Does ash dieback affect the reproductive ecology of Fraxinus excelsior L.?

Abstract

Forest tree species reproduction is a key factor in maintaining the genetic diversity of future generations and the stability of forest ecosystems. The ongoing ash dieback disease could affect the reproductive ecology of Fraxinus excelsior L. and have a major impact on the quantity and quality of pollen and seeds. In this study, we investigated pollen production and viability of pollen and seeds of ash trees with different health status from 2018 to 2022. Inflorescences were collected from 105 trees (pollen production), pollen from 125 trees (pollen viability), and seeds from 53 trees (seed quality) in two seed orchards and in one floodplain forest in southern Germany. Not all parameters were examined at every site every year. The average pollen production per tree was estimated at 471.2 ± 647.9 billion pollen grains. In addition, we found that a high number of inflorescences did not equate to high pollen production per inflorescence. Pollen production of healthy and diseased trees did not differ significantly, although only 47% of severely diseased male trees (vs. 72% for healthy trees) produced flowers. With regards to pollen viability, the TTC test showed an average viability of 73% ± 17%. Overall, there was a slight tendency for diseased trees to have less viable pollen. However, a significant difference could only be calculated for trees in the floodplain forest. The percentage of germinable seeds in 2018 was 38% in the floodplain forest and 57% in one of the seed orchards. The percentage of viable seeds (TTC test) ranged from 17 to 22% in the orchards in 2020. Non-viable seeds were usually heavily infested by insects. In general, seed quality was not significantly different between healthy and diseased trees. Our results indicate that ash dieback affects flower formation and pollen viability but not pollen production or seed quality. Nevertheless, the fact that hardly any flowering was observed, especially for trees that were seriously affected, suggests a negative effect of ash dieback on reproductive performance. Thus, severely diseased trees will transfer their genes to a smaller extent to the next generation.

Keywords

Common ash / Seed orchard / Floodplain forest / Pollen production / Viability / TTC test / Seed stratification / Phenology

References

[1]	Aber J, Neilson RP, Mcnulty S, Lenihan JM, Bachelet D, Drapek RJ (2001) Forest processes and global environmental change: predicting the effects of individual and multiple stressors. Bioscience 51:735–751. https://doi.org/10.1641/0006-3568(2001)051[0735:FPAGEC]2.0.CO;2
[2]	Antos JA, Allen GA (1999) Patterns of reproductive effort in male and female shrubs of Oemleria cerasiformis: a 6-year study. J Ecol 87:77–84. https://doi.org/10.1046/j.1365-2745.1999.00331.x
[3]	Bajc M, Aravanopoulos F, Westergren M, Fussi B, Kavaliauskas D, Alizoti P, Kiourtsis F, Kraigher H (2020) Manual for forest genetic monitoring. Silva Slovenica Publishing Centre, Slovenian Forestry Institute
[4]	Balla A, Silini A, Cherif-Silini H, Chenari Bouket A, Moser WK, Nowakowska JA, Oszako T, Benia F, Belbahri L (2021) The threat of pests and pathogens and the potential for biological control in forest ecosystems. Forests 12:1579. https://doi.org/10.3390/f12111579
[5]	Baral HO, Queloz V, Hosoya T (2014) Hymenoscyphus fraxineus, the correct scientific name for the fungus causing ash dieback in Europe. IMA Fungus 5:79–80. https://doi.org/10.5598/imafungus.2014.05.01.09
[6]	Bartsch N, R?hrig E (2016) Forest ecology: introduction for Central Europe (Wald?kologie: Einführung für Mitteleuropa). Springer Spektrum, Berlin ((in German))
[7]	Bohrerova Z, Bohrer G, Cho KD, Bolch MA, Linden KG (2009) Determining the viability response of pine pollen to atmospheric conditions during long-distance dispersal. Ecol Appl 19:656–667. https://doi.org/10.1890/07-2088.1
