Effects Of Tree Species Identity Research Articles

Effects of tree species identity and soil origin on soil nematode communities and trophic composition in coniferous and broad-leaved forests

Effects of tree species identity and soil origin on soil nematode communities and trophic composition in coniferous and broad-leaved forests

Canopy structure influences arthropod communities within and beyond tree identity effects: Insights from combining LiDAR data, insecticidal fogging and machine learning regression modelling

Forest canopies host an abundant but often neglected diversity of arthropods, which requires careful attention in times of ongoing biodiversity loss. Yet, how tree species composition interacts with canopy structure in shaping arthropod communities remains largely unknown. Here, by combining mobile laser scanning and insecticidal fogging with a machine learning algorithm, we studied which canopy architectural properties affect canopy arthropod communities in monospecific and mixed stands of broadleaved European beech and the coniferous Norway spruce and non-native Douglas fir in Germany. Evaluating the abundances and ecological guild diversity of ∼ 90,000 arthropods and 27 partly novel high-resolution structural variables, we identified vegetation volume and tree species identity as weak predictors of arthropod abundance and ecological guild diversity. In contrast, structural heterogeneity, i.e. structural complexity, vertical layering and variability of canopy gaps—which were highest in coniferous stands—were strong positive drivers. Despite this, arthropod ecological guild diversity was lower in non-native Douglas fir. Mixed stands had intermediate arthropod abundance and ecological guild diversity. Our study shows that habitat heterogeneity and tree species-identity are closely interlinked in shaping associated canopy arthropod communities. Positive effects of habitat heterogeneity on arthropod ecological guild diversity were often uncoupled from resource availability, and the key role of our novel intra-canopy gap indices suggests that they should be considered as indicators in future research on forest heterogeneity-diversity relationships. Broadleaf-conifer mixtures may be suitable to mediate negative tree-species identity effects when adapting forests to global change.

Differential effects of tree species identity on rhizospheric bacterial and fungal community richness and composition across multiple trace element-contaminated sites

Soil microbial communities play a key role in plant nutrition and stress tolerance. This is particularly true in sites contaminated by trace metals, which often have low fertility and stressful conditions for woody plants in particular. However, we have limited knowledge of the abiotic and biotic factors affecting the richness and composition of microbial communities inhabiting the rhizosphere of plants in contaminated sites. Using high-throughput amplicon sequencing, we studied the rhizospheric bacterial and fungal community structures of 14 woody plant families planted in three contrasting sites contaminated by metals (Pb, Cd, Zn, Mn, Fe, S). The rhizospheric bacterial communities in the given sites showed no significant difference between the various woody species but did differ significantly between sites. The Proteobacteria phylum was dominant, accounting for over 25 % of the overall relative abundance, followed by Actinobacteria, Bacteroidetes and Gemmatimonadetes. Site was also the main driver of fungal community composition, yet unlike bacteria, tree species identity significantly affected fungal communities. The Betulaceae, Salicaceae and Fagaceae families had a high proportion of Basidiomycota, particularly ectomycorrhizal fungi, and the lowest diversity and richness. The other tree families and the unplanted soil harboured a greater abundance of Ascomycota and Mucoromycota. Consequently, for both bacteria and fungi, the site effect significantly impacted their community richness and composition, while the influence of plants on the richness and composition of rhizospheric microbial communities stayed consistent across sites and was dependent on the microbial kingdom. Finally, we highlighted the importance of considering this contrasting response of plant rhizospheric microbial communities in relation to their host identity, particularly to improve assisted revegetation efforts at contaminated sites.

