Biological Flora of the British Isles: Crataegus monogyna

This account presents information on all aspects of the biology of Crataegus monogyna Jacq. (Hawthorn) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of the Biological Flora of the British Isles: distribution, habitat, communities, responses to biotic factors, responses to environment, structure and physiology, phenology, floral and seed characters, herbivores and disease, history and conservation. Crataegus monogyna is native to the British flora, occurring frequently in hedgerows, scrubs, thickets and woodland. It can be found throughout almost all of Europe, on all soils of medium conditions regarding pH value, nutrient and water supply. Crataegus monogyna is a deciduous shrub or rarely a small tree of 2–8 m. Its twigs and branches bear sharp thorns about 1 cm long. Crataegus monogyna is of both ornamental and ecological value. During flowering in May and June, shrubs may appear white through a multitude of flowers, presenting pollen and nectar to a variety of different insects. Starting in August, almost the whole shrub can become dark red with the huge number of small red berries (pomes) produced during fruiting. The fruit are a preferred food for many birds. Although hybridizing freely and frequently with the other native species, Crataegus laevigata (Poir.) DC., the two species are easily recognizable in natural stands in the British Isles. Elsewhere, and with the occurrence of horticultural naturalizations and many intermediate forms of hybrid origin with closely related Crataegus species (especially the similar looking one‐styled species Crataegus rhipidophylla Gand. s.l. and Crataegus × subsphaerica Gand. s.l.) expert knowledge is required to avoid misidentifications and thus inaccurate understanding of frequency and distribution–not only on continental Europe but also increasingly in the British Isles. Identification in the field is further complicated by inbreeding of horticultural stock, which suffers from a myriad of descriptions and given names at different hierarchical levels. Cultivars are commonly planted in hedges and along roadsides or for ornamental purposes. The origin of this stock is not always known, so genetic exchange with the natural populations may lead to introgression and thus genotypes that are more adapted than the local genotypes in a changed environment.


| Climatic and topographical limitations
In its natural and naturalized ranges, C. monogyna grows in humid and subhumid temperate regions (Parsons & Cuthbertson, 1992) tolerating climates that range from cool temperate (Northern Europe) to Mediterranean (southern Europe, northern Africa, Minor Asia and the Middle East) and subcontinental (eastern Europe). For its British distribution Hill et al. (2004) provide calculated mean temperatures of 3.6°C in January and 14.7°C in July respectively. The Ellenberg value for temperature (T) is 5, representing a mean annual temperature c.
6°C (5-7°C) for Central Europe (Ellenberg et al., 2001). This reflects occurrences from foothills to montane zones throughout Europe and additionally in subalpine zones of mountain ranges in Southern Europe as C. monogyna is missing in those of Northern Europe. Conolly and Dahl (1970), cited in Rodwell (1991) list C. monogyna as a part of communities with a mean annual maximum temperature of at least 26°C. Kean (2009) calculated an optimum temperature ranging from 18 to 26°C from its natural distribution by using a fitted CLIMEX model and 6 and 30°C as the lower and upper thresholds of viability. These thresholds reflect the northern and southern edge of its European distribution. Parsons and Cuthbertson (1992) consider annual rainfall of 600 mm as a minimum, while Pasiecznik (2008) gives seven consecutive months of rainfall less than 40 mm as maximum dry duration, and 400 and 1,400 mm as lower and upper limit of mean annual rainfall. The mean of the annual precipitation in its British distribution is 1,073 mm (Hill et al., 2004). The Fraxinus excelsior-Acer campestre-Mercurialis perennis woodland (Section 3) in which hawthorn grows in Britain is confined to sites with an annual rainfall of <1,000 mm and <160 wet days per year (Chandler & Gregory, 1976; Climatological Atlas of the British Isles, 1952; Ratcliffe, 1968;all cited in Rodwell, 1991). C. monogyna tolerates exposure to cold or salt-laden air (McIndoe, 2019), but especially in coastal locations, growth is heavily affected by wind. With its strong stem and flexible twigs C. monogyna is sculpted by the wind, leaning away from it (M.B. Usher, pers. comm.), providing shelter for other herbs (Stoutjesdijk & Barkman, 2014).

