Effects Of Tree Species Richness Research Articles

Leaf isotopes reveal tree diversity effects on the functional responses to the pan-European 2018 summer drought.

Recent droughts have strongly impacted forest ecosystems and are projected to increase in frequency, intensity, and duration in the future together with continued warming. While evidence suggests that tree diversity can regulate drought impacts in natural forests, few studies examine whether mixed tree plantations are more resistant to the impacts of severe droughts. Using natural variations in leaf carbon (C) and nitrogen (N) isotopic ratios, that is δ13C and δ15N,as proxies for drought response, we analyzed the effects of tree species richness on the functional responses of tree plantations to the pan-European 2018 summer drought in seven European tree diversity experiments. We found that leaf δ13C decreased with increasing tree species richness, indicating less drought stress. This effect was not related to drought intensity, nor desiccation tolerance of the tree species. Leaf δ15N increased with drought intensity, indicating a shift toward more open N cycling as water availability diminishes. Additionally, drought intensity was observed to alter the influence of tree species richness on leaf δ15N from weakly negative under low drought intensity to weakly positive under high drought intensity. Overall, our findings suggest that dual leaf isotope analysis helps understand the interaction between drought, nutrients, and species richness.

Stand biomass of Pinus sylvestris var. mongolica plantations benefits from high density monocultures in the boreal zone

Pinus sylvestris var. mongolica (P. sylvestris) plantations are extensively established in the boreal zone. Increasing stand biomass of these plantations can effectively enhance carbon stock, which is crucial for mitigating climate change. However, the current understanding of optimizing plantation strategies to maximize stand biomass is primarily derived from experiments in tropical and subtropical zones, which is difficult to extend to the boreal due to substantial climatic differences. Based on a comprehensive dataset from 1,076 sample plots of P. sylvestris plantations in the boreal zone of China, we evaluated the effects of tree species richness and stand density on tree height, diameter at breast height (DBH), and stand biomass to investigate the optimal plantation strategy. Furthermore, we examined how these effects changed with stand age and investigated their relative importance. We found that monocultures at a high stand density of 2,000–2,500 ​ha−1 were the optimal plantation strategy to maximize stand biomass (107.5 ​Mg·ha−1), and this held true at almost all stand ages. Unfortunately, this strategy resulted in low species richness and small individual trees (10.6 ​m height and 9.8 ​cm DBH), thus presenting a trade-off. In addition, as stand age increased, the effect of tree species richness on stand biomass shifted from positive to negative, but the effect of stand density was always positive. Overall, stand age had the greatest effect on stand biomass, followed by stand density and then tree species richness. Our findings reveal a distinct plantation strategy for optimizing stand biomass of P. sylvestris plantations in the boreal zone. More importantly, this study highlights that (1) maximizing stand biomass in the boreal zone may compromise tree species richness; (2) net effects of tree species richness on stand biomass are not always positive, as negative selection effects offset positive complementary effects.

Factors affecting composition of fatty acids in wild-growing forest mushrooms

ABSTRACT The importance of mushrooms as a food source is continually increasing. To investigate how environmental factors affect the nutritional value of mushrooms, we harvested them in eastern Poland, south-central Germany, and northwestern Belgium in plots with similar environmental conditions but varying in tree species composition and richness. We used gas chromatography–mass spectrometry (GC-MS) to analyze the fatty acid (FA) content of the mushrooms. Fungal species identity explained the largest part (40%) of the total variation in FA concentration and composition. Environmental factors accounted for 1–12% of variation. The concentration of FA, especially saturated fatty acids, decreased with increasing understory cover and increasing nitrogen concentration in the topsoil. The effect of tree species richness or tree species identity was negligible. Our results suggest that the nutritional value of mushrooms depends mainly on the species identity of fungi, but that their FA content is slightly higher in forests with less undergrowth and in nitrogen-poor soils.

Trophic interactions in soil micro-food webs drive ecosystem multifunctionality along tree species richness.

