• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 207
  • 160
  • 35
  • 31
  • 5
  • 4
  • 4
  • 4
  • 4
  • 4
  • 1
  • 1
  • 1
  • 1
  • 1
  • Tagged with
  • 519
  • 192
  • 84
  • 73
  • 72
  • 65
  • 58
  • 50
  • 47
  • 47
  • 41
  • 40
  • 38
  • 36
  • 35
  • 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.

Phenotypic differences between Trientalis Borealis Raf. plants from adjoining pine and oak communities in Concord, Massachusetts

Bennett, Jane C. 01 January 1983 (has links)
No description available.

Signatures of natural selection and local adaptation in Populus trichocarpa and Populus deltoides along latitudinal clines

Bawa, Rajesh K. 18 February 2013 (has links)
Trees, like many other organisms, decrease their rate of metabolic activities to cope up with harsh environments. This stage of â dormancyâ is marked by shedding of leaves and bud-set in deciduous trees. Recent studies have revealed the role of the circadian clock in synchronizing the timing of dormancy and physiology for conferring fitness in trees. To better understand the possible role of natural selection on circadian clock-related genes in climatic adaptation, I took a candidate gene approach, selecting circadian clock genes, some of which had been functionally validated, and others hypothesized, to identify signatures of natural selection in Populus trichocarpa and P. deltoides. Using both frequency spectrum based tests and tests of heterogeneity, I identified genetic variants deviating from selective neutrality. Results reveal that photoreceptors and dormancy regulator genes may have been the targets of natural selection. Nearly the same levels of selective constraints were found in different functional groups of genes irrespective of pleiotropy. Further, upstream regions of all genes showed high selective constraint, with some of them (FT-2, PIF-4, FRIGIDA) showing significantly higher variation than the other genes, hinting at the role of non-coding regulatory regions in local adaption. In some cases, the same genes in both species appeared as outliers, including PIF-6, FRI, FT-2, SRR1, TIC, and CO, which might reflect their common role in adaptation across species boundaries. All of these results indicate a complex nature of phenology regulation and local adaptation in Populus species with photoreceptors and dormancy regulator genes playing key roles. / Master of Science

Role fenologie v diferenciaci rostlinných nik: kvantitativní a komparativní pohled za využití rozsáhlého souboru druhů / Role of phenology in differentiation of plant niches: quantitative and comparative approach using a large set of species

Huang, Lin January 2018 (has links)
Niche differentiation is the most recognized species coexistence mechanism, of which, the temporal differentiation of species, i.e. phenological differentiation has gained an increasing interest of ecologists. However, as an important dimension of phenology, the phenology of growth has drawn relatively few attentions due to the lack of sufficient phenological data. Therefore, the aim of this thesis is to examine potential role of temporal differentiation of plant growth patterns in plant coexistence. I first collected data on growth phenology of 381 perennial herbs in the Botanical Garden of Charles University in Prague within one growing season (in 2015). Using this database, I addressed the general patterns of growth phenology among perennial herbs. I distinguished two contrasting growth patterns: (1) species with early peak growth had high standardized growth rates in contrast to late species, reflecting the survival under forest canopy, where species are selected to grow early and fast before the development of tree canopy which restricts their size; (2) tall species showed later peak growth than short species, associating with (asymmetric) competition for light in open habitats, where the main selection factor is for tall stature, which cannot be attained early in the season. Then, by linking...

Modeling studies of small mammal trapping phenology, and plant succession in the Kaiparowits region, Kane County, Utah

Raines, James A. 01 April 1976 (has links)
This study makes use of simple statistical models which may be manipulated for projection of changes in the Kaiparowits Region of Southern Utah. A method of determining habitat preferences of small rodents from capture-recapture trapping data is proposed. It is suggested that some aspects of animal behavior regarding trapping may also be explained. Timing of flowering of Oryzopsis hymenoides and Hilaria jamesii as related to enviromental conditions was investigated, and models of the requisite conditions proposed. It is suggested that similar models may be used to describe the timing of other phenological events. Succession on disturbed sites is considered to determine predictablility of revegetation. It was determined that early stages of succession of three habitat-types are proceeding in a manner similar to that described on other areas of the pinyon-juniper forest.

Climate warming and interannual variability of phytoplankton phenology in the Northern Red Sea

Gittings, John 12 1900 (has links)
In agreement with global patterns of climate change and increasing temperatures in the tropical oceans, the Northern Red Sea (NRS) has been warming over the last few decades. Using 18 years of remotely-sensed chlorophyll-a data (Chl-a, an index of phytoplankton biomass), we investigate the potential impacts of climate warming on phytoplankton abundance and phenology in the Northern Red Sea by exploring the mechanistic links with the regional physical environment. The results of the analysis reveal that, in accordance with other tropical ecosystems, phytoplankton biomass in the NRS will decrease in response to warmer climate scenarios. This is attributed to lower heat fluxes (heat loss to the atmosphere) during the bloom period, and enhanced vertical stratification, which prevents vertical mixing of nutrients into the euphotic layer. In addition, we show that during warmer conditions (when heat fluxes are weakened), the winter bloom initiates significantly later (by up to 10 weeks) and its duration is considerably reduced. The biological implications of alterations to phytoplankton phenology may include increased larval mortality of pelagic species, reduced recruitment, fisheries impacts and changes to community structure.