Depth-dependent effects of tree species identity on soil microbial community characteristics and multifunctionality

Soil microbes play key roles that support forest ecosystem functioning, while their community characteristics are strongly determined by tree species identity. However, the majority studies primarily focus on soil microorganisms in the topsoil, resulting in limited understanding of the linkages between tree species identity and the microbial communities that inhabit deep soils. Here we investigated the diversity, structure, function, and co-occurrence networks of soil bacterial and fungal communities, as well as related soil physicochemical properties, to a depth of two meters in dryland forests dominated by either Pinus tabuliformis, a native coniferous species, Robinia pseudoacacia, an exotic broadleaf and nitrogen-fixing species, or both. Tree species identity had stronger effects on soil multifunctionality and microbial community structure in the deep layers (80–200 cm) than in the top layers (0–60 cm). In addition, fungal communities were more responsive to tree species identity, whereas bacteria were more sensitive to soil depth. Tree species identity strongly influenced microbial network stability and complexity, with higher quantities in R. pseudoacacia than the other plantations, by affecting microbial composition and their associations. The increased in microbial network complexity and the relative abundance of keystone taxa enhance the soil multifunctionality of microbial productivity, sugar and chitin degradation, and nutrient availability and cycling. Meanwhile, the relative abundance of keystone taxa was more representative of soil multifunctionality than microbial diversity. Our study highlights that tree species identity significantly influences soil microbial community characteristics and multifunctionality, especially in deep soils, which will help us understand soil nutrients processed in plantation forest ecosystem and provide a reference for tree species selection in ecological restoration.

Tree species traits and mycorrhizal association shape soil microbial communities via litter quality and species mediated soil properties

Soils harbor a vast diversity of soil microbiota, which play a crucial role in key ecosystem processes such as litter transformation and mineralization, but how complex plant-soil interactions shape the diversity and composition of soil microbiota remains elusive. We performed amplicon sequencing of DNA isolated from mineral topsoil of six common European trees planted in multi-site common garden monoculture stands of broadleaved maple and ash associated with arbuscular mycorrhiza (AM), broadleaved beech, lime and oak associated with ectomycorrhizal fungi (ECM) and coniferous spruce associated with ECM. The main aim of this study was to evaluate the effects of tree species identity, traits and mycorrhizal associations on diversity, community structure, cohesion, and shift in the relative abundance of taxonomic and functional groups of soil bacteria, fungi and nematodes. Our results revealed that soils beneath broadleaved trees hosted higher OTU richness of bacteria, fungi, and nematodes than under Norway spruce. Broadleaved tree species associated with AM fungi showed higher cohesion of bacterial and fungal communities than broadleaved trees associated with ECM fungi, but the cohesion of nematode communities was higher under trees associated with ECM fungi than under trees associated with AM fungi. Copiotrophic bacteria, fungal saprotrophs and bacterivorous nematodes were associated with ash, maple and lime having high soil pH, and high litter decomposition indices, while oligotrophic bacteria, ectomycorrhizal fungi and fungivorous nematodes were associated with beech, oak and Norway spruce that had low soil pH and low litter decomposition indices. Tree species associated with AM fungi had a high proportion of copiotrophic bacteria and saprotrophic fungi while trees associated with ECM fungi showed a high relative abundance of oligotrophic bacteria, ECM fungi and fungivorous nematodes. The different abundances of these functional groups support the more inorganic nutrient economy of AM tree species vs the more organic dominated nutrient economy of ECM tree species. The bacterial community was indirectly affected by litter quality via soil properties, while the fungal community was directly affected by litter quality and tree species. The functional groups of nematodes mirrored the communities of bacteria and fungi, thereby indicating the main and active groups of the tree species-specific microbial communities. Our study suggested that tree species identity, traits, and mycorrhizal association substantially shape microbial communities via a direct effect of litter chemistry as well as via litter-mediated soil properties.