| Substratum
Throughout its distribution area C. monogyna is found in nearly all geological settings, whether of siliceous, mixed or calcareous constitution, for examples, granite, gneiss, silicate slate; siliceous lime, limestone shale, flysch, sandstone; lime, dolomite (Aeschimann et al., 2004). It grows on a wide range of soil textures, preferring humic and moist to dry clays and fine-grained to heavy loams (Hess F I G U R E 2 Distribution of Crataegus monogyna in Europe and adjacent regions, after Meusel et al. (1965) redrawn by Dr. Erik Welk. The standard European distribution map from Atlas Florae Europaeae, with somewhat narrower range, is provided for comparison in Figure S1 et al., 1977). Suitable soil types are the natural profiles of different degrees of maturity (e.g. rankers, brown podzolic soils and podzols, base-poor brown earths, mulls, rendzinas and brown calcareous earths; Rodwell, 1991), as well as man-made raw soils on rock waste, and on restored ground. The Ellenberg values for pH (R = 7; Hill et al., 1999) suggest soil reactions from relatively base-poor (pH 6, e.g. in brown earths) to very base-rich (pH 7 and higher, e.g. in rendzinas). C. monogyna is found on soils of any conditions from poor in nutrients to highly eutrophic, as expressed by a mean Ellenberg value for fertility (N) equal to 6 (Hill et al., 1999). Drainage conditions ranging from slightly impeded to fully drained can be tolerated, resulting in its occurrence in moist, mesic and dry conditions (Hellwig, 2006). This is again expressed by the mean Ellenberg value for moisture (F) of 5 (Hill et al., 1999). However, these values taken as single numbers reflect a general direction and not a range. C. monogyna rarely occurs on wet peat or poor acidic sands (Clapham et al., 1989). Sebald et al. (1992) mention occurrence also in rocky habitats. The preferred form of soil humus is mull. Sell and Murrell (2014) describe Crataegus monogyna as 'frequent in [British] woods, copses and hillsides, [and] the main constituent of most of [British] hedgerows'. Hedges are defined as linear manmade habitats in contrast to the natural appearance of spontaneous scrub (Tüxen, 1952;Wirth, 1993). Scrub also appears as a natural community on forest borders. Moreover, communities can be stages in secondary successions on neglected arable land, meadows and pastures as well as replacement communities after forest degradation. Rodwell (1991) described two main British plant communities containing C. monogyna. Relevés of these, the Crataegus monogyna-Hedera helix scrub (W21) and the Fraxinus excelsior-Acer campestre-Mercurialis perennis woodland (W8), contain hawthorn at least to 60% and 40% respectively.

| COMMUNITIE S
In the W21 community, besides Rubus fruticosus agg. and Hedera helix, C. monogyna is one of the constant species, which can dominate the more or less abundant Prunus spinosa as an additional spiny species of this community. Ivy commonly forms a ground carpet and in general leads to a species-poor field layer. W21 is divided into four subcommunities reflecting edaphic and climatic factors, as well as the developmental history of the vegetation. Concerning the latter factor, Crataegus-Hedera scrub is often found on abandoned arable land, whereas only the Mercurialis perennis subcommunity (W21b) derives from degenerate Carpinion or Fagion woodland on ill-draining clays and shales with stagnogleys and pelosols. Besides species of the respective woodland field layer, Mercurialis perennis as well as the nitrophilic Sambucus nigra, Urtica dioica and Galium aparine are frequent. Like the former, the Hedera helix-Urtica dioica subcommunity (W21a) has moister and more nutrient-rich situations. In addition, there are numerous Arrhenatherion species (e.g. Arrhenatherum elatius, Holcus lanatus, Heracleum sphondylium and Silene dioica).
These species are members of a preceding herbaceous vegetation that have survived after grazing and mowing were abandoned. By contrast, the Viburnum lantana subcommunity (W21d) is found in warmer places, on oligotrophic rendzinas with a higher base saturation, and has calcicolous and relict Mesobromion species (e.g. Bromus erectus, Brachypodium pinnatum s.l., Sanguisorba minor, Origanum vulgare and Teucrium scorodonia) in combination with frequent woody species like Ligustrum vulgare, Viburnum lantana, Cornus sanguinea, Clematis vitalba and Tamus communis. The Brachypodium sylvaticum subcommunity (W21c) is a kind of intermediate as its species composition is similar to the Hedera helix-Urtica dioica subcommunity but on oligotrophic rendzinas. Nitrophilic Sambucus nigra, Urtica dioica and Galium aparine are less frequent, but Brachypodium sylvaticum often occurs as a more shade-tolerant survivor of abandoned ploughland.
Fragaria vesca and Viola riviniana occur only in this subcommunity.
Depending on the biotic and abiotic conditions Crataegus-Hedera scrub can develop to Quercus-Pteridium-Rubus woodland (W10) or Fagus-Rubus woodland (W14) on base-poor soils (Watt, 1924(Watt, , 1934 as cited in Rodwell, 1991) and on base-rich but moist soils to Fagus-Mercurialis woodland (W12), locally to Taxus woodland (W13) or to W8 in areas beyond the natural dominance of beech (Brenchley & Adam, 1915;Rackham, 1975;Tansley, 1939;all cited in Rodwell, 1991 Furthermore, C. monogyna is part of many successional sequences ( Figure 3). The species is an important early invader in different grassland types characterized 'by a decline in grazing by stock or wild herbivores (rabbits) or by neglect of mowing in field corners or more inaccessible parts of the meadow' (Rodwell, 1991). When seedlings begin to survive due to abandonment of arable usage, saplings may establish scrub, which then may be taken over by colonizing trees to form different secondary woodland types (Tansley, 1939 as cited in Rodwell, 1991), sometimes via Rubus-Holcus underscrub (W24). The reverse process is also possible: As a result of cutting and/or burning the Crataegus-Hedera scrub (W21) may develop into W24 and, with a re-imposition of grazing or mowing, mainly to Arrhenatherion communities, although this is hard to manage.
In general, the range of the distribution of the C. monogyna is correlated with other species of the Southern European dry forest like Prunus spinosa, Rosa sect. Caninae or Sorbus torminalis (Meusel et al., 1965). Although C. monogyna occurs also in Quercetalia pubescentis or thermophilous Fagetalia (or even the Alno-Ulmion), and in Erico-Pinion communities, it is a constant species of the Prunetalia (Lippert, 2001). W21 relates on the one hand to the thermophilous scrubs of Berberidion, the calcicolous alliance of sub-Mediterranean-mid-European scrub, and on the other to mesophilic scrub of Carpino-Prunion, the alliance of suboceanic-mid-European scrub.

