Arbuscular Mycorrhizal Trees Research Articles

Survival Rate, Chemical and Microbial Properties of Oak Seedlings Planted with or without Oak Forest Soils in a Black Locust Forest of a Dryland

Native tree species are frequently unable to effectively grow in non-native tree cultivation scenarios. In the Loess Plateau, China, it is difficult to find native oak (Quercus liaotungensis) seedlings in non-native black locust forests. Black locust is an arbuscular mycorrhizal (AM) tree, but oak is an ectomycorrhizal (ECM) tree. Plants highly depend on their symbiotic mycorrhizal fungi to take up water, nitrogen (N) and other nutrients. We hypothesized that black locust forests would not provide ECM inoculum to oak seedlings, limiting their water and nutrient uptake, which would be improved by ECM inoculum. Here, we (1) sowed seeds, with or without oak forest soils, (2) transplanted seedlings collected in oak forests, with or without oak forest soils, and (3) planted seedlings germinated in autoclaved or unautoclaved oak forest soils. We measured the survival and growth rate for all three experiments, along with chemical properties, and root ECM colonization. Oak seeds sowed with oak forest soils had higher mycorrhizal colonization, leaf N concentrations and survival rate, and lower root δ13C than the seeds sowed without oak forest soils. Planting with oak forest soils also increased the survival rate of the germinated seedlings, but not the transplanted seedlings. Overall, our study suggested that the use of oak forest soils in the black locust forest to improve the water and N uptake of oak seedlings by providing the ECM inoculum, resulting in a high survival rate. Our study also implies that the method of sowing seeds was effortless and effectively compared to transplanting wild/nursery seedlings.

Tree species rather than type of mycorrhizal association drive inorganic and organic nitrogen acquisition in tree–tree interactions

Mycorrhizal fungi play an important role for the nitrogen (N) supply of trees. The influence of different mycorrhizal types on N acquisition in tree-tree interactions is, however, not well understood, particularly with regard to the competition for growth-limiting N. We studied the effect of competition between temperate forest tree species on their inorganic and organic N acquisition in relation to their mycorrhizal type (i.e., arbuscular mycorrhiza or ectomycorrhiza). In a field experiment, we quantified net N uptake capacity from inorganic and organic N sources using 15N/13C stable isotopes for arbuscular mycorrhizal tree species (i.e., Acer pseudoplatanus L., Fraxinus excelsior L., and Prunus avium L.) as well as ectomycorrhizal tree species (i.e., Carpinus betulus L., Fagus sylvatica L., and Tilia platyphyllos Scop.). All species were grown in intra- and interspecific competition (i.e., monoculture or mixture). Our results showed that N sources were not used complementarily depending on a species' mycorrhizal association, but their uptake rather depended on the competitor, indicating species-specific effects. Generally, ammonium was preferred over glutamine and glutamine over nitrate. In conclusion, our findings suggest that the inorganic and organic N acquisition of the studied temperate tree species is less regulated by mycorrhizal association but rather by the availability of specific N sources in the soil as well as the competitive environment of different tree species.

The impact of beech and riparian forest herbaceous plant species with contrasting traits on arbuscular mycorrhizal fungi abundance and diversity

The impact of forest herbaceous plant species identity on arbuscular mycorrhizal fungi (AMF) communities in relation to vegetation and soil chemical properties has rarely been studied in temperate forests. We thus compared abundance and diversity of AMF in two deciduous forests, in the plots with herbaceous plant species of contrasting traits (morphology, phenology, reproduction, and ecology), including Allium ursinum and Dentaria enneaphyllos in a beech forest, as well as Aegopodium podagraria and Ficaria verna in a riparian forest. Plots with a mix of herbaceous plant species and bare soil were also studied. We found low values of AMF root colonisation degrees in both forests. In total, the spores of only 13 AMF species were detected; 12 species occurred in the riparian forest, whereas 6 were found in the beech forest. AMF species composition differed between forest types. Higher AMF root colonisation degrees and soil concentrations of PLFA 16:1ω5 AMF structural lipid marker were found in the riparian forest. There were no differences between the plots in AMF abundance and species richness in both forest soils. With regard to environmental variables, the number of arbuscular mycorrhizal (AM) tree species, plant cover, moisture, the concentrations of C, Ca, K, Mg, and N had, in general, a positive effect on AMF abundance in soils. We showed that despite AMF ubiquity, these fungi can be present at low densities in some habitats, such as beech and riparian forests. Higher AMF abundance and diversity in the riparian forest were probably related to larger numbers of AM tree and herbaceous plant species in this forest. Although the herbaceous plant species had contrasting traits, they had little effect on AMF abundance and diversity.