Tree species mixing affects throughfall in a young temperate forest plantation

Rainfall partitioning is a key process in forest ecosystems, affecting evaporation, transpiration and water storage in the canopy, litter and soil. Although rainfall partitioning has been well studied in forest ecosystems, less is known about how rainfall partitioning is modified by tree species diversity. To disentangle the effects of tree species identity and diversity on rainfall partitioning, we installed 128 rain gauges in an experimental plantation, located in Zedelgem (Belgium), part of a large, multi-site forest diversity experiment in Belgium (FORBIO). With these rain gauges, we monitored throughfall for a full year, with a temporal resolution of two weeks. Based on these data, we analysed identity (i.e. whether throughfall is species-specific), non-additive (i.e. whether throughfall in mixtures is different from the expected amounts based on monocultures) and richness effects (i.e. how species richness affects throughfall). First of all, we found that rainfall partitioning highly differed between tree species, due to pronounced species identity effects. The presence of Pinus sylvestris [pine] was always associated with lower throughfall. Year-round throughfall was, for example, the lowest in mixtures of pine and Tilia cordata [lime] (43.8 ± 14.5% of the open-field rainfall) and monocultures of pine (48.6 ± 11.8%). Additionally, non-additive effects were significant, leading to a 10% decrease of rainfall reaching the forest floor in mixtures compared to the expected throughfall based on measurements in monocultures. Tree species richness effects on rainfall partitioning acted indirectly, mostly via an increased basal area in mixed stands. Our findings suggest that mixing tree species leads to a higher than expected reduction in throughfall, which lowered soil moisture in mixed plots. Whether this reduced throughfall and soil moisture affects ecosystem functioning remains to be quantified.

Tree neighbourhood-scale variation in topsoil chemistry depends on species identity effects related to litter quality

Trees have a large impact on the topsoil chemistry and the strength and direction of this impact depend on the identity of the tree species. Tree species with nutrient-poor litter have the potential to degrade soil fertility, which is characterized by high acidity, low availability of essential nutrients for plant growth, and high levels of available Al and Fe. In contrast, species with more easily decomposing, nutrient-rich litter can ameliorate the soil quality. In this study, we are investigating the effects of tree species identity on topsoil chemistry on a small spatial scale. Our study site, situated in Białowieża Forest in Poland, replicates mature monocultures and two-species mixtures, using a pool of four tree species. Soil was sampled at a metre distance to the base of a tree and in the centre along a transect between two trees. We found that the total C concentration, plant available P and base cation concentration, and C/N ratio were all larger close to the tree. The pH was unaffected by the distance. We identified clear species identity effects on this distance effect. In particular, we found that the pH and base cations were affected more negatively by the proximity to a tree nearby nutrient-poor trees in comparison with nutrient-rich trees. However, no non-additive diversity effects in mixtures could be distinguished. Our results highlight the ameliorating effects of admixing nutrient-rich species with nutrient-poor species and the importance of tree species choice in regard to the topsoil chemical composition on a small within-stand scale.

Variation in root morphology amongst tree species influences soil hydraulic conductivity and macroporosity

Natural approaches to flood risk management are gaining interest as sustainable flood mitigation options. Targeted tree planting has the potential to reduce local flood risk, however attention is generally focused on the hydrological impacts of catchment afforestation linked to generic tree features, whilst the species-specific impacts of trees on soil hydrology remain poorly understood. This study compared effects of different tree species on soil hydraulic properties. Monocultures of Alnus glutinosa (common alder), Fraxinus excelsior (European ash), Fagus sylvatica (European beech), Betula pendula (silver birch), Castanea sativa (sweet chestnut), Quercus robur (English oak) and Acer pseudoplatanus (sycamore maple) were used to determine effects of tree species identity on soil hydraulic properties (near-saturated K and soil water retention) in a sandy loam soil, North Wales, United Kingdom. The interaction of F. excelsior root properties and soil class on hydraulic conductivity was also examined in four different soils (Rendzic Leptosol, Haplic Luvisol, Dystric Fluvic Cambisol and Dystric Gleysol) across England and Wales. Fine root biomass (FRB) and morphological characteristics were determined at three depths (0–0.1, 0.1–0.2 and 0.2–0.3 m) and complemented by in situ surface measurement of soil hydraulic conductivity. Root morphological traits were closely associated with species identity and pore-size distribution, and FRB was strongly correlated with soil hydraulic conductivity (R2 = 0.64 for 0–0.1 m depth FRB; R2 = 0.69 for 0.1–0.2 m depth FRB). Fine root biomass of F. excelsior was sixfold greater than C. sativa (p < 0.001), and the frequency of 0.01 mm radius soil pores under F. excelsior was twice that of Q. robur. Near-saturated hydraulic conductivity under F. excelsior was 7.91 ± 1.23 cm day−1, double the mean rate of the other species. Soil classification did not significantly influence FRB (p = 0.056) or near-saturated hydraulic conductivity (p = 0.076) in the 0.0–0.1 m depth soil, but soil water retention varied with depth. Species-specific traits of trees should be considered in landscape design to maximise the local hydrological benefits of trees.