[8]	Bubner B, Schrader M, Jansen M, Schneck V, R?he P (2017) Conservation of the common ash (Fraxinus excelsior L.) by establishing a seed orchard consisting of clones with high resistance to ash dieback. (Erhalt der Gemeinen Esche (Fraxinus excelsior L.) durch Anlage einer Samenplantage bestehend aus Klonen mit hoher Resistenz gegenüber dem Eschentriebsterben). Presentation at the Ashtree Workshop 2017 on 24.04.2017, Laufen. (in German)
[9]	Buchner L, Eisen AK, ?ikoparija B, Jochner-Oette S (2022) Pollen viability of Fraxinus excelsior in storage experiments and investigations on the potential effect of long-range transport. Forests 13:600. https://doi.org/10.3390/f13040600
[10]	Bunce JA (2005) Seed yield of soybeans with daytime or continuous elevation of carbon dioxide under field conditions. Photosynthetica 43:435–438. https://doi.org/10.1007/s11099-005-0069-z
[11]	Carpenedo S, Raseira M, Byrne DH, Franzon RC (2017) The effect of heat stress on the reproductive structures of peach. J Am Pom Soc 71:114–120
[12]	Casti?eiras P, Vázquez-Ruiz RA, Fernández-González M, Rodríguez-Rajo FJ, Aira MJ (2019) Production and viability of Fraxinus pollen and its relationship with aerobiological data in the northwestern Iberian Peninsula. Aerobiologia 35:227–241. https://doi.org/10.1007/s10453-018-09553-z
[13]	Cipollini ML, Whigham DF (1994) Sexual dimorphism and cost of reproduction in the dioecious shrub Lindera benzoin (Lauraceae). Am J Bot 81:65–75. https://doi.org/10.1002/j.1537-2197.1994
[14]	Coker TLR, Rozsypálek J, Edwards A, Harwood TP, Butfoy L, Buggs RJA (2019) Estimating mortality rates of European ash (Fraxinus excelsior) under the ash dieback (Hymenoscyphus fraxineus) epidemic. Plants People Planet 1:48–58. https://doi.org/10.1002/ppp3.11
[15]	Comtois P (1994) Airborne pollen dispersal and survival on mount sutton (Canada). Aerobiologia 10:31–37. https://doi.org/10.1007/BF02066744
[16]	Cotrozzi L (2022) Spectroscopic detection of forest diseases: a review (1970–2020). J Forestry Res 33:21–38. https://doi.org/10.1007/s11676-021-01378-w
[17]	D’Amato G, Cecchi L, Bonini S, Nunes C, Annesi-Maesano I, Behrendt H, Liccardi G, Popov T, van Cauwenberge P (2007) Allergenic pollen and pollen allergy in Europe. Allergy 62:976–990. https://doi.org/10.1111/j.1398-9995.2007.01393.x
[18]	Damialis A, Fotiou C, Halley JM, Vokou D (2011) Effects of environmental factors on pollen production in anemophilous woody species. Trees 25:253–264. https://doi.org/10.1007/s00468-010-0502-1
[19]	Darbah JN, Kubiske ME, Nelson N, Oksanen E, Vapaavuori E, Karnosky DF (2008) Effects of decadal exposure to interacting elevated CO2 and/or O3 on paper birch (Betula papyrifera) reproduction. Environ Pollut (Barking, Essex : 1987) 155. https://doi.org/10.1016/j.envpol.2008.01.033
[20]	Dellinger AS (2020) Pollination syndromes in the 21st century: where do we stand and where may we go? New Phytol 228:1193–1213. https://doi.org/10.1111/nph.16793
[21]	Douglas GC, Pliura A, Dufour J, Mertens P, Jacques D, Fernandez-Manjares J, Buiteveld J, Parnuta G, Tudoroiu M, Curnel Y, Thomasset M, Jensen V, Knudsen MA, Foffová E, Chandelier A, Steenackers M (2013) Common Ash (Fraxinus excelsior L.). In: Paques LE (ed) Forest tree breeding in Europe: Current State-of-the-Art and Perspectives, vol 25. Springer, Dordrecht, pp 403–462
[22]	Duro A, Piccione V, Zampino D (2013) Air quality biomonitoring through pollen viability of Fabaceae. Environ Monit Assess 185:3803–3817. https://doi.org/10.1007/s10661-012-2829-0
[23]	Eisen AK, Fussi B, ?ikoparija B, Jochner-Oette S (2022) Aerobiological pollen deposition and transport of Fraxinus excelsior L. at a small spatial scale. Forests 13:424. https://doi.org/10.3390/f13030424