Arbuscular mycorrhizal trees cause a higher carbon to nitrogen ratio of soil organic matter decomposition via rhizosphere priming than ectomycorrhizal trees

Tree roots and their associated microbes can significantly influence soil organic matter (SOM) decomposition, i.e., the rhizosphere priming effect. This effect is expected to be greater in trees associated with arbuscular mycorrhizal (AM) fungi, which produce higher extracellular enzymes especially surrounding hyphae, than in trees associated with ectomycorrhizal (ECM) fungi. Here, we selected five tree species associated with AM (Juglans mandshurica Maxim. and Cunninghamia lanceolata (Lamb.) Hook.) or ECM (Picea koraiensis Nakai, Quercus mongolica Fischer ex Turcz. and Larix kaempferi (Lamb.) Carriere). We grew tree seedlings inside of cores lined with different mesh sizes to investigate how roots, hyphae and exudates influence soil carbon (C) and nitrogen (N) mineralization via the rhizosphere priming effect, using a13C natural abundance approach and a15N pool dilution method, concurrently. We found that tree seedlings significantly accelerated soil C decomposition by on average 78%, i.e., positive priming, compared to unplanted control pots. AM-associated trees induced 2.1 times greater soil C decomposition than ECM-associated trees across all mesh sizes. In contrast, gross N mineralization did not differ between tree-mycorrhizal associations. Compared to ECM counterparts, AM-associated trees had higher C- and lower N-degrading enzyme activities. Consequently, AM-associated trees induced a significantly higher C:N ratio of SOM decomposition than their ECM counterparts, which could be associated with the differences in soil enzyme activities for C and N degradation. Further, for both AM- and ECM-associated trees, we found no significant influences of mesh size on soil C decomposition, suggesting that the rhizosphere priming effect of mycorrhizal symbiosis was predominantly driven by root exudates. We conclude that SOM decomposition caused by AM-associated trees may have a higher C:N ratio than that by ECM-associated trees mainly due to differences in microbial enzyme investment. Our findings imply that tree-mycorrhizal associations are capable of modulating soil biogeochemical cycling via the rhizosphere priming effect.

Mixing tree species associated with arbuscular or ectotrophic mycorrhizae reveals dual mycorrhization and interactive effects on the fungal partners.

Recent studies found that the majority of shrub and tree species are associated with both arbuscular mycorrhizal (AM) and ectomycorrhizal (EM) fungi. However, our knowledge on how different mycorrhizal types interact with each other is still limited. We asked whether the combination of hosts with a preferred association with either AM or EM fungi increases the host tree roots’ mycorrhization rate and affects AM and EM fungal richness and community composition.We established a tree diversity experiment, where five tree species of each of the two mycorrhiza types were planted in monocultures, two‐species and four‐species mixtures. We applied morphological assessment to estimate mycorrhization rates and next‐generation molecular sequencing to quantify mycobiont richness.Both the morphological and molecular assessment revealed dual‐mycorrhizal colonization in 79% and 100% of the samples, respectively. OTU community composition strongly differed between AM and EM trees. While host tree species richness did not affect mycorrhization rates, we observed significant effects of mixing AM‐ and EM‐associated hosts in AM mycorrhization rate. Glomeromycota richness was larger in monotypic AM tree combinations than in AM‐EM mixtures, pointing to a dilution or suppression effect of AM by EM trees. We found a strong match between morphological quantification of AM mycorrhization rate and Glomeromycota richness. Synthesis. We provide evidence that the combination of hosts differing in their preferred mycorrhiza association affects the host's fungal community composition, thus revealing important biotic interactions among trees and their associated fungi.

Aboveground herbivores drive stronger plant species-specific feedback than belowground fungi to regulate tree community assembly.