Impact of tree species diversity on throughfall deposition in a young temperate forest plantation

Throughfall deposition is an important pathway via which particles, aerosols and gases can move from the atmosphere to the forest floor, which can greatly impact forest biodiversity and functioning. Although throughfall deposition biochemistry has been well studied in forest ecosystems, less is known about how throughfall deposition is modified by tree species diversity. To disentangle the effects of tree species identity and diversity on throughfall deposition, we installed rain gauges in a 10-year-old tree diversity experiment. With these rain gauges, we monitored throughfall biweekly, and performed chemical analyses monthly for all the major ions (Na+, Cl−, SO42−-S, K+, Ca2+, Mg2+, PO43−-P, NO3−-N and NH4+-N), for a period of one year. Based on these data, we analysed species identity (i.e. whether throughfall deposition depended on the tree species present in the overstorey), and diversity effects (i.e. whether throughfall deposition depended on tree species richness). We confirmed species identity effects on throughfall deposition. Presence of pine led to higher throughfall deposition of inorganic nitrogen, and lower amounts of phosphorus, calcium and magnesium reaching the forest floor. Additionally, species characteristics and stand structural characteristics, i.e. basal area and canopy cover, emerged as key factors driving throughfall deposition of sodium, chloride, nitrate, sulphate and potassium. Tree species diversity effects on throughfall deposition were also present, mostly via increased basal area and canopy cover, leading to increased throughfall deposition of inorganic nitrogen, sulphate, chloride, sodium and potassium. Our findings thus suggest that tree species diversity enhances throughfall deposition of most ions that we analysed, potentially increasing nutrient availability for below-canopy plant communities. Given that inorganic nitrogen and sulphur are major atmospheric pollutants, increased deposition in mixed plots might affect ecosystem functioning. Further research will be needed to determine the extent of this impact.

Effects of tree species identity and diversity on young tree growth in a south subtropical plantation

To select the tree species assembly model for improving the productivity in south subtropical plantations, we carried out an experiment following a random block design with eight native tree species across a richness gradient of 1, 2, 4, and 6 species. The effects of tree species diversity and species mixing with different functional identities on the young tree growth were investigated in the 5th year of the experiment. The results showed that tree growth was not positively correlated with tree species richness. The growth of fast-growing tree species (Pinus massoniana and Mytilaria laosensis) in the monoculture was 2.5-4.5 times of the valuable broadleaved tree species (Castanopsis hystrix and Erythrophleum fordii) monoculture. Tree growth was significantly increased by 51.5%-132.8% in the conifer and broadleaved tree species mixing plantations and in the fast-growing and nitrogen fixation tree species mixing plantations, when two tree species or four tree species were mixed. There was no significant difference in tree growth among different tree species mixed types, when six tree species were mixed. The contents of soil nitrogen, phosphorus and organic matter were the main factors affecting tree growth. The results indicated that young tree growth could be improved through the selecting conifer and broadleaved tree species mixing, fast-growing and nitrogen fixation tree species mixing in south subtropical plantations.