[24]	Eisen AK, Semizer-Cuming D, Jochner-Oette S, Fussi B (2023) Pollination success of Fraxinus excelsior L. in the context of ash dieback. Ann for Sci 80(1):1–21. https://doi.org/10.1186/s13595-023-01189-5
[25]	Enderle R (2019) An overview of ash (Fraxinus spp.) and the ash dieback disease in Europe. CAB Rev. https://doi.org/10.1079/PAVSNNR201914025
[26]	Enderle R, Nakou A, Thomas K, Metzler B (2015) Susceptibility of autochthonous German Fraxinus excelsior clones to Hymenoscyphus pseudoalbidus is genetically determined. Ann for Sci 72:183–193. https://doi.org/10.1007/s13595-014-0413-1
[27]	Enderle R, Fussi B, Lenz HD, Langer G, Nagel R, Metzler B (2017) Ash dieback in Germany: research on disease development, resistance and management options. In: Vasaitis R, Enderle R (eds) Dieback of European Ash (Fraxinus spp): Consequences and Guidelines for Sustainable Management. Uppsala, Sewden
[28]	F?gri K (2000) Textbook of pollen analysis, 4th edn. Blackburn Press, Caldwell, NJ
[29]	Fussi B (2020) This way the ash tree has a chance (So hat die Esche eine Chance). LWF Aktuell 126:60–61 ((in German))
[30]	Fussi B, Konnert M, Cremer E (2014) Gene flow in forest stands (Genfluss in Waldbest?nden). LWF Wissen 74:22–26 ((in German))
[31]	Fussi B, Baier R, Dobler G, Wolf H (2017) Ash dieback, quo vadis? (Eschentriebsterben, quo vadis?) AFZ – Der Wald:44–46. (in German)
[32]	FVA (2006) Ash seed orchard Emmendingen, area of origin 81107; provenances of the Schw. Alb and north of the Danube; application for approval as source material for the production of qualified reproductive material (Eschen-Samenplantage Emmendingen, Herkunftsgebiet 81107, Herkünfte der Schw. Alb und n?rdlich der Donau, Antrag auf Zulassung als Ausgangsmaterial zur Gewinnung von Qualifiziertem Vermehrungsgut) (2–8633.03/6306). (in German)
[33]	Gassner M, Schmid-Grendelmeier P, Clot B (2019) Ash pollen allergy and aerobiology. Allergo J Int 28:289–298. https://doi.org/10.1007/s40629-019-00105-6
[34]	Ge YX, Fu CX, Bhandari H, Bouton J, Brummer EC, Wang ZY (2011) Pollen viability and longevity of switchgrass (Panicum virgatum L.). Crop Sci 51:2698–2705. https://doi.org/10.2135/cropsci2011.01.0057
[35]	Goberville E, Hautekèete N-C, Kirby RR, Piquot Y, Luczak C, Beaugrand G (2016) Climate change and the ash dieback crisis. Sci Rep 6:35303. https://doi.org/10.1038/srep35303
[36]	Gottardini E, Cristofolini F, Paoletti E, Lazzeri P, Pepponi G (2004) Pollen viability for air pollution bio-monitoring. J Atmos Chem 49:149–159. https://doi.org/10.1007/s10874-004-1221-z
[37]	Gross A, Holdenrieder O, Pautasso M, Queloz V, Sieber TN (2014) Hymenoscyphus pseudoalbidus, the causal agent of European ash dieback. Mol Plant Pathol 15:5–21. https://doi.org/10.1111/mpp.12073
[38]	Hammond ME, Pokorny R, Okae-Anti D, Gyedu A, Obeng IO (2021) The composition and diversity of natural regeneration of tree species in gaps under different intensities of forest disturbance. J Forestry Res 32:1843–1853. https://doi.org/10.1007/s11676-020-01269-6
[39]	Hedhly A (2011) Sensitivity of flowering plant gametophytes to temperature fluctuations. Environ Exp Bot 74:9–16. https://doi.org/10.1016/j.envexpbot.2011.03.016
[40]	Heuertz M, Vekemans X, Hausman JF, Palada M, Hardy OJ (2003) Estimating seed vs. pollen dispersal from spatial genetic structure in the common ash. Mol Ecol 12:2483–2495. https://doi.org/10.1046/j.1365-294x.2003.01923.x
[41]	Holsinger KE, Steinbachs JE (1997) Mating Systems and Evolution in Flowering Plants. In: Iwatsuki K, Raven PH (eds) Evolution and diversification of land plants, 1st edn. Springer, Tokyo, Japan, New York, New York, pp 223–248
[42]	Iannotti O (2000) Pollen viability as a bio-indicator of air quality. Aerobiologia 16:361–365. https://doi.org/10.1023/A:1026559602547
[43]	IPCC (2023) Climate Change 2023: Synthesis Report of the IPCC sixth assessment report.
