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  • 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.
1

The Effects of Mesquite Tree Removal on Soil Microbial and Nutrient Cycling Processes in Semi-Arid Environments

Finnerty-Rae, Eileen Claire January 2009 (has links)
The expansion of mesquite (Prosopis velutina) in southeastern Arizona and attempts to control or remove mesquite have been well documented. However, removal of woody plants may affect the distribution and quantities of nutrient resources within soil including changes in carbon (C) and nitrogen (N) cycling and changes in the amounts of C and N sequestered in these soils. We hypothesized that mechanical mesquite removal would alter soil microclimate and nutrient inputs, leading to changes in soil C and N cycling and microbial communities and activities.This study was conducted at two separate semi-arid grassland sites south of Tucson, AZ during a three-year period. There were six study plots at each site, three plots from which mature mesquite trees had been mechanically removed and three plots where mature mesquite trees were left intact. We measured the soil temperature and moisture, C and N pools, N2O and CO2 fluxes, N mineralization rate, and microbial resistance and resilience to determine if short term changes in any of these parameters were taking place.No significant differences in N mineralization rate or microbial resistance or resilience were found between the soils under intact mesquite and from plots where mesquite had been mechanically removed. Soil temperature was not statistically analyzed because temperature was taken in only one plot per treatment and without within-treatment replicates, t-tests could not be performed. Statistically significant differences in soil moisture, total soil C and N, microbial biomass C and N, heterotrophic plate counts, and CO2 and N2O fluxes were found between treatments on different dates throughout the experiment. Despite this finding of some significant differences between treatments for some parameters measured, no recognizable pattern of changes was observed during this study. Over the course of the experiment, the data did not support our overall hypothesis, that short term changes in microclimate following mesquite removal would alter soil C and N cycling and microbial communities and activities.
2

Altering hydrologic regime to revgetate crusted soils on semiarid rangeland

Wentz, Amy Leigh 15 November 2004 (has links)
Dysfunctional rangelands lose nutrients and material faster than they capture or create them. The objective of this study was to determine the effectiveness of contour furrows, drill seeding, and aeration treatments in capturing overland flow, concentrating resources, and establishing perennial bunch grasses to convert dysfunctional semiarid rangeland to a functional rangeland. The site, located on the Edwards Plateau in west Texas, USA, had bare, structurally crusted soils with sparse short-grasses (Scleropogon brevifolius). The site had a low infiltration rate contributing to excess overland flow and loss of nutrients, organic matter, and soil. Contour furrows were installed with varying intra-furrow distances (0.6 to 61 m) and then broadcast seeded to determine if furrow spacing would produce a vegetative response. Portions of the intra-furrow areas were aerated and drill seeded. All seed mixes contained warm season, perennial bunch grasses (Bouteloua curtipendula, Leptochloa dubia, and Setaria leucopila). Soil beneath furrows had greater soil water content (p-value < 0.05) than intra-furrow areas. Furrow plots had greater density of seeded grasses and total vegetation (19 individuals m-2 and 191 individuals m-2, respectively) than intra-furrow plots (0 individuals m-2 and 89 individuals m-2, respectively). This study supports other findings that suggest 1.5 m to 1.8 m is optimum intra-furrow spacing. Vegetative responses to drill seeding and aeration treatments were insignificant. Observations suggest that contour furrows are effective at establishment and support of perennial vegetation by capturing and retaining water that otherwise would be lost to runoff from untreated soil.
3

Altering hydrologic regime to revgetate crusted soils on semiarid rangeland

Wentz, Amy Leigh 15 November 2004 (has links)
Dysfunctional rangelands lose nutrients and material faster than they capture or create them. The objective of this study was to determine the effectiveness of contour furrows, drill seeding, and aeration treatments in capturing overland flow, concentrating resources, and establishing perennial bunch grasses to convert dysfunctional semiarid rangeland to a functional rangeland. The site, located on the Edwards Plateau in west Texas, USA, had bare, structurally crusted soils with sparse short-grasses (Scleropogon brevifolius). The site had a low infiltration rate contributing to excess overland flow and loss of nutrients, organic matter, and soil. Contour furrows were installed with varying intra-furrow distances (0.6 to 61 m) and then broadcast seeded to determine if furrow spacing would produce a vegetative response. Portions of the intra-furrow areas were aerated and drill seeded. All seed mixes contained warm season, perennial bunch grasses (Bouteloua curtipendula, Leptochloa dubia, and Setaria leucopila). Soil beneath furrows had greater soil water content (p-value < 0.05) than intra-furrow areas. Furrow plots had greater density of seeded grasses and total vegetation (19 individuals m-2 and 191 individuals m-2, respectively) than intra-furrow plots (0 individuals m-2 and 89 individuals m-2, respectively). This study supports other findings that suggest 1.5 m to 1.8 m is optimum intra-furrow spacing. Vegetative responses to drill seeding and aeration treatments were insignificant. Observations suggest that contour furrows are effective at establishment and support of perennial vegetation by capturing and retaining water that otherwise would be lost to runoff from untreated soil.
4