Roots and rhizospheric soil microbial community responses to tree species mixtures

Below-ground processes are crucial in determining the effects of plants on ecosystem function. The root-soil interface is a highly active zone due to root exudation and nutrient uptake. However, its role in determining effects of tree species and their interactions on the soil microbial community, ecosystem function and above-ground growth is less well known.We compared the effects of tree species monocultures and their mixture on rhizospheric microbial communities, specific functional genetic markers associated with processes in the nitrogen (N) cycle, and above-ground and below-ground growth and nutrient allocation. Two pairs of tree species were grown: Pseudotsuga menziesii and Alnus rubra; Acer pseudoplatanus and Quercus robur. Tree establishment altered soil microbial composition, but after 26 months differences amongst tree species and effects of species mixture were minor, suggesting functional redundancy in microbial communities. A greater abundance of fungi, bacteria, and specifically ammonia oxidising and denitrifying bacteria in the rhizospheric soil of the N-fixing A. rubra was the most notable trend. Mixing A. rubra with P. menziesii did produce overyielding: trees grown in mixture attained a two-fold greater (Relative Yield Total 2.03 ± 0.52) above-ground biomass than in a mixture predicted from trees grown in monoculture. We did not observe strong trends in overyielding for A. psuedoplatanus and Q. robur. Inclusion of the N-fixing species A. rubra in admixture with P. menziesii promoted N cycling, and decreased the C:N ratios of leaf, branch, and root tissues but not soil C:N ratio for P. menziesii. Given the observed overyielding in the A. rubra with P. menziesii mixtures, we explored potential mechanistic links between functional genetic markers for nitrification and ammonification, however we found no statistically significant effects attributable to these genetic markers. We found root area index was significantly lower in A. rubra monocultures than in admixture with P. menziesii. For both P. menziesii and A. rubra, the number of root tips was lower in mixture than monoculture, indicating physical partitioning of soil space as a result of growing in mixture. We documented additive and synergistic effects of tree species identity on above and belowground productivity, and rhizospheric microbial community development in these four tree species.

Litter quality, mycorrhizal association, and soil properties regulate effects of tree species on the soil fauna community

Forest management, including selection of appropriate tree species to mitigate climate change and sustain biodiversity, requires a better understanding of factors that affect the composition of soil fauna communities. These communities are an integral part of the soil ecosystem and play an essential role in forest ecosystem functioning related to carbon and nitrogen cycling. Here, by performing a field study across six common gardens in Denmark, we evaluated the effects of tree species identity and mycorrhizal association (i.e., arbuscular mycorrhiza (AM) and ectomycorrhiza (ECM)) on soil fauna (meso- and macrofauna) taxonomic and functional community composition by using diversity, abundance, and biomass as proxies. We found that (1) tree species identity and mycorrhizal association both showed significant effects on soil fauna communities, but the separation between community characteristics in AM and ECM tree species was not entirely consistent; (2) total soil fauna abundance, biomass, as well as taxonomic and functional diversity were generally significantly higher under AM tree species, as well as lime, with higher litter quality (high N and base cation and low lignin:N ratio); (3) tree species significantly influenced the properties of litter, forest floor, and soil, among which litter and/or forest floor N, P, Ca, and Mg concentrations, soil pH, and soil moisture predominantly affected soil fauna abundance, biomass, and taxonomic and functional diversity. Our results from this multisite common garden experiment provide strong and consistent evidence of positive effects of tree species with higher litter quality on soil fauna communities in general, which helps to better understand the effects of tree species selection on soil biodiversity and its functions related to forest soil carbon sequestration.

