Early growth and phenological development of Finnish-origin curly birch clones under Estonian hemiboreal conditions

Introduction

Curly birch (Betula pendula var. carelica (Merckl.) Hämet-Ahti) is a rare hereditary form of silver birch valued for its highly decorative figured wood. Owing to its distinctive wood texture, curly birch has long been used for veneer, furniture, handicrafts and decorative interior products throughout northern and eastern Europe (Hagqvist & Mikkola, 2008; Niemistö et al., 2008). The characteristic figured wood develops as a result of irregular cambial activity and abnormal grain formation (Vetchinnikova & Titov, 2016, 2020). Because of its high commercial value, interest in cultivating curly birch outside natural stands has increased during recent decades (Sibul et al., 2011).

Natural populations of curly birch are concentrated mainly in Finland, the Baltic countries and north-western Russia (Vetchinnikova & Titov, 2018, 2020). In Estonia, curly birch has increasingly been planted on former agricultural land, where fertile soils provide favourable conditions for the establishment of valuable broadleaved tree species (Sibul et al., 2011). Most commercially available planting material used in Estonia originates from Finland. Despite the long tradition of curly birch cultivation in Finland, relatively little published information is available on the performance of Finnish-origin clones under Estonian hemiboreal conditions.

In northern deciduous tree species, phenological development is closely associated with climatic adaptation. The timing of bud burst, shoot elongation, growth cessation and leaf fall determines not only the length of the effective growing season but also the ability of trees to synchronise their annual growth cycle with local environmental conditions (Basler & Körner, 2012; Heide, 1993). Even relatively small differences in growth rhythm may influence annual biomass production, winter hardiness and long-term survival. Studies on silver birch have shown that provenance, photoperiodic responses and transfer of reproductive material between geographical regions can affect growth, phenology and stem quality (Myking & Heide, 1995; Viherä-Aarnio & Velling, 2008; Viherä-Aarnio et al., 2005). Although comparable information for curly birch is considerably more limited, available studies indicate substantial hereditary variation in growth rate, morphology and phenological behaviour (Hagqvist & Mikkola, 2008; Ryynänen & Ryynänen, 1986; Vetchinnikova & Titov, 2016, 2020).

Knowledge of these differences is important because early plantation establishment largely determines subsequent stand development. At the same time, vigorous juvenile growth alone cannot be regarded as the principal criterion for clone selection in curly birch. Stem quality, crown architecture, branching pattern and, ultimately, the formation of figured wood are equally important characteristics determining the silvicultural and economic value of the species (Niemistö et al., 2008). Long-term field experiments are therefore needed to identify planting material that combines good adaptation with desirable growth characteristics. The availability of suitable reproductive material has become increasingly important as interest in establishing curly birch plantations continues to expand in Estonia. Comparative field trials therefore provide valuable information for selecting planting material that is well adapted to local climatic conditions.

The present study was carried out in a comparative clone trial established at Järvselja in southern Estonia. The aim was to compare the early growth performance and phenological development of eight Finnish-origin curly birch clones during the first two growing seasons after planting under Estonian hemiboreal conditions. We hypothesised that genetically determined differences in the growth rhythm and phenology among clones would be reflected in their early growth performance after transfer to Estonia.

Materials and Methods

Study site

The study was conducted at the Järvselja Training and Experimental Forest Centre in southern Estonia (58°17′9″ N, 27°17′12″ E). The experimental plantation was established in November 2012 on former agricultural land in Agali village, Tartu County. The site belongs to the hemiboreal vegetation zone and is characterised by relatively cool winters and moderately warm summers. According to long-term climatic data (1991–2020) from the Estonian Environment Agency (2024), the mean annual air temperature is approximately 5–6°C and annual precipitation ranges between 650 and 700 mm.

The plantation occupied an area of 0.36 ha (30 m × 120 m) with a nearly homogeneous topography characterised by a gentle westward slope. The experimental layout and clone arrangement are presented in Figure 1. Prior to plantation establishment, the site had been maintained as a weed-free cultivated fallow during the preceding growing season, resulting in effective control of perennial root weeds. Immediately before planting, the entire site was deep-cultivated. During the establishment phase, perennial weeds were controlled using a glyphosate-based herbicide, and herbaceous vegetation was mown once annually in August. No fertiliser was applied during the study period.

Figure 1.

Soil conditions were verified by field excavations and were dominated by gleyic weakly podzolised soil (WRB: Albic Retisol (Gleyic)), whereas a small area along the eastern margin consisted of weakly podzolised soil (WRB: Albic Retisol).

The plantation was enclosed by a 1.5-m-high wire fence to prevent browsing damage. Individual seedlings were additionally protected from rodents using plastic collars placed around the root collar. No browsing damage, rodent damage, frost injury, major disease symptoms or mortality were recorded during the first two growing seasons.

The plantation was established as a comparative clone trial to evaluate the early growth performance and phenological characteristics of Finnish-origin curly birch clones under Estonian hemiboreal conditions.

