A comprehension of the effects planting density and nitrogen (N) fertilization have on the physiological and morphological characteristics of trees is critical for optimizing the require size and characteristics of wood products. We evaluated the growth traits and the leaf and wood characteristics of three clone poplars including Populus simonii × P. nigra ‘Xiaohei’, ‘Xiaohei-14’ and ‘Bailin-3’ under five planting densities (1666, 1111, 833, 666, and 555 tree ha−1) and four N fertilization rates (0, 100, 160, and 220 g tree−1 year−1). The results show that the clone type significantly affected all observed indicators, while planting density and N fertilization treatments had a significant effect on growth traits and leaf characteristics, but not on wood characteristics. Specifically, the clone ‘Bailin-3’ exhibited the largest annual increments in tree height and diameter at breast height (DBH), leaf width, N content, and soluble protein content. A decrease in initial planting density (from 1666 to 555 tree ha−1) led to an increased annual incremental tree height and DBH, regardless of clone type and N fertilization treatment. N fertilization treatment significantly impacted the annual increment in DBH, but not that of tree height. Further, the annual increments in tree height and DBH were positively correlated with leaf width, N content, chlorophyll content, and soluble protein content, and negatively correlated with hemicellulose content. In addition, the chlorophyll and soluble protein contents were identified as the most reliable predictors of the annual increments in tree height and DBH. Our results demonstrate the clone ‘Bailin-3’ with 555 tree ha−1 under 160 g N tree−1 yr−1 showed superior growth traits and leaf characteristics. Thus, it is recommended for future poplar silviculture of larger diameter timber production at similar sites. The results contribute to understanding of the effects of planting density and fertilization on the growth traits and the leaf and wood characteristics of three poplar clones, offering valuable guidance for the sustainable development and long-term productivity of poplar plantations.

The aim of this paper was to investigate the transcriptomic and metabolomic differences in Populus cathayana × canadasis ‘Xinlin1’ (P. cathayana × canadasis ‘Xinlin 1’) under varying irrigation and fertilization conditions. Ten-year-old P. cathayana × canadasis ‘Xinlin 1’ was selected as the test subject in this study; different irrigation and fertilization treatments were set up, and DEGs and DAMs in response to water and fertilizer regulation were identified. Transcriptomic and metabolomic profiles were analyzed from both leaves and roots. A total of 22,870 DEGs were identified in response to water and fertilizer treatments, predominantly belonging to 48 transcription factor families, including MYB, ERF, and MYB-related ones. Additionally, 2432 DAMs were detected and categorized into 18 metabolite classes, with flavonoids being the most abundant (342 metabolites), followed by terpenoids, lipids, and others. KEGG enrichment analysis revealed that DEGs and DAMs were significantly associated with pathways such as plant hormone signal transduction and starch and sucrose metabolism pathways. The levels of ABA exhibited an initial decrease followed by an increase, with several key genes, including PYR/PYL, PP2C, SnRK2, and ABF, also differentially expressed in the plant hormone signal transduction pathway. In the starch and sucrose metabolic pathways, sucrose was more hydrolyzed into D-fructose, which gradually translocated from roots to leaves. DEGs were significantly involved in sucrose synthesis and decomposition into D-fructose and 1,3-β-glucose, as well as starch synthesis and starch decomposition into cellulose dextrin, which underwent complete hydrolysis to glucose. In the starch hydrolysis process, 29 DEGs were involved, with 12 down-regulated and 17 up-regulated.

Needles play key roles in photosynthesis and branch growth in Larix olgensis. However, genetic variation and SNP marker mining associated with needle and branch-related traits have not been reported yet. In this study, we examined 131 samples of unrelated genotypes from L. olgensis provenance trails. We investigated phenotypic data for seven needle and one branch-related traits before whole genome resequencing (WGRS) was employed to perform a genome-wide association study (GWAS). Subsequently, the results were used to screen single nucleotide polymorphism (SNP) loci that were significantly correlated with the studied traits. We identified a total of 243,090,868 SNP loci, and among them, we discovered a total of 161 SNP loci that were significantly associated with these traits using a general linear model (GLM). Based on the GWAS results, Kompetitive Allele-Specific PCR (KASP), designed based on the DNA of population samples, were used to validate the loci associated with L. olgensis phenotypes. In total, 20 KASP markers were selected from the 161 SNPs loci, and BSBM01000635.1_4693780, BSBM01000114.1_5114757, and BSBM01000114.1_5128586 were successfully amplified, were polymorphic, and were associated with the phenotypic variation. These developed KASP markers could be used for the genetic improvement of needle and branch-related traits in L. olgensis.