| RE S P ON S E TO B I OTI C FAC TOR S
By applying different levels of shade to hawthorn Grubb et al. (1996) found total mortality of saplings in 0.3% daylight. In 1.6% daylight the death rate was 10%, except in saplings growing on nutrientrich soil, as sufficient nutrient supply may appear toxic in shade (Hutchinson, 1967). Williams and Buxton (1989) concluded a mechanism of avoiding shade by strong monopodial growth by observing a significantly faster shoot growth rate and only a little increase in shoot number of saplings in 66% daylight than in 16% daylight. Besides that, the number of leaves and correspondingly the total leaf area increased significantly with higher irradiance (Grubb et al., 1996). Leaf thickness increased with irradiance too, so that leaf mass per area tended asymptotically to 10.12 mg/cm 2 (Aranda et al., 2004), leading towards significant gain in total dry weight (Grubb et al., 1996;Williams & Buxton, 1989 Grubb et al. (1996) Aranda et al. (2004) found hawthorn to be the most shade-intolerant among Ilex aquifolium, F. sylvatica, Quercus petraea, Q. pyrenaica, Prunus avium, Sorbus aria and S. aucuparia. Bradshaw (1948) noted a more tree-like habit in shaded localities compared to C. laevigata (see also Thomas et al., 2021). The Ellenberg value for light (L) is 6 (Hill et al., 1999) indicating the growth of C. monogyna in partially shaded (rarely <20% daylight) to lit places.
Its thorns may directly confer the ability to compete with more palatable plants (Section 9.1).

Species
Community code according to Rodwell (1991Rodwell ( , 1992 (Grime et al., 1988). It performs better in establishing from seeds in grazed areas than many other woody species (Linhart & Whelan, 1980), although it is only moderately tolerant to grazing (Klotz et al., 2002) and normally invasion follows a relaxation in grazing pressure (Grime et al., 1988). Its thorn-bearing habit also makes C. monogyna a good nurse plant for other woody plants in scrub community succession.
Although hawthorn is intolerant to mowing (Klotz et al., 2002), it resprouts after coppicing (Michielsen et al., 2017;Sell & Murrell, 2014) and tends to sucker especially after disturbance (Bass, 1990a). It is also very tolerant to trampling (Klotz et al., 2002). Shrubs of C. monogyna are not resistant to fire (Hegi, 1923) and burn readily. They often manage to resprout (Michielsen et al., 2017), but do not benefit from fire in terms of producing seedlings and saplings (Esposito et al., 2014).

| Gregariousness
Crataegus monogyna can exist as a solitary tree or produce stands ranging from open to dense scrub. Good et al. (1990)

| Performance in various habitats
A study by Grubb et al. (1999)  Like most of its accompanying woody hedge species (Section 3), C. monogyna is a more or less light-demanding plant. In the open, it comes into full flower, but shaded by other plants, for example inside woods, it will produce fewer or even no flowers (Grime et al., 1988).

| Frost
Dirr (2010) (1946) state that C. laevigata (reported as C. oxyacantha L.) was not very much damaged by late frosts, although it appeared softer than hornbeam or birch. The authors received one report of severe damage to seedlings, in which many were killed, and a few reports of damage to the blossoms only. These findings may presumably also apply to C. monogyna. The freezing temperature of the xylem sap is −6.1 ± 0.8°C in winter and −5.5 ± 0.2°C in early spring (Lintunen et al., 2015). The authors also found the smallest diameter shrink-

| Drought
C. monogyna is more resistant to drought than the other mid-European species of the genus (Hegi, 1923). Despite that summer dryness causes earlier leaf abscission starting in July with a mean leaf longevity of 164 days (Mediavilla & Escudero, 2003

| Flooding
Frye and Grosse (1992) tested the effect of flooding 10 cm above soil surface on 1-year-old seedlings for 120 days during the growing season. They reported a decrease in growth in height (

| Salinity
Despite the Ellenberg value for salt (S) being zero (Ellenberg et al., 2001), C. monogyna grows in dunes and coastal communities and reacts to exposure to salt (NaCl) in different ways.
Responses range from toleration to severe adverse effects, depending on amount and the method of application of sodium chloride. Thompson and Rutter (1986) tested the effect of applying water with different salt concentrations (4,8,16 and 32 g/L) as spray or irrigation water. Although some young shoots died in the spraying treatment the total new biomass increased by a mean of 36% over all concentrations compared to the control. In the irrigation treatment C. monogyna did suffer from solutions of 4 or 8 g/L with 10%-20% mortality, respectively, but survivors did not show significant change in biomass. With higher concentrations the results became severe with death rates from 70% to 100% for 16 and 32 g/L respectively. Rodwell (1991) considers C. monogyna less tolerant to salt than Prunus spinosa.

