• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 852
  • 687
  • 149
  • 97
  • 19
  • 18
  • 10
  • 10
  • 9
  • 8
  • 7
  • 6
  • 4
  • 3
  • 3
  • Tagged with
  • 2205
  • 310
  • 291
  • 257
  • 254
  • 234
  • 227
  • 212
  • 143
  • 135
  • 135
  • 134
  • 132
  • 129
  • 127
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

Mechanisms of coexistence among Aporosa species in two Malaysian rain forests

Debski, Igor January 2000 (has links)
Tropical forests contain the greatest biotic diversity of any biome, and those of South-east Asia are amongst the most diverse and tree species rich. I studied the speciose understorey tree genus, Aporosa (Euphorbiaceae), at two c. 50 ha plots in Malayisa, at Lambir Hills National Park, Sarawak, and at Pasoh Forest Reserve Peninsular Malaysia, where 17 and 12 species co-occur, respectively. The aim of this study was to determine which factors influence the spatial distribution of Aporosa species, what limits their abundance, and ultimately what are the mechanisms of coexistence among the species of this genus. Using a method of second-order spatial pattern analyses based on Ripley's K function to test for non-random spatial distributions, and to test for spatial associations between species (using a bivariate method) I found that Aporosa species formed spatially distinct assemblages at both sites. A randomization procedure suggested that these assemblages were explained by biases in the distribution of species in relation to habitat types. Soil type, as determined by parent material, was an important determinant of habitat preferences, although topography and forest stmcture also accounted for some variation. I found measurable effects of position on soil and especially topographic types on mortality and recruitment rates of many Aporosa species. Growth rates over a 5-8 year period varied between species, but, perhaps due to the short time interval and the correlation between abiotic variables, they were a poor indicator of habitat preference as defined by bias in a species' distribution. Aporosa species exhibit a range of morphological, anatomical and biochemical properties, and I found evidence that both habitat partitioning and density-dependent mortality of juveniles occurred. These are both mechanisms contributing to the maintenance of species richness. A severe drought had differential effects on mortality rates of Aporosa species and may be important in determining species' spatial distributions and abundance. Therefore disturbance events are also likely to be important mechanisms, as are non-equilibrium random population fluctuations, between ecologically similar species.
32

Organic matter dynamics in relation to two forest types in Korup National Park, SW Cameroon

Njampa, Leopold Leiche January 1996 (has links)
This study presents new data on soil organic matter dynamics in relation to two forest types in a strongly seasonal lowland rainforest in Korup National Park, SW Cameroon. Organic matter dynamics at the start of the wet season were investigated by quantifying changes in the amount of the standing crop of surface organic matter and light fraction soil organic matter (LF.SOM) in five replicate plots with low (≤ 15%) basal area ectomycorrhizal trees ≥ 30 cm gbh (LEM forest type), and five with high (≥ 45 %) basal area ectomycorrhizal trees (HEM forest types). Soil samples were collected in the wet season from three soil depths: an organic-enriched layer (0 - 3 cm), 3 - 8 cm and >8 cm layers, in both HEM and LEM forest types, in 1993 (3 harvests), and in 1994 (8 harvests). LF.SOM and heavy fraction soil organic matter (HF.SOM) were separated from < 2 mm whole soil using tap water and/or saturated NaI solution (density 1.65 g cm<sup>-3</sup>). Other pools of organic matter quantified included that in the > 2 mm soil fraction and in the < 2 mm sieved whole soil. LF.SOM accounted for 10 - 40 % soil organic matter, 7 - 34 % carbon, 2 - 28 % N and 2 - 25% P of the < 2 mm whole soil across forest types and down the soil profile. The > 2 mm soil fraction accounted for 14 - 21% carbon, 11 - 20 % N, and 6 - 22% P of total soil (i.e. <2 mm whole soil + > 2 mm soil fraction). The amount of LF.SOM was not significantly different between the two forest types at all depths. Both the amount of the standing crop of surface organic matter and LF.SOM declined as the wet season progressed. However, the amount of LF.SOM declined faster (55 %) in the LEM forest than in the HEM forest (22 %). On the other hand, the amount of HF.SOM increased over the same period. A vertical gradient in LF.SOM content was observed.
33