[44]	Isabel N, Holliday JA, Aitken SN (2020) Forest genomics: Advancing climate adaptation, forest health, productivity, and conservation. Evol Appl 13:3–10. https://doi.org/10.1111/eva.12902
[45]	ISTA (2003a) International Rules for Seed Testing. The International Seed Testing Association (ISTA), Wallisellen, Switzerland.
[46]	ISTA (2003b) Working Sheets on Tetrazolium Testing: Fraxinus, 1st edn. The International Seed Testing Association (ISTA), Wallisellen, Switzerland.
[47]	Jochner S, Heckmann T, Becht M, Menzel A (2011) The integration of plant phenology and land use data to create a GIS-assisted bioclimatic characterisation of Bavaria, Germany. Plant Ecol Divers 4:91–101. https://doi.org/10.1080/17550874.2011.574739
[48]	Jochner-Oette S, Rohrer T, Eisen AK, T?nnes S, Stammel B (2021) Influence of forest stand structure and competing understory vegetation on ash regeneration—potential effects of ash dieback. Forests 12:128. https://doi.org/10.3390/f12020128
[49]	Jump AS, Marchant R, Pe?uelas J (2009) Environmental change and the option value of genetic diversity. Trends Plant Sci 14:51–58. https://doi.org/10.1016/j.tplants.2008.10.002
[50]	Kassambara A (2023) ggpubr. https://cran.r-project.org/web/packages/ggpubr/ggpubr.pdf
[51]	Kj?r ED, McKinney LV, Nielsen LR, Hansen LN, Hansen JK (2012) Adaptive potential of ash (Fraxinus excelsior) populations against the novel emerging pathogen Hymenoscyphus pseudoalbidus. Evol Appl 5:219–228. https://doi.org/10.1111/j.1752-4571.2011.00222.x
[52]	Kowalski T (2006) Chalara fraxinea sp. nov. associated with dieback of ash (Fraxinus excelsior) in Poland. For Pathol 36:264–270. https://doi.org/10.1111/j.1439-0329.2006.00453.x
[53]	Kozlowski TT (1971) Cambial Growth, Root Growth, and Reproductive Growth. Growth and development of trees, Elsevier Science, Oxford
[54]	Kr?mer S, Jonitz A, Leist N (2007) Tetrazolium Working Sheets, method development and inclusion of new species in ISTA regulations. (Die Tetrazolium Working Sheets, Methodenentwicklung und Aufnahme neuer Arten in die ISTA-Vorschriften). In: Association of plant breeders and seed merchants of Austria (ed) Conference Proceedings of the 58th Annual Meeting of the association of plant breeders and seed merchants of Austria: Advances in seed technology and testing. Yield-oriented breeding strategies for new utilisation possibilities, LFZ Raumberg - Gumpenstein, pp 67–69. (in German)
[55]	Krauss N, K?hler K-H (1985) A contribution to the knowledge on stratification and germination of ash seeds (Fraxinus excelsior L.) (Ein Beitrag zur Kenntnis über die Stratifikation und Keimung von Eschensamen (Fraxinus excelsior L.)). Flora 177:91–105 ((in German))
[56]	Laaidi M (2001) Forecasting the start of the pollen season of Poaceae: evaluation of some methods based on meteorological factors. Int J Biometeorol 45:1–7. https://doi.org/10.1007/s004840000079
[57]	LaDeau SL, Clark JS (2006) Pollen production by Pinus taeda growing in elevated atmospheric CO2. Funct Ecol 20:541–547. https://doi.org/10.1111/j.1365-2435.2006.01133.x
[58]	Larue C, Austruy E, Basset G, Petit RJ (2021) Revisiting pollination mode in chestnut (Castanea spp.): an integrated approach. Bot Lett 168:348–372. https://doi.org/10.1080/23818107.2021.1872041
[59]	Lenz H, Stra?ner L, Baumann M, Baier U (2012) Vitality score for the classification of mature ash trees. (Boniturschlüssel zur Einstufung der Vitalit?t von Alteschen). AFZ Der Wald 3:18–129
