Eucalyptus Monoculture Research Articles

Successive monoculture of Eucalyptus spp. alters the structure and network connectivity, rather than the assembly pattern of rhizosphere and bulk soil bacteria

Eucalyptus as a fast-growing timber tree plays a crucial role in maintaining wood supplication and ecosystem balance worldwide. Nonetheless, the management practices of Eucalyptus plantation in southern China have resulted in productivity decline and soil degradation due to high-intensity successive cultivation. Soil bacteria is a sensitive indicator of soil quality, how does the successive planting of Eucalyptus regulate the soil bacterial community structure for instance the abundance of oligotrophic bacteria, network complexity, and soil bacteria assembly remain ambiguity. To explicate the underlying influencing mechanisms of successive cultivation of Eucalyptus plantations on soil bacterial community structure. Four harvests of Eucalyptus plantations that underwent 0, 1, 2, and 3 times of harvesting were conducted to examine variations in the bacterial community between rhizosphere and bulk soils after successive planting of Eucalyptus. An adjacent evergreen broadleaf forest was employed as control. The present study revealed that successive planting of Eucalyptus increased and reduced the relative abundance of Chloroflexi (oligotrophic bacteria) and Proteobacteria (copiotrophic bacteria), respectively. Continuous planting of Eucalyptus did not modify the assembly pattern of soil bacterial communities, which was governed by stochastic processes. Successive planting of Eucalyptus decreased co-occurrence network complexity, and elevated the proportion of rare microorganisms in the keystone bacterial taxa. Soil particle size composition (clay, silt, sand) indirectly influenced the structure of soil bacterial communities by directly affecting pH, carbon, nutrients, and their stoichiometric ratios. Improving soil physical structure, increasing the input of soil carbon, nitrogen, and other nutrient resources, and maintaining a balanced resource allocation may be effective strategies for ensuring enhanced productivity and sustainable utilization of soil in successive Eucalyptus plantations.

Assessment of remote-sensed vegetation indices for estimating forest chlorophyll concentration

Remote-sensed vegetation indices (VIs) have emerged as essential tools for retrieving forest chlorophyll concentration. Although VIs are widely used, some concerns regarding VIs for estimating chlorophyll remain to be addressed, such as saturation effect, leaf area index (LAI) disturbance, and soil brightness influence. Currently, a systematic study on such performance evaluation of chlorophyll-related VIs considering these issues is still insufficient. This study coupled two radiative transfer models, the PROSPECT model and the LESS model, to simulate Eucalyptus monocultures with different chlorophyll content and systematically evaluated the 18 broad-band VIs’ ability in chlorophyll estimation at different scales. Our results indicate that most VIs designed for chlorophyll estimation were relatively resistant to saturation, except for SIPI and some classical VIs (e.g., NDVI and DVI), which were insensitive to chlorophyll decreases and tended to reach saturation quickly (when leaf chlorophyll content (LCC) exceeded 40 ug/cm2). The relationships between NDVI, SR, DVI, and LCC were easily influenced by soil brightness and LAI. S2REP, MTCI, TGI, TCARI, and EVI were insensitive to soil brightness when estimating LCC. Overall, S2REP was best at quantitatively retrieving chlorophyll and resisting interference from other factors. For practical applications, our study suggests that it is preferable to use S2REP for LCC estimation when the red-edge band is available; otherwise, CVI can be used instead. The judicious utilization of VI can effectively depict chlorophyll levels and improve the understanding of vegetation response to climate change. Our findings provide the necessary information for the selection of specific VIs tailored to specific vegetation parameters.

