Ysgol Gwyddorau Amgylcheddol a Naturiol

  1. Cyhoeddwyd

    Introducing a common taxonomy to support learning from failure in conservation

    Dickson, I., Butchart, S. H. M., Catalano, A., Gibbons, D., Jones, J. P. G., Lee-Brooks, K., Oldfield, T., Noble, D., Paterson, S., Roy, S., Semelin, J., Tinsley-Marshall, P., Trevelyan, R., Wauchope, H., Wicander, S. & Sutherland, W. J., Chwef 2023, Yn: Conservation Biology. 37, 1, e13967.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  2. Cyhoeddwyd

    What agricultural practices are most likely to deliver "sustainable intensification" in the UK?

    Dicks, L., Rose, D., Ang, F., Birch, N., Boatman, N., Bowles, E., Chadwick, D., Dinsdale, A., Durham, S., Elliott, J., Firbank, L., Humphreys, S., Jarvis, P., Jones, D., Kindred, D., Knight, S., Lee, M., Leifert, C., Lobley, M., Matthews, K., Midmer, A., Moore, M., Morris, C., Mortimer, S., Murray, T., Norman, K., Ramsden, S., Roberts, D., Smith, L., Soffe, R., Stoate, C., Taylor, B., Tinker, D., Topliff, M., Wallace, J., Williams, P., Wilson, P., Winter, M. & Sutherland, W., Chwef 2019, Yn: Food and Energy Security. 8, 1, e00148.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  3. Cyhoeddwyd

    The effects of winter waterlogging and summer drought on the growth and yield of winter wheat (Triticum aestivum L.)

    Dickin, E. & Wright, D., 14 Medi 2007, Yn: European Journal of Agronomy. 28, 3, t. 234-244

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  4. Cyhoeddwyd

    Effect of genotype, environment and agronomic management on β-glucan concentration of naked barley grain intended for health food use.

    Dickin, E., Steele, K., Frost, G., Edwards-Jones, G. & Wright, D., 21 Maw 2011, Yn: Journal of Cereal Science.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  5. Cyhoeddwyd

    Agronomic diversity of naked barley (Hordeum vulgare L.): a potential resource for breeding new food barley for Europe

    Dickin, E., Steele, K., Edwards-Jones, G. & Wright, D., 1 Maw 2012, Yn: Euphytica. 184, 1, t. 85-99

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  6. Cyhoeddwyd

    Growth and yield responses of UK winter wheat cultivars to winter waterlogging.

    Dickin, E., Bennett, S. & Wright, D., 1 Ebr 2009, Yn: Journal of Agricultural Science. 147, 2, t. 127-140

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  7. Cyhoeddwyd

    Cross-laboratory comparison of fluorimetric microplate and colorimetric bench-scale soil enzyme assays

    Dick, R., Dick, L., Deng, S., Li, X., Kandeler, E., Poll, C., Freeman, C., Jones, T., Weintraube, M. N. & Saxena, J., 1 Meh 2018, Yn: Soil Biology and Biochemistry. 121, t. 240-248

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  8. Cyhoeddwyd

    Dipolar stabilization of emissive singlet charge transfer excited states in polyfluorene copolymers.

    Dias, F. B., King, S., Monkman, A. P., Perepichka, I. I., Kryuchkov, M. A., Perepichka, I. F. & Bryce, M. R., 29 Mai 2008, Yn: Journal of Physical Chemistry B. 112, 21, t. 6557-6566

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  9. Cyhoeddwyd

    Microbial communities in a porphyry copper tailings impoundment and their impact on the geochemical dynamics of the mine waste

    Diaby, N., Dold, B., Pfeifer, H. R., Holliger, C., Johnson, D. B. & Hallberg, K. B., 1 Chwef 2007, Yn: Environmental Microbiology. 9, 2, t. 298-307

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  10. Cyhoeddwyd

    Taxonomy, biology and natural history of the Milos viper Macrovipera schweizeri (Werner, 1935): literature review and observations on autumnal activity and importance of residual pools

    Di Nicola, M. R., Pozzi, A., Avella, I., Mezzadri, S. & Paolino, G., 1 Tach 2022, Yn: Spixiana. 45, 1, t. 111-130

