SCHMIDT,PeterA.
Dresden University of Technology; Deutsche Dendrologische Gesellschaft, D–01640 Coswig bei Dresden, Am Wasserwerk 24, Germany, praesident@ddg-web.de,
peteraschmidt@yahoo.de
Abstract – The paper documents the recent knowledge of Crataegus in Germany. The author discusses some of the main causes of the “Crataegus problem”. Variation of taxonomically important characters, hybridization, human influence on the evolution and distribution of the hawthorns belong to the main factors in Germany. An account of species groups (agg.) and species as well as hybrid complexes (agg.) and nothospecies is given. A table allows comparing this concept with different concepts, which does not accept species but subspecies or varieties for one species group (Crataegus rhipidophylla agg.) and its hybrid complexes (Crataegus ×macrocarpa agg., Crataegus ×subspaerica agg.).
Key words: Crataegus problem, taxonomy, variation, hybridization, human influence, species groups, nothospecies, Germany
INTRODUCTION
In the last decades of 20th century numerous studies about Crataegus were published in Europe (e.g. CHRISTENSEN 1985,1992,LIPPERT 1994) and in several European countries (e. g. CINOVSKIS 1971, CHRISTENSEN 1982, GOSTYNSKA -JAKUSZEWSKA & HRABĚTOVÁ-UHROVÁ 1983, BARANEC 1986, HOLUB 1992), among others in Germany (e. g. DOLL 1976,LIPPERT 1978,SCHMIDT 1981,1995;
LOOS 1994). In the 1970th DOLL (1975, 1976) accepted for Germany numerous species (8) and hybrids (11), and he described 20 species and hybrids as new, often only from single localities in NE Germany. Other students of Crataegus (e.g.
LIPPERT 1978, 1994, MÜLLER 1994, CHRISTENSEN 1998, LOOS 1994, 2007, SCHMIDT 1981-2011,BUTTLER et al. 2014) agree with a limited number of species and nothospecies (or species of hybridogeneous origin). However, there are considerable differences about the number and taxonomic ranking of the taxa (species and nothospecies; species and subspecies / nothosubspecies with different figures about their number; species and varieties / nothovarieties; see table 1).
Unfortunately, in Germany up to now serious experimental, moleculargenetical or phytochemical, work in Crataegus is lacking. A recent study (LEINEMANN et al.
2015) of DNA for practical purposes, to be precise fingerprint method for identification of plants for seed collection and trade, deals only with a “Monogyna Group” (C. monogyna, incl. plants coming near this species and hybrids of it) and
“Laevigata Group” (C. laevigata, incl. plants coming near this species and hybrids of it). Gaps in knowledge of the reproductive system and molecular genetics, different taxonomic concepts, frequent nomenclatural changes are some reasons for the difficulties and open questions. Discussion about “The Crataegus Problem” are not new, already CAMP (1942) putted, regarding the Crataegus species in North America, the question “Why does ‘The Crataegus Problem’ exist?”. The problems in the understanding and taxonomic treatment of Crataegus are the result of a number of
factors (see BYATT 1974,LIPPERT 1978,1994,SCHMIDT 1981,1995,CHRISTENSEN
1982,1992,BARANEC 1986,HOLUB 1992,LOOS 1994), e. g.
- the species are inherently variable (e. g. heterophylly and heteroblasty),
- hybridisation, introgression, and subsequent polyploidy or even apomixis may occur,
- human influence on the population dynamic, the distribution pattern, the formation of hybrids, the evolution of the species,
- botanists have described a large number of new (micro)species and hybrids based on very slight differences (e.g. CINOVSKIS 1971, HRABĚTOVÁ-UHROVÁ
1973, DOLL 1975),
- often only herbarium studies were done without sufficient field and experimental work to determine the constancy of the distinctive character states.
Variability and characters of taxonomic importance
Crataegus plants characterized by a well-developed heterophylly and heteroblasty. The between-shoot variation (short and flowering shoots comparing with those of long shoots) and within-shoot variation are considerable. However, when these variations are accounted for, morphological characters of leaves (esp.
deeply/ shallowly/ nearly not lobed; number of pairs of lobes; serrate or crenate leaf margin, number and shape of teeth of basal lobes) and stipules (esp. entire, serrate, irregularly denticulate-serrate) can be scored for taxonomic studies and the determination of plants. Therefore, for identification of Crataegus taxa are to score only leaves of the same position: the characters of the subterminal leaves of flowering or short shoots and their stipules. To obtain all the taxonomically important morphological data for investigations in Crataegus, plant material of both flowering and fruiting time, is needed. The precise identification of plants of the native Crataegus taxa in Germany without flowers and/or fruits is mostly impossible. Firstly the number of styles per flower and/or the number of pyrenes per fruit are to determine: only 1 (C. monogyna, C. rhipidophylla agg., C. ×subsphaerica agg.); 1 and 2 (C. ×macrocarpa agg., C. ×media); only 2 and rarely 3 (C. laevigata).
