Hybrid aspen (Populus tremula L. × Populus tremuloides Michx.) plantations may produce valuable sawlogs for the growing timber market and contribute to carbon sequestration. However, environmental risks such as stem rot, the spread of which is facilitated by insect or frost damage, may reduce the proportion of valuable timber. It is important to understand the various factors affecting the spread of aspen rot to mitigate negative impacts with tree breeding. This study aimed to assess the impact of frost cracks and large poplar borer on stem rot in hybrid aspen clones in two clonal trials in Latvia. Genetic parameters for the traits were also estimated. The presence of insect passages substantially increased the probability of stem rot without distinct clonal differences. A negative and mainly insignificant correlation was observed between rot and stem cracking. The highest broad-sense heritability (H2 = 0.21) and strong site-site genotypic correlation (0.86) showed that the probability of stem rot is genetically determined in the study material. Significant differences in diameter at breast height, the presence of stem rot, and its severity were found among the clones, albeit without undesirable positive correlation between growth and presence of decay. This indicated its potential to improve both productivity and rot resistance.

Highlights • After 47 years, whole tree harvesting (WTH) increased richness of ground cover species compared to conventionally managed stands. • Higher occurrence of the oligotrophic species after WTH suggested reduction of soil nutrient content, hence formation of different plant community. • WTH, apparently, facilitated recovery of species typical for later successional stages.

The long-term (50 years) effect of whole-tree harvesting (stump harvesting) on ground vegetation in experimental drained Norway spruce (Picea abies (L.) Karst.) stands was studied. We used a chronosequence approach to assess the long-term impact of whole-tree harvesting (WTH) on stands’ ground vegetation. WTH stands were compared with four control stands with different age and with the same forest type: young stand (15 years), middle-aged stand (45 years), mature stand (110 years) and over-mature stand (140 years). Species richness was similar between the WTH stand and middle-aged stand (61 and 60 species, respectively). Shannon-Wiener diversity indices in the WTH and middle-aged stand (3.40 and 3.19, respectively) indicated that the stands were similar to each other. A community similarity analysis showed that the composition of vegetation was similar between the WTH and middle-aged stand, although some species like Lycopodium clavatum and Diphasiastrum complanatum occurred only in the WTH stand. The study showed that a period of 50 years is sufficient for ground vegetation of a typical drained spruce forest to recover after WTH management.

Use of whole tree biomass becomes increasingly more important due to rising demand for renewable energy and materials to replace fossil resources. Therefore, assessment of influence of this approach on hemiboreal forest ecosystem is essential. The aim of our study was to assess the long-term influence of full biomass removal (FBR) on the ground vegetation and soil chemical composition in Scots pine stands. Study sites were located in Vacciniosa, Myrtillosa, and Myrtillosa mel. forest types. Almost half a century from the FBR, it had no notable or significant influence on number of ground vegetation species. Significant differences in overall vegetation composition between stands established after FBR and conventional harvesting (stem-wood removal) were not found by the detrended correspondence analysis (DCA) and analysis of similarities (ANOSIM). In addition, values of Ellenberg and Düll indicators were similar and, in most cases (determined by forest type and parameter), had no significant differences between FBR and the same age control stands. Similarly, no significant differences were found between these stands in soil carbon and nitrogen pools. Thus, there had not been a negative long-term effect of FBR on the hemiboreal Scots pine ecosystem as indicated by ground vegetation and soil.

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.