Ectomycorrhizal (EcM) tree species often become more dominant than arbuscular mycorrhizal (AM) tree species in temperate forests, but they generally coexist. Theory predicts that ecological feedback mediated by aboveground herbivory and/or belowground microbes could explain these dominance/coexistence patterns. An experimental test of how aboveground/belowground organisms associated with AM/EcM trees mediate ecological feedbacks has been lacking at the community-level. By establishing AM and EcM tree sapling assemblages in mesocosms and then introducing seedlings of each type in areciprocal planting experiment, we compared seedling performance under varying sapling species (conspecifics, heterospecifics within the same and different mycorrhizal types), using traits that reflect either aboveground herbivory-mediated feedback or belowground fungal-mediated feedback or both. When examining seedling traits that reflect aboveground herbivory-mediated feedbacks (i.e., foliar damage), AM plants tended toexperience less foliar damage and EcM plants more damage under conspecific versus heterospecific saplings within the same mycorrhizal types, and aboveground herbivory-mediated feedback was species-specific rather than mycorrhizal type-specific. Conversely, when examining traits that reflect belowground fungal-mediated feedbacks, both AM and EcM plant species often exhibited mycorrhizal type-specific feedbacks (e.g., greater aboveground biomass under the same versus different mycorrhizal-type saplings) rather than species-specific feedbacks. Furthermore, tree species affected by herbivory-mediated feedback were less affected by belowground feedback, indicating that the relative importance of the feedbacks varied among plant species. Analysis of plant-associated organisms verified that the feedback outcomes corresponded with species accumulation of belowground fungi (but not of abovegroundherbivores). Thus, aboveground herbivores drive stronger plant species-specific feedback than belowground fungi to regulate temperate tree diversity.

Plastic responses of fine root morphology and architecture traits to nitrogen addition in ectomycorrhizal and arbuscular mycorrhizal tree species in an evergreen broadleaved forest.

We measured the morphology traits (specific root length, specific root surface area, root tissue density, average root diameter) and architecture traits (root fork, root fork ratio, increase rate of root length, root tip density, root fork density) of fine roots in two mycorrhiza tree species, Castanopsis faberi (ectomycorrhizal) and Schima superba (arbuscular mycorrhizal), in an evergreen broadleaved forest in the middle subtropical zone. Root bags method was used in an in situ nitrogen deposition experiment. The aim of this study was to reveal the differences in the plastic responses of fine root morphology and architecture traits to nitrogen deposition between the different mycorrhizal trees. The plastic responses of specific root length, specific root surface area and root fork to nitrogen addition decreased from the first-order root to the fourth-order root, while root tissue density showed an opposite pattern. Such a result indicated a trade-off between nutrient acquisition and resource maintenance of different fine root orders. Different mycorrhizal tree species adopted diffe-rent adaptation strategies to the variations of soil nitrogen availability. C. faberi adopted an opportuni-stic strategy, which relied on fine root to improve nutrient absorption efficiency, enhanced the capacity of space expansion and in-situ nutrient absorption to focus on rapid nutrient absorption strategy. S. superba did not change fine root morphological traits through the trade-off between nutrient absorption efficiency and root construction cost, but relied more on the complementarity between mycorrhizal fungi and fine root architecture traits for nutrient acquisition. The differences in the cost of maintaining and constructing fine root C between different mycorrhizal trees led to fine root adopting the most suitable nutrient capture strategy.

Tree‐induced alterations to soil properties and rhizoplane‐associated bacteria following 23 years in a common garden

Key functional root traits, including mycorrhizal association and root diameter, can help project ecosystem processes like root turnover and soil carbon sequestration. It is less clear, however, how such traits relate to variations in soil biology and chemistry. Here, we examined the impact of tree species with varied root traits on soil properties and rhizoplane bacterial composition, focusing specifically on mycorrhizal association type and root diameter. Within a long-term common garden in central Pennsylvania, USA, we selected three arbuscular mycorrhizal (AM) and three ectomycorrhizal (EM) tree species and assessed changes in (1) soil and leaf chemistry and (2) bacterial composition along fine absorptive roots, through 16S rRNA gene sequencing. AM trees increased soil pH and soil available nitrogen relative to EM trees, and mycorrhizal association type was significantly associated with rhizoplane-associated bacterial composition. Absorptive root diameter did not clearly explain soil variability but was associated with changes in the composition of rhizoplane-associated bacteria. Tree-mediated shifts to soil properties were linked to mycorrhizal association type, whereas rhizoplane recruitment of bacteria was linked to both mycorrhizal type and root diameter. This has implications for predicting changes in biogeochemical processes following shifts in tree species composition.