Changes in light use efficiency explains why diversity effect on biomass production is lower at high planting density in mixed-species plantations of Eucalyptus grandis and Acacia mangium

Understanding the effect of planting densities and species proportions on light absorption and light use efficiency can help to improve the management of mixed-species forest plantations. Our study aimed to disentangle the role of light interception and light use efficiency (LUE) on the biomass production of Eucalyptus grandis (E), a highly productive species in tropical conditions, and Acacia mangium (A), a N2-fixing species, in monocultures and mixed-species plantations for contrasting planting densities. A randomized block experiment set up over 4 ha in southern Brazil was intensively monitored for 14 months at mid rotation. The absorbed photosynthetically active radiation (APAR) was simulated for each tree of the experiment using the tri-dimensional MAESPA model parameterized with detailed in situ measurements of tree and foliage. LUE for stem wood production was estimated as the ratio of measured stem biomass production (SBP) and simulated APAR. The APAR of Eucalyptus trees did not significantly differ between monocultures and mixed plantations, the reduction of Eucalyptus density being compensated by an increase in light absorption of Eucalyptus individuals. The LUE of Eucalyptus trees in monoculture and mixed-species stands was found to be comparable only at low planting densities. The replacement of Eucalyptus trees with Acacia trees resulted in a reduction in Eucalyptus LUE only at high planting density. The SBP of Eucalyptus trees was mainly explained by differences in APAR, while both APAR and LUE explained the SBP of Acacia trees. The maximum stand production was obtained with monoculture of Eucalyptus at high density and no mixture reached this productivity. Reducing the proportion of Eucalyptus in mixture lead to a substantial decrease in stand production at high planting density due to a decrease in LUE, while this stand production reduction was offset at low planting density, underlying a higher diversity effect at low planting density. In the perspective of increasing diversity in forest plantations to foster multifunctionality, mixed plantations of Acacia and Eucalyptus at low planting density can be an interesting option to maintain a relatively high productivity, which is similar to Eucalyptus monocultures at the same low planting density.

Influência de plantações de eucalipto na comunidade de macroinvertebrados bentônicos em nascentes neotropicais

Abstract Aim The high sensitivity of springs to anthropic impacts is largely due to the high existing aquatic-terrestrial connectivity. The degradation of these ecosystems can be caused by land use, such as eucalyptus monoculture. The aim of this study was to test the influence of Eucalyptus urograndis plantations on the richness and diversity of the benthic macroinvertebrate community and on the environmental variables measured in springs in the Atlantic Forest domains. Methods Ten springs (5 in native forest areas and 5 in eucalyptus area) were sampled in the dry period of 2017. The organisms were screened and identified at the family level. For each spring, dissolved oxygen, temperature, pH, turbidity, electrical conductivity, total dissolved solids, total nitrogen, nitrite, nitrate, ammonia, total phosphorus, vegetation cover, granulometric characterization and organic matter were also measured. Results: The t-test showed that some environmental variables differed, with the eucalyptus areas having higher values of total dissolved solids and electrical conductivity, and lower values of coarse sand. In the present study, the richness and diversity of the benthic fauna of springs were significantly lower in eucalyptus sites than in native sites. Nonetheless, the composition of the communities did not differ. The Indicator Species Analysis (IndVal) associated Acari, Hydropsychidae, Leptoceridae, Nematoda, Psychodidae, and Tipulidae with areas of native forest. Conclusions Our results show that Brazilian springs are affected by eucalyptus monocultures reducing the richness and diversity of benthic macroinvertebrates and also changing water environmental properties. Thus, we verified that previous studies elsewhere in the world also apply to neotropical areas where eucalyptus is not native.

Impact of mixed-species forest plantations on soil mycobiota community structure and diversity in the Congolese coastal plains.