| Pollutants
Hawthorn has a moderate sensitivity to SO 2 and HF and is only slightly sensitive to NH 3 (Däßler, 1991) but shows foliar injuries due to a strong impairment by O 3 at concentrations below the European threshold AOT40 (Novak et al., 2003). Fruit yield is heavily reduced by the herbicide metsulfuron-methyl (Kjaer et al., 2006).

| Morphology
Crataegus monogyna is normally more of a shrub than a tree. Apical growth and a more tree-like shape may be suppressed by either disturbance when young or competition for light with other plants.
The shoot (Bartels, 1993). While the thorns grow sylleptically from the mid part of a long shoot, that is, during the same vegetation period as the long shoot bearing them, the leafy short shoots grow proleptically, that is, from lateral meristems in the axils of leaves or thorns with an intervening period of dormancy. The mode of branching is strongly affected by number, time and orientation of pruning, but total shoot length is not (Bannister & Watt, 1995). The authors found a cut applied horizontally in summer resulted in fewer but longer shoots, whereas a cut applied vertically in summer produced more thorn-tipped shoots. A vertical cut in winter resulted in longer shoots than one in summer and reduced the number which were thorn-tipped.
C. monogyna has a far-reaching and deep tap-root system (Bartels, 1993;Kutschera & Lichtenegger, 2013). In a study on excavated individuals, Kárász (2006) measured maximum root spread and penetration lengths. We calculated from these data mean annual horizontal and vertical increases of about 10 and about 3 cm respectively.
Leaf morphology displays considerable environmental plasticity (Gosler et al., 1994) and it also differs enormously between long and short shoots, as do the stipules. Leaves on short shoots are heteromorphic in their leaf succession. Laminae are smaller and less deeply lobed at the base of a short shoot and become larger (1.0-5.7 × 0.8-6.0 cm) with 1-3 pairs of lobes at the shoot's apex. Leaves on long shoots are slightly larger (2.2-6.2 × 2.2-6.4 cm) and more deeply lobed or even nearly dissected. Oliver (1999) shows actual compound leaves on long shoots. The lobes are frequently horizontally spreading and bear two to 16 teeth. Their stipules are conspicuous, often leaf-like and ± regularly serrate. Prior to abscission in autumn, the leaves turn yellow-orange. Taking the broad variation in some morphological traits into account, particularly from heteroblasty and heterophylly, reliable identification therefore requires short shoots and fertile material, and preferably collections of both flowers and fruit.
The authors presented in a subsequent study a range of 90 ± 22 stomata/mm 2 (Beerling & Kelly, 1997). In comparison to the findings of Salisbury (1927), they found a significant decrease in stomatal density in the 20th century which was mainly related to the increase in atmospheric CO 2 . Unicellular covering trichomes can be present primarily at the margin and along veins (Upton et al., 2011). They occur in different lengths with the longer one (up to 500-600 µm) along the veins.

| Mycorrhiza
The roots of C. monogyna show both arbuscular and ectotrophic mycorrhiza (Harley & Harley, 1987 and the literature cited therein).
In a study of root-associated fungi in sandy grassland of the Great Hungarian Plain, Kovács and Szigetvári (2002)  were reported as ectomycorrhizal fungal associates of different Crataegus species (Becker, 1956 as cited in Trappe, 1962), whereas Cenococcum graniforme (Sow.) Ferd. & Winge was mentioned as specific ectomycorrhizal associate of C. monogyna (Dominik, 1957 as cited in Trappe, 1962). Scutellospora armeniaca Błaszkowski was reported as a fungal partner of arbuscular mycorrhiza with C.
monogyna in Poland (Błaszkowski, 1992). Recently Tedersoo (2019, 2020) have questioned the mycorrhizal research conducted over 50 years ago for misinterpretation and consider that the genus Crataegus forms arbuscular mycorrhiza only (see also Thomas et al., 2021).

| Perennation: Reproduction
Phanerophyte. Reproduction is primarily by seeds, as sexual reproduction yields a large number of drupaceous pomes. Vegetative reproduction by root suckers (Hegi, 1923) is seldom seen but occurs more frequently after coppicing or disturbance. Propagation through cuttings from the current year's shoots taken in early summer is possible (Schuck, 2005). In nurseries both sexual and asexual propagation techniques are used (Lasseigne & Blazich, 2008). In horticulture, propagation from seeds is important to produce root-stocks for grafting of cultivars and related fruit genera (Bush et al., 1991;Dirr & Heuser, 2006). Micropropagation can be an alternative to current propagation practices. Iapichino and Airò (2009)

| Biochemical data
Crataegus monogyna contains a range of flavonoids, biogenic amides and triterpenic acids in leaves, flowers and fruit (Braun & Frohne, 1987). By contrast, no alkaloids or saponins were detected in the similar C. laevigata (Dau, 1941), but the pseudosaponin tormentosid is present in this species (Steinegger & Peters, 1966). Very unusually for Rosaceae, neither of the cyanogenic compounds, amygdalin or prunasin, is found in Crataegus (Hegnauer, 1973).