[60]	Liebhold AM, Brockerhoff EG, Kalisz S, Nu?ez MA, Wardle DA, Wingfield MJ (2017) Biological invasions in forest ecosystems. Biol Invasions 19:3437–3458. https://doi.org/10.1007/s10530-017-1458-5
[61]	Lindner M, Fitzgerald JB, Zimmermann NE, Reyer C, Delzon S, van der Maaten E, Schelhaas MJ, Lasch P, Eggers J, van der Maaten-Theunissen M, Suckow F, Psomas A, Poulter B, Hanewinkel M (2014) Climate change and European forests: what do we know, what are the uncertainties, and what are the implications for forest management? J Environ Manage 146:69–83. https://doi.org/10.1016/j.jenvman.2014.07.030
[62]	Lobo A, McKinney LV, Hansen JK, Kjaer ED, Nielsen LR (2015) Genetic variation in dieback resistance in Fraxinus excelsior confirmed by progeny inoculation assay. For Pathol 45:379–387. https://doi.org/10.1111/efp.12179
[63]	Luschkova D, Traidl-Hoffmann C, Ludwig A (2022) Climate change and allergies (Klimawandel und Allergien). Allergo J 31:44–53. https://doi.org/10.1007/s15007-022-5030-y
[64]	Mangla Y, Gupta CK (2015) Love in the Air: Wind Pollination Ecological and Evolutionary Considerations. In: Kaur I, Koul M (eds) Kapoor R. Plant reproductive biology and conservation. IK International, New Dehli (India)
[65]	Mar?iulynien? D, Davydenko K, Stenlid J, Shabunin D, Cleary M (2018) Fraxinus excelsior seed is not a probable introduction pathway for Hymenoscyphus fraxineus. For Pathol 48:e12392. https://doi.org/10.1111/efp.12392
[66]	McKinney LV, Nielsen LR, Hansen JK, Kj?r ED (2011) Presence of natural genetic resistance in Fraxinus excelsior (Oleraceae) to Chalara fraxinea (Ascomycota): an emerging infectious disease. Heredity 106:788–797. https://doi.org/10.1038/hdy.2010.119
[67]	McKinney LV, Nielsen LR, Collinge DB, Thomsen IM, Hansen JK, Kjaer ED (2014) The ash dieback crisis: genetic variation in resistance can prove a long-term solution. Plant Pathol 63:485–499. https://doi.org/10.1111/ppa.12196
[68]	Meier U (2001) Entwicklungsstadien mono- und dikotyler Pflanzen. BBCH-Monographie, 2nd edn. Blackwell Wissenschaftsverlag, Berlin.
[69]	Metzler B, Enderle R, Karopka M, T?pfner K, Aldinger E (2012) Development of Ash dieback in a provenance trial on different sites in southern Germany. Allgemeine Forst- Und Jagdzeitung 183:168–180
[70]	Mondal S, Srivastava A, Joshi G, Yashavantha Rao HC (2019) Asynchronous flowering in clonal seed orchards—An effective strategy for alternative management. J Plant Sci Phytopathol 3:36–41. https://doi.org/10.29328/journal.jpsp.1001029
[71]	Moore PD (1999) Pollen analysis, 2nd edn. Blackwell Science, Oxford
[72]	Morand ME, Gerber S, Frascaria-Lacoste N (2002) Limited seed dispersal in a partially scattered tree species, Fraxinus excelsior L., as revealed by parentage analysis using microsatellites. Dynamics and conservation of genetic diversity in forest ecosystems, Strasbourg
[73]	Mwangola DM, Kees AM, Grosman DM, Auekema BH (2022) Effects of systemic insecticides against emerald ash borer on ash seed resources. For Ecol Manag 511:120144. https://doi.org/10.1016/j.foreco.2022.120144
[74]	Nelson R (1997) Modeling forest canopy heights: the effects of canopy shape. Remote Sens Environ 60:327–334. https://doi.org/10.1016/S0034-4257(96)00214-3
[75]	Obeso JR (2002) The costs of reproduction in plants. New Phytol 155:321–348. https://doi.org/10.1046/j.1469-8137.2002.00477.x