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  11. Cyhoeddwyd

    The Endangered Sardinian Grass Snake: Distribution Update, Bioclimatic Niche Modelling, Dorsal Pattern Characterisation, and Literature Review

    Di Nicola, M. R., Pozzi, A., Mezzadri, S., Faraone, F. P., Russo, G., C. M. Dorne, J. L. & Minuti, G., 4 Medi 2023, Yn: Life.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  12. Cyhoeddwyd

    Rational truncation of aptamer for cross-species application to detect krait envenomation

    Dhiman, A., Anand, A., Malhotra, A., Khan, E., Santra, V., Kumar, A. & Sharma, T. K., 12 Rhag 2018, Yn: Scientific Reports. 8, 17795.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  13. Cyhoeddwyd

    Mapping to explore the challenges and opportunities for reconciling artisanal gem mining and biodiversity conservation

    Devenish, K., Goodenough, K., Jones, J. P. G., Rakoto Ratsimba, H. & Willcock, S., Medi 2023, Yn: Extractive Industries and Society. 15, 28 t., 101311.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  14. Cyhoeddwyd

    On track to achieve No Net Loss of forest at Madagascar’s biggest mine

    Devenish, K., Desbureaux, S., Willcock, S. & Jones, J. P. G., Meh 2022, Yn: Nature Sustainability . 5, 6, t. 498-508 11 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  15. Cyhoeddwyd

    One hundred priority questions for the development of sustainable food systems in sub-Saharan Africa

    Devenish, A. J., Schmitter, P., Jellason, N. P., Esmail, N., Abdi, N. M., Barron, J., Chapman, A. S. A., Chausson, A. M., Chibesa, M., Reading, U., Dugan, E., Edwards, G. I., Egeru, A., Gebrehiwot, T., Griffiths, G. H., Haile, A., Hunga, H. G., Igbine, L., Jarju, O. M., Keya, F., Khalifa, M., Ledoux, W. A., Lejissa, L. T., Loupa, P., Lwanga, J., Mapedza, E. D., Marchant, R., McLoud, T., Mukuyu, P., Musah, L. M., Mwanza, M., Mwitwa, J., Neina, D., London, U., Njogo, S., Robinson, E. J. Z., Singini, W., Umar, B. B., Wesonga, F., Willcock, S., Yang, J. & Tobias, J. A., 7 Hyd 2023, Yn: Land. 39 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  16. Cyhoeddwyd

    The role of fragmented sacred forests in biodiversity conservation in the Gamo Highlands of Ethiopia

    Desissa, D. & Healey, J., 1 Rhag 2015.

    Allbwn ymchwil: Cyfraniad at gynhadleddPapuradolygiad gan gymheiriaid

  17. Cyhoeddwyd

    Sacred groves in the Gamo Highlands of southern Ethiopia: biocultural values and threats

    Desissa, D. & Healey, J. R., 1 Meh 2012, Yn: Langscape. 2, 11, t. 44-47

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  18. Cyhoeddwyd

    Screening a genome-wide S.pombe deletion library identifies novel genes and pathways involved in genome stability maintenance.

    Deshpande, G. P., Hayles, J., Hoe, K. L., Kim, D. U., Park, H. O. & Hartsuiker, E., 1 Mai 2009, Yn: DNA Repair. 8, 5, t. 672-679

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  19. Cyhoeddwyd

    Integrating climate change adaptation into biodiversity and forestry assessments and programming: African and Latin American Resilience to Climate Change (ARCC)

    Deshmukh, I., Byers, B. & Cameron, A., Maw 2015, US Agency for International Development.