Mixed tree plantations containing nitrogen (N2)-fixing species have the potential to enhance C sequestration, soil biodiversity and forest productivity. Here, we investigated the impact of Acacia mangium and Eucalyptus urophilla x E. grandis mixed plantations in the Congolese coastal plains on soil mycobiota community structure and diversity by ITS metabarcoding sequencing and bioinformatic analysis. Higher Faith's phylogenetic diversity and Evenness' was found in Eucalyptus monoculture relative to stands containing Acacia. Differences in beta diversity were found among Eucalyptus and Acacia monoculture, and mixed-species stands highlight the effects of plant species on fungal community structure. Ascomycota, Basidiomycota and Rozellomycota phyla were predominant in all stands, with both Dikarya (Ascomycota and Basidiomycota) accounting for more than 70% in all stands. Correlation analysis revealed that sulfur (S) was the most correlated soil attribute with the three predominant phyla but also with Mucoromycota and Calcarisporiellomycota phyla, although mostly negatively correlated (4 out of 5). Phosphorus was mostly positively correlated to soil attributes (3 out of 4) and nitrogen was correlated twice, positively and negatively. Distance-based redundancy analysis revealed a positive correlation of nitrogen (p-value = 0.0019, contribution = 22%) and phosphorus (p-value = 0.0017, contribution = 19%) with soil mycobiota. A high prevalence of generalists (28% to 38%) than specialists (9% to 24%) were found among the different sites. In stands containing Acacia (pure and mixed species) the soil mycobiota harbor the prevalence of generalist strategies with the potential to withstand environmental stresses and utilize a higher number of resources against specialists in Eucalyptus stands. Stronger positive correlation between soil attributes and main fungal taxa, higher generalists' strategies and lower Faith's phylogenetic diversity and Evenness were reported in stands containing Acacia. This highlights the potential of mixed-species in preserving community stability following environmental disturbances and increasing the number of resources confirming their important ecological role in boosting the resilience of the forest ecosystems to climate and land-use (plant species as shown by PCA analysis) changes.

Advantages and limitations of tree mixtures in enhancing avian diversity in commercial timber plantations: Early insights from southern China

Commercial timber plantations, particularly monocultures, have limited capacity to support biodiversity. A widely promoted approach to enhance this capacity is the diversification of monocultures into tree mixtures, by adding “secondary” timber species while retaining the production focus on the monoculture species. However, the presumed biodiversity benefits of tree mixtures are yet to be robustly tested, because current evidence predominantly comes from mixed plantations in the general sense, which are often not under active production or differ from monocultures in other important ways. To address this gap, we assess the effectiveness of early-stage tree mixtures in enhancing the avian diversity of Eucalyptus monocultures in southern China, which is the country's most important timber production region and has been pioneering tree mixtures in large trials over the past decade. During the avian breeding and non-breeding seasons in 2021, we conducted rigorous surveys to compare the avian diversity of Eucalyptus monocultures and their tree mixtures involving slow-growing native species, against the reference baseline of mature native forests. We found that tree mixtures exhibited modest advantages over monocultures during the breeding season, and both plantation types had notable shortfalls relative to mature native forests. We additionally found that tree mixtures tended to benefit species that foraged in the medium–low strata, consumed diets of higher trophic levels, and depended more on mature forest habitat. We conclude that the biodiversity benefits of early-stage commercial tree mixtures were rather limited and more modest than typically assumed, highlighting the importance of studying tree mixtures in real-world production settings. Our study also underscores the conservation importance of protecting existing mature native forests.

A system lock-in blocks the uptake of mixed sustainable Eucalyptus plantations in Brazil

Forest plantation areas across the globe are increasing in size and in 2019 Eucalyptus monocultures in Brazil covered 6.97 million hectares (0.8%). These monocultures hardly provide ecosystem services and do not support wildlife habitats. A transition towards more sustainable Eucalyptus plantations is urgently needed to support the provisioning of ecosystem services and conserve biodiversity. This transition requires the development of sustainable business models. We studied the upscaling of sustainable Eucalyptus plantation alternatives in Brazil, by analysing the potential barriers that need to be overcome using the Technological Innovation Systems perspective and semi-structured interviews of relevant stakeholders across the Eucalyptus value chain. Although the provisioning of ecosystem services is improved by alternative plantation management, we identified four blocking mechanisms that inhibit the functioning of the innovation system and as such hamper upscaling; 1) the productivist approach by incumbent actors on plantation management that is focussing only on short-term profit maximisation and results in a strong resistance to change current practices, 2) the weak societal and governmental vision that does not put sufficient pressure to change practices, 3) additional certification to support alternatives is in its infacy to develop niche markets which also hampers the development of financial support, and 4) failed demonstration projects in the past that have led to a lack of proof of concept for further experimentation and knowledge development. These barriers combined lead to a system lock-in, resistant to change and not capable for a diffusion of sustainable alternatives. We conclude that the directionality of a sustainability transformation should be enhanced by formulating long-term goals and strong commitment of public and private actors. We discuss how a mission oriented approach could foster such directionality for the urgently needed regime transformation in Eucalyptus paper and pulp production.