Tropical geometry of curves with large theta characteristics

Deopurkar, Ashwin January 2017 (has links)
In this dissertation we study tropicalization curves which have a theta characteristic with large rank. This fits in the more general framework of studying the limit linear series on a curve which degenerates to a singular curve. We explore this when the singular curve is not of compact type. In particular we investigate the case when dual graph of the degenerate curve is a chain of g-loops. The fundamental object under consideration is a family of curves over a complete discrete valuation ring. In the first half of the dissertation we study geometry of such a family. In the third chapter we study metric graphs and divisors on them. This could be a thought of as the theory of limit linear series on a curve of non-compact type. In the fourth chapter we make this connection via tropicalization. We consider a family of curves with smooth generic fiber X η of genus g such that the dual graph of the special fiber is a chain of g loops. The main theorem we prove is that if X η has a theta characteristic of rank r then there are at least r linear relations on the edge lengths of the dual graph.
34

TERPENÓIDES VOLÁTEIS EM ÁRVORES RESINOSAS DO GÊNERO Protium (Burm. f.) NA AMAZÔNIA CENTRAL

Piva, Luani Rosa de Oliveira 14 February 2017 (has links)
Submitted by Inácio de Oliveira Lima Neto (inacio.neto@inpa.gov.br) on 2018-06-15T13:14:24Z No. of bitstreams: 2 Dissertacao_VersaoFINAL_LuaniPiva_ATA.pdf: 4417876 bytes, checksum: 11d62a30d4befaaa00307256913971c0 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) / Made available in DSpace on 2018-06-15T13:14:24Z (GMT). No. of bitstreams: 2 Dissertacao_VersaoFINAL_LuaniPiva_ATA.pdf: 4417876 bytes, checksum: 11d62a30d4befaaa00307256913971c0 (MD5) license_rdf: 0 bytes, checksum: d41d8cd98f00b204e9800998ecf8427e (MD5) Previous issue date: 2017-02-14 / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / Despite the evident importance of the Amazon forest in regulating the global climate, through processes like photosynthesis and evapotranspiration, little is known about gases synthesized in the secondary metabolism and emited by trees of this ecosystem that are involved in these processes, such as the volatile terpenoids (e.g. monoterpenes and sesquiterpenes). In the present study, from a method of collecting volatile terpenoids in situ, with a low collection time and collected air volume, combined with thermal desorption analysis with gas chromatography coupled to mass spectrometry (TD GC-MS), the composition of volatile terpenoids in tree trunks belonging to 15 species of Protium (Burm. f.) genus was determined. A library of volatile terpenoids was created and sixteen (16) monoterpenes were found, in addition to six (6) sesquiterpenes. The monoterpenes α-pinene and D-limonene were the most abundant, appearing 63 and 58 times, respectively, in 69 samples. In addition, a new record of a monoterpene for tropical tree species, the ξ-fencheno, has been reported. An explanatory analysis of the distribution of volatile terpenoids by species was also performed, from which information on the richness of compounds in the genus Protium was obtained, as well as how each species could be characterized by the higher or lower frequency of compounds reported. Results of the present study provide a basis for the beginning of filling the knowledge gap on this subject, considering that to date no work on the characterization of volatile terpenoids in tropical tree species in the Brazilian Amazon has been found. In addition, these outcomes favor the understanding of the ecological and evolutionary interactions of resins, to ensure the sustainable commercial exploration of this material, as well as contribute to its ecological conservation. In addition, they may contribute to elucidate the functions of these compounds in the tropical forests dynamics (plant-plant interactions, plant-insect interactions) as well as the forest-atmosphere interactions. / Apesar da notória importância da Floresta Amazônica na regulação do clima do planeta, por meio de processos como a fotossíntese e evapotranspiração, pouco se conhece a respeito dos gases sintetizados e emitidos pelas árvores desse ecossistema, a partir do metabolismo secundário, envolvidos nesses processos, como é o caso dos terpenóides voláteis (e.g. monoterpenos e sesquiterpenos). No presente estudo, a partir de um método de coleta de terpenóides voláteis in situ, com baixo tempo de coleta e volume de ar coletado, aliado à análise por dessorção térmica com cromatografia a gás acoplada a espectrometria de massas (TD GC- MS, na sigla em inglês), foi possível determinar a composição de terpenóides voláteis em troncos de árvores resinosas pertencentes a 15 espécies do gênero Protium (Burm. f.). Uma biblioteca de terpenóides voláteis foi criada, sendo que 16 monoterpenos foram encontrados, além de 6 sesquiterpenos. Os monoterpenos α-pineno e D-limoneno foram os mais abundantes, sendo relatados 63 e 58 vezes, respectivamente, num total de 69 amostras. Ademais, um novo registro de monoterpeno para espécie arbórea tropical, o ξ-fencheno, foi relatado. Uma análise explanatória da distribuição de terpenóides voláteis por espécie também foi realizada, a partir da qual obteve-se informações sobre a riqueza de compostos no gênero Protium, além de como cada espécie pôde ser caracterizada pela maior ou menor frequência de compostos relatada. Os resultados do presente estudo fornecem uma base para o início do preenchimento de uma lacuna de conhecimento acerca dessa temática, tendo em vista que, até o momento não foram encontrados trabalhos sobre a caracterização de terpenóides voláteis em espécies arbóreas tropicais na Amazônia brasileira. Além disso, favorecem o entendimento sobre as interações ecológicas e evolutivas das resinas, visando assegurar a exploração comercial sustentável desse material, bem como contribuir para sua conservação ecológica. Ainda, podem contribuir para a obtenção de maiores esclarecimentos sobre as funções desses compostos na dinâmica das florestas tropicais (interações planta-planta; interações planta-inseto), bem como nas interações floresta-atmosfera.
35

