Successional Gradient Research Articles

Quantification of biogeomorphic interactions between small‐scale sediment transport and primary vegetation succession on proglacial slopes of the Gepatschferner, Austria

AbstractProglacial slopes provide suitable conditions for observing the co‐development of abiotic and biotic systems. The frequency and magnitude of geomorphic processes and plant composition govern this interplay, which is described in the model of biogeomorphic succession. In high mountain environments, this model has only been tested in a limited number of studies. The study aimed to quantify small‐scale sediment transport via erosion plots along a plant cover gradient and to investigate the influence of sediment transport on plant communities. We aimed to generate quantitative data to test existing biogeomorphic models. Small‐scale biogeomorphic interactions were investigated on 30 test plots of 2 × 3 m size on proglacial slopes of the Gepatschferner (Kaunertal) in the Austrian Alps during the snow‐free summer months over three consecutive years. The experimental plots were established on slopes along a plant cover gradient. A detailed vegetation survey was carried out to capture biotic conditions, and specific sediment yield was measured at each plot. Species abundance and composition at each site reflected successional stages. Additional environmental parameters, such as terrain age, geomorphometry, grain size distribution, soil nutrients, and precipitation, were also included in the analyses. We observed two pronounced declines in geomorphic activity on plots with both above 30% and above 75% plant cover. Nonmetric multidimensional scaling showed distinct clusters of vegetation composition that mainly followed a successional gradient. Sites that were affected by high‐magnitude geomorphic events showed different environmental conditions and species communities. Quantified process rates and observed species composition support the concept of biogeomorphic succession. The findings help to narrow down a biogeomorphic feedback window.

Null‐model analysis and changes in species interactions in biocrusts along a successional gradient in the Tengger Desert, northern China

AbstractAimsIn arid regions, the succession of biological soil crusts (biocrusts) usually accompanies substantial turnover of community composition. How interspecific interactions among biocrust‐forming mosses, lichens, algae and cyanobacteria change along the successional gradient is largely unknown. According to the stress gradient hypothesis (SGH), the frequency or intensity of facilitative and competitive interactions within the community will vary along an environmental stress gradient, with an increasing role of competition relative to facilitation under more benign environmental conditions. In the process of biocrust succession, decreasing soil stress also occurs because of slow soil amelioration, which meets the assumption of the SGH. Therefore, we hypothesize that the changes in facilitative and competitive interactions within biocrust communities will follow the prediction of the SGH and that the nature of biocrust interactions will also change.LocationSand‐binding vegetation belts for the Baotou–Lanzhou railway at the southeast fringe of the Tengger Desert in the Shapotou region of the Ningxia Hui Autonomous Region, western China.MethodsWe conducted a field evaluation at five adjacent sand‐binding vegetation belts built in chronosequence to study the successional gradient. Small‐scale patterns of co‐occurrence and null models were used to evaluate the community‐level interspecific interactions among the biocrust‐forming mosses, lichens, algae and cyanobacteria. In addition, an accessory method of species‐pair co‐occurrence based on null models was also applied to potentially identify the nature of the interspecific interactions within the biocrusts and for the causal investigation of these patterns.ResultsA strong spatial segregation of co‐occurrence patterns occurred between cyanobacteria and algae and species of mosses and lichens at early succession, and occurred among species of mosses and lichens at late succession, while the weak spatial aggregation between lichen species only occurred at middle succession.ConclusionsNegative interspecific interactions were prevalent within biocrusts throughout succession. The co‐occurrence patterns at both the community and species‐pair levels are consistent, supporting the unimodal version of the SGH that species interactions shift toward competition under extremely stressed conditions. Given the lack of research on interspecific interactions in biocrusts, our study is important for obtaining a deeper understanding of biocrust succession.