Mycorrhizal type governs foliar and root multi-elemental stoichiometries of trees mainly via root traits

Multi-elemental stoichiometry can represent the biogeochemical niches of species, which can further guide community assemblage. Mycorrhizae play a key role in plant elemental acquisition. Whether and how mycorrhizal type influences the plant multi-elemental stoichiometries and elemental coordination in above- and belowground organs, however, remain unclear. We determined the concentrations of C, N, P, K, Ca, Mg, Fe, Mn, Cu, Zn, Na, Pb and Sr and some elemental ratios in the leaves and roots of arbuscular mycorrhizal (AM) trees (Phoebe bournei, Schima superba, Michelia maudiae and Cunninghamia lanceolata) and ectomycorrhizal (ECM) trees (Cyclobalanopsis glauca, Pinus massoniana and P. elliottii) in subtropical China. The effects of root traits and soil abiotic factors on elemental concentrations and ratios were reported. The elemental concentrations and ratios differed substantially between the AM and ECM trees, indicating the their biogeochemical niches’ differentiation. The C, P, Mg and Mn concentrations and the K:Mg andCa:Mg ratios for AM and the N, Mn and Na concentrations and the Fe:Mn ratio for the ECM species were strongly correlated between leaves and roots, but the correlations were positive or negative depending on elemental composition and mycorrhizal type. The root branching ratio, branching intensity and length played important roles in the plant elemental stoichiometries. The biogeochemical niche’s differentiation between the AM and ECM species emphasizes the important role of mycorrhizal type in the assemblage of co-occurring species. This study offers a new opinion for species choosing in mixed forests from the view of the biogeochemical niche’s separation.

Mycorrhizal type influences plant density dependence and species richness across 15 temperate forests.

Recent studies suggest that the mycorrhizal type associated with tree species is an important trait influencing ecological processes such as response to environmental conditions and conspecific negative density dependence (CNDD). However, we lack a general understanding of how tree mycorrhizal type influences CNDD strength and the resulting patterns of species abundance and richness at larger spatial scales. We assessed 305 species across 15 large, stem-mapped, temperate forest dynamics plots in Northeastern China and North America to explore the relationships between tree mycorrhizal type and CNDD, species abundance, and species richness at a regional scale. Tree species associated with arbuscular mycorrhizal (AM) fungi showed a stronger CNDD and a more positive relationship with species abundance than did tree species associated with ectomycorrhizal (ECM) fungi. For each plot, both basal area and stem abundance of AM tree species was lower than that of ECM tree species, suggesting that AM tree species were rarer than ECM tree species. Finally, ECM tree dominance showed a negative effect on plant richness across plots. These results provide evidence that tree mycorrhizal type plays an important role in influencing CNDD and species richness, highlighting this trait as an important factor in structuring plant communities in temperate forests.

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.

Dominant community mycorrhizal types influence local spatial structure between adult and juvenile temperate forest tree communities

Abstract Plant–soil feedback (PSF) is known to influence plant community composition, and recent work suggests that these effects may be regulated by traits related to mycorrhizal associations and phylogenetic relationships. However, there is a critical need to test the usefulness of these traits in predicting PSF outcomes in natural plant communities. To test for evidence of mycorrhizal and phylogenetic controls over PSF at both the species and community level, we examined the spatial relationship between adult and juvenile trees in stem‐mapped hardwood forest plots using point pattern analyses and linear mixed‐effect models. We found that spatial patterns of adult and juvenile trees, as well as overall adult tree recruitment, were significantly affected by the dominant mycorrhizal type of our forested communities, but were not influenced by the phylogenetic relationship between adult and juvenile trees. Additionally, PSF experienced by individual species was dependent on the mycorrhizal dominance of the surrounding community. Spatial patterns in communities dominated by arbuscular mycorrhizal trees reflected overdispersion between adult and juvenile trees (suggestive of negative PSF), while communities dominated by ectomycorrhizal trees reflected clustering (suggestive of positive PSF). Our findings indicate that PSFs are driven by the mycorrhizal associations of dominant trees, with effects of dominant community member traits on soil microorganisms and biogeochemistry ‘spilling over’ onto less abundant individuals in the community. Our research supports the use of whole‐community, mycorrhizal‐based frameworks for studying PSF in plant communities where multiple mycorrhizal types are present. A free Plain Language Summary can be found within the Supporting Information of this article.