Vertical Distribution of Mercury in Forest Soils and Its Transfer to Edible Mushrooms in Relation to Tree Species

Soil pollution by mercury (Hg) is a global problem that poses risks to natural ecosystems and to human health. Forests represent an important recipient of Hg deposition, however, so far, very little is known about the tree species identity effects on the distribution of Hg in forest soils and its accumulation in edible mushrooms. To clarify the effect on the two main Central-European commercial forest tree species, soil samples were collected from organic F+H horizons and from mineral soil depths of 0–2, 2–10, 10–20, and 20–30 cm in mature Norway spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica L.) dominated stands. Mushroom samples of the Boletaceae family were also collected at each sampling site. The highest Hg contents were found in the F+H layer and were significantly higher in spruce- (mean 0.46 ± 0.03 mg/kg) than in beech- (mean 0.29 ± 0.10 mg/kg) dominated stands. The variation in Hg contents in F+H was best predicted by pH, the overall lower soil pH in strongly acidic spruce stands might induce Hg immobilization in the F+H layer to cause a decrease in the bioavailability of Hg for Xerocomellus chrysenteron (Bull.) Šutara. In mineral soil, the Hg contents did not differ significantly between the spruce- and beech-dominated stands. The Hg content strongly correlated with the S, N, and C contents only in mineral soil; at the depths of 2–10, 10–20, and 20–30 cm, significantly also with the silt vs. sand, Alo, and Feo contents. Studied mushroom species were not Hg-contaminated and, therefore, their consumption does not pose serious health risks regardless of the forest type. The results suggest that species-related soil chemistry and mineral associations, rather than different atmospheric Hg interception by spruce vs. beech, drive the vertical distribution and accumulation of Hg in forest soils.

Growth and mortality of the oak processionary moth, Thaumetopoea processionea, on two oak species: direct and trait-mediated effects of host and neighbour species

The presence of heterospecific neighbours can affect the amount of damage caused by insect pests to focal plants. However, how plant neighbours influence herbivore performance is poorly understood, in particular when they are closely related. We tested the independent and interactive effects of tree species identity and tree neighbour type (conspecific vs. heterospecific) on the performance of a major oak pest, the oak processionary moth larvae (OPM, Thaumetopoea processionea L.) fed on Quercus robur L. and Q. petraea Liebl. We performed a factorial greenhouse experiment in which we grew two oak saplings per pot, either from the same species or from both species. We quantified growth and mortality of OPM larvae, leaf phenolic compounds, C:N ratio and bud phenology. OPM larvae performed significantly better on Q. petraea than on Q. robur , regardless of plant neighbour type. Phenolic compounds and C:N, but not phenology, differed between oak species and neighbour species identity. Only bud phenology had a significant effect on OPM performance, which was better when young larvae had access to recently unfolded leaves, regardless of oak species and neighbour identity. Although oak neighbour identity altered the expression of leaf traits, this effect had no measurable consequences on OPM performance. Our results showed that OPM performed better when feeding on Q. petraea than on Q. robur . We also found that oak neighbours altered the expression of leaf traits associated with defence, but that this effect did not cascade onto OPM performance. These findings should be replicated in the field in order to improve forest management strategies against OPM attacks.

Moderate effects of tree species identity on soil microbial communities and soil chemical properties in a common garden experiment

Knowledge about the effects of tree species on soil physicochemical and microbial properties is crucial for sustainable forest management. The evaluation of these effects in common garden experiments may be advantageous compared to natural conditions due to the reduction of confounding factors such as differences in topography, microclimate, the age of the tree stand, the admixture of other tree species or soil type. In this study, we assessed the effects of 14 tree species grown for nearly 50 years in monospecific plots in the Siemianice Experimental Forest in Poland on soil microbial community assessed by phospholipid fatty acid (PLFA) analysis, namely total, bacterial (G+, G−) and fungal biomass, fungi/bacteria ratio, G+/G− ratio, and microbial community structure, as well as on soil chemical properties, namely pH, organic C, total Ca, K, Mg, N, P, exchangeable Ca, K, Mg, N-NH4, N-NO3, S-SO4, and C/N ratio. Species richness (species number) and cover of undergrowth vegetation were also assessed. We found only moderate effects of tree species identity on soil. Significant differences were found in fungal biomass, fungi/bacteria ratio, and microbial community structure, with the highest fungal biomass under Betula pendula and Quercus rubra. Among chemical parameters, significant differences were observed in soil pH and base cations concentrations. The highest values of these parameters were under Acer pseudoplatanus, A. platanoides, and Tilia cordata. Total microbial, bacterial (G−), fungal biomass, and fungi/bacteria ratio were significantly higher under broadleaf than under coniferous species, while for G+/G− ratio the opposite was observed. Tree species with arbuscular mycorrhiza did not generally differ in their effects on soil microbial biomass from those with ectomycorrhiza. Total microbial and G+ bacterial biomass correlated positively with soil elements, mainly base cations, C, and N, while very few correlations were found between soil chemical properties and G− bacteria and fungi. Species richness and cover of undergrowth vegetation were not related to any of soil microbial properties. The lack of correlations between soil element contents, undergrowth vegetation, and some microbial groups may suggest that these microorganisms are primarily influenced by other soil/litter properties, for example secondary metabolite concentrations.

