Climatic and Soil Factors Shape the Demographical History and Genetic Diversity of a Deciduous Oak (Quercus liaotungensis) in Northern China

Exploring the connection between climatic conditions and genetic properties of Abies alba Mill., including warm and summer-dry Mediterranean environments

Abies alba Mill. is a prominent European tree species predominantly inhabiting cool and humid montane environments. However, paleoecological evidence reveals that during the Eemian and mid-Holocene, A. alba thrived in much warmer and drier climates. This capacity is nowadays reflected in cryptic meso- and sub-Mediterranean lowland populations. To link A. alba populations across diverse climates spanning from the Mediterranean lowlands to the Alpine timberline, we genotyped 421 specimens from Italy and Switzerland at 190 single-nucleotide polymorphisms (SNPs). Population genetic structure analyses indicate that isolated meso- and sub-Mediterranean lowland populations in Tuscany and Ticino align genetically with higher elevation populations in each region, suggesting that their capacity to thrive in warmer, drier conditions cannot be attributed to plantations with planting stock originating from different climates or to the occurrence of a single warm-adapted lineage showing a disjunct distribution, unless migration between Tuscany and Ticino stands occurred. Genotype-environment associations reveal that temperature seasonality, precipitation during critical seasons, and relative humidity are important for explaining the species' genetic variation. With genotype-environment and genotype-phenotype associations, we identified candidate adaptive genes potentially linked to climatic conditions and drought response. While certain adaptive alleles may have spread from Tuscany and Southern Italy or could be explained by a shared ancestry of Ticino and Tuscan populations, local adaptation may have occurred at specific loci. These findings underscore the importance of considering the hitherto overlooked lowland Mediterranean populations of A. alba to better understand the species' climatic niche and its potential for forest conservation and management under global warming.

Leaf water relations determine the trade-off between ozone resistance and stomatal functionality in urban tree species

Urban trees possess different capacities to mitigate ozone (O3) pollution through stomatal uptake. Stomatal closure protects trees from oxidative damage but limits their growth. To date, it is unclear how plant hydraulic function affect stomatal behaviour and determine O3 resistance. We assessed gas exchange and hydraulic traits in three subtropical urban tree species, Celtis sinensis, Quercus acutissima, and Q. nuttallii, under nonfiltered ambient air (NF) and elevated O3 (NF60). NF60 decreased photosynthetic rate (An) and stomatal conductance (gs) only in Q. acutissima and Q. nuttallii. Maintained An in C. sinensis suggested high O3 resistance and was attributed to higher leaf capacitance at the full turgor. However, this species exhibited a reduced stomatal sensitivity to vapour pressure deficit and an increased minimal gs under NF60. Such stomatal dysfunction did not decrease intrinsic water use efficiency (WUE) due to a tight coupling of An and gs. Conversely, Q. acutissima and Q. nuttallii showed maintained stomatal sensitivity and increased WUE, primarily correlated with gs and leaf water relations, including relative water content and osmotic potential at turgor loss point. Our findings highlight a trade-off between O3 resistance and stomatal functionality, with efficient stomatal control reducing the risk of hydraulic failure under combined stresses.

The Phylogeography of Deciduous Tree Ulmus macrocarpa (Ulmaceae) in Northern China

Disentangling how climate oscillations and geographical events significantly influence plants' genetic architecture and demographic history is a central topic in phytogeography. The deciduous ancient tree species Ulmus macrocarpa is primarily distributed throughout Northern China and has timber and horticultural value. In the current study, we studied the phylogenic architecture and demographical history of U. macrocarpa using chloroplast DNA with ecological niche modeling. The results indicated that the populations' genetic differentiation coefficient (NST) value was significantly greater than the haplotype frequency (GST) (p < 0.05), suggesting that U. macrocarpa had a clear phylogeographical structure. Phylogenetic inference showed that the putative chloroplast haplotypes could be divided into three groups, in which the group Ⅰ was considered to be ancestral. Despite significant genetic differentiation among these groups, gene flow was detected. The common ancestor of all haplotypes was inferred to originate in the middle-late Miocene, followed by the haplotype overwhelming diversification that occurred in the Quaternary. Combined with demography pattern and ecological niche modeling, we speculated that the surrounding areas of Shanxi and Inner Mongolia were potential refugia for U. macrocarpa during the glacial period in Northern China. Our results illuminated the demography pattern of U. macrocarpa and provided clues and references for further population genetics investigations of precious tree species distributed in Northern China.