[76]	Ohnishi S, Miyoshi T, Shirai S (2010) Low temperature stress at different flower developmental stages affects pollen development, pollination, and pod set in soybean. Environ Exp Bot 69:56–62. https://doi.org/10.1016/j.envexpbot.2010.02.007
[77]	Pautasso M, Dehnen-Schmutz K, Holdenrieder O, Pietravalle S, Salama N, Jeger MJ, Lange E, Hehl-Lange S (2010) Plant health and global change—some implications for landscape management. Biol Rev Camb Philos Soc 85(4):729–755. https://doi.org/10.1111/j.1469-185X.2010.00123.x
[78]	Pautasso M, Aas G, Queloz V, Holdenrieder O (2013) European ash (Fraxinus excelsior) dieback—A conservation biology challenge. Biol Cons 158:37–49. https://doi.org/10.1016/j.biocon.2012.08.026
[79]	Pliūra A, Bakys R, Suchockas V, Mar?iulynien? D, Gustien? A, Verbyla V, Lygis V (2017) Ash dieback in Lithuania: disease history, research on impact and genetic Ash dieback in Lithuania: disease history, research on impact and genetic variation in disease resistance, tree breeding and options for forest management. In: Vasaitis R, Enderle R (eds) Dieback of European Ash (Fraxinus spp.): Consequences and Guidelines for Sustainable Management, Uppsala, Sewden, pp 150–165.
[80]	Ranpal S, Sieverts M, W?rl V, Kahlenberg G, Gilles S, Landgraf M, K?pke K, Kolek F, Luschkova D, Heckmann T, Traidl-Hoffmann C, Büttner C, Damialis A, Jochner-Oette S (2022) Is pollen production of birch controlled by genetics and local conditions? Int J Environ Res Public Health 19(13):8610. https://doi.org/10.3390/ijerph19138160
[81]	Revelle W (2022) psych: Procedures for Psychological, Psychometric, and Personality Research., Northwestern University, Evanston, Illinois. https://CRAN.R-project.org/package=psych.
[82]	Rezanejad F (2007) The effect of air pollution on microsporogenesis, pollen development and soluble pollen proteins in Spartium junceum L. (Fabaceae). Turk J Botany 31:183–191
[83]	Roloff, Pietzarka (1997) Fraxinus excelsior. Enzyklop?die der Holzgew?chse 7. Erg Lfg 3/97.
[84]	Satake A, Iwasa Y (2002) The synchronized and intermittent reproduction of forest trees is mediated by the Moran effect, only in association with pollen coupling. J Ecol 90:830–838. https://doi.org/10.1046/j.1365-2745.2002.00721.x
[85]	Saumitou-Laprade P, Vernet P, Dowkiw A, Bertrand S, Billiard S, Albert B, Gouyon P-H, Dufay M (2018) Polygamy or subdioecy? The impact of diallelic self-incompatibility on the sexual system in Fraxinus excelsior (Oleaceae). Proc Biol Sci 285:20180004. https://doi.org/10.1098/rspb.2018.0004
[86]	Schirmer R (2002) Harvesting of ash seeds (Beerntung von Eschensaatgut). In: Bavarian State Institute of Forestry (ed) LWF-Wissen 34: Contributions to ash trees. Symposium about the Tree of the Year 2001, pp 21–25. (in German)
[87]	Schueler S, Schlnzen KH, Scholz F (2005) Viability and sunlight sensitivity of oak pollen and its implications for pollen-mediated gene flow. Trees 19:154–161. https://doi.org/10.1007/s00468-004-0376-1
[88]	Schumacher J, Wulf A, Leonhard S (2007) First detection of Chalara fraxinea T. Kowalski in Germany - a causative agent of a new type of damage to ash trees (Erster Nachweis von Chalara fraxinea T. Kowalski in Deutschland - ein Verursacher neuartiger Sch?den an Eschen). Deutscher Pflanzenschutzbund 59:121–123 ((in German))
[89]	Schwab A, Stammel B, Kiehl K (2018) Seed dispersal via a new watercourse in a reconnected floodplain: differences in species groups and seasonality. Restor Ecol 26:103–113. https://doi.org/10.1111/rec.12677