The Variability of Ring Characteristics within Trees as Shown by a Reanalysis of Four Ponderosa Pine

Fritts, Harold C., Smith, David G., Budelsky, Carl A., Cardis, John W. 11 1900 (has links)
No description available.
5

Etnopedolgia na unidade de produção agrícola familiar Canto da Ilha de Cima, São Miguel do Gostoso, RN / Ethnopedology in the Family Agricultural Production Unit Canto da Ilha de Cima, São Miguel do Gostoso, RN

Cunha, Marialdo Santana da 12 May 2016 (has links)
Submitted by Socorro Pontes (socorrop@ufersa.edu.br) on 2016-11-10T13:16:58Z No. of bitstreams: 1 MarialdoSC_DISSERT.pdf: 2590642 bytes, checksum: 095ef28e7ca2841fc84541491e5f6234 (MD5) / Approved for entry into archive by Vanessa Christiane (referencia@ufersa.edu.br) on 2017-01-24T14:31:49Z (GMT) No. of bitstreams: 1 MarialdoSC_DISSERT.pdf: 2590642 bytes, checksum: 095ef28e7ca2841fc84541491e5f6234 (MD5) / Approved for entry into archive by Socorro Pontes (socorrop@ufersa.edu.br) on 2017-02-03T13:13:47Z (GMT) No. of bitstreams: 1 MarialdoSC_DISSERT.pdf: 2590642 bytes, checksum: 095ef28e7ca2841fc84541491e5f6234 (MD5) / Made available in DSpace on 2017-03-21T15:06:20Z (GMT). No. of bitstreams: 1 MarialdoSC_DISSERT.pdf: 2590642 bytes, checksum: 095ef28e7ca2841fc84541491e5f6234 (MD5) Previous issue date: 2016-05-12 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The union and the exchange of knowledge between the peasantry and the academy are very important tools for environmental studies, proper management of agroecosystems and the inclusion of technologies adapted to semi-arid conditions. The aim of this study was to describe morphologically the soil and classify it according to Ethnopedological studies in the Family Agricultural Production Unit Canto da Ilha de Cima, São Miguel do Gostoso (RN). Participatory research was conducted from January to December 2015. In the exploratory phase of the research were carried out field activities, meetings and workshops in the community involving farmers, technicians, students and teachers. In the soil classification study the emic [local] and the etic [academic] knowledges were used. Eight soil profiles were excavated in five different areas of the production unit (grazing of sheep and goats, beekeeping area, agroecological cultivation of vegetables, preserved caatinga and conventional production area - the area called Old Chica). In the emic [local] classification prevailed the impressions and qualitative analyzes of morphological and physical properties of soil in an objective and inteperative way through the light of ethnopedology. Farmers classified the lands as good land, piebald ox earth and old chica land as the most important in terms of fertility for crops, differentiating those from the arisco [a sandy soil] and the piçarro [a muddy-stony soil] lands. In the etic [academic] classification was used the Brazilian System of Soil Classification. In the etic classification were identified the Typical Carbonate Haplic Cambisols soils, YELLOW Typical Eutrophic Oxisols, RED YELLOW Typical Eutrophic Latosols, YELLOW Eutrophic Acrisols Latosols, Typical Eutrophic Haplic Cambisols Tb and Typical Carbonate Haplic Vertisols. The emic [local] and etic [academic] classifications performed in this study had a close relationship with each other, since the morphological attributes characterized from different methodological criteria contributed to affirm the importance of Ethnopedology / A união e a troca de saberes entre o campesinato e a academia são instrumentos de grande relevância para estudos de ambientes, um manejo adequado dos agroecosistemas e a inserção de tecnologias adaptados as condições do semiárido. O objetivo desta pesquisa foi descrever morfologicamente o solo e classifica-lo de acordo com os estudos etnopedológicos na Unidade de Produção Agrícola Familiar Canto da Ilha de Cima, São Miguel do Gostoso (RN). A pesquisa participativa foi realizada durante o período de janeiro a dezembro de 2015. Na fase exploratória da pesquisa foram realizadas atividades de campo, reuniões e oficinas na comunidade envolvendo os agricultores, técnicos, estudantes e professores. No estudo de classificação do solo foram utilizados os conhecimentos emicista e o eticista. Foram escavados oito perfis de solo em cinco áreas distintas da unidade de produção (pastejo de ovinos e caprinos, área de apicultura, cultivo agroecológico de hortaliças, caatinga preservada e área de produção convencional - área denominada Velha Chica). Na classificação emicista prevaleceu às impressões e análises qualitativas dos atributos morfológicos e físicos dos solos de forma objetiva e inteperativa a luz da etonopedologia. Os agricultores classificaram os solos como terra boa, terra malhada do boi e terra velha chica como as mais importantes em termos de fertilidade para o cultivo agrícola, diferenciando essas das terras de arisco e de piçarro. Na classificação eticista foi utilizado o Sistema Brasileiro de Classificação de Solos. Na classificação eticista foram identificados os solos CAMBISSOLOS HÁPLICOS Carbonáticos Típicos, LATOSSOLOS AMARELOS Eutróficos típicos, LATOSSOLOS VERMELHOS AMARELOS Eutróficos típicos, LATOSSOLOS AMARELOS Eutróficos argissólicos, CAMBISSOLOS HÁPLICOS Tb Eutróficos típicos e VERTISSOLOS HÁPLICOS Carbonáticos típicos. As classificações emicista e eticista realizadas neste estudo apresentaram estreita relação entre si, uma vez que os atributos morfológicos caracterizados a partir de diferentes critérios metodológicos contribuíram para afirmar a importância da etnopedologia / 2016-11-09
6

