Low energy electron beam induced vacancy activation in GaN

Experimental evidence on low energy electron beam induced point defect activation in GaN grown by metal-organic vapor phase epitaxy(MOVPE) is presented. The GaN samples are irradiated with a 5–20 keV electron beam of a scanning electron microscope and investigated by photoluminescence and positron annihilation spectroscopy measurements. The degradation of the band-to-band luminescence of the irradiatedGaN films is associated with the activation of point defects. The activated defects were identified as in-grown Ga-vacancies. We propose that MOVPE-GaN contains a significant concentration of passive VGa-H n complexes that can be activated by H removal during low energy electron irradiation.Peer reviewe

Stabilization of Ge-rich defect complexes originating from E centers in Si1- xGex:P

Thermal evolution of vacancy complexes was studied in P-doped ([P]=10 exp 18 cm exp −3) proton irradiated Si1−xGex with Ge contents of 10%, 20%, and 30% in the range of 250–350 °C using positron annihilation spectroscopy. The radiation damage recovers in the course of anneals but the final state differs from that in as-grown samples indicating the presence of small Ge clusters in the samples, contrary to the initially random Ge distribution. The activation energy for the annealing process was estimated to be 1.4±0.3 eV and attributed to the dissociation energy of the vacancy-phosphorus-germanium (V-P-Ge) complex.Peer reviewe

A free-access online key to identify Amazonian ferns

There is urgent need for more data on species distributions in order to improve conservation planning. A crucial but challenging aspect of producing high-quality data is the correct identification of organisms. Traditional printed floras and dichotomous keys are difficult to use for someone not familiar with the technical jargon. In poorly known areas, such as Amazonia, they also become quickly outdated as new species are described or ranges extended. Recently, online tools have allowed developing dynamic, interactive, and accessible keys that make species identification possible for a broader public. In order to facilitate identifying plants collected in field inventories, we developed an internet-based free-access tool to identify Amazonian fern species. We focused on ferns, because they are easy to collect and their edaphic affinities are relatively well known, so they can be used as an indicator group for habitat mapping. Our key includes 302 terrestrial and aquatic entities mainly from lowland Amazonian forests. It is a free-access key, so the user can freely choose which morphological features to use and in which order to assess them. All taxa are richly illustrated, so specimens can be identified by a combination of character choices, visual comparison, and written descriptions

Newly discovered diversity in the tropical fern genus Metaxya based on morphology and molecular phylogenetic analyses

For a long time, the genus Metaxya was treated as monotypic with a single species, M. rostrata. A second species, M. lanosa, was described in 2001 on the basis of morphological features and rbcL gene sequences with a suggestion that the genus may contain even more species. We have now systematically compared morphological traits in a large number of Metaxya specimens collected in tropical America. We have also carried out phylogenetic analyses of 32 Metaxya and 5 outgroup specimens based on four markers of the plastid genome (rbcL, matK, and rps4 genes, and trnG-trnR intergenic spacer). Based on the morphological variation among the Metaxya specimens, we accept six distinct species, three of which we describe as new in this paper. Molecular phylogenetic analyses resolved five distinct clades. Four of these corresponded with the morphologically delineated species (M. contamanensis sp. nov., M. elongata sp. nov., M. lanosa, and M. parkeri). The fifth clade contained all the individuals of the remaining two species, of which M. rostrata was rendered paraphyletic by M. scalaris sp. nov. being nested within it. Since M. scalaris was resolved as monophyletic, and the two are clearly distinguishable morphologically, we consider them true species despite the incomplete genetic differentiation.</p

Prodromus of a fern flora for Bolivia. VIII. Marattiaceae

The Marattiaceae is a phylogenetically isolated family of eusporangiate ferns. Here we provide a synopsis to the family in Bolivia, where it is represented by five species in the genus Danaea and a single species of Eupodium

Mechanisms of electrical isolation in O+-irradiated ZnO

We have applied positron annihilation spectroscopy combined with sheet resistance measurements to study the electrical isolation of thin ZnO layers irradiated with 2 MeV O+ ions at various fluences. Our results indicate that Zn vacancies, the dominant defects detected by positrons, are produced in the irradiation at a relatively low rate of about 2000 cm exp −1 when the ion fluence is at most 10 exp 15 cm exp −2 and that vacancy clusters are created at higher fluences. The Zn vacancies introduced in the irradiation act as dominant compensating centers and cause the electrical isolation, while the results suggest that the vacancy clusters are electrically inactive.Peer reviewe