[90]	Semizer-Cuming D, Kj?r ED, Finkeldey R (2017) Gene flow of common ash (Fraxinus excelsior L.) in a fragmented landscape. PLoS ONE 12:e0186757. https://doi.org/10.1371/journal.pone.0186757
[91]	Semizer-Cuming D, Finkeldey R, Nielsen LR, Kj?r ED (2019) Negative correlation between ash dieback susceptibility and reproductive success: good news for European ash forests. Ann for Sci 76:16. https://doi.org/10.1007/s13595-019-0799-x
[92]	Semizer-Cuming D, Chybicki IJ, Finkeldey R, Kj?r ED (2021) Gene flow and reproductive success in ash (Fraxinus excelsior L.) in the face of ash dieback: restoration and conservation. Ann for Sci 78(1):1–15. https://doi.org/10.1007/s13595-020-01025-0
[93]	Shivanna KR, Rangaswamy NS (1992) Pollen biology: a laboratory manual. Springer, Berlin, Heidelberg
[94]	Smith WH (1981) Forest tree reproduction: influence of air pollutants. In: Smith WH (ed) Air Pollution and Forests: Interactions Between Air Contaminants and Forest Ecosystems. Springer, New York, NY, pp 141–152
[95]	Soldi E, Tiley A, O’Hanlon R, Murphy BR, Hodkinson TR (2022) Ash dieback and other pests and pathogens of Fraxinus on the island of Ireland. Biol Environ: Proc Royal Irish Acad 122(2):85–122. https://doi.org/10.1353/bae.2022.0007
[96]	Stanley RG, Linskens HF (1974) Pollen: Biology Biochemistry Management. Springer, Berlin, Heidelberg
[97]	Tabari KM, Lust N (1999) Monitoring of natural regeneration in a mixed deciduous forest. Silva Gandavensis 64:58–71
[98]	Talwar S, Bamel K, Prabhavathi MA (2022) Effect of high temperature on reproductive phase of plants: a review. Nat Env Pol Tech 21:1887–1892
[99]	Timerman D, Barrett SCH (2020) Influence of local density and sex ratio on pollination in an ambophilous flowering plant. Am J Bot 107:587–598. https://doi.org/10.1002/ajb2.1453
[100]	Tulik M, Yaman B, K?se N (2018) Comparative tree-ring anatomy of Fraxinus excelsior with Chalara dieback. J Forestry Res 29:1741–1749. https://doi.org/10.1007/s11676-017-0586-1
[101]	Ueno N, Suyama Y, Seiwa K (2007) What makes the sex ratio femalebiased in the dioecious tree Salix sachalinensis? J Ecol 95:951–958. https://doi.org/10.1111/j.1365-2745.2007.01269.x
[102]	van der Linde S, Perez-Sierra A, Needham RH, Combes M, McCartan SA (2021) Identification, detection and eradication of Hymenoscyphus fraxineus from ash (Fraxinus excelsior) seeds. Forestry 94:745–756. https://doi.org/10.1093/forestry/cpab017
[103]	Wada KC, Takeno K (2010) Stress-Induced Flowering. Plant Signal Behav 5:944–947. https://doi.org/10.4161/psb.5.8.11826
[104]	Wang SW, Xie BT, Yin LN, Duan LS, Li ZH, Eneji AE, Tsuji W, Tsunekawa A (2010) Increased UV-B radiation affects the viability, reactive oxygen species accumulation and antioxidant enzyme activities in maize (Zea mays L.) pollen. Photochem Photobiol 86:110–116. https://doi.org/10.1111/j.1751-1097.2009.00635.x
[105]	Wang HJ, Lin SZ, Dai JH, Ge QS (2022) Modeling the effect of adaptation to future climate change on spring phenological trend of European beech (Fagus sylvatica L.). Sci Total Environ 846:157540. https://doi.org/10.1016/j.scitotenv.2022.157540
[106]	Wickham H (2016) ggplot2: Elegant graphics for data analysis, Springer-Verlag New York. https://ggplot2.tidyverse.org.
[107]	Zhang YX, Steiner AL (2022) Projected climate-driven changes in pollen emission season length and magnitude over the continental United States. Nat Commun 13:1–10. https://doi.org/10.1038/s41467-022-28764-0

Funding

Katholische Universit?t Eichst?tt-Ingolstadt (3115)