Caracterização e desempenho econômico dos sistemas de produção de ovinos Morada Nova no estado do Ceará / Characterization and economic performance of production systems of sheep breed Morada Nova in the state of Ceará

Sousa Neto, Jaime Martins de January 2011 (has links)
SOUSA NETO, Jaime Martins. Caracterização e desempenho econômico dos sistemas de produção de ovinos Morada Nova no estado do Ceará. 2011. 84 f. Dissertação (Mestrado em zootecnia)- Universidade Federal do Ceará, Fortaleza-CE, 2011. / Submitted by Elineudson Ribeiro (elineudsonr@gmail.com) on 2016-05-17T19:07:06Z No. of bitstreams: 1 2011_dis_jmsousaneto.pdf: 1342135 bytes, checksum: 2ad71bdd3537cc268e5b6a39cd7e7dce (MD5) / Approved for entry into archive by José Jairo Viana de Sousa (jairo@ufc.br) on 2016-05-27T20:02:34Z (GMT) No. of bitstreams: 1 2011_dis_jmsousaneto.pdf: 1342135 bytes, checksum: 2ad71bdd3537cc268e5b6a39cd7e7dce (MD5) / Made available in DSpace on 2016-05-27T20:02:34Z (GMT). No. of bitstreams: 1 2011_dis_jmsousaneto.pdf: 1342135 bytes, checksum: 2ad71bdd3537cc268e5b6a39cd7e7dce (MD5) Previous issue date: 2011 / This chapter has the aim to make the technical analysis of production systems in Morada Nova sheep to complement the project "Characterization and basis for the genetic improvement of sheep Morada Nova", coordinated by Embrapa Goats and Sheep, with the financial support of Banco do Nordeste (FUNDECI / ETENE). In order to achieve this objective, primary research was conducted with data collected through the application of questionnaires to the sheep farmers of the municip ality of Morada Nova, Ceará State. To assess the technological level, the system of sheep was divided into four components: the producer management, infrastructure, production system, herd management and performance of the herds. Next, we determined the te chnological indices for each component separately and for all of them, based on its recommended technology. We found four production systems baptized by the technology index for each producer, being called "high technology gap" - level D, "regular technolog ical gap" - level C, "low technology gap" - level B and "high art " - level A. It was found that 17% of farmers fall in the level A, 33% at level B, Level C in 33% and 17% at level D. / O presente trabalho tem por objetivo principal efetuar a caracterização e avaliação do desempenho econômico dos sistemas de produção de ovinos da raça Morada Nova no Estado do Ceará. Para tanto foram aplicados questionários junto aos produtores de ovinos da raça Morada Nova no Estado do Ceará. Para caracterização técnica, dividiu-se o sistema de produção de ovinos em quatro componentes: gerenciamento do produtor, infra-estrutura do sistema de produção, manejo dos rebanhos e desempenho dos rebanhos. Em seguida, foram determinados os índices tecnológicos para cada componente, separadamente e para o conjunto deles, com base na respectiva tecnologia recomendada. Para a caracterização socioeconômica dos produtores, foram analisados sete fatores, a saber: infra-estrutura e produção de volumosos, rebanhos e manejo de produção, produção, aspectos ambientais, mão-de-obra, crédito rural e administração do empreendimento. A análise econômica foi feita utilizando-se a metodologia do Sistema Integrado de Custos Agropecuários – CUSTAGRI. Foram identificados quatro níveis tecnológicos de sistemas de produção que foram usados para a análise dos aspectos socioeconômicos, estimativas de custos e avaliação de rentabilidade. Os sistemas de produção de ovinos Morados Nova mais tecnificados mostraram-se lucrativos e capazes de viabilizar o desenvolvimento sustentável no meio rural, proporcionando a fixação do homem no campo com emprego e renda. As principais dificuldades observadas pelos produtores estão ligadas à mão-de-obra desqualificada, políticas de crédito insuficientes e falta de políticas públicas focadas na atividade.
7