Abundance of saprotrophic fungi determines decomposition rates of leaf litter from arbuscular mycorrhizal and ectomycorrhizal trees in a subtropical forest

Leaf litter decomposition is a fundamental process involved in soil carbon and nitrogen dynamics. Observational and experimental evidence suggests that leaf litter from arbuscular mycorrhizal (AM) tree species has a faster decomposition rate than leaf litter from ectomycorrhizal (ECM) tree species. This could be due to the diversity and activity of decomposer microorganisms, but remains to be confirmed in subtropical forests. In this study, we compared the effects of fungi and bacteria on decomposition rates of litter from AM and ECM subtropical trees. In the field, we conducted a fungal hyphal exclusion experiment beneath AM and ECM trees. We also used fungicides and bactericides to distinguish the contribution of bacteria and fungi to litter decomposition in a shade-house experiment. Fungi were responsible more leaf litter decay in soil around AM trees than in soil around ECM trees. When fungi were excluded, AM and ECM litters with similar chemical properties decomposed at the same rate, but the AM litters decayed more quickly when saprotrophic fungi were present. Soil around AM trees had more abundance of saprotrophic fungi and greater N-acetylglucosaminidase and acid phosphatase activities than soil around ECM trees. We conclude that the higher leaf litter decomposition rates under AM trees were due to the saprotrophic fungi in the soil microbial community.

Resolving the mycorrhizal status of important northern hemisphere trees

Knowledge about mycorrhizal associations is important for understanding mineral nutrition, stress tolerance and regeneration dynamics of trees. Here we address the mycorrhizal status of 940 species of important trees growing in ecosystems or cultivated in temperate regions of the Northern Hemisphere by resolving conflicting mycorrhizal trait information. Using 3800 observations from the FungalRoot database, we show that mycorrhizal status is highly consistent within species, genera and most families. Most contradictory mycorrhizal designations result from putative diagnosis errors, such as reported ectomycorrhizas EcM) in otherwise arbuscular mycorrhizal (AM) trees (10% of records). Furthermore, the most commonly studied species are more likely to have incorrect designations in databases due to accumulation of errors. Here we provide a definitive mycorrhizal status based on careful evaluation of records, with additional support from detailed anatomical observations and physiological data. We also identify common causes for errors, such as complex root anatomy in the Cupressaceae and Rosaceae. We also present detailed microscopic images of root structural features in trees with EcM or AM associations. Most AM roots have a suberised exodermis, which forms a permeability barrier around plant-fungus interfaces in the cortex and also protects roots from unwanted fungi. EcM short roots also have highly specialised anatomical features. Tree root atatomical features demonstrate convergent evolution that is presumably linked to more efficient and specific mycorrhiza formation. We recommend that future metastudies use corrected databases and new errors be avoided by using appropriate methodology and consistent definitions of association types.

Temperate Forests Dominated by Arbuscular or Ectomycorrhizal Fungi Are Characterized by Strong Shifts from Saprotrophic to Mycorrhizal Fungi with Increasing Soil Depth.

In temperate and boreal forests, competition for soil resources between free-living saprotrophs and ectomycorrhizal (EcM) fungi has been suggested to restrict saprotrophic fungal dominance to the most superficial organic soil horizons in forests dominated by EcM trees. By contrast, lower niche overlap with arbuscular mycorrhizal (AM) fungi could allow fungal saprotrophs to maintain this dominance into deeper soil horizons in AM-dominated forests. Here we used a natural gradient of adjacent forest patches that were dominated by either AM or EcM trees, or a mixture of both to determine how fungal communities characterized with high-throughput amplicon sequencing change across organic and mineral soil horizons. We found a general shift from saprotrophic to mycorrhizal fungal dominance with increasing soil depth in all forest mycorrhizal types, especially in organic horizons. Vertical changes in soil chemistry, including pH, organic matter, exchangeable cations, and extractable phosphorus, coincided with shifts in fungal community composition. Although fungal communities and soil chemistry differed among adjacent forest mycorrhizal types, variations were stronger within a given soil profile, pointing to the importance of considering horizons when characterizing soil fungal communities. Our results also suggest that in temperate forests, vertical shifts from saprotrophic to mycorrhizal fungi within organic and mineral horizons occur similarly in both ectomycorrhizal and arbuscular mycorrhizal forests.