Impacts of land use change on landscape structure and eco- system services at local scale: A case study in central Portugal

This study aims to evaluate land use and land cover (LULC) changes and associated impacts in the landscape structure and ecosystem services (ES) value in Lousã municipality, in Central Portugal. The results show that signif- icant changes in LULC were recorded over the study period (1974–2018). Agricultural abandonment, expansion of woodland due to the promotion of the eucalyptus (EU) monoculture (chiefly Eucalyptus globulus), the invasion of exotic species (mainly Acacia dealbata) and the increase of built-up areas are the most significant changes that have shaped the landscapes of the study area. The analysis also revealed that the mean patch size has decreased whereas the number of patches increased. The results showed that although the EU and deciduous forest (DF) increased the ES value, the overall total ES value fell around 10% between 1974 and 2018, mainly due to the decrease in the supply of agricultural goods. Studies of this kind on local rural landscapes are vital when it comes to devising appropriate land management policies for the landscape level by considering the interaction between each element for sustainable development.

Planting nitrogen-fixing trees in tropical Eucalyptus plantations does not increase nutrient losses through drainage

Planting nitrogen (N) fixing species can provide substantial levels of N for tree growth, but it can also result in large nutrient losses through deep drainage, threatening soil fertility. Nutrient losses through deep leaching have been little studied in tropical forest plantations and comprehensive studies are needed before planting these species on a large scale. We assessed nutrient fluxes in soil solutions collected in monocultures and mixed plantations of Acacia mangium and Eucalyptus grandis in Sao Paulo state (Brazil) on Ferralsols fertilized with potassium, phosphorus and lime. Soil solution sampling began after replanting the trees in an experiment that had already been conducted for six years in a first rotation. We collected soil solutions beneath the forest floor and at depths of 30, 100 and 300 cm, and determined nutrient concentrations monthly for 3.5 years. Drainage fluxes were calculated at the depth of the lysimeters by modeling the water fluxes with Hydrus 1D. The N concentrations in topsoil solutions were generally higher in Acacia than in Eucalyptus monocultures, confirming that the introduction of Acacia increases soil N availability. However, these differences were no more observed at 100 cm depth and total N leaching at 300 cm depth was low in all the treatments, with an average of 4.8 kg N ha−1 yr−1, probably due to the rapid root system development and high nutrient requirements of these fast-growing trees. Leaching fluxes of K+, Cl−, Ca2+ and Mg2+ peaked in the upper soil layers after fertilizer applications, but the fluxes drastically decreased in deep soil layers. The low fluxes of dissolved nutrients in deep gravitational solutions in our study suggest that the risks of nutrient leaching losses are low in fast-growing plantations established on deep Ferralsols. Our results show that planting N-fixing trees can enhance the N availability for plants without producing large leaching losses, which confirms the interest of mixed plantations with N-fixing trees.

Intermittent Rivers as a Challenge for Freshwater Ecosystems Quality Evaluation: A Study Case in the Ribeira de Silveirinhos, Portugal

Intermittent rivers and streams are aquatic ecosystems that lose hydrological connectivity during drought periods. Anthropogenic pressures combined with climate change accelerate the spatial and temporal intermittency of these ecosystems, promoting alterations in ecology and ecosystem functions. This study aimed to assess the ecological status of Ribeira de Silveirinhos, located in the municipalities of Valongo and Gondomar (north of Portugal), using the metrics described in the Water Framework Directive. Thus, five sampling sites were selected along Ribeira de Silveirinhos. Sampling was done in three different periods, spring of 2019 and 2020, and autumn of 2019. At each site, physical and chemical water parameters were measured, and the benthic macroinvertebrates community was characterized. Physical and chemical parameters showed low pH values at upstream sites (where the dry phase occurs and is surrounded by Eucalyptus monoculture) and high concentrations of phosphorus at the downstream sites (subjected to several anthropogenic activities, iron waters, and agriculture). The macroinvertebrate community revealed low values of specific abundance and richness, especially during the dry period. Sensitive taxa (Ephemeroptera, Plecoptera, and Trichoptera) were negatively correlated with flow intermittency. Results showed that Ribeira de Silveirinhos is classified between “poor” and “moderate” in terms of ecological status. Intermittent streams are undervalued, so their protection is at risk. Thus, it is important to consider the specific ecological functioning of these ecosystems and to adjust the planning and management models in order to guarantee ecological quality and conservation processes.