Artrópodes e atributos microbiológicos do solo em cultivo de fruteiras no Vale do Curu - CE / Arthropods and attributes soil microbiology in fruit trees en Vale do Curu-CE, Brasil

Araújo, Jackson de Lima January 2014 (has links)
ARAÚJO, Jackson de Lima. Artrópodes e atributos microbiológicos do solo em cultivo de fruteiras no Vale do Curu - CE. 2014. 79 f. Dissertação (Mestrado em agronomia)- Universidade Federal do Ceará, Fortaleza-CE, 2014. / Submitted by Elineudson Ribeiro (elineudsonr@gmail.com) on 2016-08-30T20:44:45Z No. of bitstreams: 1 2014_dis_jlaraújo.pdf: 1782360 bytes, checksum: bd8086576c989888ae34f3e1963337d2 (MD5) / Approved for entry into archive by Jairo Viana (jairo@ufc.br) on 2016-08-31T23:29:51Z (GMT) No. of bitstreams: 1 2014_dis_jlaraújo.pdf: 1782360 bytes, checksum: bd8086576c989888ae34f3e1963337d2 (MD5) / Made available in DSpace on 2016-08-31T23:29:51Z (GMT). No. of bitstreams: 1 2014_dis_jlaraújo.pdf: 1782360 bytes, checksum: bd8086576c989888ae34f3e1963337d2 (MD5) Previous issue date: 2014 / A major concern related to agricultural management systems used by the man lies on the effects on biological processes in the soil, whose action within ecosystems aiming at the farm, modifies the intensity of these processes under use of techniques that exclusively aim to maximize crop yield. One of the challenges of research in soil biology is precisely understand the impacts on the complex interactions of all soil organisms at the community level by the management that are critical in maintaining soil quality. The objective of this work was to evaluate the influence of management practices on fruit trees (mango, guava and coconut trees) in the composition and distribution of soil arthropods (edaphic macrofauna, mesofauna) and on microbial biomass, the activity and interaction with the chemical, physical and environmental attributes. The study of soil arthropods began in July 2013 and followed until May 2014, with measurements at four periods. Microbiological soil activities were evaluated by the determination of microbial soil properties (RBS, CBM, NBM, NBM / N, qCO2 and qMIC). It was found that the soil arthropods were sensitive to changes arising from different soil tillage systems, allowing it to be used as an important tool to apply as bio-indicators of the quality of edaphic system. The system with guava cultivation proved to be unstable over time in relation to the structure of the community of soil arthropods. The system with coconut tree cultivation allows for better soil quality, increase in abundance and species richness of invertebrate macrofauna. The microbial biomass carbon (MBC), microbial biomass nitrogen (NBM) and the metabolic quotient (qCO2) were the main microbiological soil properties responsible for identifying dissimilarities between areas. Correlations between microbiological and faunal characteristics of the soil were low with significant correlations only between Collembola, NBM and NBM / N. / Uma das grandes preocupações relacionadas aos sistemas de manejo agrícola adotado pelo homem reside nos efeitos sobre os processos biológicos que ocorrem no solo, cuja ação dentro dos ecossistemas visando à exploração agrícola, modifica a intensidade desses processos em virtude do emprego de práticas que objetivam exclusivamente a maximização da produção vegetal. Um dos desafios da pesquisa em biologia do solo é justamente entender os impactos do manejo sobre as complexas interações de todos os organismos edáficos ao nível de comunidade que são fundamentais na manutenção da qualidade do solo. Objetivou-se avaliar a influência dos manejos em cultivos de fruteiras (mangueira, goiabeira e coqueiro) na composição e distribuição dos artrópodes de solo (mesofauna e macrofauna edáfica) e sobre a biomassa microbiana, sua atividade e interação com os atributos químicos, físicos e ambientais. O estudo dos artrópodes edáficos teve início em julho de 2013 e seguiu até maio de 2014, com coletas realizadas em quatro períodos. As atividades microbiológicas do solo foram avaliadas mediante as determinações dos atributos microbianos do solo (RBS, CBM, NBM, NBM/N, qCO2 e qMIC). Constatou-se que os artrópodes de solo mostraram-se sensíveis as alterações ocasionadas pelos sistemas de manejos do solo, possibilitando a sua indicação como importante ferramenta para aplicar-se como bioindicadores da qualidade do sistema edáfico. O sistema com cultivo de goiabeiras mostrou-se instável ao longo do tempo em relação à estrutura da comunidade dos artrópodes de solo. O sistema com cultivo de coqueiros propicia melhores características do solo e aumento na abundância e riqueza de espécies da macrofauna invertebrada. O carbono da biomassa microbiana (CBM), nitrogênio da biomassa microbiana (NBM) e o quociente metabólico (qCO2), foram os principais atributos microbiológicos do solo responsáveis por identificar dissimilaridades entre as áreas. Correlações entre as variáveis microbiológicas e faunísticas do solo foram baixas, com correlações significativas somente entre Collembola, NBM e NBM/N.
8