    Allbwn ymchwil: Llyfr/AdroddiadAdroddiad Comisiwn

  20. Cyhoeddwyd

    Coexisting cryptic species of the Litoditis marina complex (Nematoda) show differential resource use and have distinct microbiomes with high intraspecific variability

    Derycke, S., De Meester, N., Rigaux, A., Creer, S., Bik, H. M., Thomas, W. K. & Moens, T., Mai 2016, Yn: Molecular Ecology. 25, 9, t. 2093-2110

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  21. Cyhoeddwyd

    Contrasting patterns of leaf trait variation among and within species during tropical dry forest succession in Costa Rica

    Derroire, G., Powers, J., Hulshof, C., Cardenas Varela, L. & Healey, J., 2018, Yn: Scientific Reports. 8, 285.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  22. Cyhoeddwyd

    Isolated trees as nuclei of regeneration in tropical pastures: testing the importance of niche-based and landscape factors

    Derroire, G., Coe, R. & Healey, J. R., 27 Gorff 2016, Yn: Journal of Vegetation Science. 27, 4, t. 679-691

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  23. Cyhoeddwyd

    Resilience of tropical dry forests – a meta-analysis of changes in species diversity and composition during secondary succession

    Derroire, G., Balvanera, P., Castellanos-Castro, C., Decocq, G., Kennard, D. K., Lebrija-Trejos, E., Leiva, J. A., Oden, P., Powers, J. S., Rico-Gray, V., Tigabu, M. & Healey, J. R., Hyd 2016, Yn: Oikos. 125, 10, t. 1386-1397

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  24. Cyhoeddwyd

    The Effects of Established Trees on Woody Regeneration during Secondary Succession in Tropical Dry Forests

    Derroire, G., Tigabu, M., Oden, P. C. & Healey, J. R., 1 Mai 2016, Yn: Biotropica. 48, 3, t. 290-300

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  25. Cyhoeddwyd

    Biological versus abiotic oxidation of iron in acid mine drainage waters: an important role for moderately acidophilic, iron-oxidising bacteria.

    Dennison, F., Johnson, D. B., Sen, A. M., Hallberg, K. B., Ciminelli, V. S. (gol.) & Garcia Jr, O. (gol.), 1 Ion 2001, Biohydrometallurgy: Fundamentals: Technology and Sustainable Development. 2001 gol. Elsevier, t. 493-501

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddPennod

  26. Cyhoeddwyd

    Intramolecular guanidine epoxide ring opening reactions.

    Dennis, M., Hall, L. M., Murphy, P. J., Thornhill, A. J., Nash, R., Winters, A. L., Hursthouse, M. B., Light, M. E. & Horton, P., 7 Ebr 2003, Yn: Tetrahedron Letters. 44, 15, t. 3075-3080

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  27. Cyhoeddwyd

    Identification of Armillaria species on declined oak in Britain: implications for oak health

    Denman, S., Barrett, G., Kirk, S. A., McDonald, J. & Coetzee, M. P. A., 7 Ion 2017, Yn: Forestry. 90, 1, t. 148-161

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  28. Cyhoeddwyd

    Microbiome and infectivity studies reveal complex polyspecies tree disease in Acute Oak Decline

    Denman, S., Doonan, J., Ransom-Jones, E., Broberg, M., Plummer, S., Kirk, S. A., Scarlett, K., Griffiths, A., Kaczmarek, M., Foster, J., Peace, A., Golyshin, P., Hassard, F., Brown, N., Kenny, J. G. & McDonald, J., 2018, Yn: The ISME Journal. 12, t. 386-399

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  29. Cyhoeddwyd

    Isolation studies reveal a shift in the cultivable microbiome of oak affected with Acute Oak Decline

    Denman, S., Plummer, S., Peace, A. & McDonald, J., 1 Hyd 2016, Yn: Systematic and Applied Microbiology. 39, 7, t. 484-490

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  30. Cyhoeddwyd

    Comparison and Standardization of Soil Enzyme Assay for Meaningful Data Interpretation

    Deng, S., Dick, R., Freeman, C., Kandeler, E. & Weintraub, M., Chwef 2017, Yn: Journal of Microbiological Methods. 133, February, t. 32-34

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  31. Cyhoeddwyd

    Alcanivorax borkumensis produces an extracellular siderophore in iron-limitation condition maintaining the hydrocarbon-degradation efficiency

    Denaro, R., Crisafi, F., Russo, D., Genovese, M., Messina, E., Genovese, L., Carbone, M., Ciavatta, M. L., Ferrer, M., Golyshin, P. & Yakimov, M. M., 1 Hyd 2014, Yn: Marine Genomics. 17, October, t. 43-52