Rapid biodiversity losses under global climate change threaten forest ecosystem functions. However, our understanding of the patterns and drivers of multiple ecosystem functions across biodiversity gradients remains equivocal. To address this important knowledge gap, we measured simultaneous responses of multiple ecosystem functions (nutrient cycling, soil carbon stocks, organic matter decomposition, plant productivity) to a tree species richness gradient of 1, 4, 8, 16, and 32 species in a young subtropical forest. We found that tree species richness had negligible effects on nutrient cycling, organic matter decomposition, and plant productivity, but soil carbon stocks and ecosystem multifunctionality significantly increased with tree species richness. Linear mixed-effect models showed that soil organisms, particularly arbuscular mycorrhizal fungi (AMF) and soil nematodes, elicited the greatest relative effects on ecosystem multifunctionality. Structural equation models revealed indirect effects of tree species richness on ecosystem multifunctionality mediated by trophic interactions in soil micro-food webs. Specifically, we found a significant negative effect of gram-positive bacteria on soil nematode abundance (a top-down effect), and a significant positive effect of AMF biomass on soil nematode abundance (a bottom-up effect). Overall, our study emphasizes the significance of a multitrophic perspective in elucidating biodiversity-multifunctionality relationships and highlights the conservation of functioning soil micro-food webs to maintain multiple ecosystem functions.

Plant–soil feedback is dependent on tree mycorrhizal types and tree species richness in a subtropical forest

Plant-soil feedback (PSF) is an important driver of plant species coexistence and diversity maintenance. However, it remains unclear how changes in PSF due to decline in tree species richness influence the performance of arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) tree species. A PSF experiment was established with eight target tree species (four AM and four EcM tree species) based on a subtropical forest Biodiversity-Ecosystem Functioning Experimental in China (BEF-China) platform, where soil inocula were collected beneath the canopy of target tree species individuals growing in monoculture and eight tree-species mixture plots. We hypothesized that the negative PSF strength of AM tree species would be stronger in monocultures than that in eight tree-species mixtures, resulting in better performance in highly diverse plant communities, whereas EcM tree species would benefit less from tree species richness. However, the results showed consistent PSFs of AM tree species regardless of tree species richness. In contrast, EcM tree species experienced a change from positive in monocultures to negative PSF in eight tree-species mixtures, influencing the performance of EcM tree seedlings. With high concern of the cascading effects of tree species richness on PSFs via modulating soil fungal communities, we revealed that the alterations in EcM fungal abundance, putatively pathogenic fungal diversity, and fungal co-occurrence network complexity mirrored those of PSF associated with EcM tree species, showing the changes from positive in monocultures to negative feedback in eight tree-species mixtures. Our findings highlight the differential PSFs exhibited by AM and EcM tree species in response to tree species richness, and provide insight into the potential role of fungal functional guilds and co-occurrence network complexity in shaping the PSFs of them.

The more the merrier? Perceived forest biodiversity promotes short‐term mental health and well‐being—A multicentre study

Abstract Forests can foster mental health and well‐being. Yet, the contribution of forest biodiversity remains unclear, and experimental research is needed to unravel pathways of biodiversity–health linkages. Here, we assess the role of tree species richness, both actual and perceived, and how stress reduction and attention restoration can serve as potential mediating pathways to achieve positive mental health and well‐being outcomes. We conducted an experimental, multicentric field study in three peri‐urban forests in Europe, employing a mixed design with 223 participants, that comprised 20‐min stays in forests with either low, medium or high tree species richness or a built control. Participants' short‐term mental health and well‐being and saliva cortisol as a biomarker of stress were measured before and after the intervention. Forest visits for 20 min were found to be beneficial for participants' short‐term mental health, short‐term mental well‐being, subjective stress, subjective directed attention and perceived restorativeness compared with a built environment. No differences were found for the physiological stress indicator saliva cortisol, which decreased in both the forest and the built environments. Increased perceived biodiversity—possibly linked to structural forest attributes—was significantly associated with well‐being outcomes, while no association was found for differences in actual tree species richness. Structural equation modelling indicates that higher levels of perceived biodiversity had an indirect effect on short‐term mental health and well‐being through enhancing perceived restorativeness. While we found no evidence of actual tree species richness effects, perceived biodiversity was associated with positive short‐term mental health and well‐being outcomes. Understanding these biodiversity–health linkages can inform conservation management and help develop effective nature‐based interventions for promoting public health through nature visits. Read the free Plain Language Summary for this article on the Journal blog.

