Effect of selective logging on the genetic diversity of Scaphium macropodum

We examined the effect of selective logging on the genetic diversity of Scaphium macropodum using RAPD markers via two approaches: (1) to investigate the immediate effect by studying a same population before and after logging, and (2) to determine the long term effect by comparing two regenerated stands with an adjacent unlogged stand, assuming that they were genetically identical before logging. Results showed no negative immediate impact for the first approach, probably due to the high abundance and heterogeneity of S. macropodum in the compartment investigated. However, for the latter approach, substantial genetic erosion (i.e. 31.5% reduction for Shannon diversity, H) was detected in one of the regenerated stands corresponding to its extremely low tree density for S. macropodum. This implies the possible occurrence of genetic drift and increased inbreeding due to population decline as a result of logging. However, the observed genetic differences among the three sub-populations having prevailed before logging cannot be totally discounted in the second approach. This study also demonstrates the use of tree density as a good surrogate measure of genetic diversity. The present harvesting system in Malaysia based on a general cutting limit need to be refined; the basis for determining cutting limit in a forest management unit should consider abundance of commercial species.     

择伐作业对常绿阔叶林乔木层树种结构及物种多样性的影响

研究4种不同强度择伐作业(弱度13.0%、中度29.1%、强度45.8%和极强度67.1%)对常绿阔叶林林分结构及乔木层物种多样性的影响结果表明,轻度与中度择伐对林分结构的影响较小,原林分乔木层优势树种的地位仍保持甚至略有提高;而强度择伐和极强度择伐则引起林分结构一定的变化,一些优势树种的地位削弱,而另一些树种的优势地位上升。但采伐作业不利于常绿阔叶林物种多样性的发展,尤其是林冠强度破坏对物种多样性的影响最大,而轻度与中度择伐作业有利于原有物种保持与恢复,这可能与不同强度择伐作业引起林地光照条件、温度和湿度等环境因子的不同变化有关。     

Analysis of genetic diversity in the endangered tropical tree species <Emphasis Type="Italic">Hagenia abyssinica</Emphasis> using ISSR markers

Genetic diversity within and among 12 populations of the dioecious tropical tree species Hagenia abyssinica (Bruce) J.F. Gmel. in Ethiopia was examined with eight inter simple sequence repeat (ISSR) primers. A total of 104 clearly scorable bands were generated, among which 84 (81%) were polymorphic. Jaccard similarity coefficient was calculated for pairwise comparisons among all 120 individuals and ranged from 0.30 to 0.88 while average within-population similarity ranged from 0.53 to 0.66. Within-population variability was estimated as percentage polymorphic loci (ranging from 52% to 87%), Shannon’s information index (0.30–0.50) and Nei’s genetic diversity (0.21–0.35). The highest variability values were obtained for one recently planted population and for one wild population growing in an undisturbed primary forest area. Significant overall differentiation among populations was detected by both Shannon’s information index (0.26) and G _ST (0.25). Relatedness among samples was estimated with a principal coordinate analysis, and relatedness among populations was estimated with a cluster analysis (UPGMA). A Mantel test indicated a significant association between genetic and geographic distances, and an autocorrelation analysis showed significant evidence of gene flow over distances up to 30 km. This study is the first of its kind for H. abyssinica, which has decreased recently in Ethiopia and now must be regarded as an endangered species. Both within-population and between-population diversity estimates are typical of outcrossing, longlived and late successional species, suggesting that recent anthropogenic disturbances have not yet had much impact on population genetic parameters. DNA marker data can, however, be used to identify the most suitable sites for in situ conservation and for collection of material for establishment of genebanks and plant improvement programs.     

Effects of habitat fragmentation on pollen flow and genetic diversity of the endangered tropical tree Swietenia humilis (Meliaceae)

Fragmentation of tropical forest represents a major threat to some tree populations by reducing local population size and gene flow from other populations. Both processes can decrease outcrossing rates and genetic variation in remnant stands. Despite these risks, some tree species have pollen vectors that mitigate these negative consequences for fragmented populations. In this paper, we assess both pollen flow and diversity of pollen sources in continuous forest and isolated stands of Swietenia humilis, a tropical tree species pollinated by small insects. Using seven nuclear microsatellite markers, we test the hypothesis that genetic diversity and the number of pollen donors are lower in remnant populations. Results show that allelic richness of seeds is lower in isolated populations (6.1 vs. 8.3 alleles per locus), even though adult populations do not show this difference.Pollen pool structure is greater in isolated patches (Φ_Iso = 0.26) than in continuous forest (Φ_For = 0.14), which yields estimates of the average effective number of pollen donors (N_ep) of 1.9 and 3.6 respectively. In addition, estimates of number of sires per mother indicate that isolated trees have half the number of pollen sources (4.98) than trees in the forest (9.8). Although extensive pollen movement (>2000 m) was recorded on both habitat conditions, indicating that fragmented patches are not isolated from pollen-mediated gene flow, this extensive pollen flow among trees in fragmented landscapes may not serve to counteract deleterious reproductive and genetic consequences of habitat fragmentation.     