The multiple vortex nature of tropical cyclogenesis

Sippel, Jason Allen 17 February 2005 (has links)
This thesis contains an observational analysis of the genesis of Tropical Storm Allison (2001). Using a paradigm of tropical cyclone formation as the superposition of potential vorticity (PV) anomalies, the importance of different scales of PV merger to various aspects of Allison’s formation is discussed. While only the case of Allison is discussed in great detail, other studies have also documented PV superposition on various scales, and superposition could be important for most tropical cyclones. Preceding Allison’s genesis, PV superposition on the large scale destabilized the atmosphere and increased low-level cyclonic vorticity. This presented a more favorable environment for the formation of MCV-type PV anomalies and smaller, surface-based, meso-&#946;-scale vortices. Although these vortices eventually merged to form a more concentrated vortex with stronger surface pressure gradients, the merger happened well after landfall of Allison and no strengthening ensued. The unstable, vorticity-rich environment was also favorable for the development of even smaller, meso-&#947;-scale vortices that accompanied deep convective cells within one of Allison’s meso-&#946;-scale vortices. The observations herein suggest that the meso-&#947;- scale convective cells and vortices are the respective source of PV production and building blocks for the meso-&#946;-scale vortices. Finally, this thesis discusses issues related to the multiple vortex nature of tropical cyclone formation. For instance, the tracking of developing tropical cyclones is greatly complicated by the presence of multiple vortices. For these cases, the paradigm of a single cyclone center is inappropriate and alternative tracking methods are introduced.
36

The Dynamics and Predictability of Tropical Cyclones

Sippel, Jason A. 15 January 2010 (has links)
Through methodology unique for tropical cyclones in peer-reviewed literature, this study explores how the dynamics of moist convection affects the predictability of tropical cyclogenesis. Mesoscale models are used to perform short-range ensemble forecasts of a non-developing disturbance in 2004 and Hurricane Humberto in 2007; both of these cases were highly unpredictable. Taking advantage of discrepancies between ensemble members in short-range ensemble forecasts, statistical correlation is used to pinpoint sources of error in forecasts of tropical cyclone formation and intensification. Despite significant differences in methodology, storm environment and development, it is found in both situations that high convective instability (CAPE) and mid-level moisture are two of the most important factors for genesis. In the gulf low, differences in CAPE are related to variance in quasi-geostrophic lift, and in Humberto the differences are related to the degree of interaction between the cyclone and a nearby front. Regardless of the source of CAPE variance, higher CAPE and mid-level moisture combine to yield more active initial convection and more numerous and strong vortical hot towers (VHTs), which incrementally contribute to a stronger vortex. In both cases, strength differences between ensemble members are further amplified by differences in convection that are related to oceanic heat fluxes. Eventually the WISHE mechanism results in even larger ensemble spread, and in the case of Humberto, uncertainty related to the time of landfall drives spread even higher. It is also shown that initial condition differences much smaller than current analysis error can ultimately control whether or not a tropical cyclone forms. Furthermore, even smaller differences govern how the initial vortex is built. Differences in maximum winds and/or vorticity vary nonlinearly with initial condition differences and depend on the timing and intensity of small mesoscale features such as VHTs and cold pools. Finally, the strong sensitivity to initial condition differences in both cases exemplifies the inherent uncertainties in hurricane intensity prediction. This study illustrates the need for implementing advanced data analysis schemes and ensemble prediction systems to provide more accurate and event-dependent probabilistic forecasts.
37