Resiliência à secas futuras, índices de seca e análise de vulnerabilidade em bacias hidrográficas do semiárido brasileiro.

ANDRADE, Tafnes da Silva. 11 September 2018 (has links)
Submitted by Lucienne Costa (lucienneferreira@ufcg.edu.br) on 2018-09-11T19:57:24Z No. of bitstreams: 1 TAFNES DA SILVA ANDRADE – TESE (PPGRN) 2017.pdf: 12846482 bytes, checksum: c67b2441de336d9fe46d15963d7dfc5f (MD5) / Made available in DSpace on 2018-09-11T19:57:24Z (GMT). No. of bitstreams: 1 TAFNES DA SILVA ANDRADE – TESE (PPGRN) 2017.pdf: 12846482 bytes, checksum: c67b2441de336d9fe46d15963d7dfc5f (MD5) Previous issue date: 2017-08-11 / A seca liga os fenômenos meteorológicos e a sociedade de tal forma que sua ocorrência, quase sempre, implica em consequências sociais e econômicas negativas. É um fenômeno natural cíclico comum no Semiárido nordestino do Brasil. Sua gestão deve priorizar medidas proativas tomadas antes e durante a ocorrência das secas. Obras de infraestrutura hídrica podem auxiliar nessa gestão pois contribuem para diminuição da vulnerabilidade e consequente diminuição dos impactos da seca na região. O monitoramento é outra ferramenta fundamental nesse processo pois pode integrar sistemas de alerta e orientar o momento correto para executar ações de preparação e mitigação. Diferentes índices de seca são usados no monitoramento, e deveriam considerar não apenas os fenômenos meteorológicos, mas também as atividades econômicas significativas para a região. Tomando por base o conceito de gestão do risco, este trabalho buscou identificar as relações entre a gestão do risco e a diminuição da vulnerabilidade à seca, considerando que as obras de infraestrutura e índices de secas relacionados com as atividades econômicas regionais são chaves importantes desse processo. Como resultados podemos citar que as ações de infraestrutura hídrica contribuíram para o aumento da oferta hídrica na região, que se refletiu nas baixas vulnerabilidades a seca dos municípios mais atendidos pelo programa. E que os índices de seca VCI, VHI e SPEI, possuem potencial de uso como ferramenta para aplicação de medidas de monitoramento e gestão da seca na região em regiões semiáridas, que tem na pecuária sua principal atividade econômica. / Drought connects meteorological phenomena and society in such a way that their occurrence, almost always, implies negative social and economic consequences. It is a natural phenomenon common in the Brazilian northeastern Semiarid. Its management should focus on proactive measures taken before and during the occurrence of droughts. Water infrastructure constructions may help in this management as they contribute to decrease vulnerability and consequently reduce drought impacts. Monitoring is another fundamental tool in this process because it can integrate warning systems and guide the right time to perform preparedness and mitigation actions. Different drought index are used in monitoring, and should look not only the meteorological phenomena, but also the significant economic activities for the region. Based on the concept of risk management, this work sought to identify the relationships between risk management and the reduction of vulnerabilities to drought, considering that infrastructure works and drought indexes related to regional economic activities are important keys to this process. As a result, we can mention that water infrastructure constructions contributed to the increase in the water supply in the region, which was reflected in the low vulnerability to drought in the municipalities most served by the program. And that the drought index VCI, VHI and SPEI have potential for monitoring and management drought in the semiarid region, which has livestock as its main economic activity.
9