Rapid post‐fire re‐assembly of species‐rich bryophyte communities in Afroalpine heathlands

AbstractQuestionsIn some fire‐prone ecosystems, bryophytes play a crucial role by providing the surface fuel that controls the fire return interval. Afroalpine heathlands are such an ecosystem, yet almost nothing is known about the bryophytes in this system. We do not know the level of species richness, or if there is a successive accumulation of species over time, or if some species are adapted to specific phases along the successional gradient, for example early‐successional species sensitive to competition.LocationAfroalpine heathlands in Ethiopia.MethodsWe made an inventory of all bryophytes in 48 plots of 5 m × 5 m, distributed along a chronosequence from 1 to 25 years post fire. The heathlands are located between 3500 m and 3800 m a.s.l. and are managed by traditional pasture burning with fire intervals of 8–20 years.ResultsWe found in total 111 taxa of bryophytes. Post‐fire mortality was almost 100%. The youngest plots had only a few cosmopolitan species often found after fire. Initially, species richness increased monotonically while starting to level off around 15 years after fire, when many plots had around 30 species and a high cover of Breutelia diffracta, which is a key ground‐living species, important as surface fuel. Most species were found with sporophytes, a pattern even stronger for the most frequent species.ConclusionsInterestingly, bryophyte diversity is already remarkably high by only 15 years after total eradication. The relatively slow accumulation of species in the first years after fire suggests that dispersal in space, and not time, is the major mechanism by which sites regain their diversity (i.e. spore banks play a smaller role than colonization of wind‐borne spores). This indicates that the high species richness is built up through colonization from surrounding heathlands, and perhaps also from higher‐altitude alpine grasslands and lower‐altitude forests, and that the bryophyte diversity in this system is maintained by the traditional fire and grazing management.

Plant functional traits along an old-field succession in the high tropical andes

The páramo, a high tropical Andean ecosystem, presents very distinct climatic, floristic and ecological features. Paramo plants have adapted to pronounced daily temperature fluctuations, freezing temperatures any night of the year, high incoming radiation and seasonal unfavorable water availability conditions. Long fallow agricultural systems are common in the páramo, permitting partial recuperation of the vegetation through a secondary succession. Different hypothesis related to plant traits (water relations and gas exchange characteristics or biomass allocation patterns) during succession have been proposed. Particular traits have been associated to the stages along successional gradients from early species (traits related to high resource acquisition abilities) to late species (traits linked to internal conservation of resources). Our purpose was to study plant functional responses of different herbaceous species in an old-field succession in the high tropical Andes. Plant traits (specific leaf area (SLA), maximum CO2 assimilation rates (Amax), leaf nitrogen content (Nleaf), leaf water relations and biomass allocation) were studied in grasses and forbs along different successional stages. Clear trends in SLA, Amax on a mass basis, Nleaf and aboveground/belowground biomass allocation were found along the successional gradient for grasses, while patterns were not as well-defined for forbs in the latter two traits. No patterns in leaf water relations characteristics were observed for either plant-growth forms. Strategies associated to plant traits significantly influence plant species dominance and community assembly during secondary succession in tropical alpine open environments like the páramos. Plant traits linked to rapid growth (SLA, Amax and Nleaf) dominate during early succession. In contrast to previous studies, drought resistance traits were not found to dominate in late succession stages for either grasses or forbs.

Evolutionary responses of a dominant plant along a successional gradient in a salt-marsh system