Low root functional dispersion enhances functionality of plant growth by influencing bacterial activities in European forest soils.

Current studies show that multispecies forests are beneficial regarding biodiversity and ecosystem functionality. However, there are only little efforts to understand the ecological mechanisms behind these advantages of multispecies forests. Bacteria are among the key plant growth-promoting microorganisms that support tree growth and fitness. Thus, we investigated links between bacterial communities, their functionality and root trait dispersion within four major European forest types comprising multispecies and monospecific plots. Bacterial diversity revealed no major changes across the root functional dispersion gradient. In contrast, predicted gene profiles linked to plant growth activities suggest an increasing bacterial functionality from monospecific to multispecies forest. In multispecies forest plots, the bacterial functionality linked to plant growth activities declined with the increasing functional dispersion of the roots. Our findings indicate that enriched abundant bacterial operational taxonomic units are decoupled from bacterial functionality. We also found direct effects of tree species identity on bacterial community composition but no significant relations with root functional dispersion. Additionally, bacterial network analyses indicated that multispecies forests have a higher complexity in their bacterial communities, which points towards more stable forest systems with greater functionality. We identified a potential of root dispersion to facilitate bacterial interactions and consequently, plant growth activities.

Nutrient status not secondary metabolites drives herbivory and pathogen infestation across differently mycorrhized tree monocultures and mixtures

Research aimed at understanding the mechanisms underlying the relationship between tree diversity and antagonist infestation is often neglecting resource-use complementarity among plant species. We investigated the effects of tree species identity, species richness, and mycorrhizal type on leaf herbivory and pathogen infestation. We used a tree sapling experiment manipulating the two most common mycorrhizal types, arbuscular mycorrhiza and ectomycorrhiza, via respective tree species in monocultures and two-species mixtures. We visually assessed leaf herbivory and pathogen infestation rates, and measured concentrations of a suite of plant metabolites (amino acids, sugars, and phenolics), leaf elemental concentrations (carbon, nitrogen, and phosphorus), and tree biomass. Tree species and mycorrhizal richness had no significant effect on herbivory and pathogen infestation, whereas species identity and mycorrhizal type had. Damage rates were higher in arbuscular mycorrhizal (AM) than in ectomycorrhizal (EM) trees. Our structural equation model (SEM) indicated that elemental, but not metabolite concentrations, determined herbivory and pathogen infestation, suggesting that the investigated chemical defence strategies may not have been involved in the effects found in our study with tree saplings. Other chemical and physical defence strategies as well as species identity as its determinant may have played a more crucial role in the studied saplings. Furthermore, the SEM indicated a direct positive effect of AM trees on herbivory rates, suggesting that other dominant mechanisms, not considered here, were involved as well. We found differences in the attribution of elemental concentrations between the two rates. This points to the fact that herbivory and pathogen infestation are driven by distinct mechanisms. Our study highlights the importance of biotic contexts for understanding the mechanisms underlying the effects of biodiversity on tree-antagonist interactions.