Assessing the phylogenetic relationship among varieties of Toona ciliata (Meliaceae) in sympatry with chloroplast genomes

Toona ciliata is an endangered species due to over-cutting and low natural regeneration in China. Its genetic conservation is of an increasing concern. However, several varieties are recognized according to the leaf and flower traits, which complicates genetic conservation of T. ciliata. Here, we sequenced the whole chloroplast genome sequences of three samples for each of four varieties (T. ciliata var. ciliata, T. ciliata var. yunnanensis, T. ciliata var. pubescens, and T. ciliata var. henryi) in sympatry and assessed their phylogenetic relationship at a fine spatial scale. The four varieties had genome sizes ranged from 159,546 to 159,617 bp and had small variations in genome structure. Phylogenomic analysis indicated that the four varieties were genetically well-mixed in branch groups. Genetic diversity from the whole chloroplast genome sequences of 12 samples was low among varieties (average π = 0.0003). Besides, we investigated genetic variation of 58 samples of the four varieties in sympatry using two markers (psaA and trnL-trnF) and showed that genetic differentiation was generally insignificant among varieties (Ф st = 0%-5%). Purifying selection occurred in all protein-coding genes except for the ycf2 gene that was under weak positive selection. Most amino acid sites in all protein-coding genes were under purifying selection except for a few sites that were under positive selection. The chloroplast genome-based phylogeny did not support the morphology-based classification. The overall results implicated that a conservation strategy based on the T. ciliata complex rather than on intraspecific taxon was more appropriate.

Population Structure, Genetic Diversity and Candidate Genes for the Adaptation to Environmental Stress in Picea koraiensis

Picea koraiensis is major silvicultural and timber species in northeast China, and its distribution area is an important transition zone for genus spruce migration. The degree of intraspecific differentiation of P. koraiensis is high, but population structure and differentiation mechanisms are not clear. In this study, 523,761 single nucleotide polymorphisms (SNPs) were identified in 113 individuals from 9 populations of P. koraiensis by genotyping-by-sequencing (GBS). Population genomic analysis showed that P. koraiensis was divided into three geoclimatic regions: Great Khingan Mountains climatic region, Lesser Khingan Mountains climatic region, and Changbai Mountain climatic region. Mengkeshan (MKS) population on the northern edge of the distribution area and Wuyiling (WYL) population located in the mining area are two highly differentiated groups. Selective sweep analysis showed that MKS and WYL populations had 645 and 1126 selected genes, respectively. Genes selected in the MKS population were associated with flowering and photomorphogenesis, cellular response to water deficit, and glycerophospholipid metabolism; genes selected in the WYL population were associated with metal ion transport, biosynthesis of macromolecules, and DNA repair. Climatic factors and heavy metal stress drives divergence in MKS and WYL populations, respectively. Our findings provide insights into adaptive divergence mechanisms in Picea and will contribute to molecular breeding studies.

