How do species richness and its component dependence vary along the natural restoration in extremely heterogeneous forest ecosystems?

Giant habitat heterogeneity is an important factor contributing to the high species richness (SR) in karst forests. Yet, the driving factor behind the alterations in SR patterns during natural restoration remains unclear. In this study, we established the forest dynamics plots along the natural restoration sequence (including shrub-tree mixed forest stage (SC), secondary forest stage (SG) and old-growth forest sage (OG)) in degraded karst forests to compare the SR and the dependence on its components (including total community abundance, species abundance distribution (SAD), and conspecific spatial aggregation (CSA)) among stages of natural restoration. By evaluating the degree of contribution of the components to local SR and rarefied SR, we found that the SG exhibited the highest local SR, while the rarefied SR remained increasing along the restoration sequence after controlling the sample size. At SC-SG stage, SAD and CSA contributed negatively to the differences in SR, while abundance made a positive contribution to SR differences. At SG-OG, abundance contributed positively to the difference in SR at all scales, while SAD contributed negatively at small scales. No significant contribution of CSA was found at observed scales. In addition, local SR varied more significantly with PIE than with abundance. Our research emphasizes the importance of eliminating the influence of abundance on species richness in forest ecology and management, as well as the significance of separately evaluating the components that shape the diversity patterns.

Species pool, local assembly processes: Disentangling the mechanisms determining bacterial α- and ß-diversity during forest secondary succession.

Across ecology, and particularly within microbial ecology, there is limited understanding how the generation and maintenance of diversity. Although recent work has shown that both local assembly processes and species pools are important in structuring microbial communities, the relative contributions of these mechanisms remain an important question. Moreover, the roles of local assembly processes and species pools are drastically different when explicitly considering the potential for saturation or unsaturation, yet this issue is rarely addressed. Thus, we established a conceptual model that incorporated saturation theory into the microbiological domain to advance the understanding of mechanisms controlling soil bacterial diversity during forest secondary succession. Conceptual model hypotheses were tested by coupling soil bacterial diversity, local assembly processes and species pools using six different forest successional chronosequences distributed across multiple climate zones. Consistent with the unsaturated case proposed in our conceptual framework, we found that species pool consistently affected α-diversity, even while local assembly processes on local richness operate. In contrast, the effects of species pool on ß-diversity disappeared once local assembly processes were taken into account, and changes in environmental conditions during secondary succession led to shifts in ß-diversity through mediation of the strength of heterogeneous selection. Overall, this study represents one of the first to demonstrate that most local bacterial communities might be unsaturated, where the effect of species pool on α-diversity is robust to the consideration of multiple environmental influences, but ß-diversity is constrained by environmental selection.

Changes in nitrogen and phosphorus availability driven by secondary succession in temperate forests shape soil fungal communities and function.

The soil fungal community plays an important role in forest ecosystems and is crucially influenced by forest secondary succession. However, the driving factors of fungal community and function during temperate forest succession and their potential impact on succession processes remain poorly understood. In this study, we investigated the dynamics of the soil fungal community in three temperate forest secondary successional stages (shrublands, coniferous forests, and deciduous broad-leaved forests) using high-throughput DNA sequencing coupled with functional prediction via the FUNGuild database. We found that fungal community richness, α-diversity, and evenness decreased significantly during the succession process. Soil available phosphorus and nitrate nitrogen decreased significantly after initial succession occurred, and redundancy analysis showed that both were significant predictors of soil fungal community structure. Among functional groups, fungal saprotrophs and pathotrophs represented by plant pathogens were significantly enriched in the early-successional stage, while fungal symbiotrophs represented by ectomycorrhiza were significantly increased in the late-successional stage. The abundance of both saprotroph and pathotroph fungal guilds was positively correlated with soil nitrate nitrogen and available phosphorus content. Ectomycorrhizal fungi were negatively correlated with nitrate nitrogen and available phosphorus content and positively correlated with ammonium nitrogen content. These results indicate that the dynamics of fungal community and function reflected the changes in nitrogen and phosphorus availability caused by the secondary succession in temperate forests. The fungal plant pathogen accumulated in the early-successional stage and ectomycorrhizal fungi accumulated in the late-successional stage may have a potential role in promoting forest succession. These findings contribute to a better understanding of the response of soil fungal communities to secondary forest succession and highlight the importance of fungal communities during the successional process.

Secondary succession of shrub-herb communities in the hilly area of Taihang Mountain.