Tree mycorrhizal type mediates the strength of negative density dependence in temperate forests

Abstract Recent plant–soil feedback experiments suggest that arbuscular mycorrhizal (AM) tree species experience stronger conspecific negative density dependence (CNDD) than ectomycorrhizal (EM) tree species. Yet, how these findings inform our understanding of natural systems is limited because the roles of local soil conditions, light availability and tree species abundances in influencing CNDD for AM and EM species are not clear. Here we examined seedling and sapling survival in two temperate old‐growth forests (broad‐leaved pine and spruce‐fir forests) in Northeast China, to evaluate the effects of both conspecific and heterospecific neighbour density, as well as the soil and light environments, on the survival of AM‐ and EM‐dependent trees at early life stages. While light availability increased the survival of EM seedlings, soil organic resources increased EM sapling survival in the spruce‐fir plot. AM tree species suffered stronger CNDD than did EM tree species in both plots. In the spruce‐fir plot, soil factors and light availability mediated species CNDD but their effects differed between AM and EM species, and also between seedlings and saplings. For seedlings in both plots, we found that AM species exhibited a positive relationship between species abundance and CNDD strength, whereas this relationship was negative for EM species. Synthesis. Our results provide one of the few tests of how fungal symbioses determine species responses to intraspecific and interspecific interactions and the direct effects of local environmental conditions on seedling and sapling survival. We show that mycorrhizal type mediates the strength of CNDD and its relationship with species abundance. These results suggest that tree mycorrhizal association can determine the strength of CNDD effect on both rare and common species, and these CNDD differences are likely to influence the community composition of temperate forests.

Ectomycorrhizal fungi drive positive phylogenetic plant-soil feedbacks in a regionally dominant tropical plant family.

While work in temperate forests suggests that there are consistent differences in plant-soil feedback (PSF) between plants with arbuscular and ectomycorrhizal associations, it is unclear whether these differences exist in tropical rainforests. We tested the effects of mycorrhizal type, phylogenetic relationships to overstory species, and soil fertility on the growth of tree seedlings in a tropical Bornean rainforest with a high diversity of both ectomycorrhizal and arbuscular mycorrhizal trees. We found that ectomycorrhizal tree seedlings had higher growth in soils conditioned by close relatives and that this was associated with higher mycorrhizal colonization. By contrast, arbuscular mycorrhizal tree seedlings generally grew more poorly in soils conditioned by close relatives. For ectomycorrhizal species, the phylogenetic trend was insensitive to soil fertility. For arbuscular mycorrhizal seedlings, however, the effect of growing in soils conditioned by close relatives became increasingly negative as soil fertility increased. Our results demonstrate consistent effects of mycorrhizal type on plant-soil feedbacks across forest biomes. The positive effects of ectomycorrhizal symbiosis may help explain biogeographic variation across tropical forests, such as familial dominance of the Dipterocarpaceae in southeast Asia. However, positive feedbacks also raise questions about the role of PSFs in maintaining tropical diversity.

Arbuscular Mycorrhizal Fungi Mitigate Nitrogen Leaching under Poplar Seedlings

The leaching of soil nitrogen (N) has become one of the most concerning environmental threats to ecosystems. Arbuscular mycorrhizal (AM) fungi have important ecological functions, however, their influence on soil N leaching and the mechanism of action remain unclear. We conducted a two-factor (N application level × AM inoculation) experiment on poplar, and for the first time, comprehensively analyzed the mechanism by which AM fungi influence soil N leaching. The results showed that, under optimum (7.5 mM) and high (20 mM) N levels, the nitrate (NO3−) and ammonium (NH4+) concentrations of leachate in the AM inoculated treatment (+AM) were lower than in the non-inoculated treatment (−AM), with significant reductions of 20.0% and 67.5%, respectively, under high N level, indicating that AM inoculation can reduce soil N leaching and that it is more effective for NH4+. The arbuscular and total colonization rates gradually increased, and the morphology of spores and vesicles changed as the N level increased. Under optimum and high N levels, +AM treatment increased the root N concentration by 11.7% and 50.7%, respectively; the increase was significant (p < 0.05) at the high N level, which was associated with slightly increased transpiration and root activity despite reductions in root surface area and root length. Additionally, the +AM treatment increased soil cation exchange capacity (CEC), soil organic carbon (SOC), and significantly (p < 0.05) increased the proportions of macroaggregates (but without significant change in microaggregates), causing soil total nitrogen (TN) to increase by 7.2% and 4.7% under optimum and high N levels, respectively. As the N levels increased, the relative contributions of AM inoculation on N leaching increased, however, the contributions of plant physiological and soil variables decreased. Among all of the variables, SOC had important contributions to NH4+ and total N in the leachate, while root N concentration had a higher contribution to NO3−. In conclusion, AM fungi can mitigate soil N leaching and lower the risk of environmental pollution via enhancing N interception by the inoculated fungi, increasing N sequestration in plant roots, and by improving soil N retention.