Litter decomposition is a highly complex physical and biochemical process that plays a crucial role in promoting energy transformation in forest ecosystems. This study examines the impact of different concentrations of nitrogen and compound fertilizers on the quality of litter in a plantation of Populus euramericana ‘N3016’ × Populus ussuriensis. The major components and elemental contents of litter from different decomposition layers (the undecomposed layer and semidecomposed layer) were analyzed across various months. Overall, the application of nitrogen fertilizer or compound fertilizer did not significantly alter the cellulose, lignin, or potassium (K) contents of the litter in the different decomposition layers. Nitrogen fertilizer increased the average content of undecomposed layer (U-layer) nitrogen (N) and phosphorus (P) by 0.220% and 0.009%, respectively. Compound fertilizer increased the average content of U-layer nitrogen (N) by 0.055%. These findings suggest that while fertilization can increase the initial N and P contents in litter to some extent, it has a minimal overall impact on litter quality. Future research should be focused on the effects of climatic conditions, soil properties, soil fauna, and microbial activity on litter decomposition.

Pruning is a common forest-tending method; its purpose is to promote growth and improve the overall stand quality. Poplar is a fast-growing, broad-leaved tree species with high ecological and economic value. It is a common management method to promote its growth by pruning and adjusting the spatial structure of the stand, but its potential regulatory mechanism remains unclear. In this study, transcriptome and metabolome data of different parts at all pruning intensities were determined and analyzed. The results showed that 7316 differentially expressed genes were identified in this study. In the plant hormone signal transduction pathway, candidate genes were found in eight kinds of plant hormones, among which the main expression was gibberellin, auxin, and brassinosteroid. Some candidate gene structures (beta-glucosidase, endoglucanase, hexokinase, glucan endo-1, 3-beta-D-glucosidase, beta-fructofuranosidase, fructokinase, maltase-glucoamylase, phosphoglucomutase, and sucrose) were specifically associated with starch and sucrose biosynthesis. In the starch and sucrose biosynthesis pathway, D-fructose 6-phosphate, D-glucose 1,6-bisphosphate, and glucose-1-phosphate were the highest in stems and higher in the first round of pruning than in no pruning. The bHLH plays a key role in the starch and sucrose synthetic pathway, and AP2/ERF-ERF is important in the plant hormone signal transduction pathway. These results laid a foundation for understanding the molecular mechanism of starch and sucrose biosynthesis and provided a theoretical basis for promoting tree growth through pruning.

Poplar (Populus spp.) is a keystone commercial tree species in Northeast China, valued for its high economic returns. The genotype-by-environment (G × E) interaction critically governs its growth performance and ecological adaptability, which are pivotal for ensuring the long-term sustainability and economic viability of poplar plantations. In this study, the fibrous roots of the (P. simonii × P. nigra) × P. deltoides clone planted at three distinct sites, including Lishu (named SR1), Xinmin (named SR2), and Cuohai (named SR3), were used to perform transcriptome and metabolome. Comparative analysis revealed 6246, 3455, and 3854 differentially expressed genes (DEGs) in the SR1 vs. SR2, SR1 vs. SR3, and SR2 vs. SR3 comparisons, respectively. These DEGs were functionally enriched in pathways associated with antioxidant enzyme activity, stimulus response, plant hormone signal transduction pathways, and α-linolenic acid metabolism. Metabolomic analysis identified 106, 147, and 189 significantly differentially accumulated metabolites (DAMs) across the same comparisons, primarily linked to glutathione metabolism, butanoate metabolism, and pentose–glucuronate interconversions. Notably, we identified a core regulatory module comprising 57 genes and four key metabolites within the α-linolenic acid metabolic pathway, which exhibited strong correlations with phenotypic adaptability. These findings provide mechanistic insights into poplar’s plasticity under environmental heterogeneity, offering a molecular roadmap for future breeding strategies and the sustainable expansion of poplar cultivation.