| PHENOLOGY
Budburst of C. monogyna has been recorded between mid-March and mid-April, depending on altitude and provenance (Jones et al., 2001;Vander Mijnsbrugge et al., 2015). In Britain flowering starts from late April to end of May (Gyan & Woodell, 1987a). Using a 35-year dataset, Jeffree (1960) identified the mean start of flowering as 13 May ± 8 days (mistakably referred to as C. oxyacantha). In a consecutive study with a 58-year dataset Sparks et al. (2000) clarified the ambigous usage of C. oxyacantha as C. monogyna and came to the same result with the earliest −19 days and the latest +16 days. In Central Europe, hawthorn is listed to flower in the Sorbus aucuparia-Galium odoratum phase at the end of mid spring (Dierschke, 1995), which is during May and June. In a more Mediterranean climate flowering begins earlier in April (Browicz, 1972;Guitián & Fuentes, 1992) and at more oceanic sites as early as February. In general, flowering appears about 1-2 weeks later than in C. laevigata (Clapham et al., 1989;Hegi, 1923).
A second blooming during winter is seen in C. monogyna 'Biflora' (forma praecox), famously known as the Holy Thorn of Glastonbury according to the legend of Joseph of Arimathea (Sell & Murrell, 2014).
Anthesis is at maximum between 10.2 and 18°C air temperature and is inhibited by rain (Percival, 1955).

| Floral biology
Hawthorn features flowers on any side of the individual but with significantly more on western sides than on eastern sides (Sparks & Croxton, 2007). The flowers of C. monogyna are hermaphroditic and protogynous and have five, rarely four or six, petals (Knuth, 1898).
Macreight (1837) describes the aestivation of petals as quincuncial or spirally imbricate, where two petals are outside all others, two are inside all others, and the fifth is outside on one margin and inside on the other. However, they also appear cochleate or spirally twisted, where one petal is outside all others, one petal is inside all others, and three are outside on one margin and inside on the other (A. Fichtner, pers. obs.). On the day of anthesis, the petals bend outwards and soon reveal the mature stigma. During the next 2 days, the stamens expand until they are erect to suberect before the outermost anthers begin first to dehisce and are laced with pollen. In doing so, they change colour from pink-purplish to yellowish-brown (Godet, 1984). After anthesis flowers keep blooming for 5 days (Gyan & Woodell, 1987b) and attract pollinators in two ways: visually by the well-developed white petals, and by a foetid scent. Pollinators are rewarded with nectar and pollen (Ehlers, 1960). Broughton and Wright (1998)  Pollen release takes place from 7 a.m. to 5 p.m. without any peak period in C. monogyna (Percival, 1955). The pollen is of the Crataegustype (Reitsma, 1966). The grains are tricolporate, striate, without a tectate operculum, and with fine, doubled, curved and short vallae, distributed as parallel pairs (Faegri et al., 1989). Data of length and diameter range from 23.5 to 35.4 μm and 23.8 to 36.9 μm (Eide, 1981) and 30 to 44 μm and 24 to 40 μm, respectively, with a prolate-sphe- The nectar is secreted by a ring of nectaries in the receptacle (Clapham et al., 1989), whose anatomy is described in Weryszko- Chmielewska and Konarska (1996). The amount of secretion varies diurnally with a decrease in the morning and a recovery in the afternoon, so the sugar concentration increases during the day as temperature increases and relative humidity decreases, ranging between 36% and 70% (Gyan & Woodell, 1987b). In terms of amount of sugar per flower during a day the actual reward averages 0.15 mg.
As a result of the species' RNase-based gametophytic self-incompatibility (Nettancourt, 1977;Raspé & Kohn, 2002), xenogamic cross-pollination by insects should be the rule, whereas self-pollination only occurs at failure of outcrossing (Knuth, 1898). Guitián and Fuentes (1992) and  report different flower to fruit conversion rates from 20% to 50% under natural conditions and a fruit-set limited by lack of pollinator activity. Gyan and Woodell (1987a) found autogamy with a flower to fruit conversation rate of 82% and Guitián and Fuentes (1992) observed 20% after manual self-pollination. However, further research on the questions of reproduction seems necessary, as in an ongoing study with self-and cross-pollination by hand of different Crataegus species, neither autogamy nor geitonogamy have been detected (A. Fichtner, unpubl. data).
While fresh mass of fruit yield is significantly lowered by higher cutting frequencies the percentage of dry matter content remains unaffected (Croxton & Sparks, 2002). There is no evidence for mast events in C. monogyna. Sallabanks (1992) showed higher fitness for larger and/or older plants through producing more fruit, suggesting that growing as big and as quickly as possible by delaying fruiting until later in life is an optimal fruiting strategy for hawthorn.