Using False Rings to Reconstruct Local Drought Severity Patterns on a Semiarid River

Morino, Kiyomi January 2008 (has links)
In this research, I describe the use of false rings to reconstruct local histories of seasonal drought in riparian ecosystems in semiarid regions. In tree-ring analysis, false rings are boundary-like features often formed as a response to drought within the growing season. Drought can be a common feature in hydrologic regimes of dryland rivers but in recent decades drought has been intensifying due to climate change and increasing water use by cities, agriculture and industry. Identifying when and where water availability has decreased along the river course is critical for understanding, and therefore managing, these generally endangered ecosystems. The higher density of trees compared to instrumental data make them ideal candidates for reconstructing site-specific drought patterns.The first part of this dissertation is an observational study conducted on the San Pedro River in southeastern Arizona during 2002. I used dendrometer data and local hydrological data to show that a period of negligible radial growth in cottonwood during the middle of the growing season coincided with a channel drying event. Tree-ring core samples confirmed that false-rings had formed in each of the instrumented trees. The second part of this dissertation is an experimental study designed to evaluate the effect of different levels of water stress on false-ring formation in cottonwood and willow. I showed that experimental decreases in water availability for periods as short as ten days were enough to induce false-ring formation in willow. Longer periods of reduced water availability were generally required to induce false-ring formation in cottonwood. In the final part of this dissertation, I reconstructed false-ring occurrence in Fremont cottonwoods at three sites along the San Pedro River. I infer from false-ring frequencies that the severity of summer drought has been increasing over the last four to six decades but that the drought severity varies along a hydrological gradient. Overall, the findings in this body of research confirm that false rings in riparian tree species can be used as indicators of seasonal drought and underscore the importance of identifying site-specific responses to reduced water availability along the riparian corridor.
10

Sources and Dynamics of Carbon Dioxide Exchange and Evapotranspiration in Semiarid Environments

Yepez-Gonzalez, Enrico Arturo January 2006 (has links)
Precipitation, more than any other environmental factor, controls patterns of ecosystem production and biogeochemical cycling in arid and semiarid environments. Growing-season rains in these regions are highly unpredictable as they come in intermittent pulses varying in size, frequency and spatial extent, thereby producing unique hydrological patterns that constrain the location and residence time of soil water available for biological activity. In order to understand how arid and semiarid ecosystems respond to inputs of precipitation within the context of ecosystem science and global change studies, knowledge is needed on how plants and other organisms respond as an integrated system to such environmental control. The focus of my research was to understand how the distribution of precipitation events influences the dynamics of carbon cycling in semiarid ecosystems. At a semiarid riparian woodland, measurements of CO2 exchange and evapotranspiration revealed that following precipitation events occurring soon after prolonged dry periods the efficiency of rain-use (amount of carbon gain per unit of precipitation over a specific period time) was low. Precipitation did not readily stimulate primary productivity, water was mainly lost as soil evaporation and large respiratory CO2 effluxes were observed. This commonly observed features in seasonally dry ecosystems might have profound consequences for the seasonal and annual carbon balance. In this woodland, 47% of the precipitation within a single growing season (May-October) was returned to atmosphere as soil evaporation and the CO2 efflux observed just during the first rainy month (July) was equivalent to almost 50% of the net carbon gain observed over the six-month growing season. Results from experimental irrigations in understory plots of riparian mesquite woodland revealed that the magnitude and duration of the large CO2 fluxes occurring soon after rainfall was higher in plots located under tree canopies where, relative to intercanopy plots, the amount of plant litter was higher, soil evaporation and plant photosynthetic rates were lower. Efficiency of rain-use in semiarid ecosystems during the growing season apparently was determined by the degree of coupling between gross photosynthesis and ecosystem respiration, by the fraction of precipitation lost as soil evaporation and by the water-use efficiency of the component vegetation.

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