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  32. Cyhoeddwyd

    Clay and biochar amendments decreased inorganic but not dissolved organic nitrogen leaching in soil

    Dempster, D. N., Jones, D. L. & Murphy, D. V., 25 Mai 2012, Yn: Soil & Tillage Research. 50, 3, t. 216-221

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  33. Cyhoeddwyd

    Organic nitrogen mineralisation in two contrasting agro-ecosystems is unchanged by biochar addition

    Dempster, D. N., Jones, D. L. & Murphy, D. V., Mai 2012, Yn: Soil Biology and Biochemistry. 48, t. 47-50

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  34. Cyhoeddwyd

    Decreased soil microbial biomass and nitrogen mineralisation with Eucalyptus biochar addition to a coarse textured soil

    Dempster, D. N., Gleeson, D. B., Solaiman, Z. M. & Jones, D. L., Mai 2012, Yn: Plant and Soil. 354, 1-2, t. 311-324

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  35. Cyhoeddwyd

    The role of wind in controlling the connectivity of blue mussels (Mytilus edulis L.) populations

    Demmer, J., Robins, P., Malham, S., Lewis, M., Owen, A., Jones, T. & Neill, S., 21 Ion 2022, Yn: Movement Ecology. 10, 1, 3.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  36. Cyhoeddwyd

    Dve formy vnekletochnoĭ nizkomolekuliarnoĭ ribonukleazy Bacillus sp. BCF 247. Vydelenie i kharakteristika belka

    Dement'ev, A. A., Golyshin, P. N., Riabchenko, N. F., Pustobaev, V. N. & Shliapnikov, S. V., Awst 1993, Yn: Biokhimiia (Moscow, Russia). 58, 8, t. 1258-65 8 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  37. Cyhoeddwyd

    Vnekletochnaia shchelochnaia ribonukleaza Bacillus thuringiensis var. subtoxicus

    Dement'ev, A. A., Riabchenko, N. F., Protasevich, I. I., Golyshin, P. N., Stepanov, A. I., Orlov, V. M., Pustovaev, V. N., Makarov, A. A., Moiseev, G. P. & Karpeĭskiĭ, M. I., 1 Tach 1992, Yn: Molekuliarnaia biologiia. 26, 6, t. 1338-49 12 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  38. Cyhoeddwyd

    Stream metabolism and the open diel oxygen method: Principles, practice, and perspectives

    Demars, B. O., Thompson, J. & Manson, J. R., Gorff 2015, Yn: Limnology and Oceanography: Methods. 13, 7, t. 356-374

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  39. Cyhoeddwyd

    A Decision Support System for Drinking Water Production Integrating Health Risks Assessment

    Delpla, I., Monteith, D. T., Freeman, C., Haftka, J., Hermens, J., Jones, T. G., Baures, E., Jung, A. & Thomas, O., 18 Gorff 2014, Yn: International Journal of Environmental Research and Public Health. 11, 7, t. 7354-7375

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  40. Cyhoeddwyd

    Heavy Rainfall Impacts on Trihalomethane Formation in Contrasting Northwestern European Potable Waters

    Delpla, I., Jones, T. G., Monteith, D. T., Hughes, D. D., Baures, E., Jung, A. V., Thomas, O. & Freeman, C., 10 Gorff 2015, Yn: Journal of Environmental Quality. 44, 4, t. 1241-1251

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  41. Cyhoeddwyd

    Degradation of Hydrocarbons and Heavy Metal Reduction by Marine Bacteria in Highly Contaminated Sediments

    Dell'Anno, F., Brunet, C., van Zyl, L. J., Trindade, M., Golyshin, P., Dell'Anno, A., Ianora, A. & Sansone, C., 11 Medi 2020, Yn: Microorganisms . 8, 9

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  42. Cyhoeddwyd

    Microbiome enrichment from contaminated marine sediments unveils novel bacterial strains for petroleum hydrocarbon and heavy metal bioremediation

    Dell'Anno, F., van Zyl, L. J., Trindade, M., Buschi, E., Cannavacciulo, A., Pepi, M., Sansone, C., Brunet, C., Ianora, A., de Pascale, D., Golyshin, P., Dell'Anno, A. & Rastelli, E., 15 Ion 2023, Yn: Environmental Pollution. 317, 120772.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  43. Cyhoeddwyd

    The effects of logging on forest stand structure and tree communities in the area of a new nacional wildlife refuge, Costa Rica.