The ecological responses of plant populations along a successional gradient have been intensively examined; however, the evolutionary responses received much less attention. Here, I explored genetic changes of key phenotypic traits of a dominant clonal plant (Elytrigia atherica) along a saltmarsh successional gradient by collecting samples along the successional gradient in the high and low marsh and growing them in a common environment (greenhouse). Additionally, to explore whether changes in traits are driven by abiotic (e.g. clay thickness) and biotic (e.g. grazing intensity) variables along the successional gradient, I measured these two variables in the field. I found that clay thickness (a proxy of total nitrogen) increased along the successional gradient both in the high and low marsh; grazing intensity from hares (the most important herbivores) decreased along the successional gradient in the high marsh but did not change in the low marsh. Meanwhile, I found that growth in number of leaves and ramets decreased, while rhizome length increased, along the successional gradient for E. atherica collected from the high marsh. Opposite trends were found for E. atherica collected from the low marsh. Results suggest that, in the high marsh, herbivores may overrule nutrients to drive trait changes. That is, at the early successional stages, E. atherica had higher growth in number of leaves and ramets to compensate for high-intensity grazing. In the low marsh, nutrients may be the dominant driver for trait changes. That is, at the late successional stages, E. atherica had higher growth in number of leaves and ramets but shorter rhizomes to maximize its expansion under the favorable conditions (higher nutrient availability). Results suggest that ecologically important abiotic and biotic variables such as nutrients and herbivores may also have a substantial evolutionary impact on plant populations.

Intra- and interspecific variations on plant functional traits along a successional gradient in a Brazilian tropical dry forest

The present study aimed to determine how plant functional traits change both at the intra- and interspecific levels along three successional stages (early, intermediate and late) in a Brazilian tropical dry forest (TDF). Plants in early-successional plots are expected to exhibit conservative traits to increase photoprotection and water conservation, whereas late successional plants would invest on acquisitive traits. Sampling was conducted in three plots of 50 × 20 m per successional stage, where plant species composition and structural parameters were previously determined for individuals with DBH ≥ 5 cm. From each of three plots, the five tree species with the highest importance values were selected. This resulted in a total of 13 species from seven families selected for this study. We quantified specific leaf area (SLA), leaf slenderness, polyphenols and chlorophyll contents, and leaf lifespan. Based on leaf traits, we detected two major tree functional species groups: the early-intermediate and the late successional groups. Confirming our expectations, the former was positively associated with drought tolerance traits (polyphenols and slenderness) related to conservative resource use; and the latter was positively associated with acquisitive traits (SLA). This pattern was partially corroborated at the intraspecific level for species that occurred in more than one successional stage. We found a weak phylogenetic signal for the functional grouping (only for polyphenols). The analysis of variance structure across scales indicated that a very small proportion (up to 2.8%) of the general variance in leaf traits occurred at intraspecific level suggesting that, compared to interspecific variations, trait differences among individuals due to phenotypic plasticity are low. Although further studies with a larger number of species are needed, our findings suggest a decoupling between functional and structural/species composition regeneration, contradicting several studies on secondary succession where species composition is regarded as the main indicator of the regeneration of ecological functions.

Soil resource availability, plant defense, and herbivory along a successional gradient in a tropical dry forest

Tropical dry forests (TDFs) usually occur as mosaics at different stages of natural regeneration, along which variations in soil attributes (fertility and texture) can affect plant growth and leaf traits and, consequently, leaf herbivory. In the present study, we used a chronosequence approach to determine the variations in plant growth, leaf defense, and insect herbivory in different successional species groups, following the predictions of the resource availability hypothesis: plants at early successional stages would grow faster, have lower specific leaf mass (SLM), and suffer higher herbivore damage than late successional plants. For this purpose, we measured plant diameter and collected leaves at the end of the rainy season (April) during three consecutive years in three different successional stages (early, intermediate, and late). In total, we sampled 38 plant species, from which 31 species were exclusive to a single successional stage (10 early, 13 intermediate, and eight late species). Our results did not conform to the resource availability hypothesis, as plants growing in early successional stages had higher growth rates and SLM, and lower leaf damage by herbivores. Soil fertility and texture did not affect plant growth or SLM, but leaf damage was negatively affected by SLM. Apart from being a defensive trait, SLM is also involved in the conservation of water and nutrients and photoprotection, and its variation along successional gradients in TDFs conforms to a continuum from conservative traits in hot and dry, early successional stages, to acquisitive traits in more humid late stages. Although other plant defenses must be considered, our three-year study indicates that the resource availability hypothesis, which was formulated for wet forests, should be adapted to deciduous TDFs where water availability is the main environmental filter along succession.