Drivers of population divergence and genetic variation in Elymus breviaristatus (Keng) Keng f. (Poaceae: Triticeae), an endemic perennial herb of the Qinghai-Tibet plateau

Elymus breviaristatus, a rare grass species with excellent resistance and ecological importance, is narrowly distributed on the Qinghai-Tibet plateau. Populations of E. breviaristatus are declining due to habitat fragmentation, and thus far, characteristics of genetic differentiation and adaptive responses to climate change remain poorly understood in this species. Here, we explored the genetic structure of 18 natural populations (269 individuals) in the transition zone between Tibet and the Hengduan Mountains using 15 expressed sequence tag (EST)-SSR primer pairs and identified possible barriers to gene flow that might have caused genetic discontinuities. Additional analyses were performed to identify the environmental factors affecting genetic diversity and to test whether the patterns of genetic variation among populations were more consistent with the isolation by distance (IBD) or isolation by environment (IBE) model. Multiple measures of genetic diversity revealed that intra-population genetic variation was low, while inter-population genetic variation was high. Clustering, structure, and principal coordinate analyses identified three genetic groups: (a) Eastern Qamdo, (b) Nagqu and Western Qamdo, and (c) Lhasa and Nyingchi. A clear physical barrier to gene flow was formed by the Yarlung Zangbo Grand Canyon and the Tanggula Mountains. We found that both IBD and IBE contributed to the observed patterns of genetic variation, and the IBE model played a leading role. In addition, precipitation-related variables, soil phosphorus content and soil K:P ratio significantly affected population genetic variation. Overall, our results emphasized the genetic fragility of E. breviaristatus populations and showed that this species requires attention, as future climate changes and human activities may further threaten its survival. In addition, the genetic differences among E. breviaristatus populations should be considered when formulating conservation measures for E. breviaristatus populations in the study area.

Effects of Drought Stress and Rehydration on Physiological and Biochemical Properties of Four Oak Species in China

Quercus fabri Hance, Quercus serrata Thunb, Quercus acutissima Carruth, and Quercus variabilis BL are four Chinese oak species commonly used for forestation. To ensure the survival of seedlings, we first need to understand the differences in drought resistance of the four oak species at the seedling stage, and comprehensively evaluate their drought resistance capabilities. The four oak seedlings were divided into drought-rewatering treatment group and well watered samples (control group). For the seedlings of the drought-rewatering treatment group, drought stress lasting 31 days was used, and then re-watering for 5 days. The water parameters, osmotic solutes content, antioxidant enzyme activity and photosynthesis parameters of the seedlings in the two groups were measured every 5 days. Compared with the control group, the relative water content, water potential, net photosynthetic rate, transpiration rate, and stomatal conductance levels of the four oaks all showed a downward trend under continuous drought stress, and showed an upward trend after rehydration. The soluble protein, soluble sugar, proline, peroxidase, superoxide dismutase and catalase content of the four oaks increased first and then decreased under drought stress, and then increased after rehydration. The content of glycine betaine and malondialdehyde continued to increase, and gradually decreased after rehydration. The weight of each index was calculated by principal component analysis, and then the comprehensive evaluation of each index was carried out through the membership function method. The drought resistance levels of the four oak species were as follows: Q. serrata > Q. fabri > Q. variabilis > Q. acutissima.

Genetic Differentiation and Demographic History of Three Cerris Oak Species in China Based on Nuclear Microsatellite Makers

Knowledge of interspecific divergence and population expansions/contractions of dominant forest trees in response to geological events and climatic oscillations is of major importance to understand their evolution and demography. However, the interspecific patterns of genetic differentiation and spatiotemporal population dynamics of three deciduous Cerris oak species (Q. acutissima, Q. variabilis and Q. chenii) that are widely distributed in China remain poorly understood. In this study, we genotyped 16 nuclear loci in 759 individuals sampled from 44 natural populations of these three sibling species to evaluate the plausible demographical scenarios of the closely related species. We also tested the hypothesis that macro- and microevolutionary processes of the three species had been triggered and molded by Miocene–Pliocene geological events and Quaternary climatic change. The Bayesian cluster analysis showed that Q. acutissima and Q. chenii were clustered in the same group, whereas Q. variabilis formed a different genetic cluster. An approximate Bayesian computation (ABC) analyses suggested that Q. variabilis and Q. acutissima diverged from their most common ancestor around 19.84 Ma, and subsequently Q. chenii diverged from Q. acutissima at about 9.6 Ma, which was significantly associated with the episodes of the Qinghai–Tibetan Plateau (QTP). In addition, ecological niche modeling and population history analysis showed that these three Cerris oak species repeatedly underwent considerable ‘expansion–contraction’ during the interglacial and glacial periods of the Pleistocene, although they have varying degrees of tolerance for the climatic change. Overall, these findings indicated geological and climatic changes during the Miocene–Pliocene and Pleistocene as causes of species divergence and range shifts of dominant tree species in the subtropical and warm temperature areas in China.