Synergistic effects of nitrogen and plant growth-promoting rhizobacteria inoculation on the growth, physiological traits and nutrient absorption of intercropped Eucalyptus urophylla × Eucalyptus grandis and Dalbergia odorifera

Key messageLegume/nonlegume intercropping systems equipped with moderate nitrogen (N) application and PGPR inoculation could be exploited in eucalyptus growing areas and degraded land as an ecologically sustainable system to avoid excessive fertilization and enhance nitrogen use efficiency.Hazardous nitrogenous fertilizers in eucalyptus monocultures are challenging for the balance between eucalyptus wood production and ecological service functions. To assess whether plant growth-promoting rhizobacteria (PGPR) inoculation coupled with N application may improve plant growth by increasing biomass, photosynthesis, soil nutrient supply, and nutrient uptake capacity in the intercropping system. A pot experiment was performed to evaluate the synergistic effects of N application and N-fixing PGPR on growth, physiological parameters, N accumulation and nitrogen use efficiency (NUE) in intercropped Eucalyptus urophylla × Eucalyptus grandis (E. urophylla × E. grandis) and Dalbergia odorifera (D. odorifera). N fertilization positively influenced the plant height, dry matter yield, photosynthetic characteristics, N accumulation and nitrate reductase activity of E. urophylla × E. grandis under both inoculations. The growth and physiological traits of D. odorifera improved under the N2 (6 g N pot−1) application level. E. urophylla × E. grandis inoculated with Rhizobium japonicum IOC 113-2 showed higher values for plant height, biomass accumulation, N accumulation and photosynthesis with N fertilization, but the growth and physiological parameters of D. odorifera responded differentially to different inoculation treatments. The results demonstrated that an appropriate N supply in combination with N-fixing PGPR inoculation of legumes could increase nutrient absorption, NUE and yield advantages in intercropped E. urophylla × E. grandis and D. odorifera; thus, this method could be recommended as an alternative planting system under N-limited conditions in agroecosystems.

How different farming practices influence the activity of insectivorous Neotropical bats

Recent bioacoustic studies have shown different responses of insectivorous bats to native habitat loss. We examined the activity and species/sonotypes composition of aerial insectivorous bats present in a human-modified karst landscape in Southeast Brazil, characterized by the presence of semideciduous forest, pastures and Eucalyptus globulus monocultures. Using ultrasonic detectors, we investigated activity and identified bat species and/or sonotypes in the three habitat types. We compared the activity (as a surrogate for abundance) and composition of species/sonotypes present and used Generalized Linear Models to investigated whether canopy density, understory density and food availability influence the response of bats in these habitat types. Our main results demonstrate that the variables general passes and species/sonotype richness did not differ significantly between forest and pasture, however, both variables in these two environments differed significantly from the values ​​found for eucalyptus. We conclude that, in the studied agropastoral landscape, pastures interspersed with forest areas can be used by aerial insectivorous bats during foraging. However, we also found evidence that eucalyptus monocultures, not yet mature and without an understory, have a negative impact on the species/sonotype richness and activity of Neotropical aerial insectivorous bats.