In the British Isles, Crataegus monogyna hybridizes most
notably with the other native species, C. laevigata, (Thomas et al., 2021). However, in its natural range and where it has been introduced, C. monogyna hybridizes freely with other Crataegus species. The taxonomic status of these hybrids as nothospecies, nothosubspecies or nothovarieties is debatable (Schmidt, 2017c) and, for some crosses, depends also on the acceptance of the constitution of 'C. rhipidophylla'. In Table 2 we present a system adopting the members of C. rhipidophylla s.l. (i.e. C. rhipidophylla Gand. and C. lindmanii Hrabětová) as species (Schmidt, 2017a) and medlar, formerly treated as genus Mespilus, as Crataegus germanica (L.) O. Kuntze (Lo et al., 2007;Talent et al., 2008).
The unstable graft-chimera +Crataegomespilus dardarii Simon-Louis ex Bellair is of horticultural origin only (Byatt et al., 1977;Hegi, 1923;Schneider, 1906). The first and best-known example was developed as a graft of C. germanica on C. monogyna at the nursery of Simon-Louis in Bronvaux near Metz, France. Depending on the number of epidermal layers there are two cultivars (Bergann & Bergann, 1984;Fitschen et al., 1994): +Crataegomespilus dardarii 'Dardarii' with one layer of medlar, resembling C. germanica and +Crataegomespilus dardarii 'Asnieresii' with two layers of medlar resembling C. monogyna. Byatt et al. (1977) came to the conclusion of 'dynamic rather than static random arrangements of parental tissues' leading to a series of transitional stages in different branches on individuals.

| Seed production and dispersal
There are typically five to eight fruits per corymb, each drupaceous pome bearing one nutlet (Bojňanský & Fargašová, 2007). Khadivi-Khub et al. (2015) provided data on number of nutlets ranging from one to four (mean 3.11) in Iran. These counts have to be regarded with suspicion as the pictures of leaves, fruit and nutlets of the studied plants presented suggest that some were probably incorrectly allocated to C. monogyna. Each nutlet holds one seed enclosed in a lignified endocarp that may act to protect the seed and to retard germination (Bewley & Black, 1982). The presence of an endosperm was denied by Ascherson and Graebner (1906)

but was described for
Crataegus species by Aldasoro et al. (2005) and Talent and Dickinson (2007b). The seeds show embryo dormancy (Dickinson, 1985).
Mean fruit mass has been measured as 675.4 mg for fresh and 294.7 g for dried fruit, respectively, resulting in 171.0 mg per dried pulp and 123.7 mg per dry seed (Herrera, 1987). The thousand-seed weight varies with authorities from 55 g (Hrynkiewicz-Sudnik et al., 1987as cited in Bujarska-Borkowska, 2002 to 98 g (Royal Botanic Gardens Kew, 2019) and 280 g (Kheloufi et al., 2019).
The flesh of the fruit contains starch and is mealy without any special flavour. Specific pulp constituents are provided in Table 3.
Although it is not really poisonous it contains some slightly toxic compounds such as saponins (Rossiiskaya et al., 1989) so only a small number of fruit are consumed by an individual each time (Barnea et al., 1993). Therefore, they can remain on the trees for more than 9 months (Bass, 1990a) representing, in case of need, a food supply for overwintering that lasts all winter (Barnea et al., 1993). On the other hand, birds consistently reject fruit infested by insects (Manzur & Courtney, 1984).

| Viability of seeds: Germination
Under natural conditions seeds of C. monogyna usually start germinating in the second spring after seed-set (Flemion, 1938as cited in Lasseigne & Blazich, 2008 and can take up to 6 years to germinate (Christensen, 1992a); thus they require cold stratification to break embryo dormancy (Brinkman, 1974as cited in Lasseigne & Blazich, 2008. Increasing temperatures during spring will induce secondary dormancy if the seeds have not germinated after their cold stratification during winter (Bujarska-Borkowska, 2002).
Therefore, according to Phipps (1998;pers. comm. as cited in Lasseigne & Blazich, 2008) and Davies et al. (2017), a double dormancy of embryo and endocarp dormancy can be postulated for C.

monogyna.
For propagation purposes, fruit are collected best in October when they are fully ripe, either readily from the ground or handpicked from the plants (Brinkman, 1974 as cited in Lasseigne & Blazich, 2008). To store seeds, fruit need to be either dried at TA B L E 3 Pulp constituents of C. monogyna expressed in relation to dry mass of pulp except for WCF (Herrera, 1987) Borkowska, 2002) or macerated and floated to remove the pericarp (Munson, 1986). After air-drying, the seeds should be stored under refrigerated conditions to remain viable for 2 to 3 years (Dirr & Heuser, 2006). St. John (1982), however, noted decreased seed viability after storage for 2 years. This may be the reason why no persistent seed bank in nature has been reported (Grime et al., 1988), although Kheloufi et al. (2019) found 20%-25% of seeds dormant and only about 5% dead after cold stratification under natural-like conditions.
To overcome seed dormancy, manifold methods are published. Deno (1993) found a germination rate of 55% after a coldwarm-cold-warm-cold cycle of 3 months each at 21°C or 4°C respectively. A germination rate of 80% was gained after acid scarification and cold stratification at 2-4°C (St. John, 1982). The ground-breaking study of Bujarska-Borkowska (2002) lists different thermal regimes of pre-treatments which led to a germination rate of 90%: Fully ripe fruit dried to a moisture content of 10% at room temperature should experience 16 weeks at 25°C or cyclical 20-30°C, the latter at either 16 + 8 or 24 + 24 hr per day followed by 14-18 weeks at 3°C.