Post-harvest forest herb layer demography: General patterns are driven by pre-disturbance conditions

Timber harvesting constitutes extensive anthropogenic disturbance in temperate forests, producing a broad range of ecological impacts that most often enhance the demographic processes of vegetation. This study monitored post-harvest herb layer demography over a 6-year period in mesic Dinaric fir-beech forests (Slovenia), a vascular plant diversity hotspot among European forests. Three experimental harvesting intensities, i.e. full harvest (FH), partial harvest (PH) and a control treatment (NH), were each applied over a circular area of 4000 m2 and replicated three times at each of three study sites. Vegetation sampling was conducted before harvesting (in 2012), and two (2014) and six (2018) years following it, in a 400 m2 circular plot positioned in the centre of each treatment area. We focused on identifying general demographic patterns and evaluating the effects of various pre-disturbance abiotic and biotic predictors on compositional responses to disturbance. Two years after harvest (2012–2014), compositional shifts were larger than those in the next 4-year period (2014–2018), confirming the general theoretical prediction that species turnover rate decreases along a successional gradient. The degree of compositional shifts in gaps (FH) and thinned stands (PH) was affected by local abiotic factors (geomorphology of karst sinkholes) and community attributes, such as pre-harvest species richness. Our results indicate that compositional stability is positively associated with pre-disturbance species richness. Over the whole study period, increases in plot-level species richness (alpha diversity) and overall enrichment of the species pool (gamma diversity) were accompanied by compositional convergence, i.e. a decline in floristic dissimilarity (beta diversity) between and within study sites. However, the observed tendency towards homogenization was rather weak and would have been even more pronounced if the demographic type of persistent resident species had not shown a high degree of resistance, thus leaving a strong imprint on post-harvest vegetation development by preserving the forest characteristics of the herb layer community.

The role of deterministic succession during forest development within a southern African savanna

AbstractWoody encroachment can lead to a switch from open savannas to dense woodlands or forests. This has implications for both the composition of ecological communities and the provision of ecosystem services such as nutrient cycling and grazing capacity. The patterns and underlying drivers responsible for woody encroachment are not fully understood. Here, we investigate the underlying determinants of bush clump formation (a form of encroachment) in a South African savanna and explore whether bush clump succession is driven by deterministic (i.e., predictable changes in species composition) or stochastic (i.e., random) processes. Specifically, we test (1) whether the similarity in species composition of saplings and trees differs among small and large clumps, (2) which environmental factors are driving succession, and (3) whether forest specialization of tree and sapling species within bush clumps increases with the successional gradient. Similarity in species composition between saplings in small clumps and trees in large clumps was higher than similarity between trees in small clumps and trees in large clumps. Furthermore, temperature, soil moisture, relative humidity, and light intensity were related to changes in species composition along the successional gradient. As expected, forest specialization of trees increased with increasing clump area indicating that late‐successional bush clumps have more forest‐type species. The directional changes of species found along the successional gradient suggest a deterministic process of succession driven by changes in local environmental conditions during clump formation.

Biogeochemical stoichiometry of soil and plant functional groups along a primary successional gradient following glacial retreat on the eastern Tibetan plateau

