Estonian Forests Research Articles

Habitat indicators for cavity-nesters: The polypore Phellinus pini in pine forests

Foresters and arborists have long used fruit-bodies of heart-rot fungi as signs of advanced live-tree decay, but such usage has not been elaborated for forest conservation. I analysed relationships between a heart-rot fungus, tree-cavity supply, and cavity-nesting bird assemblage in wet hemiboreal Scots pine (Pinus sylvestris) forests in Estonia. The focal species, Phellinus pini, is the main heartwood decayer of live pines; it typically forms fruit-bodies at the stage of advanced decay on old trees. I found that the pine wetlands had few tree-cavities (mostly in snags) and cavity-nesting birds (woodpeckers being almost absent) despite abundant snag supply. Only one fruit-body of P. pini was found on cavity-tree but stand-scale abundance of the fruit-bodies correlated well with cavity-nester densities. Multifactor models indicated that cavity formation, not tree death, was the limiting process for secondary cavity-nesters, and P. pini could indeed be used (in combination with old-pine abundance) for assessing their habitat quality. This fungus could also serve as an educational flagship species to bridge conservation biology and forest pathology, and its fruit-bodies can signal trees to be retained at harvesting in pine forests. The conclusion is that there is hope for developing practical indicators to manage for the hidden decay processes that govern tree-cavity development.

Semi-naturally managed forests support diverse land snail assemblages in Estonia

Snails are an integral part of most forest ecosystems where their sensitivity to timber harvesting depends on insufficiently understood interactions of multiple factors. Some snail species have been reported to indicate natural forest conditions, and deadwood-rich old growth stands are known to be particularly species rich. Other studies demonstrate rich snail fauna in young forests and hypothesize that clear-cutting is less detrimental on wet soils. We investigated forest management impacts on snail assemblages in hemiboreal Estonian forests. We sampled 100 stands using a balanced design of five site types and four management stages (old growth; mature forest; retention cuts and clear-cuts 4–19years post harvest). The material comprised ca. 12,500 live individuals and included 52 species of terrestrial shelled snails. Snail assemblages differed mostly among site types, and less among management stages. Dry boreal site type with a thick moss carpet, pine stands and little woody debris had the most distinct, impoverished assemblages. Harvesting-caused assemblage shifts were restricted to nutrient-rich and wet sites, but the assemblages recovered during maturing of the regrowth. We did not detect ‘life-boating’ function of solitary retention trees on cutover sites. One species, Vertigo alpestris, preferred old growth and abundant coarse woody debris appeared to be a reason for that. We conclude that seminatural forest management approach can sustain relatively intact snail assemblages in humid temperate climate and on calcium poor to moderately rich sites. The clearest long-lasting impact is the decline of some wetland species after draining.

Seasonal Differences in Forest Height Estimation From Interferometric TanDEM-X Coherence Data

This paper demonstrates the use of X-band bistatic synthetic aperture radar (SAR) interferometric coherence for retrieving tree height of coniferous and deciduous forests during leaf-off season in early spring and leaf-on period in summer. TerraSAR-X add-on for Digital Elevation Measurements (TanDEM-X) HH and VV polarization channel coherence images were studied for over 249 ha of forests in Estonia and compared against light detection and ranging (LiDAR) and forest registry field inventory data. Strong correlation was found between interferometric coherence magnitude and LiDAR measured average tree height, especially for winter period. The regression models show the strongest correlation between pine stand heights and single-polarization interferometric coherence, where the correlation coefficients ( ${{\text{r}}^{2}}$ ) range between 0.75 and 0.97. The highest correlation for mixed deciduous tree stands was found during leaf-off period with ${{\text{r}}^{2}}$ ranging from 0.87 to 0.94, whereas leaf-on period resulted in ${{\text{r}}^{2}}$ from 0.58 to 0.75. Strong correlations were also found for spruce trees with ${{\text{r}}^{2}}$ between 0.54 and 0.83. Moreover, a simple semiempirical model based on random volume over ground model framework was constructed to describe the relation between the forest height and interferometric coherence. Also, the seasonal variability of the correlation was studied. Our results demonstrate that under Northern-European conditions, seasonal changes have a significant effect for deciduous trees as standard deviations dropped from 1.34–1.78 m during leaf-off conditions to 2.22–3.16 m for leaf-on conditions. Thus, height estimation of deciduous stands requires leaf-off conditions for accurate coherence-based height retrieval. Correlation coefficients for pine stands were unvarying across different weather conditions and least affected by the season. The observed strong sensitivity of interferometric coherence to forest height makes it feasible for estimating canopy height for boreal and deciduous forests in both summer and winter conditions. The estimation algorithm works best for coniferous forests.

Mycocoenological methods based on investigations in the Estonian forests

Mycocoenological methods based on investigations in the Estonian forests

Biomass resource and environmental effects of Norway spruce (Picea abies) stump harvesting: An Estonian case study

Stump removal is a novel potential source of woody biomass for renewable energy in the Nordic and Baltic countries. In Estonia, a prospective resource of Norway spruce (Picea abies) stumps is available for bioenergy. The main aims of the present study were to estimate the biomass and volume of harvested spruce stumps; also to evaluate nutrient and carbon loss from a site due to stump removal; and to estimate the effect of stump harvesting on soil respiration.In this pilot study an allometric model was elaborated for estimation of the biomass and volume of harvested stumps. The volume of harvested spruce stumps reached up to 130m3ha−1 and potential energy content amounted to 290MWhha−1. The average calorific value of different stump and root fractions varied between 19.0 and 19.3kJg−1 and there was no significant difference between the stump fractions. The weighted average density of the harvested spruce stumps was 430kgm−3.Fertile automorphic forest types were selected for this pilot study and the nitrogen (N) storage removed from clear-cuts with stumps was not appreciable, being 1.2–4.4%, in comparison with soil N storages. Removed phosphorus (P) accounted for 8–28% of total soil lactate soluble P and removed potassium (K) accounted for 6–23% of the soil K storage. The loss of carbon (C) made up 11–33% of the soil C storage. However, the concentration of lactate soluble P evidently reflected the balance between solubilized and assimilated P in the soil rather than actual P limitation. Hence, at fertile sites stump removal does not caused considerable N and C depletion, however, removal of P and K may be appreciable compared with corresponding soil storage values.Stump harvesting did not increase soil respiration intensity. Total annual soil respiration at the Oxalis site type was 4.9 and 5.1tCha−1yr−1 in the uprooted and control plots, respectively. At the Myrtillus site type the respective values were lower, 3.7 and 3.9tCha−1yr−1. Also soil heterotrophic respiration in the Oxalis site type was practically the same at the uprooted and control sites, being 3.3–3.2tCha−1yr−1. However, in the Myrtillus site type the values of the heterotrophic respiration efflux was higher from the control plots than from the uprooted plot, at 2.5 and 1.8Cha−1yr−1, respectively.Norway spruce stumps are a potential bioenergy resource in Estonian forests and stump harvesting at fertile autotrophic sites does not cause problems in terms of depletion of nutrients and carbon from the ecosystem.

Assessment of tree mortality on the Estonian Network of Forest Research Plots/ Puude väljalangevuse hindamine metsa kasvukäigu püsiproovitükkidel

Abstract Assessment of tree mortality provides deeper understanding of forest structure and functioning. This enables evaluation of stand sustainability and provides information on stand productivity, diversity and health condition. Tree mortality can be assessed by spatiotemporal patterns as well as by studying the processes and causes of mortality. Tree mortality is caused by specific disturbance agents or by the complex effect of various disturbances. The purpose of this study is to examine tree mortality in Estonian forests, determine the causes of tree death, and estimate how different management regimes affect tree mortality and its causes. This study is based on 360 sample plots measured in 2003-2005 and re-measured in 2008-2010. The sample plots were divided into recently managed (RM, 146 plots) and low intensity/unmanaged (LU, 214 plots) plots based on forest management regime. In total, 53,990 trees were measured, of which 20,020 were located on RM and 33,970 on LU plots. The tree mortality for 5-year period was 3.4% on RM plots and 8.0% on LU plots. The main cause of tree mortality in RM stands was insect damage, which attributes to 29.8% of tree mortality, whereas in LU stands the main cause was tree competition, which attributes to 45.4% of tree mortality. The analysis of tree mortality indicated that an increase in relative tree diameter in both stand types contributes to an increase in mortality due to insect, wind or fungal damage and diseases. Opposite results were received with respect to competition - the smaller the tree relative diameter, the more probable it is that a tree will die because of competition with neighbouring trees. The analysis of game damage and other causes of tree mortality showed that these were not dependent on the relative diameter of trees. The analysis of the overall probability of tree mortality revealed that relatively smaller trees have a higher probability of mortality than larger trees.