| Seedling morphology
Germination is epigeal and the cotyledons are green, ovate and fleshy. The first leaves are smaller and less dissected compared to the subsequent ones (Lippert, 1995). Seedling morphology is shown in Figure 4. The seedlings may reach heights of 40 cm after one year (Köpp, 1987as cited in Schuck, 2005.

| Animal feeders or parasites
Its thorns render C. monogyna relatively unpalatable and, therefore, not a preferred diet of grazing mammals, though donkeys (Equus asinus asinus L.;  and, in winter, Highland cattle (Bos primigenius taurus L.;  in Belgium, sheep (Bos ovis L.) in North Wales (Good et al., 1990) and feral goats (Capra hircus L.) in Somerset (Smith & Bullock, 1993) were found to graze on C. monogyna-containing dune scrub. C. monogyna also has had reduced cover due to grazing by Przewalski's horses (Equus ferus przewalskii Poljakow) and red deer (Cervus elaphus L. ;Hanauer et al., 2012). Bushes may persist in a heavily grazed state at 30-50 cm height, as they are too thorny to be finished off by grazing animals, and seem stuck in this state because the young shoots are eaten down when they emerge above the thorny mass (C.D. Preston, pers. comm.). Seedlings and young, non-thorny plants are grazed by the European rabbit (Oryctolagus cuniculus (L.)) which led to proliferation of hawthorn scrub after introduction of myxomatosis in the 1950s (Thomas, 1960). The foliage is reported to be relatively unpalatable to garden snails (Cornu aspersum (Müller)), but serves a large insect fauna (Wratten et al., 1981). A list of associated insects, their food sources and their preferences is given in Table 4. Further information is available from Biological Records Centre (2019) and Ellis (2001 ongoing). The Palaearctic and Crataegus-exclusive psyllid Cacopsylla peregrina (Foerster) was introduced to North America probably with nursery root-stocks and is potentially injurious to the native Crataegus flora (Wheeler & Stoops, 2001). Edwards and Wratten (1985) suggest a mechanism of inducible defence against foliar predation by insects for C. monogyna. Figure 5 shows the only recorded higher plant parasite of hawthorn from Europe, Viscum album L. (Buhr, 1964;Kubus, 1998).

| Plant diseases
In contrast to other woody species C. monogyna is affected by a huge number of diseases (Kehr & Butin, 2003as cited in Schuck, 2005).
An overview of fungal associates is provided in Table 5. A noteworthy and harmful disease is fire blight, induced by the bacterium Erwinia amylovora (Burrill) Winslow et al. (Lippert, 1995).
Introduced from the United States to Europe (southern England) in 1957 (Crosse et al., 1958), this disease spread across the continent (France 1978, Switzerland 1989, Germany 1990, Yugoslavia 1990 II, III and IV and Ipswichian interglacial substages I, II, III respectively (Godwin, 1984). There is no evidence of presence during glacial stages for the whole genus, but it is supposed to have returned readily to the British Isles if it was excluded in these periods (Godwin, 1984). The oldest subfossil evidence for C.
monogyna on the British Isles at Ballybetagh, Dublin, Ireland is dated in the Boreal (Flandrian zone V; Godwin, 1984) and the one on con- Hence, hawthorn was planted extensively across the continent as part of wind-break hedges and on clearance cairns (Hegi, 1923).
An ideal hedge for these purposes has an A-shape of 2-m height.
A special form of hedges are the 'knicks' of Central European lowlands (Weber, 2003). Hawthorn among other shrubs was planted and either bent down and interwoven or partially cut down and laid crosswise to keep stock enclosed. For this purpose, it was even introduced by settlers in Australia (Bass, 1990a). As planting material has been grown from cuttings or uniform seeds in nurseries, hedgerows may be genetically uniform for long distances. However, planted shrubs may have been derived from diverse locations and the local genotypes seem no longer to be grown in Britain (Sell & Murrell, 2014). Whereas Schwarz (1899) surprisingly considered C.
monogyna to be less frequent than C. laevigata, C. monogyna is now the most frequent species of hawthorn across Europe and its natural distribution remains somewhat unclear (Batkó, 1946). In periods of agricultural depression, C. monogyna has been the major colonist of abandoned, agriculturally marginal, heavy clay, arable land and pasture in eastern England and such stands are illustrated by Tansley (1939); they tend to be cleared when prosperity returns. The extent of hedges declined dramatically in the 20th century when they were cut down either for farmland consolidation or to reduce the risk of epidemic spreading of fire blight (Lippert, 1995).
Individual hawthorns are mentioned as landmarks in Anglo-Saxon boundary charters, most of which date from the 10th century, and in Wiltshire they are mentioned twice as frequently as any other tree or shrub (Grose, 1947). The first botanical reference to hawthorn in Britain is the mention in Turner's (1562). Herball of 'our common hawthorn' (Pearman, 2017); Turner also mentions the thorn at Glastonbury that is green all winter.