Infestation intensity by the invasive oak lace bug, Corythucha arcuata (Say) in mixed and pure oak stands

AbstractThere has been accumulating evidence for effects of tree species composition on herbivory with many examples of lower damage by specialist feeders in tree species rich forests. In a joint study in five Central and Southeastern European countries, we studied the effect of tree species richness on infestation intensity of the oak lace bug, Corythucha arcuata (Say) (Heteroptera, Tingidae), an invasive pest on oak trees that has spread rapidly across the Balkan Peninsula and Central Europe. Intensity of infestation by C. arcuata on oaks was assessed on three or four study plots with high (pure stands) and three or four plots with low percentage of oak (mixed stands) in each country. Ordinal regression analysis showed that intensity of infestation of trees by C. arcuata differed between countries; no significant effect of stand type (mixed or pure) on infestation levels was detected. When analysing the percentage of trees in the highest infestation class, stand type had a significant effect with more intensive infestation in pure stands. We conclude that mixed stands will not prevent severe infestation but may help mitigating the impact of established C. arcuata populations.

Predation pressure by arthropods, birds, and rodents is interactively shaped by tree species richness, vegetation structure, and season

Tree species richness, forest structure, and seasonal fluctuations between rainy and dry seasons can strongly affect trophic interactions in forest ecosystems, but the inter- and scale dependence of these variables remains unclear. Using artificial caterpillars (~18,000 replicates), we analyzed predation pressure by arthropods, birds, and rodents along a tree species richness gradient across seasons in a subtropical tree diversity experiment (BEF-China). The aim of the study was to test if forest structure, in addition to tree species richness, has an effect on predation pressure and to further specify which structural variables are important in driving predation. We assessed the effects of tree species richness and forest structure at the plot and local neighborhood levels. We also included fine-scale placement covariates, plot size, and topographical covariates of the study site. Forest structure and tree species richness independently and interactively affected predation pressure. The spatial scale was an important determinant for tree species richness and structural effects, extending from within plot scales to the overall heterogeneity of the plots’ surrounding environment. For example, the effect of branch density in the local neighborhood depended on both surrounding tree species richness and plot-level vegetation density. Similarly, visibility-enhancing factors increased attacks by arthropods (lack of branches in close surroundings) and by birds (open area), depending on the surrounding vegetation. A comparison of structural measures showed that predation pressure can be addressed in much greater detail with multiple specific structural features than with overall forest complexity. Seasonal change also affected predation pressure, with foliage being a stronger attractant in spring, but also by presumable topography-driven study plot differences in sun exposure and humidity between rainy and dry seasons. Our study demonstrates that predation pressure is not simply a function of tree species richness or structure but is shaped by the interplay of structural elements, spatial scale, and seasonal dynamics along gradients of tree species richness and forest structure. The structural and seasonal effects are important to take into account when addressing how current and future biodiversity loss may change top‐down control of herbivory and overall ecosystem functioning.

