Tree diversity promotes predatory wasps and parasitoids but not pollinator bees in a subtropical experimental forest

From regional to global scales, anthropogenic environmental change is causing biodiversity loss and reducing ecosystem functionality. Previous studies have investigated the relationship between plant diversity and functional insect communities in temperate and also in tropical grasslands and forests. However, few studies have explored these dynamics in subtropical forests. Here, cavity-nesting Hymenoptera and associated parasitoids were collected across a controlled tree diversity experiment in subtropical China to test how predatory wasps, bees and parasitoids respond to tree species richness. Abundance and species richness of predatory wasps and parasitoids were positively correlated with tree species richness, while bee abundance and bee species richness were unrelated to tree species richness. Our results indicate that tree species richness increases the abundance and species richness of important communities such as predators and parasitoids. Moreover, the results highlight the importance of subtropical forests in maintaining abundance and species richness of key functional insect groups.     

Tree diversity promotes predator but not omnivore ants in a subtropical Chinese forest

1. Epigeic ants are functionally important arthropods in tropical and subtropical forests, particularly by acting as predators. High predation pressure has been hypothesised to be a mechanism facilitating high diversity across trophic levels. 2. In this study, standardised pitfall traps were used in a highly diverse subtropical forest to test if and how ant species richness is related to tree species richness and a comprehensive set of other environmental variables such as successional age, soil properties or elevation. 3. A total of 13 441 ant individuals belonging to 3839 species occurrences and 71 species were collected, of which 26 species were exclusive predators and 45 species were omnivores. 4. Occurrence and species richness of total and omnivore ants were positively related to soil pH. Predator ant occurrence was unrelated to all environmental variables tested. 5. The species richness of predator ants increased with tree species richness but decreased with leaf functional diversity and shrub cover. Elevation negatively influenced only total ant species richness. 6. The evenness of predators increased with tree species richness, while the evenness of all ants decreased with shrub cover. Omnivore ant evenness decreased with tree evenness, but increased with successional age. 7. The results highlight the value of diverse forests in maintaining species richness and community evenness of a functionally important predator group. Moreover, the results stress the importance of analysing trophic groups separately when investigating biodiversity effects.     

Tree diversity promotes generalist herbivore community patterns in a young subtropical forest experiment

Stand diversification is considered a promising management approach to increasing the multifunctionality and ecological stability of forests. However, how tree diversity affects higher trophic levels and their role in regulating forest functioning is not well explored particularly for (sub)tropical regions. We analyzed the effects of tree species richness, community composition, and functional diversity on the abundance, species richness, and beta diversity of important functional groups of herbivores and predators in a large-scale forest biodiversity experiment in south-east China. Tree species richness promoted the abundance, but not the species richness, of the dominant, generalist herbivores (especially, adult leaf chewers), probably through diet mixing effects. In contrast, tree richness did not affect the abundance of more specialized herbivores (larval leaf chewers, sap suckers) or predators (web and hunting spiders), and only increased the species richness of larval chewers. Leaf chemical diversity was unrelated to the arthropod data, and leaf morphological diversity only positively affected oligophagous herbivore and hunting spider abundance. However, richness and abundance of all arthropods showed relationships with community-weighted leaf trait means (CWM). The effects of trait diversity and CWMs probably reflect specific nutritional or habitat requirements. This is supported by the strong effects of tree species composition and CWMs on herbivore and spider beta diversity. Although specialized herbivores are generally assumed to determine herbivore effects in species-rich forests, our study suggests that generalist herbivores can be crucial for trophic interactions. Our results indicate that promoting pest control through stand diversification might require a stronger focus on identifying the best-performing tree species mixtures.     

Multitrophic effects of experimental changes in plant diversity on cavity-nesting bees, wasps, and their parasitoids

Plant diversity changes can impact the abundance, diversity, and functioning of species at higher trophic levels. We used an experimental gradient in grassland plant diversity ranging from 1 to 16 plant species to study multitrophic interactions among plants, cavity-nesting bees and wasps, and their natural enemies, and analysed brood cell density, insect diversity (species richness), and bee and wasp community similarity over two consecutive years. The bee and wasp communities were more similar among the high (16 species) diversity plots than among plots of the lower diversity levels (up to 8 species), and a more similar community of bees and wasps resulted in a more similar community of their parasitoids. Plant diversity, which was closely related to flower diversity, positively and indirectly affected bee diversity and the diversity of their parasitoids via increasing brood cell density of bees. Increasing plant diversity directly led to higher wasp diversity. Parasitism rates of bees and wasps (hosts) were not affected by plant diversity, but increased with the diversity of their respective parasitoids. Decreases in parasitism rates of bees arose from increasing brood cell density of bees (hosts), whereas decreasing parasitism rates of wasps arose from increasing wasp diversity (hosts). In conclusion, decreases in plant diversity propagated through different trophic levels: from plants to insect hosts to their parasitoids, decreasing density and diversity. The positive relationship between plant diversity and the community similarity of higher trophic levels indicates a community-stabilising effect of high plant diversity.     