The effect of stand structure and tree species composition on epiphytic bryophytes in mixed deciduous-coniferous forests of Western Hungary

The effect of tree species, stand structure, landscape and historical variables was studied on the species composition, species richness and cover of epiphytic bryophyte assemblages in mixed deciduous-coniferous forests of Western Hungary. Stand and tree level assemblages were analyzed by ordinations and generalized linear modeling in 35 70-110 year old stands of different management regimes.Bryophytes showed a considerable preference to different host trees, so that stand level diversity of bryophyte assemblages was determined mainly by tree species diversity, and their composition by tree species composition. Cover and diversity of epiphytic bryophytes were the highest on oaks (Quercus petraea and Quercus robur), and the lowest on Scotch pine (Pinus sylvestris). The presence of sapling (shrub) layer increased, whereas a large number of medium sized trees decreased bryophyte species richness in this study. Tree size was much less influential which is explained by the lack of large, veteran trees. Forest management maintaining tree species diversity, structural heterogeneity and temporal continuity of the stands could considerably contribute to the conservation of this organism group. Selective cutting is more appropriate for these conservational purposes than shelterwood management system.     

Impact of litter species diversity on decomposition processes and communities of soil organisms

The effect of plant species diversity and the C/N ratio of litter on soil processes were analysed in mesocosms in a three year field experiment. Plots 0.5 × 0.5 m with a depth of 15 cm containing sand mixed with loam were used to compare five natural and one artificial litter type (polypropylene string). Natural litters were either monospecific (I: Dactylis glomerata; II: Festuca rubra and III: Trifolium pratense) or were species mixtures (IV: mixture of three species I, II and III; V: mixture of twelve species, IV and nine other meadow plants). Differences among treatments in the litter decomposition rate, humic acid content and nematode density depended on the litter quality (C:N ratio) in most cases. By contrast, most of the differences among treatments found in the substratum below the litter cover resulted from litter diversity. The largest increase of carbon and nitrogen amount during growing season was found under litter mixtures (IV, V) and the highest fulvic acid content under the most diverse litter (V). Similarly, the production of algae in the substratum also significantly increased with litter diversity. Higher taxonomic diversity of Nematoda and Collembola and the most mature community of nematodes were observed under the most complex litter. Epigeic macrofauna, both dwelling in and penetrating the litter, did not differ significantly between experimental treatments. However, the highest share of predators was found in the treatment with the richest plant species diversity. In general, the results suggest that the decomposition of diverse plant litter enhances humus acid accumulation in soils. It is likely that algae participate in the process of humus formation.     

Chemical composition and diversity influence non-additive effects of litter mixtures on soil carbon and nitrogen cycling: Implications for plant species loss

Diverse plant litter mixtures frequently decompose differently than expected compared to the average of the component species decomposing alone, and it remains unclear why decomposition may respond non-additively to diversity. Here, we hypothesized that litter chemical composition and chemical diversity would be important determinants of the strength and direction (synergistic versus antagonistic) of non-additive soil carbon (C) and nitrogen (N) cycling responses to litter mixtures. To test this, we performed a soil incubation experiment using litter mixtures comprised of up to four plant species, and we measured three components of decomposition: respiration, net N mineralization, and microbial biomass N accumulation. We used nine chemical traits to calculate the chemical composition and diversity of the litter mixtures. First, we found that respiration responded as the average of the individual species in the mixture (i.e. additively), rather than non-additively as initially predicted. Second, litter mixtures stimulated significantly more net N immobilization than expected in 64% of cases, and non-additive responses were highly dependent on mixture chemical composition, and were influenced to a lesser degree by chemical diversity. Specifically, concentrations of tannins and certain low molecular weight phenolics in the mixtures were positively correlated with greater N immobilization than expected. Non-additive N mineralization responses were poorly correlated with traditional measures of litter chemistry like N concentration, C:N, lignin:N, and phenolic:N. Our results also show that non-additive N mineralization responses were affected by loss of some species significantly more than others, and the effects of species loss could depend on 1) whether a species contains compounds with strong effects on non-additive responses; and 2) whether those compounds are also found in other species. Finally, litter mixtures stimulated more microbial biomass N than expected in 45% of cases, but non-additive responses were only weakly dependent on the litter chemistry variables that we measured.