Nitrogen-fixing tree species modulate species richness effects on soil aggregate-associated organic carbon fractions

Soil aggregation is an important mechanism shaping soil organic carbon (SOC) fractions, but the effects of tree species richness on soil aggregate-associated organic carbon fractions are poorly understood. In this study, a manipulative experiment was conducted to examine whether and how tree species richness (1, 2, 4 and 6) together with or without the presence of nitrogen (N)-fixing tree species affect soil aggregate-associated organic carbon. Soil aggregates were classified into four fractions: large macroaggregates (>2 mm), small macroaggregates (0.25–2 mm), microaggregates (0.053–0.25 mm), and silt and clay (<0.053 mm). And then large and small macroaggregates were further separated into coarse (c-iPOC), fine intra-aggregate particulate organic carbon (f-iPOC) and mineral associated organic carbon (MAOC), and microaggregates were divided into f-iPOC and MAOC. We observed that the effects of tree species richness on soil aggregate-associated organic carbon fractions were depended on the presence or absence of N-fixing tree species. In the presence of N-fixing tree species, organic carbon contents of small macroaggregate, microaggregate and silt and clay fractions were increased across the gradient of tree species richness (+10.4%, +31.3% and + 26.7% from monocultures to 6-species mixture), and the similar responses occurred for c-iPOC within small macroaggregates (+42.9%) and f-iPOC within microaggregates (+50.0%). In the absence of N-fixing tree species, however, only f-iPOC within large macroaggregates (+60.0%) had a positive linear relationship with tree species richness. Structural equation modeling was then used to dissect the mechanism underlying the positive effects of tree species richness on the aggregate-associated organic carbon fractions, indicating either direct or indirect effects by increasing root length density. Our study highlights the important roles of N-fixing tree species in shaping tree species richness effects on soil aggregate-associated organic carbon fractions, and therefore, N-fixing tree species should be preferentially considered in mixed-species plantations to enhance long-term SOC stabilization.

Nitrogen-fixing tree species rather than tree species diversity shape soil nematode communities in subtropical plantations

Positive relationships between tree species diversity and soil communities are reported in many studies. However, the effects of tree species richness and nitrogen (N)-fixing tree species on soil fauna communities are not yet fully understood. Here, we conducted a field experiment to investigate changes in soil nematode communities under a gradient of tree species richness (e.g., 1, 2, 4 and 6 tree species) with N-fixing tree species (absence or presence). We found that N-fixing tree species strongly affected soil nematode trophic groups, and significantly increased the abundance of omnivore-predators and structure index via promoting more trophic links and multi-trophic interactions, leading to a more stable and complex soil food web. In the presence of N-fixing tree species, tree species richness significantly increased the abundance and relative abundance of fungivores and the channel index, with the shift in soil energy channel through increased the C/N ratio of leaf litter. In contrast, tree species richness detrimentally affected the abundance of plant parasites due to the decline of leaf litter N content under the presence of N-fixing tree species. The results revealed that the bottom-up effects of N-fixing tree species on soil nematode community structure were crucial drivers in shaping nematode communities in subtropical plantations. Our study suggests that N-fixing tree species is more important than tree species richness to regulate soil nematode community structure and soil energy channel, and therefore, should be preferentially considered in multispecies forest plantations.

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 species richness and N-fixing tree species enhance the chemical stability of soil organic carbon in subtropical plantations

Tree species diversity enhances productivity and soil carbon storage in subtropical forests. However, the effects of tree species diversity and N-fixing tree species on the chemical stability of soil organic carbon (SOC) have been poorly understood. In this study, a manipulative experiment involving varying tree species richness (i.e., one, two, four and six species) and the inclusion of N-fixing trees was conducted to investigate the effects of tree species richness and the presence of N-fixing tree species on the chemical stability of SOC. Biological traceability was used to separate SOC into plant- vs. microbial-derived components, and then assessed by four measures of the chemical stability, including the cyclic/acyclic lipid ratio, average carbon chain length (ACL), Pielou's evenness of acyclic lipids and Pielou's evenness of cyclic lipids. We found that tree species richness was positively correlated with the cyclic/acyclic lipid ratio, ACL and evenness of acyclic lipids. In the presence of N-fixing tree species, tree species richness significantly increased both plant- and microbial-derived lipids, resulting in a higher cyclic/acyclic lipid ratio through stimulated root-microorganism interactions. Plant-derived lipids were positively correlated with the cyclic/acyclic lipid ratio, ACL and evenness of acyclic lipids, suggesting the important role of plant-derived components in shaping the chemical stability of SOC. This study reveals that the chemical stability of SOC is not only a function of tree species richness, but also manifested by the presence of N-fixing tree species. These results demonstrate that mixed plantations with N-fixing trees as an appropriate silvicultural option will be beneficial to the enhancement of the chemical stability of SOC through increased soil recalcitrant C components.

