Volume 75 (2021): Issue 4 (August 2021)

Climate forecasts indicate that in the future the climate will become more and more suitable for growth of broad-leaved tree species in Latvia. It is therefore very important to recognise and understand the growth potential of these broad-leaved stands in Latvia and the factors affecting the increment of growing stock. The results of the study showed high volume yield, as already 60- to 80-year-old stand stock could reach 500–600 m³·ha^-1, and in later years even 700–1000 m³·ha^-1. Similarly, the results of our study showed that the increment of the stock and growth potential of individual forest elements varied significantly between the various broad-leaved species. Significant factors were the age of the forest stand or forest element (a set of trees of the same species, in one layer and belonging to one generation), the indicators of competition (basal area and/or relative density) and the indicators of the social status of the forest element, namely, the ratio of the mean diameter of the forest element and the mean diameter of the dominant species of the upper layer of the stand and the sum of the basal area of the larger forest elements). No significant differences were found in growing stock and growth potential between monospecies and mixed stands and between simple and complex stands, neither at the level of the stands, nor at the level of the individual forest element.

In this study, oak specific chloroplast simple sequence repeat (SSR) markers were used to analyse Latvian and Belarusian oak (Quercus robur L.) population provenance and genetic structure. Chloroplast haplotypes were compared between Latvian and Belarusian pedunculate oak, and several common haplotypes were identified. The SSR haplotypes were compared to previously reported PCR-RFLP haplotypes, and haplotypes from the A (eastern European) and C (central European) lineages were identified. C lineage haplotypes were only found in the western region of Latvia. Haplotypes shared between Latvia and Belarus were all from the A lineage. Despite the much smaller population size of oak in Latvia in comparison to Belarus, the level of genetic diversity identified using the chloroplast SSR markers is similar. Provenance trials will need to be established in order to determine the suitability of Belarusian oak reproductive material for deployment in Latvia. Based on the results from this comparison of chloroplast haplotypes, as well as climatic similarities, it is probable that Belarusian oak material will be better suited to the eastern regions of Latvia compared to the western regions of Latvia.

Acer platanoides L. (Norway maple) is the most widespread native maple species in Europe, with a distribution from south and central Europe to northern Europe and Scandinavia. Acer platanoides is widespread throughout the territory of Latvia, and is mainly found in mixed broadleaf and conifer stands. The genetic diversity and differentiation of Latvian A. platanoides populations was analysed. Sampled populations were located throughout the territory of Latvia, and were selected to represent a range of ecological conditions, with differing levels of anthropogenic impact. A total of 496 individuals from 21 populations were analysed with eight microsatellite markers, which were developed from related Acer species. The obtained molecular data revealed a moderate level of polymorphism, and the analysed Latvian A. platanoides populations were moderately differentiated. This study provides an initial assessment of the genetic diversity and differentiation of Latvian A. platanoides populations, and is also one of the first reports of the analysis of A. platanoides populations using microsatellite markers. The results can be utilised to define A. platanoides genetic resource stands to ensure conservation of a wide range of germplasm.

Changing climatic conditions are transforming the ecological and silvicultural roles of broadleaf tree species in northern Europe. Small-leaved lime (Tilia cordata Mill.) is distributed throughout most of Europe, and is a common broadleaf species in Latvia. This species can tolerate a broad range of environmental and ecological conditions, including temperature, water availability, and soil types. The aim of this study was to assess the genetic diversity and differentiation of Latvian T. cordata populations using nuclear microsatellite markers developed for Tilia platyphyllos. After testing of 15 microsatellite markers, Latvian T. cordata samples were genotyped at 14 micro-satellite loci. Latvian T. cordata populations had high genetic diversity, and were not overly isolated from each other, with moderate gene flow between populations. No highly differentiated populations were identified. Vegetative reproduction was identified in most analysed populations, and almost one-third of analysed individuals are of clonal origin. T. cordata has high timber production potential under the current climatic and growth conditions in Latvia, and therefore this species has potential for use in forestry, as well as playing a significant role in maintaining biodiversity and other ecosystem services.

The succession of semi-natural xeric calcareous grassland plant communities toward deciduous forest communities is poorly studied in Latvia. There is insufficient knowledge on the natural transformation of dry calcareous grasslands of Festuco-Brometea into thermophilous quasi-climax oak forest communities of Quercetea pubescentis that are very rare in Latvia. In this paper, a geobotanical study is presented that included studies of soils, tree age, spatial pattern of trees, vegetation composition of different succession stages, and analysis of environmental factors. The study was conducted in the Abava Valley, West Latvia, an area that is unique with species-rich xeric calcareous Festuco-Brometea grasslands, thermophilous oak forests dominated by Quercus robur, and intermediate secondary succession stages with young to medium-aged Juniperus communis and Pinus sylvestris formations.