The role of pathogens in determining plant recruitment and distribution patterns in a western Amazonian floodplain

Alvarez-Loayza, Patricia C. January 2009 (has links)
Thesis (Ph. D.)--Rutgers University, 2009. / "Graduate Program in Ecology and Evolution." Includes bibliographical references.
38

Quantification of tropical cyclone steering /

Kong, Che-wing. January 1984 (has links)
Thesis (Ph. D.)--University of Hong Kong, 1984.
39

Tropical Hurwitz spaces

Katz, Brian Paul 01 February 2012 (has links)
Hurwitz numbers are a weighted count of degree d ramified covers of curves with specified ramification profiles at marked points on the codomain curve. Isomorphism classes of these covers can be included as a dense open set in a moduli space, called a Hurwitz space. The Hurwitz space has a forgetful morphism to the moduli space of marked, stable curves, and this morphism encodes the Hurwitz numbers. Mikhalkin has constructed a moduli space of tropical marked, stable curves, and this space is a tropical variety. In this paper, I construct a tropical analogue of the Hurwitz space in the sense that it is a connected, polyhedral complex with a morphism to the tropical moduli space of curves such that the degree of the morphism encodes the Hurwitz numbers. / text
40

Investigation of Variations and Impacts of Tropical Cyclone Precipitation in Texas (1950-2009)

Zhu, Laiyin 16 December 2013 (has links)
This dissertation examines the causes of variations in tropical cyclone precipitation (TCP) and the relationship between TCP and river discharge in Texas. The dissertation has three major objectives: 1) investigate the spatial and temporal variations of TCP in Texas from 1950 to 2009, 2) construct seasonal statistical forecast models for TCP and identify the primary factors controlling TCP in Texas, and 3) examine how TCP contributes to the extreme precipitation and river discharge in watersheds surrounding the city of Houston. An automated extraction method is developed to identify TCP from 60 years of precipitation data from Cooperative Observing Network gauges. Texas receives an average of 123.5 mm of TCP/year, which is ~13% of the state’s mean annual precipitation. September is the month with the most TCP, and it receives an average of 18.5 mm. Long-term trends (>50 years) in TCP are evident at some locations, but there are no statistically significant long-term trends in aggregated annual TCP metrics. Despite the lack of long-term trends, TCP metrics show some spectral power at periodicities of ~2-3 years, ~5-8 years, and >10 years. Areas within 400 km of the coast have higher risk of extreme daily TCP (>100 mm), but inland Texas can also occasionally experience extreme TCP. In some areas in southeastern Texas the probability of receiving >100 mm of daily TCP in any given year is ~0.30 (i.e., daily TCP exceeds 100 mm, on average, 1 out every 3 years). The best seasonal forecast models of TCP can explain >20% variance based on three or fewer predictors. ENSO is the most important control of TCP in Texas. La Niña, the major driver in all TCP models, reduces the vertical wind shear in the Caribbean and tropical Atlantic and therefore generates more precipitating storms in Texas. Maximum Potential Velocity (MPV) in the Gulf of Mexico and vorticity in the Atlantic Hurricane Development Region (MDR) are also important predictors of TCP and they can increase the R2 by ~0.2. The negative relationship between MPV and vorticity with the TCP are due to the fact that TCs with weaker wind speed and slower translation speed tend to contribute much more to both extreme and total TCP. Sea level pressure in the Gulf of Mexico, SST in the Caribbean and North Atlantic Oscillation are also identified as useful predictors in some of the models. TCP is associated with many of the annual maximum discharge events in watersheds near Houston. Urbanization can significantly increase river discharge generated by TCP. Both the annual maximum discharge and 90 percentile discharge have increased significantly in many watersheds in Houston. Although no long-term trend can be observed in the TCP and TCP-related extreme discharge, there may be an increased risk of floods from TCP because of the statistically significant increases in annual maximum discharge that have been observed. There are also increased uncertainties in flood risk because extreme precipitation, including TCP, is projected to become more variable in the future.

Page generated in 0.0463 seconds