Phylogeography of Prunus armeniaca L. revealed by chloroplast DNA and nuclear ribosomal sequences

To clarify the phytogeography of Prunus armeniaca L., two chloroplast DNA fragments (trnL-trnF and ycf1) and the nuclear ribosomal DNA internal transcribed spacer (ITS) were employed to assess genetic variation across 12 P. armeniaca populations. The results of cpDNA and ITS sequence data analysis showed a high the level of genetic diversity (cpDNA: H_T = 0.499; ITS: H_T = 0.876) and a low level of genetic differentiation (cpDNA: F_ST = 0.1628; ITS: F_ST = 0.0297) in P. armeniaca. Analysis of molecular variance (AMOVA) revealed that most of the genetic variation in P. armeniaca occurred among individuals within populations. The value of interpopulation differentiation (N_ST) was significantly higher than the number of substitution types (G_ST), indicating genealogical structure in P. armeniaca. P. armeniaca shared genotypes with related species and may be associated with them through continuous and extensive gene flow. The haplotypes/genotypes of cultivated apricot populations in Xinjiang, North China, and foreign apricot populations were mixed with large numbers of haplotypes/genotypes of wild apricot populations from the Ili River Valley. The wild apricot populations in the Ili River Valley contained the ancestral haplotypes/genotypes with the highest genetic diversity and were located in an area considered a potential glacial refugium for P. armeniaca. Since population expansion occurred 16.53 kyr ago, the area has provided a suitable climate for the population and protected the genetic diversity of P. armeniaca.

Localized environmental heterogeneity drives the population differentiation of two endangered and endemic Opisthopappus Shih species

BACKGROUND

Climate heterogeneity not only indirectly shapes the genetic structures of plant populations, but also drives adaptive divergence by impacting demographic dynamics. The variable localized climates and topographic complexity of the Taihang Mountains make them a major natural boundary in Northern China that influences the divergence of organisms distributed across this region. Opisthopappus is an endemic genus of the Taihang Mountains that includes only two spatially partitioned species Opisthopappus longilobus and Opisthopappus taihangensis. For this study, the mechanisms behind the genetic variations in Opisthopappus populations were investigated.

RESULTS

Using SNP and InDel data coupled with geographic and climatic information, significant genetic differentiation was found to exist either between Opisthopappus populations or two species. All studied populations were divided into two genetic groups with the differentiation of haplotypes between the groups. At approximately 17.44 Ma of the early Miocene, O. taihangensis differentiated from O. longilobus under differing precipitation regimes due to the intensification of the Asian monsoon. Subsequently, intraspecific divergence might be induced by the dramatic climatic transformation from the mid- to late Miocene. During the Pleistocene period, the rapid uplift of the Taihang Mountains coupled with violent climatic oscillations would further promote the diversity of the two species. Following the development of the Taihang Mountains, its complex topography created geographical and ecological heterogeneity, which could lead to spatiotemporal isolation between the Opisthopappus populations. Thus the adaptive divergence might occur within these intraspecific populations in the localized heterogeneous environment of the Taihang Mountains.

CONCLUSIONS

The localized environmental events through the integration of small-scale spatial effects impacted the demographic history and differentiation mechanism of Opisthopappus species in the Taihang Mountains. The results provide useful information for us to understand the ecology and evolution of organisms in the mountainous environment from population and species perspective.