The distribution of sweet cherry (Prunus avium L. syn. Cerasus avium Moench.) in natural woodlands of Latvia was studied. The growth, vitality and stem quality of the first-generation plantations established in Latvia in different agricultural soils using the planting stock of local origin was assessed. In woodlands, sweet cherry is found in 11 Forest Resource Monitoring plots in the administrative regions of Madona, Kuldīga, Alsunga, Saldus, Cīrava, and Kazdanga. In the last decade, interest in sweet cherry as a wood producer has increased and plantations with a total area of 21.92 ha were established in 22 localities (Priekule, Kazdanga, Durbe, Talsi, Ventspils, Kuldīga, Tukums, Saldus, Dobele, Bauska, Cēsis, Pārgauja, and Krāslava). Survey was made of sweet cherry plantations established between 1997 and 2019, using both local and foreign origin clonal stock (Danish clone Truust and Swedish clones nos. 9 and 10) in different agricultural soils (typic sod-calcareous soil; sod-podzolic soil; stagnogley sod-podzolic soil) in the districts of Priekule, Kuldīga, Dobele, Bauska, Ķegums, and Smiltene. In plantation forests with optimal growing conditions (typical sod-calcareous soil; pH 6 to 7), 22-year-old sweet cherries of local origin had height (H) of 15.1 m, the diameter at breast hight (DBH) — 16.4 cm and stem volume 157.7 dm³ on average. The maximum H and DBH observed for the largest trees reached 17.6 m and 25.3 cm, respectively. The Danish clone Truust and the Swedish clones nos. 9 and 10 of sweet cherries were promising for growing in Latvian climatic conditions. Selection of perspective clones for establishment of second-generation plantations was made for sweet cherries of local origin.

Beech is expected to expand its range into the territory of Latvia due to climate change, but we lack information on its stem and wood quality as a measure of its economic importance. We studied external stem quality traits in beech stands with age of 69 to 131 years. Nine trees were cut and sampled to determine internal wood quality and the proportion of peelable cylinder. Tree value was calculated according to buckling by dimensions and buckling accounting for stem and wood defects. Trees of the second generation (69 years) had a significantly lower incidence of frost cracks (22.0% vs. 36.2%), epicormic branches (50.5 % vs. 65.8%), and forks (15.8% vs. 23.5%) as compared to trees from the first generation (115–131 years). All sampled trees had discolouration in the height of the first log. The proportion of the peelable cylinder was positively linked to tree height and ranged from 12% at the stump level to 33.4% at the tree height of 30 m. All analysed phenotypic traits decreased stand monetary value: frost cracks by 6% to 8%, all external traits by 16% to 20%, all external and internal traits by 29% to 33%.

At the beginning of the 21^st century, in extensively managed mature and over-mature conifer stands (Pinus sylvestris and Picea abies) in Latvia, establishment of native deciduous broad-leaved (Acer platanoides, Tilia cordata, Quercus robur and Ulmus laevis), as well as alien broad-leaved species (Acer pseudoplatanus and Tilia platyphyllos) has been observed. The establishment of deciduous broad-leaved species in coniferous stands is a process known as nemoralisation of coniferous stands, which is characterised by a large number of seedlings of broad-leaved species, young trees and the formation of a subcanopy layer in stands. Based on the age measurements of individuals of broad-leaved species trees, the establishment of some individuals into the studied coniferous stands occurred 40–50 years ago — in the 1970s–1980s. However, the predominant age of alien broad-leaved tree species in the study plots was 15–25 years, i.e. the introduction of large numbers of seedlings of these species in the coniferous stands occurred at the end of the 20^th century. In this manner, in the final phase of the conifer replacement process, stable, mostly shade tolerant, mixed stands consisting of 3–4 species have formed, in which spruce is often found together with broad-leaved species.

Broad-leaved trees play an important role in supporting epiphyte richness in deciduous forests. In this study we tested which broad-leaved forest stands in terms of tree composition best predicted biodiversity in regard to bryophyte species number and their functional traits. The bryoflora was surveyed in 70 stands differing in dominant broad-leaved tree species. One circular plot with diameter 30 meters was established in each stand situated in different parts of Latvia. Fifty-three of the plots were located in stands identified as protected habitats of the European Union. In total, 82 bryophyte species were recorded. At plot level the total number of species ranged between 6 and 38 species. Using principal component analysis (PCA), the ordination axes explained 41% of the variation in composition of bryophytes. The PCA axes were related to tree species composition. The presence of broad-leaved trees, especially ash, lime, and oak, was connected with higher species richness. Generalised linear models confirmed that epiphyte richness, both at the total and indicator species levels, was higher in ash stands. From the functional traits, all expected life-strategy and life-form categories were recorded. The most represented life-forms were smooth mats, tall turfs, and rough mats, while the dominant life-strategy was perennial stayers, followed by competitive perennials and long-lived shuttles. Functional trait richness was highest in Fraxinus excelsior, Tilia cordata, and Quercus robur forest stands.