Di-nitrogen (N2)-fixing plants are crucial for vegetation development following glacial retreat; however, there is currently limited understanding of how the shifting of N2-fixing and non N2-fixing plant groups may influence the stoichiometric composition of the developing plant community and associated soils. Carbon (C), nitrogen (N), and phosphorus (P) concentrations and elementary stoichiometry were investigated in soil and the N2-fixing and non-N2-fixing plant communities across a 127-yr glacial primary succession chronosequence on the eastern slope of the Qinghai-Tibetan Plateau. Soil organic C (SOC), total N (TN), and available P (AP) gradually increased along succession, but total P (TP) was greatest at the 37-yr site, resulting in an increasing trend of soil N:P and a decreasing trend of N:AP ratios along the chronosequence. N2-fixing species had higher foliar N and P concentrations than non-N2-fixing species and exhibited a higher leaf N:P ratios in early stages (37 yr and younger) of the succession. Elementary concentration and C, N, P stoichiometry in plant leaves were strongly associated with both N2-fixing and non-N2 fixing plant biomasses, while those in soils were related only with the biomass of non-N2 fixing species. The community-weighted leaf N:P ratios increased from 14.0 at 7-yr to 19.9 at 59-yr and then decreased to 12.9 at 127-yr after glacial retreat, suggests the ecosystem as a whole is not N- or P-limited at the beginning of primary succession, but may become P-limited at the middle stages and then shift towards N limitation at the late successional stages. The results confirm that both N2-fixing and non-N2-fixing species can overcome N limitation in the early primary succession and highlights the different roles of pioneer N2-fixing and non-N2-fixing species in mediating vegetation and soil C, N and P stoichiometry with primary succession following glacial retreat.

Soil physicochemical properties drive the variation in soil microbial communities along a forest successional series in a degraded wetland in northeastern China.

The Sanjiang Plain is the biggest freshwater wetland locating in northeastern China. Due to climate change and human activities, that wetland has degraded to a successional gradient from the original flooded wetland to dry shrub vegetation and a forest area with lower ground water level, which may result in changes in soil microbiologic structure and functions. The present study investigated the microbial diversity and community structure in relation to soil properties along that successional gradient. The soil physico‐chemical properties changed significantly with degradation stage. The Shannon diversity index of both soil bacteria (5.90–6.42) and fungi (1.7–4.19) varied significantly with successional stage (both p < .05). The community structures of soil bacteria and fungi in the early successional stages (i.e., the wetland) were significantly determined by water content, total nitrogen, and available nitrogen concentrations in soils, while those in the later successional stages (i.e., forests) were significantly structured by soil organic carbon, soil pH, and available phosphorus concentrations. These results suggest that the soil microbial structure is mainly determined by soil properties rather than by plant community such as plant species composition along successional stages.

Ecosystem services supply and interactions along secondary tropical dry forests succession

Secondary forests will likely dominate future tropical landscapes, and the ecosystem services they provide to humanity will be particularly relevant. However, few empirical studies provide quantitative evidence of how the supply of ecosystem services change along secondary forest succession. The need for such information is particularly pressing for tropical dry forests because of their extensive global coverage and a high degree of disturbance. Here we examine the changes in the potential supply of ecosystem services and the interactions among them along secondary tropical dry forest succession for a site on the Mexican Pacific coast. Using data from a chronosequence of twelve sites followed over ten years, we characterized changes in five ecosystem services along a successional gradient from abandoned farmland to old-growth forests: forage, multiple forest resources, microclimate regulation, carbon storage and carbon sequestration. The long-term recovery trajectory for each service was modelled using linear and non-linear mixed models. Interactions among them were examined over different timeframes using Spearman correlations. The results showed rapid non-linear recovery of multiple forest resources provision, microclimate regulation and carbon storage within the first two decades of succession. Carbon sequestration increased slightly over time, while the supply of forage showed no clear trend. The strength of the interactions among pairs of services changed through successional time, being particularly higher in farmlands and old-growth forests. Our findings suggest that adaptive forest management practices that foster the natural regeneration of tropical dry forests are cost-effective mechanisms to recover the supply of critical ecosystem services for local actors by meeting their livelihood needs as well as those for the global community through climate change mitigation. Maintaining tropical dry forest resilience will highly depend on the maintenance of diversity in terms of species and ecosystem services.