Monitoring the effects of field exposure of acetamiprid to honey bee colonies in Eucalyptus monoculture plantations

Eucalyptus plantations occupy 26 % of Portuguese forested areas. Its flowers constitute important sources for bees and beekeepers take advantage of this and keep their honey bee colonies within or near the plantations for honey production. Nonetheless, these plantations are susceptible to pests, such as the eucalyptus weevil Gonipterus platensis. To control this weevil, some plantations must be treated with pesticides, which might harm non-target organisms. This study aimed to perform a multifactorial assessment of the health status and development of Apis mellifera iberiensis colonies in two similar landscape windows dominated by Eucalyptus globulus plantations - one used as control and the other with insecticide treatment. In each of the two selected areas, an apiary with five hives was installed and monitored before and after a single application of the insecticide acetamiprid (40 g a.i./ha). Colony health and development, resources use, and pesticide residues accumulation were measured. The results showed that the application of acetamiprid in this area did not alter the health status and development of the colonies. This can be explained by the low levels of residues of acetamiprid detected only in pollen and bee bread samples, ~52 fold lower than the sublethal effect threshold. This could be attributed to the low offer of resources during and after the application event and within the application area, with the consequent foraging outside the sprayed area during that period. Since exposure to pesticides in such complex landscapes seems to be dependent on the spatial and temporal distribution of resources, we highlight some key monitoring parameters and tools that are able to provide reliable information on colony development and use of resources. These tools can be easily applied and can provide a better decision-taking of pesticide application in intensive production systems to decrease the risk of exposure for honey bees.

EVALUATION OF THE INFLUENCE OF THE MATRIX IN BIODIVERSITY CONSERVATION: EFFECTS ON SEMIDECIDUOUS TROPICAL FORESTS IN SOUTHEASTERN BRAZIL

Deforestation process results in a large archipelago of small forest fragments isolated from each other. This is especially important in the Atlantic Forest, a biome with great human occupation and which has been severely deforested. Thus, there is a necessity for verify the effects that different matrices can exert on these remnants. In this study we verified the effect that sugarcane, pasture and Eucalyptus matrices can have on forest remnants. It was hypothesized that monoculture of Eucalyptus could protect the fragments given their size and relative longevity. The fragments evaluated presented clear signs of edge effect. The forest fragments surrounded by Eucalyptus interface presented greater species richness, lower dominance and a lower incidence of pioneer species indicating a degree of protection and a more advanced stage of succession. The use of trees in the surrounding matrix of fragments can be implemented to provide a stable and continuum forest condition, increasing the protection of these remnants with ecological benefits.

Root-soil facilitation in mixed Eucalyptus grandis plantations including nitrogen-fixing species

The success of mixed plantation systems is ultimately the net result of positive and negative interactions, including belowground interactions, among the respective species. Despite increasing knowledge on positive interactions on biomass and productivity in mixed-species plantations, relatively little is known about the mechanisms underlying belowground interactions. Based on biodiversity data from four mixed Eucalyptus grandis plantations, we determined the optimal ecological facilitative interactions of mixed plantation, i.e., E. grandis and Alnus formosana. We then investigated in situ root exudation of E. grandis using a cuvette-based method and analysed root patterns, defence chemicals, bacterial communities, and biochemical properties of rhizosphere soils in mixed E. grandis with alder. Compared with pure Eucalyptus plantations, the fine root production by E. grandis was higher in subsoil layers (40–80 cm) in mixed plantations with alder. Autotrophic root respiration and associated enzymes were also increased in mixed plantations. Root exudates and defence allelochemicals of E. grandis changed in the presence of alder. Compared with the pure Eucalyptus plantations, E. grandis roots in mixed plantations reduced the release of potential allelochemicals, such as phenolic acids, flavonoids, unsaturated lactone, and glycosides. Methyl jasmonate, a common signal chemical, was significantly decreased in mixed-species plantations. The bacterial community of E. grandis rhizosphere soil was improved in mixed stands and recruited more nitrifying, N-fixing, and cellulose-decomposing bacteria, such as family Nitrosomonadaceae, genera MND1, Marmoricola, and Rikenellaceae RC9 gut groups. Thus, belowground ecological facilitative interactions occurred in mixed plantations with alder, which were due to E. grandis altering its rooting pattern, reducing the levels of released allelochemicals, recruiting more beneficial bacteria, and improving the biochemical properties of rhizosphere soil. This mechanism may be useful in reforestation programs for Eucalyptus monocultures that are suffering from problems associated with low biodiversity and reduced soil fertility.