Impacts of Synoptic Weather on the Ice Phenology of Maine Lakes, 1955-2005

Greene, Timothy Robert 05 June 2018 (has links)
The cryosphere has been shown to be particularly adept as a proxy for climate change by various studies. Accordingly, historical records from the field of ice phenology have been harvested by climate scientists for the express purpose of studying the temporal variation of ice phenomena, namely freeze-up and ice-out. Ice-out records from 20 lakes in Maine, U.S.A. were collected and clustered by z-score for this thesis. Rather than attempt to relate ice-out to spring air temperature or global teleconnections/oscillations, the Spatial Synoptic Classification (SSC) method was used to encapsulate several meteorological variables that could have a bearing on ice-out variation. The balance between occurrence of relatively cool Moist Polar (MP) and relatively warm Dry Moderate (DM) weather-types during the winter-spring "superseason" was found to be a synoptic barometer of whether ice-out would occur seasonably early or late. The significance of this is predicated upon the finding that quantity of DM days has steadily risen at the expense of MP days during the latter-half of the twentieth-century, in accordance with observed climatic warming during the same period. The remaining SSC weather-types, most notably omnipresent Dry Polar (DP), remained generally stable during the historical record in Maine, further undergirding the significance of the DM-MP relationship. / Master of Science

Observing and modeling climate controls and feedbacks on vegetation phenology at local-to-continental scales

Moon, Minkyu 13 October 2020 (has links)
Vegetation phenology controls seasonal variation in ecosystem processes and exerts important controls on land-atmosphere exchanges of carbon, water, and energy. However, the ecological processes and interactions between climate and vegetation that control phenology and associated feedbacks to the atmosphere are not fully understood. In this dissertation, I use remote sensing in combination with climate and ecological data to improve understanding of biophysical controls and feedbacks between vegetation phenology and the atmosphere in temperate forest ecosystems of North America. In the first part of this dissertation, I evaluate the agreement and characterize the similarities and differences between land surface phenology products from two remote sensing instruments (MODIS and VIIRS) that are designed to provide long-term continuity of land surface phenology measurements at global scale. Results from this analysis indicate that the VIIRS land surface phenology product provides excellent continuity with the MODIS record despite subtle differences between each instrument and the algorithms used to generate each product. In the second part of this dissertation, a state-space Bayesian modeling framework is applied to seventeen years of MODIS and daily weather data to improve understanding of what controls the timing of springtime phenology in deciduous forests of temperate and boreal North America. Results show that photoperiod is more important in warmer regions than in colder regions, which contradicts a widely held hypothesis that photoperiod provides a key safety mechanism preventing early leaf-out during springtime. In the final part of this dissertation, I use a physically-based attribution method to quantify the relative importance of covarying surface biophysical and atmospheric variables in modifying the surface energy balance during springtime. Results show that the widely observed decrease in the Bowen ratio that occurs with leaf emergence is not solely attributable to changes in surface resistance caused by increasing leaf area during spring. Rather, observed changes in the Bowen ratio reflect the combined effects of changes in surface properties and atmospheric conditions. The results from this dissertation provide an improved foundation for long-term studies focused on observing and modeling springtime vegetation phenology and associated feedbacks to the atmosphere in deciduous forest ecosystems at local-to-continental scales.

The Ecology and Evolution of Pollinator-mediated Interactions Among Spring Flowering Plants

Hensel, Lisa E 06 September 2011 (has links)
Pollinator sharing in mixed species communities is expected to significantly contribute to mating patterns in contemporary populations but may also affect the evolutionary trajectory of traits associated with plant mating. In this thesis, I considered how the spring environment and pollinator sharing may contribute to the widespread convergence in traits among spring flowering species using comparative biology. The proposed correlation between a spring flowering phenology and white or light floral colour, fleshy fruits, woody growth forms and understory occupation is confirmed. In addition, I examined the effects of pollinator responses to community and population traits to determine the relative importance of inter- and intraspecific interactions in pollinator mediated reproductive success of a spring flowering species, Trillium grandiflorum. In this study, the reproductive success of T. grandiflorum was pollen limited. However, the magnitude of pollen limitation was influenced only by intraspecific density and varied independently of community diversity. The results of this thesis contribute significantly to our understanding of pollinator-mediated interactions in spring flowering communities but also highlight future avenues of investigation.

The control of Stoebe vulgaris encroachment in the Hartbeesfontein area of the North West Province / J.P. Wepener

Wepener, Jean-Pierre January 2007 (has links)
Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2008.

The control of Stoebe vulgaris encroachment in the Hartbeesfontein area of the North West Province / J.P. Wepener

Wepener, Jean-Pierre January 2007 (has links)
Thesis (M. Environmental Science)--North-West University, Potchefstroom Campus, 2008.

Page generated in 0.059 seconds