Tree species influences soil microbial community diversity but not biomass in a karst forest in southwestern China

中国西南喀斯特森林树种对土壤微生物群落多样性和生物量的影响陆地生态系统中植物种对土壤微生物群落结构的影响不一,而喀斯特生态系统中植物种对土壤微生物群落结构影响的研究尚未见报道。本研究利用磷酸脂肪酸(PLFA)法,分析了黔中高原型喀斯特常绿落叶阔叶混交林5个优势树种—窄叶石栎(Lithocarpus confinis Huang)、圆果化香(Platycarya longipes Wu)、滇鼠刺(Itea yunnanensis Franch.)、安顺润楠(Machilus cavaleriei H. Lév.)、云贵鹅耳枥(Carpinus pubescens Burkill)—与土壤理化性质对土壤微生物群落组成和生物量的影响。在测试的212个土壤样品 中共检测出132种PLFA,每个样品土壤微生物平均PLFA数量和生物量分别为65.97和11.22 µg g^–1。土壤表层(0–10 cm)的土壤微生物PLFA数量与下层(10–20 cm)土壤接近,但前者土壤微生物生物量显著高于后者(P < 0.05)。树种影响土壤微生物PLFA数量,但对土壤微生物生物量没有影响。云贵鹅耳枥附近的土壤微生物PLFA数量显著高于其他树种(P < 0.05),而其他树种土壤微生物PLFA数量接近。土壤微生物 生物量与表层土壤的理化性质无显著相关,但与下层土壤的有机碳、全氮和全磷含量呈显著正相关 (P < 0.05)。总之,黔中高原型喀斯特森林真菌/细菌生物量比率低,微生物总生物量低,但微生物群落多样性高。树种对土壤微生物群落多样性产生影响。     

Horizontal, but not vertical canopy structure is related to stand functional diversity in a subtropical slope forest

The aim of this study was to analyse the relation of horizontal and vertical canopy structure to tree functional diversity of a highly diverse subtropical broad-leaved slope forest, stratified for different successional stages. This is of particular interest because many key ecosystem processes and functions are related to the arrangement of forest canopies. We assessed the effect of stand-related functional diversity (FD_Q, measured as Rao’s quadratic entropy of leaf traits), together with other environmental variables on horizontal [measured as relative crown projection areas (CPA_r)] and vertical [relative crown overlap, coefficients of variation (CV) of crown positioning variables] structure of the upper canopy at the local neighbourhood level. The analyses with mixed effects models revealed a negative relation (p = 0.025; estimate −0.07) between FD_Q and CPA_r. No significant effect of FD_Q on vertical canopy structure has been found (p > 0.05). The findings are discussed with regard to resource partitioning and niche differentiation of canopy and sub-canopy species. Successional stage positively impacted the CV of crown length (p = 0.019; estimate 0.03) but did not affect other response variables. The sloping terrain strongly influenced vertical canopy structure as revealed by the significant effect of slope inclination on CV of crown length (p = 0.004; estimate −0.05) and of slope aspect on CV of mean crown height (p = 0.036; estimate −0.03). The high complexity of vertical crown positioning depending on the heterogeneous sloping terrain of the study area may have obscured relations of FD_Q to vertical canopy structure.     

Trap-nest occupation by solitary wasps and bees (Hymenoptera: Aculeata) in a forest urban remanent

Temporal variation of solitary wasps and bees, nesting frequency, mortality, and parasitism were recorded from a remanent forest in Belo Horizonte, MG, Brazil. Wasps and bees were collected in trap-nests placed in areas with 25, 100, and 400 m2, from February to November 2004. The 137 trap-nests collected contained 11 species of wasps and bees. Wasps occupied most nests (75%). Occupation peaks occurred in March (25%) and September (26%); in June, the lowest occupation (2%) was observed. Except for Trypoxylon (Trypargilum) lactitarse Saussure, no significant correlation was found between number of occupied nests, and temperature and rainfall means. In the nests, 48% of the immature specimens died; 13% of the nests were parasitized. Total death and parasitism rates of wasps and bees differed significantly.     