Introduction: To document the successional processes of shrub-herb communities after large-scale human disturbance, and understand how changing environmental conditions affect species replacement in semi-humid hilly areas. Methods: Utilizing the established permanent plots in the hilly area of Taihang Mountain, we evaluated temporal patterns of vegetation and soil following grass-to-shrub succession. Results and Discussion: Along secondary succession, Vitex negundo var. heterophylla gradually dominated in dry sunny slope and shared the dominance with Leptodermis oblonga in shaded slope. Herbaceous dominant species in shrub-herb communities switched from Themeda japonica, Bothriochloa ischaemum, Artemisia sacrorum, and Cleistogenes chinensis in 1986 census to B. ischaemum and A. sacrorum in 2008 census, but herb was no longer dominant in 2020 census. As succession progresses, species dominance increased while richness decreased generally, and herb cover and aboveground biomass decreased, whereas shrub height, cover, and aboveground biomass increased significantly. Soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), and total potassium (TK) in topsoil increased significantly while pH declined by 1.04 units over the past three decades. Plant communities transitioned from perennial herbs to shrub-herb and then shrub communities, and V. negundo var. heterophylla dominated in the succession of shrub-herb communities. Climate and soil properties, combined with plant attributes, together drive post-disturbance secondary succession. From a management perspective, the tight coupling between vegetation and soil under local climatic conditions should be considered to improve the fragile ecosystem in the hilly area of Taihang Mountain.

Germination and seedling growth of Calluna vulgaris is sensitive to regional climate, heathland succession, and drought.

The coastal heathlands of Northwest Europe are highly valued cultural landscapes, that are critically endangered due to land use and climatic changes, such as increased frequency and severity of drought events. Our study is the first to assess how the germination and early seedling growth of Calluna vulgaris respond to drought. In a factorial design field experiment, we exposed maternal plants to three in-situ drought treatments (control, 60%, 90% roof coverage), across three successional stages after fire (pioneer, building, mature), and two regions (60°N, 65°N). Seeds from 540 plants within the experiment were, weighed, and exposed to five water potentials, ranging from -0.25 to -1.7 MPa, in a growth chamber experiment. We recorded germination (percentage, rate), seedling growth (above- vs. belowground allocation), and seedling functional traits (specific leaf area [SLA], specific root length [SRL]). Overall variation in germination between regions, successional stages, and maternal drought treatments was largely mediated by variation in seed mass. Plants from the northernmost region had higher seed mass and germination percentages. This is indicative of higher investment in seeds, likely linked to the populations' absence of vegetative root sprouting. Seeds from the mature successional stage germinated to lower final percentages than those from earlier successional stages, especially when the maternal plants had been exposed to drought (60% and 90% roof coverage). Exposure to reduced water availability decreased germination percentage and increased the time to 50% germination. Seedlings fully developed in the range -0.25 to -0.7 MPa, with increased root:shoot and lower SRL during reduced water availability, suggesting a resource-conservative response to drought during the early stages of development. Our results thus suggest a sensitivity to drought during the germination and seedling life-history stages that may reduce Calluna's ability to re-establish from seeds as the incidence and severity of droughts are projected to increase under future climates.

Pioneer Arabidopsis thaliana spans the succession gradient revealing a diverse root-associated microbiome.

BACKGROUND: Soil microbiomes are increasingly acknowledged to affect plant functioning. Research in molecular model species Arabidopsis thaliana has given detailed insights of such plant-microbiome interactions. However, the circumstances under which natural A. thaliana plants have been studied so far might represent only a subset of A. thaliana's full ecological context and potential biotic diversity of its root-associated microbiome. RESULTS: We collected A. thaliana root-associated soils from a secondary succession gradient covering 40 years of land abandonment. All field sites were situated on the same parent soil material and in the same climatic region. By sequencing the bacterial and fungal communities and soil abiotic analysis we discovered differences in both the biotic and abiotic composition of the root-associated soil of A. thaliana and these differences are in accordance with the successional class of the field sites. As the studied sites all have been under (former) agricultural use, and a climatic cline is absent, we were able to reveal a more complete variety of ecological contexts A. thaliana can appear and sustain in. CONCLUSIONS: Our findings lead to the conclusion that although A. thaliana is considered a pioneer plant species and previously almost exclusively studied in early succession and disturbed sites, plants can successfully establish in soils which have experienced years of ecological development. Thereby, A. thaliana can be exposed to a much wider variation in soil ecological context than is currently presumed. This knowledge opens up new opportunities to enhance our understanding of causal plant-microbiome interactions as A. thaliana cannot only grow in contrasting soil biotic and abiotic conditions along a latitudinal gradient, but also when those conditions vary along a secondary succession gradient. Future research could give insights in important plant factors to grow in more ecologically complex later-secondary succession soils, which is an impending direction of our current agricultural systems.

