Literatura científica selecionada sobre o tema "Tress gradient hypothesis"

We addressed the hypothesis that intraspecific genetic variation in plant traits from different sites along a distance/elevation gradient would influence the communities they support when grown at a new site. Answers to this hypothesis are important when considering the community consequences of assisted migration under climate change; i.e., if you build it will they come?. We surveyed arthropod communities occurring on the foundation riparian tree species Populus angustifolia along a distance/elevation gradient and in a common garden where trees from along the gradient were planted 20–22 years earlier. Three major patterns were found: 1) In the wild, arthropod community composition changed significantly. Trees at the lower elevation site supported up to 58% greater arthropod abundance and 26% greater species richness than more distant, high elevation trees. 2) Trees grown in a common garden sourced from the same locations along the gradient, supported arthropod communities more similar to their corresponding wild trees, but the similarity declined with transfer distance and elevation. 3) Of five functional traits examined, leaf area, a trait under genetic control that decreases at higher elevations, is correlated with differences in arthropod species richness and abundance. Our results suggest that genetic differences in functional traits are stronger drivers of arthropod community composition than phenotypic plasticity of plant traits due to environmental factors. We also show that variation in leaf area is maintained and has similar effects at the community level while controlling for environment. These results demonstrate how genetically based traits vary across natural gradients and have community-level effects that are maintained, in part, when they are used in assisted migration. Furthermore, optimal transfer distances for plants suffering from climate change may not be the same as optimal transfer distances for their dependent communities.

A prediction of the hypothesis that niche processes control plant species distributions across edaphic gradients is that pairwise comparisons of related species on contrasting soils should show consistent patterns of trait divergence. This hypothesis was tested in lowland Bornean rain forest by combining measurements of leaf traits, soil nutrients and the distribution of tree species in the Dipterocarpaceae across a nutrient gradient. Nine species were studied, comprising four related pairs in Shorea (phylogenetic independent contrasts; PICs) and one habitat generalist (Dipterocarpus sublamellatus). Lamina area, specific leaf area (SLA), leaf nitrogen (N) and phosphorus (P) were measured for ∼10 saplings of each species; habitat associations were defined as a continuous function of soil nutrients and categorically in relation to substrate. Species distributions and traits varied significantly with soil nutrients. When all species were pooled, SLA and leaf P increased significantly with species' distribution across the nutrient gradient; leaf N showed the same trend. Trait shifts with habitat were stronger and more pervasive when habitat was defined categorically – in all four PICs, rich-soil specialists had larger leaves, higher SLA, leaf N and P, and lower N:P ratios. Trait shifts with habitat at least partly reflect intrinsic differences between species. Within Shorea, variation in lamina area and N:P ratio were more strongly related to phylogeny than habitat, whereas the reverse held for SLA and leaf P. Phylogeny also influenced the extent of trait divergence between related species on different soils, and patterns of trait correlation within lineages. Results support the hypothesis that niche processes influence the distribution of species and traits in lowland tropical tree communities, and highlight the value of phylogenetic information for increasing the power of comparative studies.

Abstract. Latitudinal and altitudinal gradients can be utilized to forecast the impact of climate change on forests. To improve the understanding of how these gradients impact forest dynamics, we tested two hypotheses: (1) the change of the tree growth–climate relationship is similar along both latitudinal and altitudinal gradients, and (2) the time periods during which climate affects growth the most occur later towards higher latitudes and altitudes. To address this, we utilized tree-ring data from a latitudinal gradient in Finland and from two altitudinal gradients on the Tibetan Plateau. We analysed the latitudinal and altitudinal growth patterns in tree rings and investigated the growth–climate relationship of trees by correlating ring-width index chronologies with climate variables, calculating with flexible time windows, and using daily-resolution climate data. High latitude and altitude plots showed higher correlations between tree-ring chronologies and growing season temperature. However, the effects of winter temperature showed contrasting patterns for the gradients. The timing of the highest correlation with temperatures during the growing season at southern sites was approximately 1 month ahead of that at northern sites in the latitudinal gradient. In one out of two altitudinal gradients, the timing for the strongest negative correlation with temperature at low-altitude sites was ahead of treeline sites during the growing season, possibly due to differences in moisture limitation. Mean values and the standard deviation of tree-ring width increased with increasing mean July temperatures on both types of gradients. Our results showed similarities of tree growth responses to increasing seasonal temperature between latitudinal and altitudinal gradients. However, differences in climate–growth relationships were also found between gradients due to differences in other factors such as moisture conditions. Changes in the timing of the most critical climate variables demonstrated the necessity for the use of daily-resolution climate data in environmental gradient studies.

Changes in age structure, spatial distribution and intraspecific interactions across environmental gradients often reflect adaptations of plant populations to their environment. Our study explored the growth status of the Picea schrenkiana population on the north slope of the eastern Tianshan Mountains and tested the stress gradient hypothesis (SGH) against changes in the age structure and spatial pattern of P. schrenkiana populations along the environmental gradient. We sampled the forests at eight elevational locations, comprising a total of 24 plots of 30 × 30 m area from 1800 to 2500 m a.s.l. in the Jiangbulake region. By scanning the 3D structure of the forests and sampling tree rings in each plot, we precisely determined the spatial location and diameter of the breast height (DBH) of each P. schrenkiana individual. By fitting the DBH-age power model and g(r) function of the point pattern, we examined the age structure, spatial patterning and intraspecific interactions of local P. schrenkiana populations within each plot and their correlation with habitat parameters. The results indicate that (1) juveniles dominate the overall population density, age structure and spatial patterning of the P. schrenkiana population. Trees of low–middle elevations represent younger forests with faster growth and better regeneration, while trees at high elevations form older forests with slower growth and poorer regeneration. (2) The aggregated population patterns and positive intraspecific interactions occur mostly at medium elevations (2000 and 2100 m a.s.l.). (3) Population density, aggregation intensity and intraspecific interaction strength are strongly and positively correlated (p < 0.01). Our results did not fit the SGH but support a hump-shaped hypothesis that proposes that facilitation is stronger under medium stress along the elevational gradient. This study validates the spatial point pattern testing of the SGH of different types. We recommend the implementation of more intensive forest closure measures, together with a reduction in the harvesting intensity of trees to ensure the sustainable regeneration of P. schrenkiana forests in the eastern Tianshan Mountains.

Many hypotheses have been proposed to explain elevational species richness patterns; however, evaluating their importance remains a challenge, as mountains that are nested within different biogeographic regions have different environmental attributes. Here, we conducted a comparative study for trees, shrubs, herbs, and ferns along the same elevational gradient for 22 mountains worldwide, examining the performance of hypotheses of energy, tolerance, climatic variability, and spatial area to explain the elevational species richness patterns for each plant group. Results show that for trees and shrubs, energy-related factors exhibit greater explanatory power than other factors, whereas the factors that are associated with climatic variability performed better in explaining the elevational species richness patterns of herbs and ferns. For colder mountains, energy-related factors emerged as the main drivers of woody species diversity, whereas in hotter and wetter ecosystems, temperature and precipitation were the most important predictors of species richness along elevational gradients. For herbs and ferns, the variation in species richness was less than that of woody species. These findings provide important evidence concerning the generality of the energy theory for explaining the elevational species richness pattern of plants, highlighting that the underlying mechanisms may change among different growth form groups and regions within which mountains are nested.

Longleaf pine - wiregrass (Pinus palustris Mill. - Aristida stricta Michx.) woodlands occupy sites ranging from deep, xeric sandhills to the edge of wetlands in the southeastern United States. Aboveground net primary productivity (ANPP) of the overstory and understory were determined for three replicate sites of three site types (xeric, intermediate, and wet-mesic) that span a wide environmental gradient. In addition, soil moisture (at 30 and 90 cm) and N mineralization (in situ buried bag incubations) were measured through an annual cycle. Longleaf pine - wiregrass ecosystems varied by nearly twofold in ANPP across complex gradients. Overstory and understory and total (overstory and understory) ANPP were positively correlated to soil moisture at 30 and 90 cm. The proportion of understory ANPP relative to the total ANPP did not increase across the environmental gradient as predicted by hypotheses that invoke niche differentiation in rooting habits of grasses and trees. Contrary to expectations, cumulative net N mineralization was negatively related to soil moisture. All ANPP estimates were significantly and negatively related to cumulative N-mineralization. Further work is needed to explore the mechanisms by which soil moisture regulates productivity across space, time, and for individual species. Additional experimentation through resource addition would allow for investigations into multiple resource limitations and how resource limitations vary depending on gradient position.

Trees and other plants growing in stressful environments can display adaptive strategies such as sprouting, which is considered to be a functional trait for the persistence niche. For example, inselbergs are rocky outcrops that impose limitations on many plant forms through selective pressure to adapt to these environments. The present research tested the hypotheses that environmental harshness enhances sprouting, and that multi-stemmed trees have different persistence dynamics compared with single-stemmed trees. We sampled vegetation in 2006 and 2011 across a soil depth gradient in three areas: riparian forest, inselbergs and a corridor between the first two areas. Trees with a diameter at the breast height (dbh) ≥5 cm were sampled, and sprouts were counted. Results showed that the inselberg had more sprouted individuals and a larger amount of sprouts than the other areas. Thus, sprouting as a survival strategy was efficient in dealing with the environmental harshness and may be considered an adaptive strategy of trees to enable them to persist in such environments. The dynamic of multi-stemmed trees differed from single-stemmed ones, and the inselberg was distinct from the other areas. Over the 5 years of the present study, sprouting proved to be a strategy of persistence in this habitat, a finding that is in accordance with our hypothesis. Factors contributing to the environmental harshness, such as soil depth, may be the cause of sprouting in trees.

Understanding how genetic identity influences community structure is a major focus in evolutionary ecology, yet few studies examine interactions among organisms in the same trophic level within this context. In a common garden containing trees from a hybrid system (Populus fremontii S. Wats. × Populus angustifolia James), we tested the hypothesis that the structure of establishing understory plant communities is influenced by genetic differences among trees and explored foliar condensed tannins (CTs) and photosynthetically active radiation (PAR) as mechanisms. Several findings support our hypothesis: (i) Understory biomass and cover increase along the genetic gradient from P. angustifolia to P. fremontii. (ii) Along the same hybridization gradient, species richness decreases and species composition shifts. (iii) Populus foliar CT concentrations and PAR decrease from P. angustifolia to P. fremontii. (iv) Understory species richness increases with foliar CTs; however, biomass, cover, and composition show no relationship with CTs, and no understory variables correlate with PAR. (v) Structural equation modeling suggests that foliar CTs are a primary mechanism linking overstory tree genetics with understory richness. Using an experimental system dominated by naturally colonizing exotic species, this study demonstrates that a genetic gradient created by tree hybridization can influence understory plants.

The dichotomy of the earliest ecosystems into deltaic and floodplain forests was a long-standing view in paleobotany. The morphological traits such as nonbranching rootlets, bifurcating rhizomes, and bulbous bases of the primitive trees such as Eospermatopteris and lycopsids were considered adaptations to the lowland deltaic environments. In contrast, the traits of Archaeopteris trees such as wood, hierarchical branching networks of roots, and true leaves are an adaptation to the upland floodplain environments. The discovery of the Town of Cairo Highway Department (TCHD) fossil site in Upstate New York, where all major clades occupied a floodplain environment casts doubt on the validity of the environmental partition by the earliest trees at the higher taxonomic levels. This study aims to test the hypothesis of the environmental partition at the local scale by reconstructing the fossilized forest-floor landscape, the changes in the landscape over time, and the distribution patterns of the trees along the local environmental gradient at the TCHD site. To reconstruct the fossilized forest floor and to determine the environmental variations at the local scale, seven parallel cross-sections were drawn from south to north at THCH. The outcrop at the quarry floor measured 300 m in the north-south, and varied around 100–150 m in east-west direction. Primary sedimentary structures, the thickness of the sedimentary deposits that formed the forest floor and the surrounding quarry walls, paleosol features, and mega-fossils were measured, recorded, described, and mapped. Up to 3 meters deep drill-cores were extracted from the forest floor. The data was used to correlate the sedimentary deposits, and reconstruct the preserved landscape. Three dominant landscape features including an abandoned channel, an old-grown forest, and a local depression were recognized. These landscape features influenced greatly the pattern of local drainage, slope-gradients, patterns and durations of seasonal water pooling, paleosol developments, fossil distribution, and depositional environments. There is no evidence of environmental partitioning by trees at higher taxonomic levels at the local scale. The size and morphology of the root systems didn’t determine the distribution of the trees along the local environmental gradient such as drainage patterns but played important roles in tree stabilization. Forests would go through self-thinning as they matured. Upon comparison, it was found that the forests in unstable environments showed greater resiliency compared to forests established in the stable environments.

Among the hypotheses invoked to explain high species richness in tropical forests, the niche differentiation hypothesis has received observational and experimental support (Ashton 1969, Chesson 2000, Clark et al. 1999, Souza & Martins 2004, Svenning 2001, Terborgh & Mathews 1999). Habitat specialization with regard to edaphic factors and topography has been observed in several plant groups including trees, lianas, shrubs, ferns and palms (Clark et al. 1999, Ibarra-Manriquez & Martinez-Ramos 2002, Svenning 1999, Tuomisto & Ruokolainen 1993). Treefall gaps and light gradients have also been found to be important ecological factors affecting plant distribution and niche differentiation in the understory (Chazdon 1986, Poorter & Arets 2003, Terborgh & Mathews 1999). It is suggested that such habitat heterogeneity may not only maintain biodiversity but also may lead to the origin of new species through the process of parapatric speciation (Gentry 1989, Haffer 1997, Patton & Smith 1992).

Les interactions entre plantes dans les milieux impactés par les métaux/métalloïdes dépendent de nombreux facteurs et sont encore très peu connues. Tout d’abord elles semblent dépendre du niveau de pollution métallique du milieu, mais aussi des stratégies fonctionnelles végétales des plantes interagissant. D’autre part, les plantes peuvent avoir plusieurs types d’effets sur leur environnement proche, effets s’exprimant à des temporalités différentes. En effet, les plantes ont un effet immédiat via leur canopée et racines sur les ressources et le microclimat à proximité. Aussi, au cours d’une saison de végétation, les plantes peuvent avoir des effets liés à la production de la litière et sa décomposition dans les sols sous leur canopée. A plus long-terme, lorsque ce cycle de production/décomposition est répété au fil des années, les plantes vont avoir un effet lié à la dynamique de la matière organique dans les sols. Dans cette thèse, notre objectif principal était de différencier ces effets, et de comprendre comment les stratégies fonctionnelles végétales des plantes pouvaient influencer les différents effets en jeu le long de gradients de pollution métallique. Nous avons étudié ces effets pendant trois années (entre 2020 et 2022) dans une ancienne vallée minière des Pyrénées Ariégeoises (Sentein, France). Dans cette zone d’étude, nous avons étudié les interactions entre plantes par des méthodes observationnelles et de transplantations de cibles avec contrôle de la présence de canopée et/ou de la litière des plantes, sur trois sites d’étude : un terril avec une pollution homogène et deux zones de résidus miniers avec des pollutions hétérogènes créant un gradient de pollution. Le long des gradients étudiés, les effets de canopée et de prélèvement racinaire ont suivi l’Hypothèse du Gradient de Stress, passant de la compétition à la facilitation avec l’augmentation de la pollution. Cette facilitation était d’autant plus forte que les espèces produisant l’effet sont dites « exploitatrices » (en lien avec l’exploitation des ressources du sol et Leaf Economic Spectrum), et bénéficiait le plus aux plantes les moins tolérantes aux métaux. Les effets positifs étaient surtout liés à l’amélioration des conditions microclimatiques lors d’épisodes chauds et secs en été. Concernant les effets liés à la production et décomposition des litières, des effets négatifs sur les plantes cibles ont été démontrés, suggérant des effets dits d’allélopathie élémentaire, et liés à la forte concentration en éléments métalliques dans les litières en décomposition. Ces effets négatifs de litière étaient maximums dans les milieux les moins pollués où les plantes métallophytes accumulatrices (qui ont des fortes teneurs en métaux dans leurs feuilles) et les plantes moins tolérantes aux métaux interagissaient. Ils étaient particulièrement marqués pour les cibles sensibles à la pollution métallique. Les résultats de cette thèse donnent des pistes potentielles pour utiliser la facilitation dans un cadre de phyto-management de milieux pollués par les métaux/métalloïdes, en prenant en compte explicitement les stratégies fonctionnelles végétales des plantes en interactions et le niveau de pollution en jeu. Des résultats obtenus pendant la canicule de 2022 nous donnent aussi une bonne vision des évolutions attendues des différents effets impliqués dans les interactions entre plantes dans les écosystèmes métallifères dans un contexte de changement climatique<br>Plant-plant interactions have been overlooked in metal/metalloids-impacted environments and are likely driven by several factors whose influence is barely known. First, plant-plant interactions depend on the level of metal pollution, but also on the functional plant strategies of the interacting plants. Furthermore, plants can have several type of effects on their immediate environment, acting at different timescales. Plants canopy and roots have an instantaneous influence on the microclimate and available resources in their immediate vicinity. Then, during a growing season, the production of litter and its decomposition beneath their canopy can influence soil chemical and physical properties. In the longer term, when this cycle of litter production/decomposition is repeated over the years, the dynamics of the organic matter will influence soil conditions even more. In this thesis, our main objective was to delineate these effects, and to understand how plant functional strategies can influence these various effects along metal pollution gradients. We studied these effects during three consecutive years (from 2020 to 2022) in a former mining valley in the French Pyrenees (Sentein, Ariège, France). In this area, we studied interactions between plants using observational and target transplantation methods controlling for the presence of plant canopy and/or plant litter, in three study sites: a slag heap with homogeneous pollution and two mine tailings areas with heterogeneous pollution creating a gradient of pollution. Along these gradients, short-term canopy and root-uptake effects followed the Stress Gradient Hypothesis, switching from competition to facilitation as pollution increased. This facilitation was stronger when the species producing the effect were acquisitive (in relation with soil resources and the Leaf Economic Spectrum), and benefits more the low metal-tolerant plants. These positive effects were mainly due to the improvement of micro-climatic conditions during hot and dry episodes in summer. Concerning the effects linked to litter production and decomposition, negative effects on target plants were found, suggesting the so-called “elemental allelopathic” effects, in relation with the high concentration of metallic elements in the decomposing litter. These negative litter effects were more important in the least polluted environments, where metal-accumulating metallophyte plants (which have high concentration of metals in their leaves) and less metal-tolerant plants interacts. They were particularly marked for targets sensitive to metal pollution. The results of this thesis give important perspectives regarding the use of facilitation for the phyto-management of metals/metalloids-polluted environments, given that the functional strategies of interacting plants and the level of pollution involved are explicitly considered. Additionnaly, the results obtained during the 2022 heatwave provide useful insights regarding the expected evolution of the different effects driving plant interactions in metalliferous ecosystems in a climate change context

AbstractUnderstanding tree and stand growth dynamics in the frame of climate change calls for large-scale analyses. For analysing growth patterns in mountain forests across Europe, the CLIMO consortium compiled a network of observational plots across European mountain regions. Here, we describe the design and efficacy of this network of plots in monospecific European beech and mixed-species stands of Norway spruce, European beech, and silver fir.First, we sketch the state of the art of existing monitoring and observational approaches for assessing the growth of mountain forests. Second, we introduce the design, measurement protocols, as well as site and stand characteristics, and we stress the innovation of the newly compiled network. Third, we give an overview of the growth and yield data at stand and tree level, sketch the growth characteristics along elevation gradients, and introduce the methods of statistical evaluation. Fourth, we report additional measurements of soil, genetic resources, and climate smartness indicators and criteria, which were available for statistical evaluation and testing hypotheses. Fifth, we present the ESFONET (European Smart Forest Network) approach of data and knowledge dissemination. The discussion is focussed on the novelty and relevance of the database, its potential for monitoring, understanding and management of mountain forests toward climate smartness, and the requirements for future assessments and inventories.In this chapter, we describe the design and efficacy of this network of plots in monospecific European beech and mixed-species stands of Norway spruce, European beech, and silver fir. We present how to acquire and evaluate data from individual trees and the whole stand to quantify and understand the growth of mountain forests in Europe under climate change. It will provide concepts, models, and practical hints for analogous trans-geographic projects that may be based on the existing and newly recorded data on forests.