Soil organic carbon stability in forests: Distinct effects of tree species identity and traits.

Rising atmospheric CO2 concentrations have increased interest in the potential for forest ecosystems and soils to act as carbon (C) sinks. While soil organic C contents often vary with tree species identity, little is known about if, and how, tree species influence the stability of C in soil. Using a 40year old common garden experiment with replicated plots of eleven temperate tree species, we investigated relationships between soil organic matter (SOM) stability in mineral soils and 17 ecological factors (including tree tissue chemistry, magnitude of organic matter inputs to the soil and their turnover, microbial community descriptors, and soil physicochemical properties). We measured five SOM stability indices, including heterotrophic respiration, C in aggregate occluded particulate organic matter (POM) and mineral associated SOM, and bulk SOM δ15 N and ∆14 C. The stability of SOM varied substantially among tree species, and this variability was independent of the amount of organic C in soils. Thus, when considering forest soils as C sinks, the stability of C stocks must be considered in addition to their size. Further, our results suggest tree species regulate soil C stability via the composition of their tissues, especially roots. Stability of SOM appeared to be greater (as indicated by higher δ15 N and reduced respiration) beneath species with higher concentrations of nitrogen and lower amounts of acid insoluble compounds in their roots, while SOM stability appeared to be lower (as indicated by higher respiration and lower proportions of C in aggregate occluded POM) beneath species with higher tissue calcium contents. The proportion of C in mineral associated SOM and bulk soil ∆14 C, though, were negligibly dependent on tree species traits, likely reflecting an insensitivity of some SOM pools to decadal scale shifts in ecological factors. Strategies aiming to increase soil C stocks may thus focus on particulate C pools, which can more easily be manipulated and are most sensitive to climate change.

Tree species identity surpasses richness in affecting soil microbial richness and community composition in subtropical forests

Plant interactions and feedbacks with soil microorganisms play an important role in sustaining the functions and stability of terrestrial ecosystems, yet the effects of tree species diversity on soil microbial community in forest ecosystems are still not well understood. Here, we examined the effects of tree species richness (1–12 species) and the presence of certain influential tree species (sampling effect) on soil bacterial and fungal communities in Chinese subtropical forests, using high-throughput Illumina sequencing for microbial identification. We observed that beta rather than alpha diversities of tree species and soil microorganisms were strong coupled. Multivariate regression and redundancy analyses revealed that the effects of tree species identity dominated over tree species richness on the diversity and composition of bacterial and fungal communities in both organic and top mineral soil horizons. Soil pH, nutrients and topography were always identified as significant predictors in the best multivariate models. Tree species have stronger effect on fungi than bacteria in organic soil, and on ectomycorrhizal fungi than saprotrophic fungi in mineral topsoil. Concluding, tree species identity, along with abiotic soil and topographical conditions, were more important factors determining the soil microbial communities in subtropical forests than tree diversity per se.

Tree species diversity indirectly affects nutrient cycling through the shrub layer and its high-quality litter

Tree species effects on biogeochemical cycles are well studied, but the interactive effects of tree species in mixtures remain poorly understood. We studied how tree species identity and species diversity affect nutrient cycling in mature forests. In a Belgian platform of 53 forest plots varying in tree species diversity and composition, we sampled the return of carbon, nitrogen and base cations via leaf litterfall and their stocks in the forest floor and topsoil. Tree species identity effects were clear; diversity effects were weak or absent. The leaf litter input from shrub species depended on the composition of the tree canopy and had a higher quality than the litter of the trees. Monocultures of pedunculate oak had the highest input of litter from shrub species; in mixtures with beech, however, this input was disproportionally low. We found indirect effects of tree species diversity on nutrient cycling, via effects of the tree species composition on the abundance and composition of the shrub layer. This is particularly important in forests consisting of tree species with low leaf litter quality, because nutrient cycling may benefit from the presence of shrub species with higher leaf litter quality.