Unraveling the genetic diversity and structure of Quercus liaotungensis population through analysis of microsatellite markers

Background

Quercus liaotungensis Koidz. is an ecologically and economically important tree species widely distributed in Northern China. However, the effective assessment, utilization, and protection of Q. liaotungensis resources remain unexplored.

Methods

In total, 120 samples obtained from 12 Q. liaotungensis populations of Northern China were investigated for genetic diversity and structure using 19 simple sequence repeat (SSR) primer pairs.

Results

The total number of alleles detected was 293, the average number of effective allele (Ne) was 6.084, the genetic differentiation coefficient (Fst) was 0.033, and the mean observed heterozygosity (Ho) and expected heterozygosity (He) were 0.690 and 0.801, respectively. Moreover, analysis of molecular variance (AMOVA) showed a 5.5% genetic variation among 12 Q. liaotungensis populations, indicating that a high level of genetic diversity and a low degree of genetic differentiation among Q. liaotungensis populations. STRUCTURE and cluster analysis divided the 12 Q. liaotungensis populations into the following three subpopulations: Bashang Plateau subpopulation (SH), Liaodong Peninsula subpopulation (NC), and Loess Plateau subpopulation (other 10 populations). The cluster analysis based on 19 climatic factors was consistent with the genetic structure. A positive correlation was found between genetic distance and geographical distance (r = 0.638, p = 0.028) by the Mantel test, and two boundaries were found among the 12 Q. liaotungensis populations by the Barrier analysis, indicating that Q. liaotungensis populations existed isolated by geographical distance and physical barrier.

Conclusion

This study suggests that geographical isolation, physical barrier, climatic types, and natural hybridization promote the formation of genetic structures, which can contribute to future protection and genetic improvement of Q. liaotungensis.

Genetic, Morphological, and Environmental Differentiation of an Arid-Adapted Oak with a Disjunct Distribution

The patterns of genetic and morphological diversity of a widespread species can be influenced by environmental heterogeneity and the degree of connectivity across its geographic distribution. Here, we studied Quercus havardii Rydb., a uniquely adapted desert oak endemic to the Southwest region of the United States, using genetic, morphometric, and environmental datasets over various geographic scales to quantify differentiation and understand forces influencing population divergence. First, we quantified variation by analyzing 10 eastern and 13 western populations from the disjunct distribution of Q. havardii using 11 microsatellite loci, 17 morphological variables, and 19 bioclimatic variables. We then used regressions to examine local and regional correlations of climate with genetic variation. We found strong genetic, morphological and environmental differences corresponding with the large-scale disjunction of populations. Additionally, western populations had higher genetic diversity and lower relatedness than eastern populations. Levels of genetic variation in the eastern populations were found to be primarily associated with precipitation seasonality, while levels of genetic variation in western populations were associated with lower daily temperature fluctuations and higher winter precipitation. Finally, we found little to no observed environmental niche overlap between regions. Our results suggest that eastern and western populations likely represent two distinct taxonomic entities, each associated with a unique set of climatic variables potentially influencing local patterns of diversity.

Spatial Genetic Structure and Demographic History of the Dominant Forest Oak Quercus fabri Hance in Subtropical China

