soils on Flysch are less acidic, better supplied with nutrients, and less sandy than soils developed on Molasse. Species composition did not affect total stores (forest floor and 0–50 cm mineral soil) of carbon or nitrogen on Molasse, however, paired samples tests indicated that mean total stores were significantly higher both for carbon and nitrogen for pure spruce stands than for mixed species stands on Flysch. Carbon stores of the whole soil profile were best predicted by admixture of spruce, stores of phosphorus, sodium and calcium as well as stand age on Flysch; on Molasse, the only given predictor was aluminum storage, indicating that acidic soil conditions favor accumulation of organic carbon. On Flysch, total nitrogen stores were best explained by stores of sulfur and of the sum of base cations.

Direct seeding of beech ( Fagus sylvatica L.) in Norway spruce ( Picea abies [L.] Karst.) stands—effects of canopy density and fine root biomass on seed germination

Direct seeding of deciduous trees below the canopy of conifer stands has proven to be a very economical method for converting pure stands into mixed stands. However, many direct seedings of beech seeds provided unsatisfactory results without a clear explanation for this lack of success. In order to identify the main factors determining seed germination under the canopy of pure Norway spruce stands, three different treatments of direct seeding were tested. These treatments were: (i) direct seeding without additional measure (control), (ii) direct seeding and liming, and (iii) direct seeding covered with beech leaf litter. The experiment consists of two stands. Direct seeding was carried out on six plots per treatment and stand. Each plot includes 81 sample units of 0.5 m 2 upon each of which 15 g of seeds were sown. The germinative capacity of the seeds was pre-determined (68%). Germination of the seeds was recorded repeatedly from May to September 1997. On the related sample units canopy density was measured and fine root biomass of the overstorey trees was estimated. There was a significant difference in seedling number between the two stands. The number of seedlings did not increase by liming (coincident with seeding). In contrast, the covering of seeds with leaf litter caused a remarkable and significant increase in the number of germinated seeds. As ANCOVA showed, models including canopy density or estimated fine root biomass of the overstorey trees explained 72 and 52% of the variation in mean seedling number per plot. As canopy density as well as estimated fine root biomass determine the water content of the soil, it can be assumed that the germination of beech seeds originating from direct seedings under the canopy of pure spruce stands depends to a great extent on soil moisture at the time when the direct seeding is carried out, as well as the weeks following seeding. For this reason, direct seedings in very dense stands are not recommended against the background of this study.