Dynamics of soil specific enzyme activities and temperature sensitivities during grassland succession after farmland abandonment

The influence of plant secondary succession on soil enzyme activities has been increasingly recognized recently. However, the characteristics of specific enzyme activities and temperature sensitivity (Q10) in the rhizosphere remain elusive. We collected rhizosphere and bulk soil samples from a secondary successional series (0, 7, 12, 17, 22, and 32 years after farmland abandonment, and a natural grassland reference) in a typical semi-arid ecosystems on the Loess Plateau of China. The potential activity of soil enzymes, including β-1,4-glucosidase (BG), β-1,4-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP), and alkaline phosphatase (AP), was assayed at 37, 20 and 4 °C and expressed as activity per unit of soil organic carbon (specific enzyme activity). Q10 values were calculated at 20–37 °C and 4–20 °C, respectively. The specific enzyme activities of BG, NAG, LAP and AP in bulk soil increased to a maximum at the 17-year site and then significantly decreased in older grasslands. In the rhizosphere, specific enzyme activities increased and then decreased along the successional gradient with significantly higher value at the 7-year site and the 17-year site. The Q10 values of BG activity in bulk soil and the four enzyme’s activities in rhizosphere soil were changed significantly along the successional gradients. In general, the Q10 values at the two temperature ranges differed in bulk soil, but not in the rhizosphere soil. Along the plant successional gradient, the rhizosphere effects of specific enzyme activities gradually changed to negative values in the oldest sites. Our results indicate that plant secondary succession had different impacts on the enzyme activity and characteristics in the rhizosphere and bulk soil, and highlights the importance of temperature sensitivity and rhizosphere effects of enzyme activity along plant secondary successional gradients.

Shifts of sediment bacterial community and respiration along a successional gradient in a typical karst plateau lake wetland (China)

Bacteria are important regulators of carbon cycling in lakes and are central to sediment ecosystem processes. However, the sediment microbial communities and their respiratory responses to the lake wetland succession are poorly understood. In this study, we collected sediment samples from four different succession points (the Potamogeton lucens zone, the Scirpus tabernaemontani zone, the Scirpus triqueter zone, and the Juncus effusus zone) in the Caohai Wetland of the Guizhou Plateau (China). The bacterial communities at these succession points were studied using a high-throughput sequencing approach. The sediment microbial respiration (SR) was measured using static chambers in the field and basal respiration (BR) was determined in the laboratory. The results show that the dominant bacterial taxa in the sediment was Proteobacteria (34.7%), Chloroflexi (17.8%), Bacteroidetes (7.3%), Acidobacteria (6.6%), and Cyanobacteria (6.1%). Principal coordinate analysis showed that the microbial community structure differs significantly at different sampling points along the successional gradient, indicating that the bacterial community structure is sensitive to the lake wetland succession. Different hydrological regimes and soil characteristics such as NH 4 + -N, Fe2+, Mn2+, and sediment organic carbon (SOC) content may be important factors responsible for the differences in the sediment microbial characteristics of the different successional stages in the Caohai wetland. Additionally, it was found that the SR increased significantly from the P. lucens zone to the J. effusus zone, but BR had the opposite response. The shifts in the bacterial community structure along the successional gradient may be the main reason for the observed differences in sediment respiration.

Structural changes of vegetation and its association with microclimate in a successional gradient of low thorn forest in northeastern Mexico

Understanding how microclimate and vegetation are associated during secondary succession is of primary importance for plant conservation in the face of the increasing land cover modification. However, these patterns are still unstudied for many plant communities. This study aimed to evaluate the structure (species richness, Shannon's diversity index, Simpson´s dominance index, abundance of each species, average height of species, species cover (%), species composition, and indicator values) of a low thorn forest fragment and to analyze its relation with microclimate along a successional gradient. Four stages of succession were delimited by the analysis of Landsat images, in the state of Tamaulipas, northeast Mexico. Statistical models incorporated species richness, diversity indices, abundance, height, and cover, as variables for searching differences between stages, or to evaluate microclimate associations. A total of 70 species, 54 genera, and 27 families were determined. Height of tree layer was the most important variable for discrimination of the successional stages. Conserved areas differed floristically from other stages, associated mainly with the lowest values of wind speed originated by tree layer characteristics. A significant association between species and microclimate was found, being wind speed and relative humidity the most important variables. Some species, due to their high importance values and their patterns of association with microclimate, may be considered as key taxa for low thorn forest, which is a threatened semitropical community in northeast Mexico. Conserved and late successional areas account for climatic regulation of this plant community, and the importance of these forest patches may be considered when establishing biodiversity protection areas.