Variations in the soil micro-food web structure and its relationship with soil C and N mineralization during secondary succession of subalpine forests.

The soil micro-food web is an important network of belowground trophic relationships and it participates directly and indirectly in soil ecological processes. In recent decades, the roles of the soil micro-food web in regulating ecosystem functions in grasslands and agroecosystems have received much attention. However, the variations in the soil micro-food web structure and its relationship with ecosystem functions during forest secondary succession remain unclear. In this study, we investigated how forest secondary succession affected the soil micro-food web (including soil microbes and nematodes) and soil carbon and nitrogen mineralization across a successional sequence of "grasslands - shrublands - broadleaf forests - coniferous forests" in a subalpine region of southwestern China. With forest successional development, the total soil microbial biomass and the biomass of each microbial group generally increased. The significant influences of forest succession on soil nematodes were mainly reflected in several trophic groups with high colonizer-persister values (particularly bacterivore3, herbivore5 and omnivore-predator5) that are sensitive to environmental disturbance. The increases in the connectance and nematode genus richness, diversity, and maturity index indicated an increasingly stable and complex soil micro-food web with forest succession, which was closely related to soil nutrients, particularly the soil carbon contents. Additionally, we found that the soil carbon and nitrogen mineralization rates also exhibited generally increasing trends during forest succession, which had significant positive correlations with the soil micro-food web composition and structure. The path analysis results indicated that the variances in ecosystem functions induced by forest succession were significantly determined by soil nutrients and soil microbial and nematode communities. Overall, these results suggested that forest succession enriched and stabilized the soil micro-food web and promoted ecosystem functions via the increase in soil nutrients, and the soil micro-food web played an important role in regulating ecosystem functions during forest succession.

Legacy Effects of Phytoremediation on Plant-Associated Prokaryotic Communities in Remediated Subarctic Soil Historically Contaminated with Petroleum Hydrocarbons.

Phytoremediation of petroleum hydrocarbons in subarctic regions relies on the successful establishment of plants that stimulate petroleum-degrading microorganisms, which can be challenging due to the extreme climate, limited nutrients, and difficulties in maintaining sites in remote locations. A long-term phytoremediation experiment was initiated in Alaska in 1995 with the introduction of grasses and/or fertilizer to petroleum hydrocarbon (PHC)-contaminated soils that were subsequently left unmanaged. In 2011, the PHC concentrations were below detection limits in all soils tested and the originally planted grasses had been replaced by volunteer plant species that had colonized the site. Here, we sought to understand how the original treatments influenced the structure of prokaryotic communities associated with plant species that colonized the soils and to assess the interactions between the rhizospheric and endophytic communities of the colonizing vegetation 20 years after the experiment was established. Metataxonomic analysis performed using 16S rRNA gene sequencing revealed that the original type of contaminated soil and phytoremediation strategy influenced the structure of both rhizospheric and endophytic communities of colonizing plants, even 20 years after the treatments were applied and following the disappearance of the originally planted grasses. Our findings demonstrate that the choice of initial phytoremediation strategy drove the succession of microorganisms associated with the colonizing vegetation. The outcome of this study provides new insight into the establishment of plant-associated microbial communities during secondary succession of subarctic areas previously contaminated by PHCs and indicates that the strategies for restoring these ecosystems influence the plant-associated microbiota in the long term. IMPORTANCE Subarctic ecosystems provide key services to local communities, yet they are threatened by pollution caused by spills and disposal of petroleum waste. Finding solutions for the remediation and restoration of subarctic soils is valuable for reasons related to human and ecosystem health, as well as environmental justice. This study provides novel insight into the long-term succession of soil and plant-associated microbiota in subarctic soils that had been historically contaminated with different sources of PHCs and subjected to distinct phytoremediation strategies. We provide evidence that even after the successful removal of PHCs and the occurrence of secondary succession, the fingerprint of the original source of contamination and the initial choice of remediation strategy can be detected as a microbial legacy in the rhizosphere, roots, and shoots of volunteer vegetation even 2 decades after the contamination had occurred. Such information needs to be borne in mind when designing and applying restoration approaches for PHC-contaminated soils in subarctic ecosystems.