Does the Introduction of N2-Fixing Trees in Forest Plantations on Tropical Soils Ameliorate Low Fertility and Enhance Carbon Sequestration via Interactions Between Biota and Nutrient Availability? Case Studies From Central Africa and South America

Plant and/or crop growth rely on nutrient dynamics driven by specific soil biota in different environments. This mini-review aims to provide an overview of interactions between soil organisms, nutrient dynamics, and C sequestration. To this end, we investigated published results from three forest plantations (eucalyptus monocultures and mixed plantations with N2-fixing acacia) on tropical nutrient-poor soils. One case study is located in Central Africa (Congolese coastal plains) and two others in South America (Southeastern Brazil). Overall, the studies showed that soil biota activity exerted positive effects on (i) C accretion, as both soil carbon and belowground and aboveground biomass are driven and enhanced by soil biota; and (ii) on nutrient dynamics and biogeochemical cycles in nutrient-poor soil of tropical ecosystems, which are boosted following C accumulation. On the other hand, the pedoclimatic environment may potentially impact soil functioning of mixed-species plantations through its influence on the composition and activity of bacterial communities. Regardless of the potential risk of acacia invasiveness, benefits such as pulp, fuelwood, electric pole and non-timber products supply, have been reported in Central Africa. We, therefore, conclude that including N2 fixing trees in forestry plantations as reported in this mini-review helps strengthen the links between soil biota, nutrient and SOC dynamics in mixed-species plantations on tropical nutrient-poor soils.

Soil carbon stock and stability under Eucalyptus-based silvopasture and other land-use systems in the Cerrado biodiversity hotspot

During the past few decades, commercial silvopastoral systems (SPS) with exotic Eucalyptus (hybrid) trees have become popular in the Brazilian Cerrado (savanna). With the increasing awareness about the role of carbon (C) storage in soils as a climate-change mitigation strategy and the relationship between the nature of soil aggregates and the soil's carbon sequestration potential, it is important to understand the influence of such SPS systems on soil organic carbon (SOC) storage. We studied C content in three aggregate size classes in six land-use systems on Oxisols in Minas Gerais, Brazil. The systems were planted forest, native secondary forest, managed pasture, and three 8-year-old SPS, differing in their tree-planting configurations. Eucalyptus hybrid was the tree in SPS and planted forest treatments, and Urochloa decumbens was the grass in SPS and pasture treatments. From each treatment, replicated soil samples were collected from four depth-classes (0–10, 10–30, 30–60, and 60–100 cm), fractionated by wet sieving into the three aggregate-size classes, 2000 to 250 μm, 250 to 53 μm, and <53 μm size classes representing macroaggregates, microaggregates, and silt + clay, respectively, and their C contents determined. Down to 1 m, total SOC stock values ranged from 260 Mg ha−1 under pasture to 167 Mg ha−1 under native forest, with 174 Mg ha−1 for Eucalyptus plantation and about 195 Mg ha−1 for the three SPS. Compared to the degraded native forest, the pasture system had significantly higher SOC in the whole soil and the aggregate size fractions, especially in the lower soil-depth classes. The lower SOC stock of Eucalyptus hybrid SPS compared to open pasture differs from the general trend of SPS having higher stock. Given that the Cerrado biome is a biodiversity hotspot, the use of native nitrogen-fixing trees, of which there are several, is worth investigating. In addition, the conversion from Eucalyptus monocultures to SPS could be considered as a strategy to increase the SOC stock.

Influence of different tree species on autotrophic and heterotrophic soil respiration in a mined area under reclamation