Assessing recovery of alpine spoil heaps by vascular plant, bryophyte, and lichen functional traits

Functional traits are linked to ecosystem processes and services and therefore relevant in recovery assessment. However, traits of bryophytes and lichens, important components of many ecosystems, have received less attention than those of vascular plants. We explored the use of functional traits of multiple important organism groups in recovery assessment. We combined data on traits and species composition for vascular plants, bryophytes, and lichens from four alpine spoil heaps and their undisturbed surroundings in western Norway, collected at three time‐points spanning more than two decades. We studied changes in community‐weighted mean (CWM) trait values and distribution of trait‐category optima over time. We analyzed temporal variation in joint functional trait composition using the ordination regression‐based approach (ORBA) to predict time to recovery. We observed functional shifts along the successional gradient for all organism groups, e.g. from wind‐dispersed propagules shortly after disturbance to vegetative reproduction at later successional stages. Over time, the similarity between dispersal‐related traits of vascular plants and bryophytes on the spoil heaps and in their surroundings increased, indicating that propagule influx is important in alpine restoration. The joint functional trait composition of all spoil heaps converged towards that of their surroundings: one spoil heap had recovered 34 years after construction, while the predicted time to recovery for the other three was 59–74 years. Our results indicate that inclusion of multiple organism groups improves trait‐based recovery assessments and time‐to‐recovery predictions. Further development of trait databases is essential for future use of joint functional trait composition in recovery assessment.

Soil microbial interactions modulate the effect of Artemisia ordosica on herbaceous species in a desert ecosystem, northern China

Soil microbes are essential for enhancing the performance (i.e., recruitment and growth) of individual plants, thereby influencing the structure and diversity of plant communities. However, how soil microbes drive the plant interspecific interactions remains largely unexplored, particularly in harsh desert ecosystems. To address this knowledge gap, we examined the community composition and species interaction networks of soil bacteria using high-throughput 16S rRNA gene sequencing along a successional gradient of an Artemisia ordosica community in the Mu Us Desert, northern China. We also explored how A. ordosica affects the soil bacteria, especially keystone species, and herbaceous plants with community succession. Our main goal is to determine how soil bacteria regulate the interactions of A. ordosica with herbaceous plants. The results showed that soil bacterial community composition and main taxonomic groups were primarily influenced by the presence of A. ordosica along the successional gradient. The phylogenetic molecular ecological network analysis identified nine keystone species, with five species categorised as module hubs and four species as connectors. We found that the effects of A. ordosica on module hubs were highly correlated with its effects on soil nitrogen content and biomass of specific herbaceous plants, resulting in an increase of soil fertility and herbaceous under the shrub. These results indicated that soil bacterial interactions might be a crucial driver affecting plant interspecific interactions. Although our results highlighted the importance of bidirectional interactions between plants and soil microbes from the perspective of biotic interactions, their contribution to plant community succession still needs further confirmation through more robust evidence from future studies. However, overall our findings advanced the current understanding of plant-soil interactions on the processes of ecological restoration and desertification reversion, especially in stressful desert ecosystems.