Alterations of ecosystem nitrogen status following agricultural land abandonment in the Karst Critical Zone Observatory (KCZO), Southwest China.

Background: Secondary succession after agricultural land abandonment generally affects nitrogen (N) cycle processes and ecosystem N status. However, changes in soil N availability and NO3 - loss potential following secondary succession are not well understood in karst ecosystems. Methods: In the Karst Critical Zone Observatory (KCZO) of Southwest China, croplands, shrub-grass lands, and secondary forest lands were selected to represent the three stages of secondary succession after agricultural land abandonment by using a space-for-time substitution approach. The contents and 15N natural abundance (δ 15N) of leaves, soils, and different-sized aggregates at the three stages of secondary succession were analyzed. The δ 15N compositions of soil organic nitrogen (SON) in aggregates and soil to plant 15N enrichment factor (EF = δ 15Nleaf -δ 15Nsoil), combined with soil inorganic N contents and δ 15N compositions were used to indicate the alterations of soil N availability and NO3 -loss potential following secondary succession. Results: Leaf N content and SON content significantly increased following secondary succession, indicating N accumulation in the soil and plant. The δ 15N values of SON also significantly decreased, mainly affected by plant δ 15N composition and N mineralization. SON content in macro-aggregates and soil NH4 + content significantly increased while δ 15N values of NH4 + decreased, implying increases in SON stabilization and improved soil N availability following secondary succession. Leaf δ 15N values, the EF values, and the (NO3 --N)/(NH4 +-N) ratio gradually decreased, indicating reduced NO3 - loss following secondary succession. Conclusions: Soil N availability improves and NO3 - leaching loss reduces following secondary succession after agricultural land abandonment in the KCZO.

Rapid bacterial and fungal successional dynamics in first year after chaparral wildfire.

The rise in wildfire frequency and severity across the globe has increased interest in secondary succession. However, despite the role of soil microbial communities in controlling biogeochemical cycling and their role in the regeneration of post-fire vegetation, the lack of measurements immediately post-fire and at high temporal resolution has limited understanding of microbial secondary succession. To fill this knowledge gap, we sampled soils at 17, 25, 34, 67, 95, 131, 187, 286, and 376 days after a southern California wildfire in fire-adapted chaparral shrublands. We assessed bacterial and fungal biomass with qPCR of 16S and 18S and richness and composition with Illumina MiSeq sequencing of 16S and ITS2 amplicons. Fire severely reduced bacterial biomass by 47%, bacterial richness by 46%, fungal biomass by 86%, and fungal richness by 68%. The burned bacterial and fungal communities experienced rapid succession, with 5-6 compositional turnover periods. Analogous to plants, turnover was driven by "fire-loving" pyrophilous microbes, many of which have been previously found in forests worldwide and changed markedly in abundance over time. Fungal secondary succession was initiated by the Basidiomycete yeast Geminibasidium, which traded off against the filamentous Ascomycetes Pyronema, Aspergillus, and Penicillium. For bacteria, the Proteobacteria Massilia dominated all year, but the Firmicute Bacillus and Proteobacteria Noviherbaspirillum increased in abundance over time. Our high-resolution temporal sampling allowed us to capture post-fire microbial secondary successional dynamics and suggest that putative tradeoffs in thermotolerance, colonization, and competition among dominant pyrophilous microbes control microbial succession with possible implications for ecosystem function.

Ecological integrity of tropical secondary forests: concepts and indicators.

Naturally regenerating forests or secondary forests (SFs) are a promising strategy for restoring large expanses of tropical forests at low cost and with high environmental benefits. This expectation is supported by the high resilience of tropical forests after natural disturbances, yet this resilience can be severely reduced by human impacts. Assessing the characteristics of SFs and their ecological integrity (EI) is essential to evaluating their role for conservation, restoration, and provisioning of ecosystem services. In this study, we aim to propose a concept and indicators that allow the assessment and classification of the EI of SFs. To this end, we review the literature to assess how EI has been addressed in different ecosystems and which indicators of EI are most commonly used for tropical forests. Building upon this knowledge we propose a modification of the concept of EI to embrace SFs and suggest indicators of EI that can be applied to different successional stages or stand ages. Additionally, we relate these indicators to ecosystem service provision in order to support the practical application of the theory. EI is generally defined as the ability of ecosystems to support and maintain composition, structure and function similar to the reference conditions of an undisturbed ecosystem. This definition does not consider the temporal dynamics of recovering ecosystems, such as SFs. Therefore, we suggest incorporation of an optimal successional trajectory as a reference in addition to the old-growth forest reference. The optimal successional trajectory represents the maximum EI that can be attained at each successional stage in a given region and enables the evaluation of EI at any given age class. We further suggest a list of indicators, the main ones being: compositional indicators (species diversity/richness and indicator species); structural indicators (basal area, heterogeneity of basal area and canopy cover); function indicators (tree growth and mortality); and landscape proxies (landscape heterogeneity, landscape connectivity). Finally, we discuss how this approach can assist in defining the value of SF patches to provide ecosystem services, restore forests and contribute to ecosystem conservation.