Abstract Among the 79 taxa of Microchiroptera in Australasia, frequency of treecavity roosting increases as mean annual temperature decreases and latitude increases. This gradient suggests there may be significant thermal benefits to tree-cavity roosting in cold climates. We explore the causes and consequences of tree-cavity roosting during summer months in Chalinolobus tuberculatus, a species that occurs at the southern limit (highest latitude) of this gradient. Five geographically distinct populations are compared. C. tuberculatus selected the oldest and largest trees for maternity roosting and avoided roosting under bark and in caves and buildings, despite the abundance of these sites. It also selected small, well-insulated cavities that accrue significant energy conservation benefits compared with other potential roosts (the ‘‘thermal hypothesis’’). Reproductive females selected roosts that reach maximum temperatures late in the day and retain high temperatures through the night, thus benefiting nonvolant young. Productivity and survival were significantly higher in populations that selected well-insulated roosts. We propose that selection favors smaller, rather than larger, roosting group sizes in this cold, temperate climate. Smaller groups of bats that use relatively small, well-insulated cavities have higher survival rates than larger groups that use larger, less insulated cavities. C. tuberculatus formed behaviorally, though not geographically, isolated subgroups. All colonies exhibited extreme roost-site lability on a daily basis, but strong long-term philopatry among pools exceeding 100 roosts. Most roosts were used once per year but date of reuse was similar each year. Strict temporal philopatry suggests that bats do not switch roosts in response to daily variability in weather conditions. The thermal hypothesis suggests that development of grouping behavior may be an incidental response to physiological constraints on thermoregulation and reproduction. Nevertheless, social interdependence would increase the probability that clusters are large enough on any one day to be thermally beneficial and individuals could improve the reproductive success of other relatives within the group. We conclude by outlining hypotheses that could test the general applicability of findings to tree-cavity roosting bats.

We propose to develop a Deep Learning (DL) framework based on the paradigm of Genetic Programming (GP). The hypothesis is that GP non-parametric and non-differentiable learning units (abstract syntax trees) have the same learning and representation capacity to Artificial Neural Networks (ANN). In an analogy to the traditional ANN/Gradient Descend/Backpropagation DL approach, the proposed framework aims at building a DL alike model fully based on GP. Preliminary results when approaching a number of application domains, suggest that GP is able to deal with large amounts of training data, such as those required in DL tasks. However, extensive research is still required regarding the construction of a multi-layered learning architecture, another hallmark of DL.

Abstract The objective of the study is to build a robust Recurrent Neural Network system using Long-Short-Term-Memory (LSTM) to predict future vibrations during drilling operations. This provides a reliable solution to the complex problem of modeling several forms of vibrations encountered downhole. This accurate prediction system can be readily integrated into advisory/warning systems giving drillers the potential to save time, improve safety, and increase efficiency in drilling operations. High-frequency downhole drilling data onshore fields, obtained from a major O&amp;G service provider, was used to train and validate the models. First, multiple classification algorithms such as Logistic Regression, KNN, Decision Trees, Random Forest were utilized to identify the presence and severity of Stickslip, Whirl, and other drill-string vibrations. LSTM-RNN was then used instead of traditional RNN intended for sequential data, to resolve the vanishing gradient problem. LSTM-RNN architecture was built to predict vibrations a)10 seconds and b) 30 seconds into the future. Results of the traditional classification models confirmed the hypothesis that dysfunctions can be successfully identified based on real-time downhole drilling data. 98% accuracy was obtained in successfully identifying torsional vibrations during drilling. A total of 101 parameters including measured and derived variables are available in the dataset. Modeling was performed with 14 features and vibrations were predicted. The RNN model was trained on data from multiple wells that encountered vibrations during drilling. The models were able to predict vibrations 10 seconds into the future with an MSE of 0.02 and 30 seconds into the future with reasonable accuracy and MSE of 0.10. Avoiding excessive vibrations will result in fewer trips by increasing longevity and reducing malfunctions of downhole electronics, the drill-string, and the BHA. Reduced NPT means drilling complex wells efficiently in less time which in turn directly translates to lower costs for the company. In addition to significant cost benefits, automated technology predicting anomalies and reacting in real-time translates to improved safety because it would now require fewer operators at risk on the rig floor. The work opens up avenues for a sophisticated advisory/warning system and effective ‘look-ahead’ drilling processes in the future.