Oak trees (Quercus L.) are important models for estimating abiotic impacts on the population structure and demography of long life span tree species. In this study, we generated genetic data for 17 nuclear microsatellite loci in 29 natural populations of Quercus fabri to estimate the population genetic structure. We also integrated approximate Bayesian computation (ABC) and ecological niche analysis to infer the population differentiation processes and demographic history of this oak species. The genetic analyses indicated two genetic clusters across the 29 populations collected, where most approximately corresponded to the intraspecific differentiation among populations from western and eastern China, whereas admixed populations were mainly found in central mountains of China. The best model obtained from hierarchical ABC simulations suggested that the initial intraspecific divergence of Q. fabri potentially occurred during the late Pliocene (ca. 3.99 Ma) to form the two genetic clusters, and the admixed population group might have been generated by genetic admixture of the two differentiated groups at ca. 53.76 ka. Ecological analyses demonstrated clear differentiation among the Q. fabri population structures, and association estimations also indicated significant correlations between geography and climate with the genetic variation in this oak species. Our results suggest abiotic influences, including past climatic changes and ecological factors, might have affected the genetic differentiation and demographic history of Q. fabri in subtropical China.

Demographic history and genetic differentiation of an endemic and endangered Ulmus lamellosa (Ulmus)

BACKGROUND

Ulmus lamellosa (one of the ancient species of Ulmus) is an endemic and endangered plant that has undergone climatic oscillations and geographical changes. The elucidation of its demographic history and genetic differentiation is critical for understanding the evolutionary process and ecological adaption to forests in Northern China.

RESULTS

Polymorphic haplotypes were detected in most populations of U. lamellosa via DNA sequencing. All haplotypes were divided into three phylogeographic clades fundamentally corresponding to their geographical distribution, namely THM (Taihang Mountains), YM (Yinshan Mountains), and YSM (Yanshan Mountains) groups. The YSM group, which is regarded as ancestral, possessed higher genetic diversity and significant genetic variability in contrast to the YSM and YM groups. Meanwhile, the divergence time of intraspecies haplotypes occurred during the Miocene-Pliocene, which was associated with major Tertiary geological and/or climatic events. Different degrees of gene exchanges were identified between the three groups. During glaciation, the YSM and THM regions might have served as refugia for U. lamellosa. Based on ITS data, range expansion was not expected through evolutionary processes, except for the THM group. A series of mountain uplifts (e.g., Yanshan Mountains and Taihang Mountains) following the Miocene-Pliocene, and subsequently quaternary climatic oscillations in Northern China, further promoted divergence between U. lamellosa populations.

CONCLUSIONS

Geographical topology and climate change in Northern China played a critical role in establishing the current phylogeographic structural patterns of U. lamellosa. These results provide important data and clues that facilitate the demographic study of tree species in Northern China.

Framework Phylogeny, Evolution and Complex Diversification of Chinese Oaks

Oaks (Quercus L.) are ideal models to assess patterns of plant diversity. We integrated the sequence data of five chloroplast and two nuclear loci from 50 Chinese oaks to explore the phylogenetic framework, evolution and diversification patterns of the Chinese oak's lineage. The framework phylogeny strongly supports two subgenera Quercus and Cerris comprising four infrageneric sections Quercus, Cerris, Ilex and Cyclobalanopsis for the Chinese oaks. An evolutionary analysis suggests that the two subgenera probably split during the mid-Eocene, followed by intergroup divergence within the subgenus Cerris around the late Eocene. The initial diversification of sections in the subgenus Cerris was dated between the mid-Oligocene and the Oligocene-Miocene boundary, while a rapid species radiation in section Quercus started in the late Miocene. Diversification simulations indicate a potential evolutionary shift on section Quercus, while several phenotypic shifts likely occur among all sections. We found significant negative correlations between rates of the lineage diversification and phenotypic turnover, suggesting a complex interaction between the species evolution and morphological divergence in Chinese oaks. Our infrageneric phylogeny of Chinese oaks accords with the recently proposed classification of the genus Quercus. The results point to tectonic activity and climatic change during the Tertiary as possible drivers of evolution and diversification in the Chinese oak's lineage.