Secondary Succession Altered the Diversity and Co-Occurrence Networks of the Soil Bacterial Communities in Tropical Lowland Rainforests.

The characteristics of plant and soil bacterial communities in forest ecosystems have been reported, but our understanding of the relationship between plant communities and soil bacteria in different stages of secondary tropical rainforest succession is still poor. In June 2018, three different natural successional stages of tropical lowland rainforests, early (33 years), early-mid (60 years), and mid successional stage (73 years), in Hainan Island, China, were selected for this study. By conducting field investigation and 16S rRNA gene high-throughput sequencing, the composition and diversity of tree communities, the niche overlap of tree species with legumes among tree species, and the diversity and composition of soil bacterial communities and co-occurrence networks within communities across the successional stages were investigated. The results showed that plant richness and species diversity increased significantly during the secondary succession of tropical lowland rainforests. The order of positive correlations between nitrogen-fixing legumes and other species in plant communities was early-mid > mid > early successional stage. Soil nutrient content and soil bacterial richness were highest in the early-mid stages of succession, followed by mid and early stages of succession. Organic matter (OM), total nitrogen (TN), alkali nitrogen (AN), and available phosphorus (AP) had a stronger positive impact on soil bacterial communities. Co-occurrence network analysis showed that with the advancement of rainforests succession, the negative correlation between soil bacterial species decreased, and the community stability increased. Overall, as a result of tropical lowland rainforest secondary natural succession, the richness and diversity of plant communities increased, which altered the living conditions of nitrogen-fixing legumes and the soil properties, and the network complexity of soil bacterial communities increased with the rising of rainforest soil nutrient content.

Increased precipitation and nitrogen addition accelerate the temporal increase in soil respiration during 8-year old-field grassland succession.

Ecological succession after disturbance plays a vital role in influencing ecosystem structure and functioning. However, how global change factors regulate ecosystem carbon (C) cycling in successional plant communities remains largely elusive. As part of an 8-year (2012-2019) manipulative experiment, this study was designed to examine the responses of soil respiration and its heterotrophic component to simulated increases in precipitation and atmospheric nitrogen (N) deposition in an old-field grassland undergoing secondary succession. Over the 8-year experimental period, increased precipitation stimulated soil respiration by 11.6%, but did not affect soil heterotrophic respiration. Nitrogen addition increased both soil respiration (5.1%) and heterotrophic respiration (6.2%). Soil respiration and heterotrophic respiration linearly increased with time in the control plots, resulting from changes in soil moisture and shifts of plant community composition from grass-forb codominance to grass dominance in this old-field grassland. Compared to the control, increased precipitation significantly strengthened the temporal increase in soil respiration through stimulating belowground net primary productivity. By contrast, N addition accelerated temporal increases in both soil respiration and its heterotrophic component by driving plant community shifts and thus stimulating soil organic C. Our findings indicate that increases in water and N availabilities may accelerate soil C release during old-field grassland succession and reduce their potential positive impacts on soil C accumulation under future climate change scenarios.

Nutrients and consumers impact tree colonization differently from performance in a successional old field.