Effects of tree species diversity and genotypic diversity on leafminers and parasitoids in a tropical forest plantation

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Cavity-nesting bees and wasps (Hymenoptera: Aculeata) in a semi-deciduous Atlantic forest fragment immersed in a matrix of agricultural land

Cavity-nesting bees and wasps (Hymenoptera: Aculeata) have been showed to be suitable models to investigate the effects of forest fragmentation and human land use. Those studies are particularly pertinent when considering fragmented ecosystems such as the Atlantic semi-deciduous forest in Brazil. We investigated the changes in composition, nest abundance, and mortality of cavity-nesting bees and wasps over edge-center forest fragment immersed in a matrix of agricultural lands in southeastern Brazil. Trap-nests (bamboo canes and cardboard tubes) were set in nine sampling stations in three different zones in the study site: three sampling stations at the forest edge; three at 250 m away from the edge and another three at 500 m away from the edge. Nests were monitored monthly for 2 years (from June 2011 to May 2013). A total of 942 nests (706 built by 16 bee species; 236 from 18 wasp species) were collected in the fragment. A significant change over gradient edge-center was observed on the parameters analyzed. The Non-metric Multidimensional Scale analysis showed that 72% of the species (host and natural enemies) that present more than ten records were associated with the forest edge. Furthermore, the highest values of the abundance of nests, parasitism, and mortality were recorded in the edge. In spite of being surrounded by farmlands, the fragment hosts a great diversity of trap-nesting wasp and bee species. Our findings demonstrate that forest edges are important habitats to maintain communities of cavity-nesting bees and wasps.     

南亚热带常绿阔叶林不同大小和发育阶段林隙的树种多样性研究

分析了南亚热带常绿阔叶林不同大小和发育阶段林隙内树种多样性的变化规律.结果表明,在南亚热带常绿阔叶林中,多样性指数在<400m²的林隙中变化不大,但在400～500m²的林隙中达到最大,而在500～600m²的林隙中最小,在>600m²的林隙中又有所增大.树种多样性指数随林隙年龄的变化趋势是中间高两端低,即在20～50年期间的多样性最大,其次是20年以下的,50年以后的多样性相对最小.林隙更新层中树种多样性指数在500～600m²的林隙中达到最大,在>600m²和200～300m²的林隙中最小.林隙更新层树种多样性指数在林隙形成最初的10年内达到最大值,但随着林隙年龄的增加,总体上表现出下降趋势,在30～40年和50～60年左右又分别形成两个相对的峰值.物种丰富度的变化趋势总体上与树种多样性指数相一致.不同大小和发育阶段的林隙通过其生态因子的改变,对不同树种的更新起到了不同的作用,从而使得不同大小和发育阶段的林隙中树种的多样性特征不同.林隙是维持南亚热带常绿阔叶林树种多样性的一个重要机制.     

Density dependence is not very prevalent in a heterogeneous subtropical forest

There is a growing body of evidence demonstrating that tree survival is influenced by negative density‐dependence, but it is still controversial how the effect may vary with life‐stage, and to what extent it plays a role in regulating tree survival in heterogeneous subtropical forests. In this study, we investigated density‐dependent effects on tree survival of six tree species in a 5‐ha subtropical forest in eastern China. The roughly 45 000 individuals in the forest were fully censused in 2003 and 2008. For each of these species, we used an inhomogeneous pair‐correlation function to quantify the change in spatial distribution for different size classes, and a case‐control design to study seedling–adult associations in 2003. Autologistic regression was used to determine the influence of neighborhood factors on individual survival from 2003 to 2008. We found that seedlings of five species were repulsed by distance to nearest conspecific adults in terms of their survival, consistent with predictions of the Janzen–Connell mechanism. By contrast, only the least shade‐tolerant Schima superba had a negative relationship with individual survival and conspecific distance‐weighted basal area. This suggests that the Janzen–Connell effect is only prevalent at the early seedling stage or seed‐to‐seedling phase. The strength of clustering significantly declined at sapling–pole and pole–adult transitions for Sycopsis sinensis and at seedling–sapling transition for Cleyera pachyphylla. Correlations between individual survival and conspecific abundance for these species were consistent with trends in the strength of clustering. These results suggest that density dependence plays a limited role in individual survival and species spatial structure beyond the early seedling stage (i.e. after true leaves growing) in this forest. In addition, this study indicates that including individuals from early life‐stages and factoring out potential confounding factors such as habitat preference are important in studies that seek evidence for density dependence in forest trees.     