Temporal Soundscape Patterns in a Panamanian Tree Diversity Experiment: Polycultures Show an Increase in High Frequency Cover

In this ecoacoustic study we used the setting of a tropical tree diversity planted forest to analyze temporal patterns in the composition of soundscapes and to test the effects of tree species richness on associated biodiversity measured as acoustic diversity. The analysis of soundscapes offers easy, rapid and sustainable methods when assessing biodiversity. During the last years the quantification of regional or global acoustic variability in sounds and the analysis of different soundscapes has been evolving into an important tool for biodiversity conservation, especially since case studies confirmed a relationship between land-use management, forest structure and acoustic diversity. Here we analyzed soundscapes from two seasons (dry and rainy season) and aurally inspected a subset of audio recordings to describe temporal patterns in soundscape composition. Several acoustic indices were calculated and we performed a correlation analysis and a non-metric multidimensional scaling analysis to identify acoustic indices that: (i) were complementary to each other and such represented different aspects of the local soundscapes and (ii) related most strongly to differences in acoustic composition among tree species richness, season and day phase. Thus, we chose “High Frequency Cover,” “Bioacoustic Index,” and “Events Per Second” to test the hypothesis that acoustic diversity increases with increasing tree species richness. Monocultures differed significantly from polycultures during night recordings, with respect to High Frequency Cover. This index covers sounds above 8 kHz and thus represents part of the orthopteran community. We conclude that increasing tree species richness in a young tropical forest plantation had positive effects on the vocalizing communities. The strongest effects were found for acoustic activity of the orthopteran community. In contrast to birds, orthopterans have smaller home ranges, and are therefore important indicator species for small scale environmental conditions.

Climatic conditions, not above- and belowground resource availability and uptake capacity, mediate tree diversity effects on productivity and stability

Tree species diversity promotes multiple ecosystem functions and services. However, little is known about how above- and belowground resource availability (light, nutrients, and water) and resource uptake capacity mediate tree species diversity effects on aboveground wood productivity and temporal stability of productivity in European forests and whether the effects differ between humid and arid regions. We used the data from six major European forest types along a latitudinal gradient to address those two questions. We found that neither leaf area index (a proxy for light uptake capacity), nor fine root biomass (a proxy for soil nutrient and water uptake capacity) was related to tree species richness. Leaf area index did, however, enhance productivity, but negatively affected stability. Productivity was further promoted by soil nutrient availability, while stability was enhanced by fine root biomass. We only found a positive effect of tree species richness on productivity in arid regions and a positive effect on stability in humid regions. This indicates a possible disconnection between productivity and stability regarding tree species richness effects. In other words, the mechanisms that drive the positive effects of tree species richness on productivity do not per se benefit stability simultaneously. Our findings therefore suggest that tree species richness effects are largely mediated by differences in climatic conditions rather than by differences in above- and belowground resource availability and uptake capacity at the regional scales.

Little effect of tree species richness on within‐ and between‐plot variability in soil chemical properties in a young plantation forest