Successful colonization and growth of trees within herbaceous communities may result from different interactions with the herbaceous community. First, colonizing trees compete against larger, established herbs, while subsequent growth occurs among similarly sized or smaller herbs. This shift from colonization to growth may lead three drivers of community dynamics-nutrients, consumers, and herbaceous diversity-to differentially affect tree colonization and, later, tree performance. Initially, these drivers should favor larger, established herbs, reducing tree colonization. Later, when established trees can better compete with herbs, these drivers should benefit trees and increase their performance. In a 4-year study in a southeastern US old field, we added nutrients to, excluded aboveground consumers from, and manipulated initial richness of, the herbaceous community, and then allowed trees to naturally colonize these communities (from intact seedbanks or as seed rain) and grow. Nutrients and consumers had opposing effects on tree colonization and performance: adding nutrients and excluding consumers reduced tree colonization rate, but later increased the size of established trees (height, basal diameter). Adding nutrients and excluding consumers also restricted tree colonization to earlier years of study, which partially explained the effect of nutrient addition on plant size. Together, this shows differing impacts of nutrients and consumers: factors that initially limited tree colonization also resulted in larger established trees. This suggests that succession of grasslands that are either eutrophied or have diminished consumer pressure may experience lags and pulses in woody encroachment, leading to an extended period of herbaceous dominance followed by accelerated woody growth.

Vertical and horizontal light heterogeneity along gradients of secondary succession in cool- and warm-temperate forests.

Aims: Light availability varies drastically in forests, both vertically and horizontally. Vertical light heterogeneity (i.e., patterns of light attenuation from the forest canopy to the floor) may be related to light competition among trees, while horizontal light heterogeneity (i.e., variations in light intensity at a given height within forests) may be associated with light-niche partitioning among tree species. However, light heterogeneity in vertical and horizontal directions and their associations with forest structure are rarely studied to date. Here we report the first comprehensive study to compare the vertical and horizontal light heterogeneity in differently aged forests in two forest types. Location: Twelve forest stands of different ages in cool-temperate forests (consisting of deciduous broad-leaved trees) and five of different ages in warm-temperate forests (evergreen conifer and deciduous broad-leaved trees) in Japan. Methods: We measured vertical light profiles at 1-m intervals from the understorey (1 m above the ground) to the top canopy (12-22 m depending on stands) at 16 locations for each stand (20 m × 20 m). We also measured structural parameters (diameter at breast height, height, and crown dimensions) for all major trees in these stands. Results: Along the secondary successional gradients, the vertical and horizontal light heterogeneity changed in a systematic manner in both forests. The vertical light attenuation rate was steeper in early succession and more gradual in late succession, and the horizontal light heterogeneity was relatively small in early succession and more pronounced in late succession. The vertical light attenuation rate was different between the two forest types; the light intensity dropped more sharply from the canopy surface in the cool-temperate forests due to the crown being vertically shorter and denser (i.e., higher leaf density per unit volume). Conclusion: In early succession, a steeper light attenuation rate is likely related to the strong light competition among co-occurring trees and thus a self-thinning process. In late succession, the high spatial light heterogeneity in forests (i.e., larger horizontal light heterogeneity and gradual light attenuation rate) may allow more species to partition light, and thus may enhance species coexistence and diversity.

Soil carbon storage is related to tree functional composition in naturally regenerating tropical forests.

Regenerating tropical forests are increasingly important for their role in the global carbon cycle. Carbon stocks in above-ground biomass can recover to old-growth forest levels within 60-100 years. However, more than half of all carbon in tropical forests is stored below-ground, and our understanding of carbon storage in soils during tropical forest recovery is limited.Importantly, soil carbon accumulation does not necessarily reflect patterns in above-ground biomass carbon accrual during secondary forest succession, and factors related to past land use, species composition and soil characteristics may influence soil carbon storage during forest regrowth.Using tree census data and a measure of tree community shade tolerance (species-specific light response values), we assessed the relationship between soil organic carbon stocks and tree functional groups during secondary succession along a chronosequence of 40- to 120-year-old naturally regenerating secondary forest and old-growth tropical forest stands in Panama.While previous studies found no evidence for increasing soil C storage with secondary forest age, we found a strong relationship between tree functional composition and soil carbon stocks at 0-10 cm depth, whereby carbon stocks increased with the relative influence of light-demanding tree species. Light demanding trees had higher leaf nitrogen but lower leaf density than shade-tolerant trees, suggesting that rapid decomposition of nutrient-rich plant material in forests with a higher proportion of light-demanding species results in greater accumulation of carbon in the surface layer of soils. Synthesis. We propose that soil carbon storage in secondary tropical forests is more strongly linked to tree functional composition than forest age, and that the persistence of long-lived pioneer trees could enhance soil carbon storage as forests age. Considering shifts in tree functional groups could improve estimates of carbon sequestration potential for climate change mitigation by tropical forest regrowth. Read the free Plain Language Summary for this article on the Journal blog.

Los bosques tropicales en regeneración son cada vez más importantes por su papel en el ciclo global del carbono. Las reservas de carbono en la biomasa aérea pueden recuperarse hasta los niveles de los bosques maduros en un plazo de 60 a 100 años. Sin embargo, más de la mitad de todo el carbono en los bosques tropicales se almacena bajo tierra, y nuestra comprensión del almacenamiento de carbono en los suelos durante la recuperación de los bosques tropicales es limitada.Es importante señalar que la acumulación de carbono en el suelo no refleja necesariamente los patrones de acumulación de carbono en la biomasa aérea durante la sucesión de bosques secundarios y los factores relacionados con el uso pasado del terreno, la composición de especies y las características del suelo pueden influir en el almacenamiento de carbono en el suelo durante la regeneración del bosque.Usando datos del censo de árboles y una medida de la tolerancia a la sombra de la comunidad de árboles (valores de respuesta a la luz específicos de la especie), evaluamos la relación entre las reservas de carbono orgánico del suelo y los grupos funcionales de los árboles durante la sucesión secundaria a lo largo de una cronosecuencia de 40 a 120 años bosques secundarios de regeneración natural y rodales de bosques tropicales primarios en Panamá.Mientras que estudios previos no encontraron evidencia de un aumento del almacenamiento de C en el suelo con la edad del bosque secundario, encontramos una fuerte relación entre la composición funcional de los árboles y las reservas de carbono del suelo a 0­10 cm de profundidad, por lo que las reservas de carbono aumentaron con la influencia relativa de especies de árboles que demanda de luz. Los árboles que requieren luz tenían más nitrógeno en las hojas pero menor densidad de hojas que los árboles tolerantes a la sombra, lo que sugiere que la descomposición rápida del material vegetal rico en nutrientes en los bosques con una mayor proporción de especies que requieren luz da como resultado una mayor acumulación de carbono en la capa superficial de los suelos. Síntesis. Proponemos que el almacenamiento de carbono en el suelo en los bosques tropicales secundarios está más fuertemente relacionado con la composición funcional de los árboles que con la edad del bosque, y que la persistencia de árboles pioneros de larga vida podría mejorar el almacenamiento de carbono en el suelo a medida que los bosques envejecen. La consideración de los cambios en los grupos funcionales de los árboles podría mejorar las estimaciones del potencial de secuestro de carbono para la mitigación del cambio climático mediante la regeneración de los bosques tropicales.

Loss of soil microbial residue carbon by converting a tropical forest to tea plantation.

Land-use change can lead to profound changes in the storage of soil organic carbon (SOC) in the tropics. Soil microbial residues make up the majority of persistent SOC pools, yet the impact of land-use change on microbial residue C accumulation in the tropics is not well understood. Here, we investigated how the conversion of tropical primary montane rainforest to secondary forest and the conversions of secondary forest to Prunus salicina plantation and tea plantation, influence the accumulation of soil microbial residue C (indicated by amino sugars). Our results showed that the secondary forest had a higher SOC than that of the primary forest (+63%), while they had no difference in microbial residue C concentration, indicating a relatively slow microbial-derived C accrual during secondary succession. Moreover, the P. salicina plantation and tea plantation had lower SOC than the secondary forest (-53% and -57%, respectively). A decrease in fungal biomass (-51%) resulted in less fungal and total residue C concentrations in the tea plantation than in the secondary forest (-38% and -35%, respectively), indicating microbial-derived C loss following the forest conversion. The change in microbial residue C depended on litter standing crop rather than soil nutrient and root biomass. Litter standing crop affected microbial residue C concentration by regulating fungal biomass and hydrolytic enzyme activities. Taken together, our results highlight that litter-microbe interactions drive microbial residue C accumulation following forest conversions in the tropics.

Non-additive biotic interactions improve predictions of tropical tree growth and impact community size structure.

Growth in individual size or biomass is a key demographic component in population models, with wide-ranging applications from quantifying species performance across abiotic or biotic conditions to assessing landscape-level dynamics under global change. In forest ecology, the responses of tree growth to biotic interactions are widely held to be crucial for understanding forest diversity, function, and structure. To date, most studies on plant-plant interactions only examine the additive competitive or facilitative interactions between species pairs; however, there is increasing evidence of non-additive, higher-order interactions (HOIs) impacting species demographic rates. When HOIs are present, the dynamics of a multispecies community cannot be fully understood or accurately predicted solely from pairwise outcomes because of how additional species "interfere" with the direct, pairwise interactions. Such HOIs should be particularly prevalent when species show non-linear functional responses to resource availability and resource-acquisition traits themselves are density dependent. With this in mind, we used data from a tropical secondary forest-a system that fulfills both of these conditions-to build an ontogenetic diameter growth model for individuals across 10 woody-plant species. We allowed both direct and indirect interactions within communities to influence the species-specific growth parameters in a generalized Lotka-Volterra model. Specifically, indirect interactions entered the model as higher-order quadratic terms, i.e., non-additive effects of conspecific and heterospecific neighbor size on the focal individual's growth. For the whole community and for four out of 10 focal species, the model that included HOIs had more statistical support than the model that included only direct interactions, despite the former containing a far greater number of parameters. HOIs had comparable effect sizes to direct interactions, and tended to further reduce the diameter growth rates of most species beyond what direct interactions had already reduced. In a simulation of successional stand dynamics, the inclusion of HOIs led to rank swaps in species' diameter hierarchies, even when community-level size distributions remained qualitatively similar. Our study highlights the implications, and discusses possible mechanisms, of non-additive density dependence in highly diverse and light-competitive tropical forests.

Functional recovery of secondary tropical forests.

One-third of all Neotropical forests are secondary forests that regrow naturally after agricultural use through secondary succession. We need to understand better how and why succession varies across environmental gradients and broad geographic scales. Here, we analyze functional recovery using community data on seven plant characteristics (traits) of 1,016 forest plots from 30 chronosequence sites across the Neotropics. By analyzing communities in terms of their traits, we enhance understanding of the mechanisms of succession, assess ecosystem recovery, and use these insights to propose successful forest restoration strategies. Wet and dry forests diverged markedly for several traits that increase growth rate in wet forests but come at the expense of reduced drought tolerance, delay, or avoidance, which is important in seasonally dry forests. Dry and wet forests showed different successional pathways for several traits. In dry forests, species turnover is driven by drought tolerance traits that are important early in succession and in wet forests by shade tolerance traits that are important later in succession. In both forests, deciduous and compound-leaved trees decreased with forest age, probably because microclimatic conditions became less hot and dry. Our results suggest that climatic water availability drives functional recovery by influencing the start and trajectory of succession, resulting in a convergence of community trait values with forest age when vegetation cover builds up. Within plots, the range in functional trait values increased with age. Based on the observed successional trait changes, we indicate the consequences for carbon and nutrient cycling and propose an ecologically sound strategy to improve forest restoration success.

Forest structure drives changes in light heterogeneity during tropical secondary forest succession.

Light is a key resource for tree performance and hence, tree species partition spatial and temporal gradients in light availability. Although light distribution drives tree performance and species replacement during secondary forest succession, we yet lack understanding how light distribution changes with tropical forest development.This study aims to evaluate how changes in forest structure lead to changes in vertical and horizontal light heterogeneity during tropical forest succession.We described successional patterns in light using a chronosequence approach in which we compared 14 Mexican secondary forest stands that differ in age (8-32 years) since agricultural abandonment. For each stand, we measured vertical light profiles in 16 grid cells, and structural parameters (diameter at breast height, height and crown dimensions) for each tree.During succession, we found a rapid increase in stand size (basal area, crown area and length) and stand differentiation (i.e. a gradual leaf distribution along the forest profile), which leads to fast changes in light conditions and more light heterogeneity. The inflection points of the vertical light gradient (i.e. the absolute height at which 50% relative light intensity is attained) rapidly moved towards higher heights in the first 20 years, indicating that larger amounts of light are intercepted by canopy trees. Light attenuation rate (i.e. the rate of light extinction) decreased during succession due to slower accumulation of the crown area with height. Understorey light intensity and heterogeneity slightly decreased during succession because of an increase in crown size and a decrease in lateral gap frequency. Understorey relative light intensity was 1.56% at 32 years after abandonment.Synthesis. During succession, light conditions changed linearly, which should lead to a continuous and constant replacement of species. Especially in later successional stages, stronger vertical light gradients can limit the regeneration of light-demanding pioneer species and increase the proportion of shade-tolerant late-successional species under the canopy. These changes in light conditions were largely driven by the successional changes in forest structure, as basal area strongly determined the height where most light is absorbed, whereas crown area, and to a lesser extent crown length, determined light distribution.