Contrasting conifer species productivity in relation to soil carbon, nitrogen and phosphorus stoichiometry of British Columbia perhumid rainforests

Abstract. Temperate rainforest soils of the Pacific Northwest are often carbon (C) rich and encompass a wide range of fertility, reflecting varying nitrogen (N) and phosphorus (P) availability. Soil resource stoichiometry (C : N : P) may provide an effective measure of site nutrient status and help refine species-dependent patterns in forest productivity across edaphic gradients. We determined mineral soil and forest floor nutrient concentrations across very wet (perhumid) rainforest sites of southwestern Vancouver Island (Canada) and employed soil element ratios as covariates in a long-term planting density trial to test their utility in defining basal area growth response of four conifer species. There were strong positive correlations in mineral soil C, N, and organic P (Po) concentrations and close alignment in C : N and C : Po both among and between substrates. Stand basal area after 5 decades was best reflected by mineral soil and forest floor C : N, but in either case included a significant species–soil interaction. The conifers with ectomycorrhizal fungi had diverging growth responses displaying either competitive (Picea sitchensis) or stress-tolerant (Tsuga heterophylla, Pseudotsuga menziesii) attributes, in contrast to a more generalist response by an arbuscular mycorrhizal tree (Thuja plicata). Despite the consistent patterns in organic matter quality, we found no evidence for increased foliar P concentrations with declining element ratios (C : Po or C : Ptotal) as we did for N. The often high C : Po ratios (as much as 3000) of these soils may reflect a stronger immobilization sink for P than N, which, along with ongoing sorption of PO4-, could limit the utility of C : Po or N : Po to adequately reflect P supply. The dynamics and availability of soil P to trees, particularly as Po, deserves greater attention, as many perhumid rainforests were co-limited by N and P, or, in some stands, possibly P alone.

Determinants of leaf area index and understorey light availability in New Zealand old‐growth forests

AbstractAimGlobal studies show strong climatic control of leaf area index (LAI) and related ecosystem properties. However, alternative states stabilized by positive feedbacks have the potential to produce appreciable differences in LAI in a given environment. On certain sites in New Zealand, ectomycorrhizal (EM) southern beech forest (Nothofagaceae) appear to form a stable alternative to more species‐rich conifer‐broadleaved forest dominated by arbuscular mycorrhizal (AM) trees. We asked which climate variables best predict LAI of New Zealand's remaining old‐growth forests, and if stands dominated by Nothofagaceae have lower LAI and better‐lit understories than conifer‐broadleaved forests, when climate and other site factors are held constant.LocationNew Zealand.TaxonSeventy‐two tree species belonging to 37 angiosperm, conifer and pteridophyte families, the most frequent occurrences being Cunoniaceae, Cyatheaceae, Dicksoniaceae, Lauraceae, Nothofagaceae, Podocarpaceae and Violaceae.MethodsWe quantified overstorey composition of 45 old‐growth stands, and used the LAI‐2200C Plant Canopy Analyzer to measure LAI and understorey light. We used generalized linear mixed models to determine how these variables were influenced by environment and by dominance of Nothofagaceae.ResultsLAI ranged from 3.5 to 6.9, and increased linearly with mean annual temperature (MAT); percentage light transmission to the understorey spanned an order of magnitude and decreased exponentially with increasing MAT. However, at a given MAT, LAI of stands dominated by Nothofagaceae averaged 0.8–0.9 units lower than conifer‐broadleaf stands, resulting in twofold higher understorey light levels in the former. Soil water deficits also significantly depressed LAI, and increased understorey light; atmospheric water deficits had the opposite effect.Main conclusionsTemperature is the best climatic predictor of LAI and understorey light in New Zealand's remaining old‐growth forests. However, the lower LAI and canopy light interception in southern beech stands presumably reflects the light‐demanding nature of Nothofagaceae, and their apparent ability to inhibit invasion by more shade‐tolerant AM trees with heavier crowns. This inhibitory effect may reflect reduced N supply to AM plants because of suppression of N mineralization by EM fungi, as observed in some EM systems elsewhere.