AbstractTrees impact the chemical properties of the soil in which they grow. When planting forests, the choice of tree species and species combinations is thus expected to partly determine the spatial variation in soil properties, even in the early stages of forest development. We made use of a biodiversity‐ecosystem functioning experiment in Belgium, FORBIO (FORest BIOdiversity and Ecosystem Functioning). The studied site is situated on former agricultural land on loamy sandy soil and replicates tree species richness (1–4 species) and composition. Soil samples (0–10 cm) were taken at five locations in 1, 2 and 4 species plots. We measured the total C and N concentration, the Olsen P, the pH‐H2O and the available Ca, Mg and K concentration. We hypothesised that the within‐plot spatial variability would increase with species richness and that the between‐plot variability would decrease with species richness. We found little evidence to support our hypothesis. We only found a smaller within‐plot variability in monocultures than mixtures for Ca, pH and C:N. Potential reasons for the little evidence for the effect of tree species richness may include the fact that the forest is only 9 years old, the agricultural land‐use history and the soil texture. Further research in sites with different soil properties or different land‐use history is needed to generalise these results.HighlightsVariation in soil chemistry was studied in young forest plots varying in tree species richness.Larger within‐ and a smaller between‐plot variation in mixtures than monocultures was expected.Larger within‐plot variation in mixtures was only found for for Ca, pH and C:N.Little effect of tree species richness may be due to young age, agricultural land‐use history or soil texture.

Distinct effects of host and neighbour tree identity on arbuscular and ectomycorrhizal fungi along a tree diversity gradient

Plant diversity and plant-related ecosystem functions have been important in biodiversity-ecosystem functioning studies. However, biotic interactions with mycorrhizal fungi have been understudied although they are crucial for plant-resource acquisition. Here, we investigated the effects of tree species richness and tree mycorrhizal type on arbuscular (AMF) and ectomycorrhizal fungal (EMF) communities. We aimed to understand how dissimilarities in taxa composition and beta-diversity are related to target trees and neighbours of the same or different mycorrhizal type. We sampled a tree diversity experiment with saplings (~7 years old), where tree species richness (monocultures, 2-species, and 4-species mixtures) and mycorrhizal type were manipulated. AMF and EMF richness significantly increased with increasing tree species richness. AMF richness of mixture plots resembled that of the sum of the respective monocultures, whereas EMF richness of mixture plots was lower compared to the sum of the respective monocultures. Specialisation scores revealed significantly more specialised AMF than EMF suggesting that, in contrast to previous studies, AMF were more specialised, whereas EMF were not. We further found that AMF communities were little driven by the surrounding trees, whereas EMF communities were. Our study revealed drivers of mycorrhizal fungal communities and further highlights the distinct strategies of AMF and EMF.

Microclimate and land surface temperature in a biodiversity enriched oil palm plantation

Agroforestry options such as mixed-species tree planting and natural regeneration in oil palm plantations may alleviate negative effects of forest loss on biodiversity and ecosystem functioning. The effects of agroforestry on microclimate and land surface temperatures (LST) remain largely unknown despite their central role in controlling abiotic and biotic factors and in buffering climate at a larger scale. We assessed spatial and temporal microclimate and LST variability in a biodiversity enrichment experiment, in which tree islands have been planted in an oil palm plantation in Sumatra (Indonesia). Four years after establishment of the experiment, we measured microclimate and LST using mini microclimate sensors and drone-recorded thermal images. We examined experimental effects of tree species richness (0, 1, 2, 3 or 6), plot size (25 m2, 100 m2, 400 m2, 1600 m2) and stand structural complexity on microclimate and LST. Diurnal patterns showed ambient air temperature peaks and relative humidity (RH) minima at 3 pm, whereas diurnal soil temperatures peaked around 6 pm. The lowest LST were observed from oil palm canopy leaves and the highest from bare soils and understorey vegetation (including trees). Spatial and temporal ranges of ambient air temperature were smaller than LST ranges, and average ambient air temperature and LST were positively correlated. Tree species diversity had no overall significant effect neither on microclimate nor LST, but humidity was higher in planted tree islands compared to natural regeneration only. Smaller plots were characterized by higher mean air, soil and LST, compared to larger plots. Structurally complex plots were associated with low mean and maximum values of ambient air temperature, soil temperature and LST and high mean and minimum RH. Still, conditions were hotter and drier in several experimental plots compared to conventional oil palm plantations, considering a higher transpiration in the latest. We conclude that stand structural complexity and tree island size control microclimate and LST in the experimental oil palm agroforests, but alleviating the harsh microclimate conditions in oil palm plantations might take longer to occur.