Tree species diversity alters plant defense investment in an experimental forest plantation in southern Mexico

How plant species diversity affects traits conferring herbivore resistance (e.g., chemical defenses), as well as the mechanisms underlying such effects, has received little attention. One potential mechanism for the effect of diversity on plant defenses is that increased plant growth at high diversity could lead to reduced investment in defenses via growth–defense trade‐offs. We measured tree growth (diameter at breast height) and collected leaves to quantify total phenolics in 2.5‐year‐old plants of six tropical tree species (N = 597 plants) in a young experimental plantation in southern Mexico. Selected plants were classified as monocultures or as polycultures represented by mixtures of four of the six species examined. Tree species diversity had a significant negative effect on total phenolics, where polycultures exhibited a 13 percent lower mean concentration than monocultures. However, there was marked variation in the effects of diversity on defenses among tree species, with some species exhibiting strong reductions in phenolic levels in mixtures, whereas others were unresponsive. In addition, tree species diversity had no effect on growth, nor was the negative effect of diversity on chemical defenses mediated by a growth–defense trade‐off. These results demonstrate that tree diversity can alter investment in chemical defenses in long‐lived tree species but that such effect may not always be under strong control by plant endogenous resource allocation trade‐offs. Regardless of the underlying mechanism, these findings have important implications for predicting effects on consumers and ecosystem function.     

Local-scale spatial diversity patterns of ectomycorrhizal fungal communities in a subtropical pine-oak forest

This study aimed to analyze spatial patterns of soil ectomycorrhizal fungal (EMF) communities at the local scale in a subtropical pine-oak forest located in the Nearctic-Neotropical transition in central Mexico, to underpin biodiversity conservation strategies in forest fragments of this region. We used a spatially-explicit nested square sampling design with the same sampling representativeness at all scales and replicated three times. We detected 674 EMF OTUs within 19,200 m² and 65 OTUs on average per sample. Seventy percent of OTUs were detected in only 1–4 samples. Average community similarity was below 5%, showed minor change within 14 and 339 m distance and increased with the spatial grain used to compare the data. We found a high species-area relationship and beta diversity coefficients for soil fungi indicating that, at the local scale, increasing area by a constant factor of four represented an increase in OTU richness by a factor of two.     

西双版纳热带森林地区不同生境蜜蜂的物种多样性研究

本文研究了西双版纳热带森林地区不同生境蜜蜂的物种多样性。共鉴定蜜蜂总科5科、22属、77种。8个生境的物种丰富度指数D_MA的取值范围在1.914～7.877之间,多样性指数H′的范围是0.608～1.398,均匀度指数J_sw为0.716～0.956,其中保护区、雨林和灌丛拥有较高的物种丰富度指数和多样性指数,橡胶林的最小,而均匀度指数以曼养广龙山最大,农田最小。根据不同生境物种的相似性系数C进行系统聚类,8个生境在相似性系数0.169的水平时全部聚合在一起。傣族龙山在保持雨林的蜜蜂物种多样性中可以起一定的作用,但很有限,且当龙山面积越小,受人为干扰越严重时,物种多样性的损失就越大。雨林一旦被开垦为橡胶林或农田,雨林中原有的许多蜜蜂将会消失,物种多样性显著减少。     

Functional diversity versus species diversity: relationships with habitat heterogeneity at multiple scales in a subtropical evergreen broad-leaved forest

Understanding the relationship between functional and species diversity as well as their association with habitat heterogeneity can help reveal the mechanisms of species coexistence in ecological communities. However, these interactions have been poorly studied in subtropical forests. In this paper, we evaluated functional diversity (as measured by Rao’s Q) and traditional species diversity (based on Simpson’s index) in a 24 ha forest plot in a subtropical evergreen broad-leaved forest (EBLF) in China. We compared the sensitivities of functional and species diversity to topographic variables (elevation, convexity, slope and aspect) at multiple spatial scales based on 10 × 10, 20 × 20, 40 × 40 and 50 × 50 m quadrats. Functional and species diversity were found to have different distribution patterns along a topographical gradient, with functional diversity better explained by topography than was species diversity using a spatial autocorrelation regression error model. Furthermore, functional diversity had a significantly greater association with topographic variables than species diversity in both adult and young trees; in both cases, the strength of the diversity-habitat association increased with quadrat size. We conclude that functional diversity reflects a greater diversity-habitat association in EBLF than does species diversity, and that the association depends on the spatial scale and life stages of the woody plants under evaluation.