Dead wood in clearcuts of semi-natural forests in Estonia: site-type variation, degradation, and the influences of tree retention and slash harvest

Semi-natural forests, which naturally regenerate after timber harvesting, provide distinct opportunities for dead wood (DW) management for biodiversity. We described DW pool and sources of its variation during the first decade after final felling in Estonia, hemiboreal Europe. Depending on forest type, the mean post-harvest volumes of above-ground DW ranged from 70 to 119 m3 ha−1. Final felling generally did not reduce downed coarse woody debris (CWD) because many sawn logs were left on-site, and soil scarification was rarely used. However, subsequent decay of downed CWD appears to be accelerated due to the increased ground contact of logs, so that even the relatively small inputs from live retention trees observed (5 m3 ha−1 per decade) can be ecologically significant. While final felling greatly reduced snag abundance, the mortality of retained live trees generally balanced their later losses. The volumes of downed fine woody debris in conventional cutover sites were roughly double that of pre-harvest forests. Slash harvest caused an approximately twofold reduction in downed DW and resulted in CWD volumes that were below mature-forest levels. The results indicate that the habitat quality of cutovers critically depends both on the retention and on the post-harvest management of biological legacies. In Estonia, the necessary improvements include more careful retention of snags in final felling, selecting larger retention trees, focusing slash harvest on the fine debris of common tree species, and providing snags of late-successional tree species.

MERIS GPP/NPP product for Estonia: I. Algorithm and preliminary results of simulation / MERIS’e GPP/NPP tulem Eesti jaoks: I. Algoritm ja mudelarvutuste esialgsed tulemused

Abstract. A light use efficiency (LUE) type model named EST_PP to simulate the yearly gross primary production (GPP) and net primary production (NPP) of Estonian land on a 1 km2 grid is described. The model is based on MERIS (MEdium Resolution Imaging Spectrometer) satellite images to describe the fraction of photosynthetically active radiation (fAPAR) and leaf area index (LAI) as well as meteorological reanalysis datasets on 11 km2 grid produced by Estonian Meteorological Institute (EMHI) and Tartu University (TU) by means of the HIRLAM (High Resolution Limited Area Model) numerical weather prediction model. The land cover map of Estonia needed for the model was derived using DMCii (Disaster Monitoring Constellation International Imaging) SLIM-6-22 (Surrey Linear Imager - 6 channel - 22 m resolution) images and ancillary information. The EST_PP model was run for the period from years 2003 to 2011. The results of GPP and NPP simulation are compared with the available global MODIS (Moderate Resolution Imaging Spectroradiometer) GPP/NPP product and with the Estonian statistical data on yearly volume increment in forests and on yield of agricultural crops. The NPP simulation results on coniferous and deciduous forests are compared with the data from tree ring analyses from different counties. These comparisons show us that the simulated country average yearly NPP values for Estonian forests agree reasonably well with the indirect estimates from other sources, taking into account the rather high uncertainty of the model predictions, uncertainty of forest inventory-based estimates and limited representativity of existing tree ring data. However, problems arise with the ability of present versions of EST_PP and MODIS NPP models to adequately simulate the regional differences of productivity and of variability of productivity in different years. The model needs some modification and the basic LUE principles to be tested in Estonia. Nevertheless, the MODIS NPP and EST_PP models offer additional possibilities to map yearly productivity and carbon sequestration by Estonian vegetation. There is a perspective to add the model-simulated NPP values into the national inventory datasets.

Testing the structural–function hypothesis of eggshell maculation in the Great Tit: an experimental approach

The function of eggshell pigmentation has long been the subject of much speculation. Most of the proposed hypotheses have been based on a signalling function. According to the recently proposed structural–function hypothesis, birds use the pigment protoporphyrin to strengthen the eggshell in response to dietary calcium (Ca) shortage. The aim of the study reported here was to test this hypothesis in a Ca-provisioning experiment involving Great Tit Parus major populations breeding in Ca-poor coniferous and Ca-rich deciduous Estonian forests, respectively, over a 2-year period. Ca-supplemented birds laid eggs with ‘darker’ pigmentation than birds in the unsupplemented (control) group, which is at odds with the structural–function hypothesis. Although eggs were smaller and had thinner shells in Ca-poor habitat, eggshell maculation did not differ between the habitats. Eggshells were more pigmented in the year when they were also thinner, but their thickness was not related to their maculation within each year. Although the results of our study neither prove nor disprove the structural–function hypothesis, they do suggest that the relationship between eggshell pigmentation and Ca availability cannot be explained simply by this hypothesis.

Forest land-use history affects the species composition and soil properties of old-aged hillock forests in Estonia

Decisions regarding forest typology, management and protection are often based on the structures of present-day forests, ignoring their successional history. Forests growing on kames, eskers and various moraine hillocks common in regions with Holocene glaciation are good examples of this approach. In Estonia, these forests locally persist as fragments of continuous primary forest, but usually they are situated on former slash-and-burn areas (bushlands) or reforested agricultural land. Our aim was to elucidate the strength of the effect of long-term land-use history on the present-day vegetation compositions of mature hillock forests and their soil chemistry. It appeared that even the mature secondary hillock forests are still distinct from historically continuous stands in terms of species composition. We discovered connections between stand history and species content in hillock forests as well as transformed soil properties. The carbon and nitrogen contents in the humus horizons of secondary forests are lower while their carbon–nitrogen ratios are higher than in continuous forests. The relationship between vegetation and stand history is demonstrated by the higher proportions of anthropophytic and apophytic species in the herb layer of the secondary forests. The presence of species that are tolerant of anthropogenic impact on the secondary hillock forests floor can also be partly explained by the effect of different species in the tree and shrub layers, gaps in the tree canopy, and the boundary effect caused by the small areas of forest patches, neighboring grasslands or fields. The extinction debt in secondary communities should also be considered.

Aspen-inhabiting Aphyllophoroid fungi in a managed forest landscape in Estonia

In northern Europe, the biologically rich European aspen (Populus tremula) tends to disappear both from intensively managed and strictly protected forests, thus representing a biodiversity conservation challenge. This study describes the distribution of Aphyllophoroid fungi inhabiting aspen in a hemiboreal mixed-forest landscape where clear-cutting was usually followed by natural regeneration. In a 4-km2 area in Estonia, 181 individual forest stands and cut areas were surveyed. Sixty-one percent of the 2831 live aspen trees found were concentrated in 11% of the area. Thirty-six polypore species were recorded on aspen, with old stands and naturally created deadwood hosting most species. Among the records of 14 pre-defined aspen-specific basidiomycetes, Phellinus tremulae comprised about 95%. Six species (Ceriporiopsis aneirina, Clavicorona pyxidata, Funalia trogii, Phellinus populicola, Rigidoporus corticola, Steccherinum pseudozilingianum) were infrequent to rare, but widely distributed, inhabiting various stand-age classes, and mostly occurring outside “key habitats”. All the seven species absent are of conservation concern and frequent in Estonian old-growth forests. Thus, rotational management with natural aspen regeneration only provided habitat for some aspen-specific species. For sustaining all species, such landscapes should probably be complemented with stands managed using modified thinning and partial harvesting techniques in aspen-containing mixed stands.

Effects of forest continuity and tree age on epiphytic lichen biota in coniferous forests in Estonia

Epiphytic lichen biota on Picea abies and Pinus sylvestris in Estonia was studied. Twenty-one spruce and 21 pine sample plots were located in old forests with long forest continuity, and 12 spruce and 12 pine sample plots in young first-generation forests (<100 years). Altogether 103 lichen species were recorded on the 330 sampled trees. Lichen species richness per plot was significantly higher in old forests in case of both tree species; 31 lichen species, including red-listed and protected species, were found only in old forests. Tree age had a positive effect on lichen species richness on tree stem in old and young spruce forests and in young pine forests. Tree age also had an effect on the presence of several species. Both tree age and forest continuity affected lichen species composition. Arthonia leucopellaea, Chrysothrix spp. and Lecanactis abietina were found in at least every third old spruce or pine forest and in no young forests, and can be regarded as good indicators of old coniferous forests with long continuity in Estonia.

Economic impact of enlarging the area of protected forests in Estonia

The article deals with the economic impact of increasing strictly protected forest areas from the current 8.2% level up to 10% from the total forest area in Estonia. Whereas there is no concrete selection of areas added in the group of strictly protected forests, the impact on annual lost income due to forest management not carried out in protected forests was assessed according to two different assumptions: 1. changing the status of forests without management restrictions (formal commercial forests) into the strictly protected ones; 2. changing the status of forests with management restrictions (formal protection forests) into the strictly protected ones. Achieving the target set by the Estonian Forest Development Programme, increasing the share of strictly protected forests up to 10%, an additional 39,000 ha of forest land will be out of management. Increasing the conservation area by 1.8% by forests without management restrictions, the reduction in potential net revenue will be 2% per year. Achieving the goal by conserving the forests with management restrictions, the total income will decrease by 0.76–1.9% per year. To realize the goal set by the Estonian Forestry Development Programme until 2010 and 2020 to enlarge the protected forest area up to 10% from one side, and the current economic recession on the other side, the economic solution could be enlarging the conservation on forests with management restrictions.

Re-evaluation of stand indicators for the assessment of the representativity status of the Natura 2000 habitat type forests

Habitat inventories and monitoring require the evaluation of the status of various charac- teristics of these habitats - indicator traits. In the case study of Estonian forests in Viru County (Natura 2000 Annex I forest habitat types *9010, *9080, and *91D0), the survey data of old-growth and natural forests were used to test for the efficiency of the indicator set that has been suggested for the evaluation of the habitat's representativity status. The data from expert-graded forests (121 stands) were alternatively clustered with k-means clustering, their characteristics were tested for indicator power with discriminant analysis, and the resulting efficient set of characteristics was clustered again for an updated classification. In studying the differences between analyses of expert grading and cluster system, we found that different characteristics had different weights in forest classification. In addition to the standard structural characteristics, signs of anthropogenic activity and landscape pattern proved to be of importance. From the testing for various precision scales of classification, we concluded that different indicator traits of structure and composition are required, and the three-grade system appears to be practical for the purpose of avoiding over-interpretation. We found that additional studies are needed to define reasonable indicator traits for wet and swampy forests, and also for forests on unproductive soils.

Indicators of Structural and Habitat Natural Quality in Boreo-Nemoral Forests along the Management Gradient

We aimed to quantify the effects of anthropogenic disturbances on forest structure and diversity in deciduous and mixed boreo-nemoral stands, conditioned on potential confounding factors. Based on these results, we created a statistically-supported indicator list of stand “naturalness”. We surveyed 50 quantitative and qualitative characteristics of a stand, the understorey and forest floor, and several widely accepted biodiversity indicators in 171 forests in Estonia. Multi-factorial GLM and GLIM analyses showed that many forest structural characteristics were confounded by forest site-type specificity, stand age and/or biogeography. Near-natural old-growth forests had higher proportions of deciduous trees, a larger amount of coarse woody debris, a higher frequency of logs in each decay class, a denser understorey, and were more homogeneous in the horizontal pattern of each layer than mature managed forests. By improving light conditions, forest management indirectly increased herb layer richness and coverage and the proportion of graminoids. The critical easy-to-apply set of indicators to assess forest ecosystem quality includes the amount and type of dead wood, the canopy closure of a stand and the presence of specially-shaped trees, specific epiphytic indicator lichens, mosses and wood-dwelling insects, and forest site-type specific herb layer species.

Classification Structure of Floodplain Forests in Estonia: A Comparison of Two Classification Approaches

Flooded forests in 79 sample areas were studied in various parts of Estonia. Using the principal component analysis and cluster analysis, six community types were established: (i) Tilia cordata—Mercurialis perennis, (ii) Ulmus laevis—Allium ursinum, (iii) Populus tremula—Convallaria majalis, (iv) Alnus incana—Cirsium oleraceum, (v) Alnus glutinosa—Filipendula ulmaria and, (vi) Alnus glutinosa—Carex acutiformis. The species composition of these types partly overlaps but the abundance proportions of species are clearly different and all types have several significant indicator species. Another classification scheme was established by TWlNSPAN analysis. Following the phytosociological approach, the communities belong to Querco—Fagetea, Alno—Ulmion (Pruno—Fraxinetum, Ficario—Ulmetum, Pruno padi—Alnetum incanae, the latter subdivided further into P.p.—A.i. var. Frangida alnus and P.p.—A.i. var. Urtica dioica), and to Alnetea glutinosae, Alnion (Carici elongatae—Alnetum, represented with two subassociations: C....