Results

Relationship between vegetation period length and growth

Clear differences in vegetation period length and annual height increment became evident among clones during the first two growing seasons covered by this study (Figure 2). Vegetation period length (K1–S3) showed a positive relationship with annual height increment during the 2014 growing season. The Pearson correlation coefficient was r = 0.64, although the relationship was not statistically significant (p = 0.084). Because the analysis was based on clone means (n = 8), the observed relationship should be interpreted with caution.

Figure 2.

Discussion

The results of this study demonstrate that the early growth and phenological development of curly birch in Estonia are strongly influenced by genetic differences among clones. Although all studied clones originated from southern and eastern Finland and were exposed to identical environmental conditions after planting, their performance under Estonian hemiboreal conditions differed considerably. The clearest differences emerged in annual height increment and vegetation period length. These differences became evident surprisingly early, after only two growing seasons, suggesting that the observed variation primarily reflects genetically determined differences in growth rhythm rather than short-term environmental effects.

Among the studied material, clones Pertunmaa E-10524 (KL4) and Kerimäki E-8306 (KL2), both originating from the South Savo region of Finland, exhibited the most vigorous growth during the second growing season after planting. Both clones also had the longest vegetation periods and a relatively late leaf fall, which was associated with stronger height growth. The longer growing period probably contributed to greater annual height increment and biomass accumulation. In northern deciduous tree species, growth rhythm and growth cessation are closely linked with climatic adaptation and seasonal carbon assimilation dynamics (Basler & Körner, 2012; Heide, 1993). Similar patterns have been reported for silver birch, where growth rhythm and vegetation period length depend strongly on provenance and photoperiodic responses (Viherä-Aarnio et al., 2005). The relationship was not entirely straightforward, however. Clone Juhlavisa 3 Padasjoki E-11398 (KL6) also achieved one of the greatest annual height increments despite having the shortest vegetation period. Apparently, the duration of the vegetation period alone does not determine annual growth. The intensity of growth during the active growing period may be equally important. These results suggest that a shorter vegetation period does not necessarily imply reduced productivity if growth during that period is sufficiently intensive.

The positive relationship between vegetation period length and annual height increment was weaker than expected. Clones Kerimäki E-8306 (KL2), Pertunmaa E-10524 (KL4) and Hartola Visakuningatar E-3648 (KL8), which had longer vegetation periods, generally also exhibited greater height growth, although the correlation was not statistically significant. This result should be interpreted cautiously because the analysis was based on the mean growth characteristics of only eight clones. Nevertheless, the observed tendency is biologically plausible and agrees with the general understanding that a longer period of active carbon assimilation may contribute to greater annual biomass production in deciduous trees (Basler & Körner, 2012). At the same time, a longer vegetation period should not automatically be interpreted as a sign of better adaptation. Under hemiboreal conditions, the timing of growth cessation and adequate shoot maturation before winter are at least as important as the overall duration of active growth. During the two years covered by the present study, no frost damage was observed, including in the best-growing clones, suggesting that the relatively late autumn phenology recorded for these clones did not reduce their short-term adaptation under the prevailing climatic conditions. Whether the same relationship persists during years with an earlier onset of autumn frosts remains unknown.

Earlier studies on Betula species likewise show that growth rhythm is strongly genetically controlled. Previous investigations carried out in Finland have demonstrated substantial hereditary variation in the growth rate, morphology and phenology of curly birch (Hagqvist & Mikkola, 2008; Ryynänen & Ryynänen, 1986). The present results support these observations by demonstrating that genetically determined differences among Finnish-origin clones became detectable already during the establishment phase under Estonian conditions. Successful cultivation of curly birch therefore depends not only on site conditions but also on the careful selection of reproductive material originating from suitable geographical regions.

Clone Hartola Visakuningatar E-3648 (KL8) deserves separate attention. At planting, this clone consisted of older, two-year-old plants and was considerably taller than all other planting stock. Nevertheless, its height increment during the first growing season was the lowest recorded in the trial. By the second growing season this difference had largely disappeared and the clone recovered well. From a plantation management perspective, this observation is noteworthy because larger planting stock is often assumed to provide a competitive advantage during plantation establishment. The additional analysis showing a significant negative relationship between initial planting height and first-year height increment supports the interpretation that the larger planting stock experienced a more pronounced establishment-related transplant shock. This relationship disappeared during the second growing season, suggesting that the effect was temporary rather than genetically determined. Temporary reductions in early growth following plantation establishment have also been reported for fast-growing deciduous trees on former agricultural land (Johansson, 2013).

Several factors may contribute to such a response. Larger planting stock generally has a greater transpiring surface and therefore requires more rapid re-establishment of root function after planting. Former agricultural soils also differ biologically from long-established forest soils, particularly in the abundance of ectomycorrhizal fungi (Laas et al., 2011). Mycorrhizal colonisation was not investigated in the present study and therefore remains only one possible explanation. Other physiological processes associated with root regeneration after planting may also have contributed to the observed response. From a practical silvicultural perspective, however, the results suggest that, under weed-free conditions and on a well-prepared planting site, smaller container-grown planting stock may establish at least as successfully as larger plants during the first years after planting.

The comparatively modest early growth of clones KL3, KL5 and KL7 may indicate that their growth rhythm is less suited to Estonian hemiboreal conditions. Even relatively small differences in spring bud burst or the timing of autumn leaf fall may influence cumulative biomass production in northern deciduous trees (Basler & Körner, 2012). Planting material transferred from different geographical regions may therefore respond differently to local photoperiodic and temperature regimes. Interestingly, none of these clones performed consistently poorly in all measured traits, indicating that the observed differences probably reflect several interacting physiological processes rather than a single phenological characteristic. Curly birch itself is characterised by considerable phenotypic and growth-form variability throughout its distribution range, reflecting both hereditary and ecological influences (Vetchinnikova & Titov, 2016, 2020).

At the same time, rapid juvenile growth of curly birch should not be regarded as the principal criterion for clone selection. The economic value of the species depends primarily on stem straightness, crown architecture, branch characteristics and, above all, on the rate and extent of figured wood formation (Hagqvist & Mikkola, 2008; Niemistö et al., 2008). Field observations made during this study also revealed clone-specific differences in crown habit. Some clones produced more competing leaders or intensive branching than others, traits that inevitably increase the need for formative pruning during the establishment phase and consequently raise management costs. From this perspective, clones combining vigorous early growth with favourable crown architecture appear particularly promising for further evaluation. Nevertheless, the study covered only the first two growing seasons after planting, and these observations should therefore be regarded as preliminary until long-term stem quality, crown development and figured wood formation have been evaluated. Consequently, a clone that grows rapidly during the first years after planting does not necessarily produce more valuable timber in the future. Likewise, somewhat slower juvenile growth does not automatically imply lower economic value.

Changes in root collar diameter generally followed the same pattern as height growth. Clones producing the greatest height increments also tended to develop thicker stems, although the relationship was not statistically significant. This is expected in a young plantation where height growth still dominates biomass allocation and radial growth becomes increasingly important only later in stand development, when competition among trees intensifies (Hynynen et al., 2010). At the same time, one of the characteristic features of young curly birch, compared with silver birch of the same age, is the relatively rapid and vigorous thickening of the root collar. This characteristic may therefore provide additional information when evaluating the early development and vitality of planting stock.

Projected climate warming in northern Europe is expected to prolong the growing season, potentially increasing the growth potential of curly birch under hemiboreal conditions. At the same time, warmer autumns may modify the balance between growth and adaptation by delaying growth cessation and shoot maturation, particularly in planting material originating from more southerly provenances. Although no frost injury was observed during the present study, future climatic conditions may alter the relative performance of different provenances. Understanding how planting material from different geographical origins responds to these changes will therefore become increasingly important when selecting reproductive material for plantation establishment.

The present study has several limitations. The experiment was conducted at only one site and covered only the early developmental phase of the plantation. Most clones were represented by a single planting row, making it impossible to separate clone effects completely from potential row-position effects, despite the relatively homogeneous site conditions. In addition, clone Hartola Visakuningatar E-3648 (KL8) differed from the remaining planting stock because it consisted of older plants. This characteristic should therefore be regarded as a property of the planting material rather than of the clone itself. Weather conditions during individual years may also have influenced both growth and phenological timing. Long-term monitoring is therefore necessary to determine whether the observed differences among clones remain stable during later stages of stand development.

Despite these limitations, the present trial provides one of the first comparative evaluations of curly birch clones originating from different geographical regions of Finland under Estonian hemiboreal conditions. Clear differences in both growth and phenological development became evident already during the establishment phase. The observation that the two best-performing clones originated from the South Savo region is noteworthy and further emphasises the need to compare reproductive material from different geographical regions under identical growing conditions. Future comparative trials should therefore be established on different soil types and should also include curly birch clones originating from more southerly geographical regions. Only such a network of long-term experiments will allow reliable evaluation of provenance effects on growth, climatic adaptation, winter hardiness, stem quality and, ultimately, the formation of high-quality figured wood under Estonian conditions. Although the field measurements reported in this study were completed more than a decade ago, comparative field trials of Finnish-origin curly birch clones remain rare in Estonia. The present dataset therefore continues to provide valuable baseline information for evaluating clone performance and for designing future provenance and clone trials under changing climatic conditions.

Conclusion

Clear differences among Finnish-origin curly birch clones became evident already during the first two growing seasons after planting under Estonian hemiboreal conditions. Significant variation was observed in annual height increment and root collar diameter, while clones also differed in phenological development, indicating differences in growth rhythm and climatic adaptation among clones.

Clones exhibiting greater height increment generally tended to have longer vegetation periods, although the relationship between vegetation period length and annual height increment was not statistically significant. The results therefore indicate that Finnish-origin curly birch clones differ in their early growth responses after transfer to Estonian growing conditions.

Because the study covered only the first two growing seasons after planting, the observed differences should not be regarded as a final assessment of clone suitability. Long-term monitoring will be required to determine whether these early differences persist and how they are related to stem quality, winter hardiness and the formation of figured wood.