Landscape Features and Climatic Forces Shape the Genetic Structure and Evolutionary History of an Oak Species (Quercus chenii) in East China

Major topographic features facilitate intraspecific divergence through geographic isolation. This process may be enhanced by environmental isolation along climatic gradients, but also may be reduced by range shifts under rapid climatic changes. In this study, we examined how topography and climate have interacted over time and space to influence the genetic structure and evolutionary history of Quercus chenii, a deciduous oak species representative of the East China flora. Based on the nuclear microsatellite variation at 14 loci, we identified multiple genetic boundaries that were well associated with persistent landscape barriers of East China. Redundancy analysis indicated that both geography and climate explained similar amounts of intraspecific variation. Ecological differences along altitudinal gradients may have driven the divergence between highlands and lowlands. However, range expansions during the Last Interglacial as inferred from approximate Bayesian computation (ABC) may have increased the genetic diversity and eliminated the differentiation of lowland populations via admixture. Chloroplast (cp) DNA analysis of four intergenic spacers (2,866 bp in length) identified a total of 18 haplotypes, 15 of which were private to a single population, probably a result of long-term isolation among multiple montane habitats. A time-calibrated phylogeny suggested that palaeoclimatic changes of the Miocene underlay the lineage divergence of three major clades. In combination with ecological niche modeling (ENM), we concluded that mountainous areas with higher climatic stability are more likely to be glacial refugia that preserved higher phylogenetic diversity, while plains and basins may have acted as dispersal corridors for the post-glacial south-to-north migration. Our findings provide compelling evidence that both topography and climate have shaped the pattern of genetic variation of Q. chenii. Mountains as barriers facilitated differentiation through both geographic and environmental isolation, whereas lowlands as corridors increased the population connectivity especially when the species experienced range expansions.

Genetic diversity and structure of baobab (Adansonia digitata L.) in southeastern Kenya

Baobab (Adansonia digitata L.) is an iconic tree of African savannahs. Its multipurpose character and nutritional composition of fruits and leaves offer high economic and social potential for local communities. There is an urgent need to characterize the genetic diversity of the Kenyan baobab populations in order to facilitate further conservation and domestication programmes. This study aims at documenting the genetic diversity and structure of baobab populations in southeastern Kenya. Leaf or bark samples were collected from 189 baobab trees in seven populations distributed in two geographical groups, i.e. four inland and three coastal populations. Nine microsatellite loci were used to assess genetic diversity. Overall, genetic diversity of the species was high and similarly distributed over the populations. Bayesian clustering and principal coordinate analysis congruently divided the populations into two distinct clusters, suggesting significant differences between inland and coastal populations. The genetic differentiation between coastal and inland populations suggests a limited possibility of gene flow between these populations. Further conservation and domestications studies should take into consideration thegeographical origin of trees and more attention should be paid to morphological characterization of fruits and leaves of the coastal and inland populations to understand the causes and the impact of the differentiation.

Spatial patterns and interspecific associations among trees at different stand development stages in the natural secondary forests on the Loess Plateau, China

Quercus wutaishansea populations on the Loess Plateau are currently becoming more dominant in natural secondary forests, whereas Pinus tabulaeformis is declining. In the present paper, the diameter class (instead of age) was used to classify the different growth stages as juvenile, subadult, or adult, and the univariate function g(r) was used to analyze the dynamic changes in spatial patterns and interspecific associations in three 1-ha tree permanent plots on the Loess Plateau, NW China. Our results suggested that the niche breadth changed with the development stage. The diameter distribution curve was consistent with the inverted "J" type, indicating that natural regeneration was common in all three plots. There was a close relationship between the spatial pattern and scale, which showed significant aggregation at small distances, and became more random as distance increased, but in the Pinus + Quercus mixed forests, the whole species were aggregated at distances up to 50 m. The degree of spatial clumping decreased from juvenile to subadult and from subadult to adult. The spatial pattern also differed at different growth stages, likely due to strong intraspecific competition. Associations among different growth stages were positively correlated at small scales. Our study is important to the understanding of the development of the Q. wutaishansea forests; thus, the spatial dynamic change features should be received greater attention when planning forest management and developing restoration strategies on the Loess Plateau.