Physiological Condition of Incubating and Brood Rearing Female Great TitsParus majorin Two Contrasting Habitats

Little is known about the physiological causes and consequences of habitat choice decisions in birds. We compared size-corrected body mass, hematocrit, plasma albumin, beta- and gamma-globulin and triglyceride concentrations, as well as the albumin/globulin ratio of female Great Tits breeding in deciduous and coniferous forests in Estonia. Females were sampled during incubation and during the chick-rearing period. Whereas the nest box occupation rate and clutch size were higher in the deciduous habitat, the female nutritional condition during incubation was better in the coniferous habitat. This habitat difference in condition was not explained by the differences in the start of egg-laying or clutch size. Females lost more mass from incubation to the chick-rearing stage in the coniferous than in the deciduous forests. Although the values of most blood parameters changed from incubation to the chick-rearing stage, no habitat-related change was observed. There were no habitat differences in the number and quality of fledglings or in female condition during the chick-rearing period. Our results indicate that incubating female Great Tits are in a worse nutritional state in the preferred deciduous habitat fragments than those in the non-preferred coniferous habitat. However, habitat-related differences in condition during incubation seem to be unrelated to habitat-specific reproductive decisions.

Floodplain forests in Estonia: Typological diversity and growth conditions

Flooded forests in 79 sample areas were studied throughout the country. Using multivariate methods, six community types were established: (i)Tilia cordata-Mercurialis perennis, (ii)Ulmus laevis-Allium ursinum, (iii)Populus tremula-Convallaria majalis, (iv)Alnus incana-Cirsium oleraceum, (v)Alnus glutinosa-Filipendula ulmaria and, (vi)Alnus glutinosa-Carex acutiformis. The species composition of these types is partly overlapping but the abundance proportions of species are clearly different and all types have several significant (P < 0.05) indicator species. Communities of the first type grow mainly on temporarily overmoistened Dystri-Gleyic Arenosols on floodplain terraces but sometimes also in lower areas close to a riverbed on Eutric Gleysols or Gleyic Fluvisols. The second and third types embrace communities on levees where mainly Eutric Gleysols or Gleyic Fluvisols are represented. Communities of the fourth type are characteristic of low levees or flat areas with Mollic Fluvisols or Eutric Gleysols. The communities of the fifth and sixth types represent floodplain backswamp black alder forests on Eutric Gleysols, Molli-Histic and Histic Fluvisols. The convergence of communities, i.e., a situation where plant communities growing in various hydrological conditions and on different soils can have similar structure and belong to one community type is rather conspicuous for floodplain forests. In the conditions of Estonia, like in the Central European and Scandinavian countries, it is reasonable to consider as floodplain forest not only those growing on Fluvisolssensu stricto but all forests that are almost annually regularly covered with flood-water at least for a couple of weeks; the amount of alluvial sediments is not a decisive criterion in this case.

Can forest certification improve forest management? Case study of the FSC certified Estonian State Forest Management Centre

SUMMARY This paper analyses environmental, social and economic impacts of FSC certification in Estonian state forests. The rapid expansion in forest certification is causing the need to evaluate actual costs and benefits of certification at the organizational and macroeconomic levels. The authors have used results of stakeholder interviews, comprehensive questionnaires among the staff of the certified State Forest Management Centre and a field inventory of managed forests. Results reveal that certification has increased the reputation of the certified organization and the international reputation of Estonian forestry in general. FSC certification has helped to improve several social and environmental aspects of forest management. On the other hand certification has not reduced the share of clear-cut free forestry or increased the preference for mixed stands, as hoped by many stakeholders.

Comparative modeling of stand development in Scots pine dominated forests in Estonia

In general, forests in Estonia are characterized by great variability, not only in protected areas but in commercial forests as well. The data needed for the derivation and calibration of growth models can be obtained by continuous observation of permanent growth plots (also known as longitudinal studies) or by establishing chronosequences with temporary plots distributed over a wide range of growing sites, densities and ages (also known as cross-sectional studies). A compromise may be achieved by a system of “interval plots” (also known as a short-time series: series which covers a short time). Since the measurement interval is a period of undisturbed growth, it is possible to measure change rates as in a longitudinal study and at the same time cover a wide range of initial conditions as in a cross-sectional study. Numerous models of stand growth have been derived from re-measured sample plots. This study, which uses the data of 142 five-year intervals from 134 unmanaged Scots pine stands, compares six different model combinations involving algebraic difference equations and fixed time-step increment equations. New stand-level diameter and basal area increment equations and a tree survival model which showed close correspondence with the existing stand-level model for Estonia were developed. The main advantage of the use of algebraic difference equations over the fixed-step increment equations is the ability to use flexible time steps. However, the projection intervals should not deviate too much from the time steps of the measurement data. An important constraint when using the algebraic difference equations is to avoid long-term predictions in one projection sequence.