Relative importance of tree species richness, tree functional type, and microenvironment for soil macrofauna communities in European forests.

Soil fauna communities are major drivers of many forest ecosystem processes. Tree species diversity and composition shape soil fauna communities, but their relationships are poorly understood, notably whether or not soil fauna diversity depends on tree species diversity. Here, we characterized soil macrofauna communities from forests composed of either one or three tree species, located in four different climate zones and growing on different soil types. Using multivariate analysis and model averaging we investigated the relative importance of tree species richness, tree functional type (deciduous vs. evergreen), litter quality, microhabitat and microclimatic characteristics as drivers of soil macrofauna community composition and structure. We found that macrofauna communities in mixed forest stands were represented by a higher number of broad taxonomic groups that were more diverse and more evenly represented. We also observed a switch from earthworm-dominated to predator-dominated communities with increasing evergreen proportion in forest stands, which we interpreted as a result of a lower litter quality and a higher forest floor mass. Finally, canopy openness was positively related to detritivore abundance and biomass, leading to higher predator species richness and diversity probably through trophic cascade effects. Interestingly, considering different levels of taxonomic resolution in the analyses highlighted different facets of macrofauna response to tree species richness, likely a result of both different ecological niche range and methodological constraints. Overall, our study supports the positive effects of tree species richness on macrofauna diversity and abundance through multiple changes in resource quality and availability, microhabitat, and microclimate modifications.

Tree species richness and water availability interact to affect soil microbial processes

The objective of this study was to examine the interactive influence of tree diversity and water availability on microbial functioning in surface soil (0–10 cm). The study was conducted at two field experiments (ages 4.2 and 9.0 years, respectively) of the TreeDivNet platform that established plantations with a gradient of tree species richness exposed to high and low water availability treatments. Soil microbial biomass, activity, functional diversity and functional identity were estimated using the MicroResp™ method. Soil organic carbon and several other variables potentially associated with soil processes (soil moisture, soil and air temperature, soil pH, soil C:N ratio, tree biomass) were also measured. Our results indicate that tree species richness significantly increased soil microbial biomass and, to a lesser extent, soil microbial activity in the high water availability treatment, at both trial sites. However, tree species richness effects on microbial processes were not expressed in the low water availability treatment, contrary to the stress-gradient hypothesis. Tree species richness and water treatment interaction impacted microbial biomass and activity through their effects on microbial functional diversity and identity at one site and air temperature at the other site. In conclusion, the mechanisms through which tree diversity creates bottom-up effects on biological soil processes depend on environmental conditions and time following the establishment of tree communities. Future research relating diversity and water availability effects on soil microbial processes should consider functional traits of tree species and a larger range of trophic levels including herbivores and soil fauna to better understand biodiversity-soil ecosystem function relationships in forest ecosystems.

The significance of tree-tree interactions for forest ecosystem functioning

Global change exposes forest ecosystems to many risks including novel climatic conditions, increased frequency of climatic extremes and sudden emergence and spread of pests and pathogens. At the same time, forest landscape restoration has regained global attention as an integral strategy for climate change mitigation. Owing to unpredictable future risks and the need for new forests that provide multiple ecosystem services, mixed-species forests have been advocated for this purpose. However, the successful establishment of mixed forests requires intrinsic knowledge of biodiversity's role for forest ecosystem functioning. In this respect, a better understanding of tree-tree interactions and how they contribute to observed positive tree species richness effects on key ecosystem functions is critical. Here, we review the current knowledge of the underlying mechanisms of tree-tree interactions and argue that positive net biodiversity effects at the community scale may emerge from the dominance of positive over negative interactions at the local neighbourhood scale. In a second step, we demonstrate how tree-tree interactions and the immediate tree neighbourhood's role can be systematically assessed in a tree diversity experiment. The expected results will improve predictions about the effects of tree interactions on ecosystem functioning based on general principles. We argue that this knowledge is urgently required to guide the design of tree species mixtures for the successful establishment of newly planted forests.