Taxonomy and evolutionary history of the neotropical fern genus Salpichlaena (Blechnaceae)

Salpichlaena is a distinctive fern genus characterised by 2-pinnate climbing fronds with indeterminategrowth. The number of species in the genus has been a matter of debate. Taxonomic studies are made difficult by within-frond variability in pinna morphology and size, and by herbarium material being incomplete. We systematically documented 62 morphological traits in 283 herbarium specimens and sequenced 52 Salpichlaena and 11 outgroupspecimens. DNA sequences included plastid genes (rbcL, rpoC1 and rps4), intergenic spacers (rps4-trnS, trnH- psbA and trnG-trnR) and a nuclear gene (pgiC). Phylogenetic analyses based on the plastid markers divided the samples into six major clades. We recognise the three deepest clades as distinct species (S. hookeriana, S. papyrussp. nov. and S. volubilis), and each of the four shallower clades as a subspecies of S. volubilis. Furthermore, wesuggest that a group of specimens, placed into different clades in the plastid and nuclear trees and showing mixedmorphological characters, represent a fourth species of hybrid origin (S. hybrida sp. nov.). The most important di- agnostic characters are: degree of lamina reduction in fertile pinnules; pinna/pinnule apex incisions, pinna/pinnule margin thickness and lamina texture in sterile pinna/pinnules; presence or absence of foliar buds; shape of scales;and the appearance of the abaxial surface of the lamina (uniform or with stomata on small white protuberances).Each of the four species can be identified by several diagnostic characters, and their geographical ranges are broadand partly overlapping. In contrast, the subspecies are mostly allopatric and their morphological limits are diffuse.</div

Differences in topographic and soil habitat specialization between trees and two understorey plant groups in a Costa Rican lowland rain forest

Two core questions in plant community ecology are to what extent the distributions of species are structured by local environmental conditions, and whether taxa differ in this regard. We compared the distributions of trees, Melastomataceae and ferns on soil and topographic gradients in a Costa Rican lowland rain forest (trees and ferns 983 plots, Melastomataceae 277 plots). To test whether these plant groups differed in the prevalence or type of habitat specialization, we calculated species' environmental optima and tolerances on each gradient. Habitat specialization was defined as a significantly biased optimum, or a narrow tolerance, relative to values obtained under spatially restricted randomizations of species occurrences. Within plant groups, we also asked whether the dispersion of species optima differed from random expectation on each gradient. Fern optima were over-dispersed on multiple gradients, implying considerable interspecific habitat partitioning, and tree optima were over-dispersed in relation to topographic position. Habitat specialization was more prevalent in the two predominantly understorey groups than in trees (75% of Melastomataceae species, 81-87% of ferns, 57-58% of trees). Species optima of Melastomataceae and ferns also tended towards lower landscape positions than did those of trees, perhaps reflecting a higher proportion of drought-sensitive species in these two groups

Environmentally driven extinction and opportunistic origination explain fern diversification patterns.

Combining palaeontological and neontological data offers a unique opportunity to investigate the relative roles of biotic and abiotic controls of species diversification, and the importance of origination versus extinction in driving evolutionary dynamics. Ferns comprise a major terrestrial plant radiation with an extensive evolutionary history providing a wealth of modern and fossil data for modelling environmental drivers of diversification. Here we develop a novel Bayesian model to simultaneously estimate correlations between diversification dynamics and multiple environmental trajectories. We estimate the impact of different factors on fern diversification over the past 400 million years by analysing a comprehensive dataset of fossil occurrences and complement these findings by analysing a large molecular phylogeny. We show that origination and extinction rates are governed by fundamentally different processes: originations depend on within-group diversity but are largely unaffected by environmental changes, whereas extinctions are strongly affected by external factors such as climate and geology. Our results indicate that the prime driver of fern diversity dynamics is environmentally driven extinction, with origination being an opportunistic response to diminishing ecospace occupancy