AbstractPlanting trees is one of the most effective activities in recovering soil organic carbon (SOC) stocks of degraded areas, but we still lack information on how different tree species can influence soil respiration, one of the main sources of carbon dioxide (CO2) to the atmosphere. This study aimed to explore the influence of different tree species on the autotrophic and heterotrophic components of the total soil respiration in a bauxite mining area under reclamation. We analyzed the soil CO2 efflux under five treatments: (i) monoculture of clonal Eucalyptus; (ii) monoculture of Anadenanthera peregrina (L.); (iii) a mixed plantation of 16 native tree species; (iv) a mined area without vegetation cover as a control site; and (v) a natural forest cover as a reference site. This design allowed exploring the soil CO2 dynamics in a gradient of recovery, from a degraded area to natural vegetation. Additionally, we measured soil temperature, moisture, and soil attributes. Soil CO2 efflux increased with increasing tree species cover in the rainy months. There was no significant change in CO2 efflux among the tree species. However, heterotrophic soil respiration contributed to 64% of total soil CO2 efflux and was associated with litter decomposition. Among the abiotic variables, increases in soil moisture had the most influence on CO2 efflux. Therefore, these results help to understand the factors that underpin the loss of SOC and show that afforestation with different tree species can recovery the soil biological activity by improving litter deposition and is a promising way to restore soil quality in degraded areas.

Faster recovery of soil biodiversity in native species mixture than in Eucalyptus monoculture after 60years afforestation in tropical degraded coastal terraces.

Afforestation is an effective method to restore degraded land. Afforestation methods vary in their effects on ecosystem multifunctionality, but their effects on soil biodiversity have been largely overlooked. Here, we mapped the biodiversity and functioning of multiple soil organism groups resulting from diverse afforestation methods in tropical coastal terraces. Sixty years after afforestation from bare land (BL), plant species richness and the abundance of plant litter (398±85gm-2 ) and plant biomass (179±3.7tha-1 ) in native tree species mixtures (MF) were restored to the level of native forests (NF; 287±21gm-2 and 243.0±33tha-1 , respectively), while Eucalyptus monoculture (EP) only successfully restored the litter mass (388±43gm-2 ) to the level of NF. Soil fertility in EP and MF was increased but remained lower than in NF. For example, soil nitrogen and phosphorus concentrations in MF (1.2±0.2gkg-1 and 408±49mgkg-1 , respectively; p<0.05) were lower than in NF (1.8±0.2gkg-1 and 523±24mgkg-1 , respectively; p<0.05). Soil biodiversity, abundance (except for nematodes), and community composition in MF were similar or greater than those in NF. In contrast, restoration with EP only enhanced the diversity of microbes and mites to the level of NF, but not for other soil biota. Together, afforestation with native species mixtures can end up restoring vegetation and most aspects of the taxonomic and functional biodiversity in soil whereas monoculture using fast-growing non-native species cannot. Native species mixtures show a greater potential to reach completely similar levels of soil biodiversity in local natural forests if they are received some more decades of afforestation. Multifunctionality of soil biotic community should be considered to accelerate such processes in future restoration practices.

Effects of Understory or Overstory Removal on the Abundances of Soil Nematode Genera in a Eucalyptus Plantation.

In south China, eucalyptus plantations typically consist of a single-species overstory (a eucalyptus monoculture) and a dense understory of a dominant fern species. In the current study, we assessed the effects of four treatments [control (CK), understory removal (UR), tree removal (TR), and all-plant removal (PR)] on the abundances of soil nematode genera, which can provide insight into the ecological functions of understory plants and trees. Soil nematodes were sampled six times (once before and five times after treatments were implemented) at 0–5 and 5–10 cm soil depths. The temporal dynamics of nematode genera were analyzed by the principle response curves (PRC) method. At 0–5 cm depth, the abundances of most nematode genera rapidly increased shortly after vegetation removal but then gradually decreased; the effects of UR were stronger than the effects of TR. The results might be explained by the pulsed input of plant debris to soil and its subsequent depletion. At 5–10 cm depth, the nematode communities were relatively unaffected by vegetation removal within the first 162 days, but the abundances of most genera sharply decreased on day 258 and then sharply increased on day 379 (the last sampling time). The results indicated that most nematode genera, even r-selected genera, were sensitive to vegetation removal in the upper soil layer and that understory vegetation can greatly affect soil nematode communities and presumably soil food webs. The nematode genera Prismatolaimus and Diphtherophora may be good indicators of the effects of vegetation removal. The results increase our understanding of the relationships between soil nematode genera and forest plant communities and of how soil biota is affected by forest management practices.