| Uses
Being a 'common and abundant wild species with a wide distribution area', hawthorn is not only an object in the European ethnomedicinal flora (Pardo-de-Santayana et al., 2015), but also used in many other ways. Regarding its medicinal uses C. monogyna homeopathic remedies such as infusions or tonic extracts made from leaves, flowers and fruit-Folia Crataegi cum floribus and Fructus Crataegi-are applied in case of cardiovascular diseases (Lippert, 1995). As a treatment of chronic congestive heart failure stage II, as defined by the New York Heart Association, its usage is supported by clinical data and the support of cardiac and circulatory functions data ( A light brown or yellow dye for garments was yielded by decocting leaves, bark and roots (Pojarkova, 1939). C. monogyna is suitable for living fences as it tolerates clipping very well. Clipped twigs and branches were used in graduation towers to enrich the salinity (Ehlers, 1960) and due to its density and hardness the wood was used for lathe work, tool handles, sticks, etc. (Pojarkova, 1939). Among the variation of this species, there are several horticulturally selected forms for ornamental purpose (Jablonski, 2020;Krüssmann, 1976). C. monogyna is a preferred stock for grafting other slow growing pomoid Rosaceae (apple, pear, quince, medlar, Japanese medlar; Pojarkova, 1939;Schretzenmayr & Hermann, 1990). Fresh fruit have been used for stewed fruit and jelly or as part of fruit cakes or were fed to pigs (Düll & Kutzelnigg, 2016;Pojarkova, 1939;Schuck, 2005). In time of need they were also dried and milled to extend flour (Düll & Kutzelnigg, 2016). This use is testified to by several regional German vernacular names of Crataegus species, for example, 'Mehlkübeli' (Bavarian Franconia), 'Mâelfässer' (Thuringia) and 'Mehlfässli' (Switzerland; Hegi, 1923). Roasted pyrenes have replaced coffee (Witt, 1995as cited in Schuck, 2005.

| CON S ERVATI ON AND MANAG EMENT
Crataegus monogyna is classified as of 'Least Concern' on the IUCN red list in Europe (Rivers et al., 2017). However, as a component of hedges it suffered from their condemnation in the 1840s 'for taking up so much land, making the use of machinery difficult, acting as weed magazines and asylums of pests, impoverishing the soil and preventing the free circulation of air' (Sheail, 2005). Although opinions have changed since, hedges are still locally threatened by either farmland consolidation (Schubert et al., 2001) or mismanagement and neglect. Watt (1994, 1995) summed up that hedgerow management originated from the understanding of growth stimulation of woody shrubs by repeated cuttings while uncut shrubs will naturally grow more tree-like than getting bushy.
The principal methods of cutting are 'plashing' (laying), coppicing and trimming (Churchward & Shea, 2005 and the literature cited therein). A variety of techniques for each method is available and these occur in various combinations. Most hedges are cut by flails.
Although hawthorn branches are likely more damaged by them than by finger-bar cutters or circular saws, this technique favours branching further from the cut end (Semple et al., 1994). The ideal shape of hedges is controversial, for example, Maclean (1992) considered the formation of mulch by accumulation of trimmings at the bottom of a flat-topped hedge as detrimental to wildlife and thus favoured an A-shaped or rectangular hedge with a chamfered top, while Deane (1989) and others considered that an A-shaped hedge may be detrimental to insects. Although the A-shape is not self-shading, ground flora may be shaded out.
An unpublished study by Hooper (1992; as cited in Barr & Stuart, 2005) concluded three points for wildlife and landscape benefiting by hedge management: (a) management to produce as large a volume of woody growth as is compatible with farming operations; (b) hedge bottom management to produce an herbaceous, grassy strip about a metre wide on either side; and (c) hedge-top management to allow sapling trees to grow. As part of the majority of hedges, hawthorn provides important nesting shelter for many species of bird through its dense thorny foliage and serves as a food plant for more than 100 insect species. Furthermore, hawthorns produce pollen and nectar for pollinating insects and provide abundant fruit as food, especially for thrushes over a long period, lasting into winter.

ACK N OWLED G EM ENTS
Support in drawing the distribution map by Dr. Erik Welk, Halle (Saale), Germany is gratefully acknowledged. We also thank Dr.
Abdenour Kheloufi, Batna, Algeria for providing pictures of germinating seeds and Dr. Theresa Reimann, Erlangen, Germany for drawing them as well as Dr. Walter Welß, Erlangen, Germany for access to his private library. Open access funding enabled and organized by Projekt DEAL.