Indicators of integrative recovery of vegetation, soil and microclimate in successional fields of a tropical dry forest

Secondary succession involves dynamic feedback among vegetation, environmental conditions and biota. Nevertheless, the recovery of tropical forests after anthropic management is often evaluated based solely on vegetation cover, which is easier to measure and requires less equipment and expertise than the assessment of soil and microclimate, but does not always reflect synchronic and integrative recovery. Given the convenience of measuring vegetation properties, this study aimed to identify the vegetation traits that could best inform on the progress of integrative recovery through the exploration of correlations among changes in soil, vegetation and microclimate over the course of succession and by the selection of the most informative and suitable vegetation indicators. We measured vegetation, soil and microclimate properties along a successional gradient in the tropical dry forest region of the coast of Jalisco, Mexico. Bi- and multi-variate analyses revealed clear correlation between changes in soil and microclimate components. Bivariate and stepwise regressions indicated that basal area, litterfall, plant richness and litter mass were the vegetation properties that were the most related to soil and microclimate recovery. Basal area was the most closely related to soil and microclimate recovery, and it was also related to the recovery of soil P, a key soil property. However, litter mass, which was highly correlated with basal area, is a less time-consuming alternative measurement that may be more manageable for assessing many sites. Litter mass is also a key variable connecting aboveground and belowground recovery processes, making it a particularly useful variable for evaluating integrative recovery. Since all indicators can be affected by natural or human disturbance, the choice of indicators to be used in each evaluation should be cautious and, whenever possible, consider the history of the plot. We discuss and suggest the complementary use of several indicators when disturbance has occurred during secondary succession.

Population-wide shifts in herbivore resistance strategies over succession.

As a strategic cost-saving alternative to constitutive resistance, induction of resistance against herbivores in plants can be especially beneficial when enemies are scarce or variable in abundance. Although probably describing the two ends of a continuum, constitutive and induced resistance strategies have long been observed to trade off within species. Examining these traits among populations along a successional gradient can help explain how temporally variable environments can maintain genetic variation and how ecosystem processes are affected by shifting plant resistance trait expression over time. Here we leverage large experimental plots that represent a chronosequence of succession up to 15yr in combination with common garden experiments to examine changes in the selective environment and genetic differences in tall goldenrod's (Solidago altissima) constitutive and induced resistance. We show that resistance against a specialist herbivore Trirhabda virgata was inducible in the plants originating from midsuccession, which coincides with the largest loads of herbivores. The flavonoid compound content of the leaves varied with successional stage of the population of origin, which is indicative of constitutive differences in secondary metabolite production. Finally, there was a clear trade-off between constitutive and induced resistance. Our study indicates that selection for resistance traits within a population can be highly variable over time and likely result in genetically determined shifts of resistance strategies over relatively short time periods via genotype sorting.

Symbiosis between river and dry lands: Phycobiont dynamics on river gravel bars

River gravel bars are dynamic and heterogeneous habitats straddling the transition between aquatic and terrestrial environments. Periodic flooding, low nutrient concentrations, frost, lack of stable sites, drought, and ground surface heat significantly influence the biota of these habitats. Mutualistic symbiosis may be a successful strategy for organisms to survive and proliferate under such harsh conditions. The lichen genus Stereocaulon was selected as a model symbiotic system from among the organisms living on river gravel bars. The goal of the current study was to determine the effect of this dynamic environment on phycobiont (i.e., green eukaryotic photobiont) community structure. We analyzed 147 Stereocaulon specimens collected in the Swiss Alps using Sanger sequencing (fungal internal transcribed spacer (ITS) rDNA, algal ITS rDNA, and algal actin type I gene) and analyzed 8 selected thalli and 12 soil samples using Illumina metabarcoding (ITS2 rDNA). Phytosociological sampling was performed for all 13 study plots. Our analyses of communities of phycobionts, lichens, bryophytes, and vascular plants indicated a gradual change in the phycobiont community along a successional gradient. The particularly large phycobiont diversity associated with Stereocaulon mycobionts included algae, here reported as phycobionts for the first time. Each of the two Stereocaulon mycobiont operational taxonomic units had a distinct pool of predominant phycobionts. The thalli selected for Illumina metabarcoding contained a wide range of additional algae, i.e., they showed algal plurality.