    Delgado, D., Finegan, B., Gormley, L., Healey, J. R., Solano, D. & Zamora, N., 1 Ion 2005.

    Allbwn ymchwil: Cyfraniad at gynhadleddPapur

  44. Cyhoeddwyd

    Ant-mediated ecosystem functions on a warmer planet: effects on soil movement, decomposition and nutrient cycling

    Del Toro, I., Ribbons, R. R. & Ellison, A. M., 23 Ebr 2015, Yn: Journal of Animal Ecology. 84, 5, t. 1233-1241

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  45. Cyhoeddwyd

    Microbial β-glucosidases from cow rumen metagenome enhance the saccharification of lignocellulose in combination with commercial cellulase cocktail

    Del Pozo, M. V., Fernández-Arrojo, L., Gil-Martínez, J., Montesinos, A., Chernikova, T. N., Nechitaylo, T. Y., Waliszek, A., Tortajada, M., Rojas, A., Huws, S. A., Golyshina, O., Newbold, C. M., Polaina, J., Ferrer, M. & Golyshin, P., 12 Medi 2012, Yn: Biotechnology for Biofuels. 5, t. 73

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  46. Cyhoeddwyd

    Multilevel comparison of Indian Naja venoms and their cross-reactivity with Indian polyvalent antivenoms

    Deka, A., Bhatia, S., Santra, V., Bharti, O., Lalremsanga, H., Martin, G., Wüster, W., Owens, J. B., Graham, S., Doley, R. & Malhotra, A., 1 Ebr 2023, Yn: Toxins. 15, 4, 24 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  47. Cyhoeddwyd

    Phylogeography of Biomphalaria glabrata and B-pfeifferi, important intermediate hosts of Schistosoma mansoni in the New and Old World tropics.

    Dejong, R. J., Morgan, J. A., Wilson, W. D., Al-Jaser, M. H., Appleton, C. C., Coulibaly, G., D'Andrea, P. S., Doenhoff, M. J., Haas, W., Idris, M. A., Magalhaes, L. A., Mone, H., Mouahid, G., Mubila, L., Pointier, J. P., Webster, J. P., Zanotti-Magalhaes, E. M., Paraense, W. L., Mkoji, G. M. & Loker, E. S., 1 Tach 2003, Yn: Molecular Ecology. 12, 11, t. 3041-3056

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  48. Cyhoeddwyd

    Optimising the detection of marine taxonomic richness using environmental DNA metabarcoding: the effects of filter material, pore size and extraction method

    Deiner, K., Lopez, J., Bourne, S., Holman, L. E., Seymour, M., Grey, E. K., Lacoursiere-Roussel, A., Li, Y., Renshaw, M. A., Pfrender, M. E., Rius, M., Bernatchez, L. & Lodge, D. M., 2 Tach 2018, Yn: Metabarcoding and Metagenomics. 2, 15 t., e28963.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  49. Cyhoeddwyd

    Environmental DNA metabarcoding: transforming how we survey animal and plant communities

    Deiner, K., Bik, H. M., Machler, E., Seymour, M., Lacoursiere-Roussel, A., Altermatt, F., Creer, S., Bista, I. & Lodge, D. M., Tach 2017, Yn: Molecular Ecology Resources. 26, 21, t. 5872-5895

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  50. Cyhoeddwyd

    Medically important snakes and snakebite envenoming in Iran

    Dehghani, R., Monzavi, S. M., Mehrpour, O., Shirazi, F. M., Hassanian-Moghaddam, H., Keyler, D. E., Wüster, W., Westerström, A. & Warrell, D. A., 23 Mai 2023, Yn: Toxicon. 230, 26 t., 107149.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid