Dynamic Rheological Behavior of Low Water-to-binder Ratio Mortars under Large Amplitude Oscillatory Shear(LAOS)

This work focuses on the dynamic rheological behavior of low water-to-binder ratio cement mortars blended with fly ash microspheres（FAM） or silica fume（SF）. The initial slump flow of each group has been controlled at similar values by adjusting the superplasticizer dosages. With the help of a coaxial cylinder rheometer, the dynamic rheological behaviors of these mortars are investigated by frequency sweeping in the range of 0-2 Hz under large amplitude oscillatory shear（LAOS）. Based on the systematical elaboration of dynamic rheological testing theory, the experimental data are processed according to Lissajous plot fitting to reveal the viscoelastic characteristics. The nonlinearity of response signals is further assessed with Fourier transform（FT） analysis. The parameters, storage modulus G＇, loss modulus G＂ and relative amplitude I3/I1 are proposed to clarify the influences of FAM and SF on the stability and energy consumption of local structures and nonlinearity of response torques. The hydration characteristics of various groups well confirmed the rheological phenomenon. This study is beneficial for the preparation and optimization of flow state concrete such as pumping concrete and self-compacting concrete.

Drivers of the differentiation between broad-leaved trees and shrubs in the shift from evergreen to deciduous leaf habit in forests of eastern Asian subtropics

In eastern Asian subtropical forests,leaf habit shifts from evergreen to deciduous broad-leaved woody plants toward higher latitudes.This shift has been largely explained by the greater capacity of deciduous broad-leaved plants to respond to harsh climatic conditions(e.g.,greater seasonality).The advantages of deciduous leaf habit over evergreen leaf habit in more seasonal climates have led us to hypothesize that leaf habits would shift in response to climate changes more conspicuously in forest canopy trees than in forest understory shrubs.Furthermore,we hypothesize that in the forests of the subtropics,plants at higher latitudes,regardless of growth form,would better tolerate seasonal harsh climates,and hence show less differentiation in leaf habit shift,compared to those at lower latitudes.To test these two hypotheses,we modelled the proportion of deciduous broad-leaved species and the incidence of deciduous and evergreen broad-leaved species in woody angiosperm species compositions of ten largesized forest plots distributed in the Chinese subtropics.We found that the rate of leaf habit shift along a latitudinal gradient was higher in forest trees than in forest shrubs.We also found that the differentiation in leaf habit shift between trees and shrubs is greater at lower latitudes(i.e.,warmer climates)than at higher latitudes(i.e.,colder climates).These findings indicate that specialized forest plants are differentially affected by climate in distinct forest strata in a manner dependent on latitudinal distribution.These differences in forest plant response to changes in climate suggest that global climate warming will alter growth forms and geographical distributions and ranges of forests.

Coarse woody debris features of a warm temperate deciduous broad-leaved forest,northern China

Stocking and structural composition of a deciduous broad-leaved forest were determined to predict coarse woody debris quantity by quantifying the empirical relationships between these two attributes.The most ecologically significant families by stem density were Salicaceae,Betulaceae,Fagaceae,and Aceraceae.P opulus davidiana was the most dominant species followed by B etula dahurica,Quercus mongolica,and Acer mono.The four species accounted for 69.5%of total stems.Numerous small-diameter species characterized the coarse woody debris showing a reversed J-shaped distribution.The coarse debris of P.davidiana,B.dahurica,and Q.mongolica mainly comprised the 10–20 cm size class,whereas A.mono debris was mainly in the 5–10 cm size class.The spatial patterns of different size classes of coarse woody debris were analyzed using the g-function to determine the size of the tree at its death.The results indicate that the spatial patterns at the 0–50 m scale shifted gradually from an aggregated to a random pattern.For some species,the larger coarse debris might change from an aggregated to a random distribution more easily.Given the importance of coarse woody debris in forest ecosystems,its composition and patterns can improve understanding of community structure and dynamics.The aggregation pattern might be due to density dependence and self-thinning effects,as well as by succession and mortality.The four dominant species across the different size classes showed distinct aggregated distribution features at different spatial scales.This suggests a correlation between the dominant species population,size class,and aggregated distribution of coarse woody debris.

Fine-scale activity patterns of large-and medium-sized mammals in a deciduous broadleaf forest in the Qinling Mountains,China

The composition of animal species and interactions among them are widely known to shape ecological communities and fine-scale(e.g.,<1 km)monitoring of animal communities is essential for understanding the relationships among animals and plants.Although the co-existence of large-and medium-sized species has been studied across different scales,research on fine-scale interactions of herbivores in deciduous broadleaf forests is limited.Camera trapping of large-and medium-sized mammals was carried out over a 1 year period within a 25 ha deciduous broadleaf forest dynamics plot in the Qinling Mountains,China.Fourteen species of large-and medium-sized mammals,including six carnivores,six ungulates,one primate and one rodent species were found.Kernel density estimations were used to analyse the diel or 24 h activity patterns of all species with more than 40 independent detections and general linear models were developed to explore the spatial relationships among the species.The combination of overlapping diel activity patterns and spatial associations showed obvious niche separation among six species:giant panda(Ailuropoda melanoleuca David),takin(Budorcas taxicolor Hodgson),Reeves’s muntjac(Muntiacus reevesi Ogilby),tufted deer(Elaphodus cephalophus Milne-Edwards),Chinese serow(Capricornis milneedwardsii David)and wild boar(Sus scrofa Linnaeus).Long-term fine-scale monitoring is useful for providing information about the co-existence of species and their interactions.The results demonstrate the importance for fine-scale monitoring of animals and plants for improving understanding of species interactions and community dynamics.

The Spatial-Temporal Heterogeneity of Understory Light Availability in a Temperate Forest of North China

The spatial-temporal variation of understory light availability has important influences on species diversity and community assembly.However,the distribution characteristics and influencing factors of understory light availability have not been fully elucidated,especially in temperate deciduous,broad-leaved forests.In this study,the understory light availability was monitored monthly(May–October)in a temperate deciduous,broad-leaved forest in Henan Province,China.Differences in the light availability among different months and habitat types were statistically analyzed using Kruskal–Wallis method,respectively.Partial least squares path modeling(PLS-PM)was used to explore the direct and/or indirect effects of stand structure,dominant species and topographic factors on the light environment.Results showed that there were differences in light environments among the four habitat types and during the studied six months.The PLS-PM results showed that the stand structure and the dominant species were negatively correlated with the light environment,and the path coefficient values were−0.089(P=0.042)and−0.130(P=0.004),respectively.Our result indicated that the understory light availability exhibit a distinct spatial and temporal heterogeneity in temperate deciduous,broad-leaved forest of north China.The characteristics of woody plant community,especially the abundance of one of the dominant plant species,were the important factors affecting the understory light availability.

Functional traits of tree species with phylogenetic signal co-vary with environmental niches in two large forest dynamics plots

Aims While using phylogenetic and functional approaches to test the mechanisms of community assembly,functional traits often act as the proxy of niches.However,there is little detailed knowledge regarding the correlation between functional traits of tree species and their niches in local communities.We suggest that the co-varying correlation between functional traits and niches should be the premise for using phylogenetic and functional approaches to test mechanisms of community assembly.Using functional traits,phylogenetic and environmental data,this study aims to answer the questions:(i)within local communities,do functional traits of co-occurring species covary with their environmental niches at the species level?and(ii)what is the key ecological process underlying community assembly in Xishuangbanna and Ailaoshan forest dynamic plots(FDPs)?Methods We measured seven functional traits of 229 and 36 common species in Xishuangbanna and Ailaoshan FDPs in tropical and subtropical China,respectively.We also quantified the environmental niches for these species based on conditional probability.We then analyzed the correlations between functional traits and environmental niches using phylogenetic independent contrasts.After examining phylogenetic signals of functional traits using Pagel’sλ,we quantified the phylogenetic and functional dispersion along environmental gradients within local tree communities.Important Findings For target species,functional traits do co-vary with environmental niches at the species level in both of the FDPs,supporting that functional traits can be used as a proxy for local-scale environmental niches.Functional traits show significant phylogenetic signals in both of the FDPs.We found that the phylogenetic and functional dispersion were significantly clustered along topographical gradients in the Ailaoshan FDP but overdispersion in the Xishuangbanna FDP.These patterns of phylogenetic and functional dispersion suggest that environmental filtering plays a key role in structuring local tree assemblages in Ailaoshan FDP,while competition exclusion plays a key role in Xishuangbanna FDP.

Large-diameter trees and deadwood correspond with belowground ectomycorrhizal fungal richness

Background: Large-diameter trees have an outsized influence on aboveground forest dynamics, composition, and structure. Although their influence on aboveground processes is well studied, their role in shaping belowground fungal communities is largely unknown. We sought to test if (i) fungal community spatial structure matched aboveground forest structure;(ii) fungal functional guilds exhibited differential associations to aboveground trees, snags, and deadwood;and (iii) that large-diameter trees and snags have a larger influence on fungal community richness than smaller-diameter trees. We used MiSeq sequencing of fungal communities collected from soils in a spatially intensive survey in a portion of Cedar Breaks National Monument, Utah, USA. We used random forest models to explore the spatial structure of fungal communities as they relate to explicitly mapped trees and deadwood distributed across 1.15 ha of a 15.32-ha mapped subalpine forest. Results: We found 6,177 fungal amplicon sequence variants across 117 sequenced samples. Tree diameter, dead-wood presence, and tree species identity explained more than twice as much variation (38.7% vs. 10.4%) for ectomy-corrhizal composition and diversity than for the total or saprotrophic fungal communities. Species identity and dis-tance to the nearest large-diameter tree (≥ 40.2 cm) were better predictors of fungal richness than were the identity and distance to the nearest tree. Soil nutrients, topography, and tree species differentially influenced the composition and diversity of each fungal guild. Locally rare tree species had an outsized influence on fungal community richness. Conclusions: These results highlight that fungal guilds are differentially associated with the location, size, and species of aboveground trees. Large-diameter trees are implicated as drivers of belowground fungal diversity, particularly for ectomycorrhizal fungi.

Differential impacts of adult trees on offspring and non-offspring recruits in a subtropical forest

An important mechanism promoting species coexistence is conspecific negative density dependence(CNDD),which inhibits conspecific neighbors by accumulating host-specific enemies near adult trees.Natural enemies may be genotype-specific and regulate offspring dynamics more strongly than non-offspring,which is often neglected due to the difficulty in ascertaining genetic relatedness.Here,we investigated whether offspring and non-offspring of a dominant species,Castanopsis eyrei,suffered from different strength of CNDD based on parentage assignment in a subtropical forest.We found decreased recruitment efficiency(proxy of survival probability)of offspring compared with non-offspring near adult trees during the seedling-sapling transition,suggesting genotype-dependent interactions drive tree demographic dynamics.Furthermore,the genetic similarity between individuals of same cohort decreased in late life history stages,indicating genetic-relatedness-dependent tree mortality throughout ontogeny.Our results demonstrate that within-species genetic relatedness significantly affects the strength of CNDD,implying genotype-specific natural enemies may contribute to population dynamics in natural forests.

Large-diameter trees, snags, and deadwood in southern Utah, USA

Background:The epidemic Dendroctonus rufipennis(spruce beetle)outbreak in the subalpine forests of the Colorado Plateau in the 1990s killed most larger Picea engelmannii(Engelmann spruce)trees.One quarter century later,the larger snags are beginning to fall,transitioning to deadwood(down woody debris)where they may influence fire behavior,regeneration,and habitat structure.Methods:We tracked all fallen trees≥1 cm in diameter at breast height(1.37-m high)and mapped all pieces of deadwood≥10-cm diameter and≥1 m in length within 13.64 ha of a high-elevation mixed-species forest in the Picea–Abies zone annually for 5 years from 2015 through 2019.We examined the relative contribution of Picea engelmannii to snag and deadwood pools relative to other species and the relative contributions of large-diameter trees(≥33.2 cm at this subalpine site).We compared spatially explicit mapping of deadwood to traditional measures of surface fuels and introduce a new method for approximating vertical distribution of deadwood.Results:In this mixed-species forest,there was relatively high density and basal area of live Picea engelmannii 20 years after the beetle outbreak(36 trees ha^(−1) and 1.94 m^(2) ha^(−1)≥10-cm diameter)contrasting with the near total mortality of mature Picea in forests nearby.Wood from tree boles≥10-cm diameter on the ground had biomass of 42 Mg ha^(−1),7 Mg ha^(−1) of Picea engelmannii,and 35 Mg ha^(−1) of other species.Total live aboveground biomass was 119 Mg ha^(−1),while snag biomass was 36 Mg ha^(−1).Mean total fuel loading measured with planar transects was 63 Mg ha^(−1) but varied more than three orders of magnitude(0.1 to 257 Mg ha^(−1)).Planar transects recorded 32 Mg ha^(−1) of wood≥7.62-cm diameter compared to the 42 Mg ha^(−1) of wood≥10-cm diameter recorded by explicit mapping.Multiple pieces of deadwood were often stacked,forming a vertical structure likely to contribute to active fire behavior.Conclusion:Bark beetle mortality in the 1990s has made Picea an important local constituent of deadwood at 20-m scales,but other species dominate total deadwood due to slow decomposition rates and the multi-centennial intervals between fires.Explicit measurements of deadwood and surface fuels improve ecological insights into biomass heterogeneity and potential fire behavior.

The importance of large-diameter trees to the creation of snag and deadwood biomass

Background:Baseline levels of tree mortality can,over time,contribute to high snag densities and high levels of deadwood(down woody debris)if fire is infrequent and decomposition is slow.Deadwood can be important for tree recruitment,and it plays a major role in terrestrial carbon cycling,but deadwood is rarely examined in a spatially explicit context.Methods:Between 2011 and 2019,we annually tracked all trees and snags≥1 cm in diameter and mapped all pieces of deadwood≥10 cm diameter and≥1 m in length in 25.6 ha of Tsuga heterophylla/Pseudotsuga menziesii forest.We analyzed the amount,biomass,and spatial distribution of deadwood,and we assessed how various causes of mortality that contributed uniquely to deadwood creation.Results:Compared to aboveground woody live biomass of 481 Mg ha^(−1)(from trees≥10 cm diameter),snag biomass was 74 Mg ha^(−1) and deadwood biomass was 109 Mg ha^(−1)(from boles≥10 cm diameter).Biomass from large-diameter trees(≥60 cm)accounted for 85%,88%,and 58%,of trees,snags,and deadwood,respectively.Total aboveground woody live and dead biomass was 668 Mg ha^(−1).The annual production of downed wood(≥10 cm diameter)from tree boles averaged 4 Mg ha^(−1) yr^(−1).Woody debris was spatially heterogeneous,varying more than two orders of magnitude from 4 to 587 Mg ha^(−1) at the scale of 20 m×20 m quadrats.Almost all causes of deadwood creation varied in importance between large-diameter trees and small-diameter trees.Biomass of standing stems and deadwood had weak inverse distributions,reflecting the long period of time required for trees to reach large diameters following antecedent tree mortalities and the centennial scale time required for deadwood decomposition.Conclusion:Old-growth forests contain large stores of biomass in living trees,as well as in snag and deadwood biomass pools that are stable long after tree death.Ignoring biomass(or carbon)in deadwood pools can lead to substantial underestimations of sequestration and stability.

The effect of tree size,neighborhood competition and environment on tree growth in an old-growth temperate forest

Aims Understanding the controls influencing tree growth is central to forest ecology.Although many factors such as tree size,neighborhood competition and environmental variables,have been proposed as being important in explaining patterns of tree growth,but their relative contributions are still subject to debate.We aimed to examine the relative importance of tree size,local abiotic conditions and the density and identity of neighbors on tree growth in an old-growth temperate forest in northeast china.Methods We used linear mixed models with data from a 25 ha(500×500 m)broad-leaved Korean pine(Pinus koraiensis)mixed forest perma-nent plot to examine the relative importance of these local drivers on tree growth at three organizational levels(community,guild and species).Subplot was included as a random effect to account for spatial autocorrelation in growth of trees located within the same subplot,and species was included as a random effect to account for variation among species in growth.Important Findings Tree size was typically the most important predictor of growth,followed by neighborhood competition and then soil nutrients.The correlation of tree size to growth varied from strongly positive for large trees and medium trees to slightly positive for small trees.The effect of neighbor-hood competition on growth showed weak negative density dependence as indicated by slightly negative effect of total basal area of neighbors and the proportion of conspecific neighbors.Environmental factors influenced growth of very common species,smaller tree-size classes and shade-tolerant species.We concluded that the relative importance of variables driving patterns of tree growth varied greatly among tree size classes,shade tolerance and abundance classes in this temperate for-est.These results provide critical information for future studies of forest dynamics and offer insight into forest management in this region.

Fine-scale habitats influence tree species assemblage in a miombo forest

Aims Relationships between local habitat heterogeneity and tree commu-nities in miombo woodlands have been very little studied.While some studies have addressed this topic at broad scales and based on few environmental parameters,this study aims at(i)detecting fine-scale habitats(≤10 ha)on the basis of a detailed characterisation of soil explicitly considering past anthropogenic disturbances,and an exhaustive census of the tree community,and at(ii)searching for indicator tree species corresponding to the resulting habitats.Methods The study was carried out in the miombo woodland of Mikembo Forest Reserve,Upper Katanga,The Democratic Republic of the congo.A complete census of the tree community was conducted in a 10-ha forest dynamics plot comprising 160 adjacent quadrats of 25×25 m,with a total of 4604 trees(diameter at breast height>10 cm).Thirty-six physicochemical soil parameters were measured.Studying the frequency distribution of soil charcoal content allowed identifying local signature of past human agriculture in the soil.Two strategies were used to define habitats:(i)a combination of principal component analysis(PcA)on soil variables and Ward clustering and(ii)multivariate regression trees(MRT)to search for key soil parameters allowing the best prediction of species composition.Tree-habitat associations were tested by means of a robust statistical framework combining the IndVal index and torus randomisations.Important Findings The forest contained 82 tree species and a significant proportion of wet miombo species(e.g.Marquesia macroura).We detected a strong east-west edaphic gradient driven by soil texture;most chemical soil parameters followed this pattern.Five habitats were identified based on soil factors and floristic composition.Nine indicator species of these habitats were found.The key soil fac-tors discriminating habitats were total calcium,available forms of phosphorus and clay content.Even though past agricultural practices were successfully detected in soils,they did not display any significant influence neither on habitat differentiation nor on the associated tree communities.Based on an unprecedented large number of soil parameters,fine-scale soil heterogeneity and niche partitioning were shown to contribute to the variabil-ity of the floristic composition in this forest.Our results indicated that considering the most variable environmental parameters,as in PcA,is a poor manner for defining habitats.In contrast,combining MRT with the IndVal index and torus randomisation has proved to be a much more robust and sensitive approach to highlight tree-habitat associations at this scale.The common dichotomous viewpoint of considering deterministic and neutral effects as acting at broad and fine scales,respectively,is not confirmed when measuring suitable environmental variables,even in a case where the physical environment does not exhibit strong heterogeneity.

Large-diameter trees dominate snag and surface biomass following reintroduced fire

The reintroduction of fire to landscapes where it was once common is considered a priority to restore historical forest dynamics,including reducing tree density and decreasing levels of woody biomass on the forest floor.However,reintroducing fire causes tree mortality that can have unintended ecological outcomes related to woody biomass,with potential impacts to fuel accumulation,carbon sequestration,subsequent fire severity,and forest management.In this study,we examine the interplay between fire and carbon dynamics by asking how reintroduced fire impacts fuel accumulation,carbon sequestration,and subsequent fire severity potential.Beginning pre-fire,and continuing 6 years post-fire,we tracked all live,dead,and fallen trees≥1 cm in diameter and mapped all pieces of deadwood(downed woody debris)originating from tree boles≥10 cm diameter and≥1 m in length in 25.6 ha of an Abies concolor/Pinus lambertiana forest in the central Sierra Nevada,California,USA.We also tracked surface fuels along 2240 m of planar transects pre-fire,immediately post-fire,and 6 years post-fire.Six years after moderate-severity fire,deadwood≥10 cm diameter was 73 Mg ha^(−1),comprised of 32 Mg ha^(−1) that persisted through fire and 41 Mg ha^(−1) of newly fallen wood(compared to 72 Mg ha^(−1) pre-fire).Woody surface fuel loading was spatially heterogeneous,with mass varying almost four orders of magnitude at the scale of 20 m×20 m quadrats(minimum,0.1 Mg ha^(−1);mean,73 Mg ha^(−1);maximum,497 Mg ha^(−1)).Wood from large-diameter trees(≥60 cm diameter)comprised 57%of surface fuel in 2019,but was 75%of snag biomass,indicating high contributions to current and future fuel loading.Reintroduction of fire does not consume all large-diameter fuel and generates high levels of surface fuels≥10 cm diameter within 6 years.Repeated fires are needed to reduce surface fuel loading.