Because of the possible intra-individual variation in some taxa, several flowers or fruits of more than one shoot are to score. In herbarium sheets the plant material often is not complete, one of the reasons for differences in identification of type material (e.g. C. rhipidophylla, see below). The two species of C. rhipidophylla agg. (C.
rhipidophylla / C. lindmanii) and their hybrids (nothospecies C. ×macrocarpa / C.
×calycina, C. ×subsphaerica / C. ×domicensis) can be exactly identified only by characters of fruits (size, shape, colour of fruits) and the sepals of the persistant fruit calyx (erect, spreading or reflexed sepals; shape of sepals). This fact causes problems for mapping of the flora of a country or provinces, for vegetation surveying etc., if fruiting material is not available. From this practical reason author have published papers with a different ranking of the taxa in the C. rhipidophylla agg.
and the hybrid complexes C. ×macrocarpa agg. and C. ×subsphaerica agg. (see table 1, concept A and B). He used sometimes the taxonomic category subspecies (e.g.
SCHMIDT 1995, 2000, 2005), because this concept allows the identification of
non-fruiting plants as species (s. l.) or nothospecies (s. l.), but taxa, which can be identified only if fruits are available were ranked at subspecies level. The author is being aware that this proceeding is a simple practical one and not in accordance with the general concept of allopatric subspecies as geographical or ecological races.
Hybridization
A range of different reproductive and variation patterns found in the genus Crataegus, from ordinary sexual outbreeders through polytypic forms with various cytotypes, to polyploid species and hybrid complexes, (facultative) apomixis may occur (PHIPPS – MUNIYAMMA 1980, CHRISTENSEN 1992). Hybridization, introgression, and subsequent polyploidy play an important role in evolution of the genus in Europe (e.g. LIPPERT 1978,CHRISTENSEN 1992,LOOS 1994). Backcrosses of a parental species with the hybrid (e.g. C. laevigata × C. ×macrocarpa) and combinations of these found (e.g. the questionable C. “palmstruchii”, see below 1.2).
HOLUB (1992) therefore described not only hybrids, but mentioned also
“polyhybrids” (result of backcrossing), “superhybrids” (crossings of hybrids) and introgressants. DANIHELKA et al. (2012) listed for Czech Republic also 7 hybrid combinations between a species and a nothospecies. Of course, if thinking about the limited characters for identification, the lack of knowledge in the genetic structure of populations the identification of plants of these hybrid complexes seems to be dubious. Because of forming hybrid swarms with numerous nothomorphs, boundaries between such nothotaxa scarcely are to find. Hybrids were and are be formed not only occasionally, but hybridization and subsequent introgression may result in the formation of extensive hybrid complexes (LIPPERT
1978, SCHMIDT 1981, CHRISTENSEN 1982). Hybrid plants are mostly vigorous.
High vitality enables fertile hybrids to get dominant in hawthorn stands. After dispersal, they are locally or regionally more frequent as the parents (e.g.
CINOVSKIS 1971 for Baltic region, NETPHYD et al. 2013 for Germany). It appears that in Germany only a very few external barriers to hybridization between the sympatric taxa exist. The ecological barriers between more shade-tolerant and light-demanding species were removed already in the Middle Ages (see below: Human influence). The differences in flowering time of C. laevigata and C. monogyna is the only remarkable barrier. If these two species grow in the same altitudinal range in equal ecological conditions C. laevigata flowers 1-2 weeks before C. monogyna. Only occasionally, in regions where a mosaic of sites (e.g. with northern and southern slopes) exists, plants of both species may flower at the same time. That is the reason why their hybrid C. ×media is less frequent, although plants of this nothospecies also were being plant in hedges.
Human influence
In Germany in the Middle Ages the woodlands were used for pasturing and logged for settlements and agriculture (fields, meadows). The former continuous woodlands were been fragmented, and often only forest islands survived. The cultural landscape with open forests (pasture woodland), wood islands, and a high degree of ecotones (forest and scrub edges) and agricultural fields, hedges, and
other open sites or scrubs was been created. The hawthorns could spread, ecological barriers disappeared, the more shade-tolerant species (C. laevigata, C.
rhipidophylla agg.) of the forests came into contact with more light-demanding species of the former natural forest edges and shrub ecosystems (C. monogyna).
Hybrids in the altered or disturbed landscape were formed (C. ×macrocarpa, C.
×subsphaerica), could evolve and spread. Later hybridization took place not only at ecotones, because after extending in areas, where the species now were sympatric, also back-crossings were possible. Intermediates could form continuous interbreeding populations. In the Industrial Age many hedges and the ecotones of forest edges were removed, and also hawthorn plants or even stands were cut. The population structures again were severely influenced by human activities, so in some regions hybrids left, and one of the parental species disappeared. On the other hand, centuries ago, hedges also were been planted, using hawthorns (esp. C.
monogyna, broad ecological spectrum, lesser demands of site conditions). Some decades ago planting of hedges again started, often using non-autochtonous plant material from nurseries, among others hybrids (esp. C. ×subsphaerica, ecology similar to C. monogyna). Some hybrids are rare (C. ×domicensis), others are frequent. Recently hybridogenous populations (esp. C. ×macrocarpa) locally are more frequent than (one of) the parent species.
Account of the Crataegus species in Germany (see table 1)
The account follows the acceptance of species and nothospecies in the C.
rhipidophylla agg., and its hybrid complexes (concept A in table 1). In table 1 you find also the different opinions of authors who accept no (notho)species but (notho)subspecies (concept 2) or (notho)varieties in C. rhipidophylla agg. and its hybrid groups. In the following account, only synonyms of species level given, for others see table 1. According the concept A, which most of the recent students of Crataegus in Germany follow, we have 3 species groups (agg.: 1, 2, 3) with 4 species (1.1, 2.1, 3.1., 3.2) and 3 nothospecies groups (agg.: 1×2, 1×3, 2×3) with 5 hybrids (1×2, 1×3.1, 1×3.2, 2×3.1, 2×3.2). Because populations as results of hybridogenous processes may stabilize, some other consider nothospecies as a species (e.g. BUTTLER et al. 2014). Hybrids between species of a species group (3.1×3.2) and backcrossing between species and nothospecies (see remarks 1.2) may also occur.
- Species groups and species 1 C. laevigata agg./ s.l.
1.1 C. laevigata (POIR.) DC. (C. laevigata s. str.)
Subatlantic floristic element, extending from W- and Central Europe to S-Scandinavia, the Baltic region, Romania, Hungary and Italy.
1.2 C. palmstruchii auct.
Unsolved problem: According CHRISTENSEN (1992) the lectotype of C. palmstruchii LINDMAN belongs to C. laevigata s.str. Some other authors (e.g. GOSTYNSKA -JAKUSZEWSKA &HRABĚTOVÁ-UHROVÁ 1983): accepted a taxon “palmstruchii” (or C. laevigata subsp. palmstruchii), slightly different of C. laevigata s.str., but mentioned
(e. g. SCHMIDT 2005, 2011), whether it may belong to a hybrid of C. ×macrocarpa agg., or being a result of backcrossing C. laevigata × C. ×macrocarpa. The same problem is to C. walokochiana (HRABĚTOVÁ)SOÓ (C. laevigata subsp. walokochiana (HRABĚTOVÁ)HOLUB), a name which also was in use for a hybrid 1.1×1.2.
2 C. monogyna agg./s.l.
2.1 C. monogyna JACQ. (C. monogyna s.str., incl. C. alemanniensis et C. subborealis CINOVSKIS)
A widespread European species, extending from Europe to N Africa, Turkey and the Caucasus Region. In Germany only subsp. monogyna (incl. subsp. nordica FRANCO, Syn. C. orientobaltica CINOVSKIS).
3 C. rhipidophylla agg. (C. curvisepala agg., C. rosiformis agg.)
3.1 C. rhipidophylla GAND. (C. rhipidophylla s.str., C. curvisepala LINDMAN
nom. illegit., C. praemonticola HOLUB)
Subatlantic to subcontinental floristic element, extending from Central to E and SE Europe, Turkey and the Caucasus Region.
Unsolved problem: because of nomenclatural reasons or different taxonomic treatments, several names were used for this species (C. curvisepala LINDMAN – C.
rosiformis JANKA – C. praemonticola HOLUB – C. rhipidophylla GAND.), one replaced the other during some decades. Recently again authors changed the name, which mostly was used last time (C. rhipidophylla). Although CHRISTENSEN (1992), the great student of Old World hawthorns, checked the holotype, the type according French authors (TISON &FOUCAULT 2014) does not belong to that species. They identified the type as a hybrid (C. ×subsphaerica GAND.), and call the species C.
rosiformis JANKA, a name, which already was used earlier (CHRISTENSEN 1985) but replaced by C. rhipidophylla by the same author (CHRISTENSEN 1992). Now again a discussion started about the correctness of the name C. rosiformis, because the identification of the not complete type specimen of JANKA is also uncertain. If botanists who have checked and will check again the types of the mentioned names will find no agreement, the only name without any doubt would be C. praemonticola (used by Czech authors, e.g. HOLUB 1992).
3.2 C. lindmanii HRABĚTOVÁ
Subatlantic floristic element, extending from Central Europe to S-Scandinavia, the Baltic region, Hungary and Romania.
3.1 × 3.2 C. ×dunensis CINOVSKIS: this name is in use for plants, which are intermediates between C. rhipidophylla and C. lindmanii.
- Hybrid complexes and nothospecies 1 × 2 C. ×media BECHST.
1 × 3 C. ×macrocarpa agg. (C. ×calycina agg.)
1 × 3.1 C. × macrocarpa HEGETSCHW. (C. ×macrocarpa s.str., C. ×schumacheri RAUNK., C. ×pseudoxyacantha CINOVSKIS, C. ×uhrovae SOÓ)
1 × 3.2 C. ×calycina PETERM. (C. calciphila HRABĚTOVÁ)
2 × 3 C. ×subsphaerica agg. (C. ×kyrtostyla agg., C. heterodonta agg.)
2 × 3.1 C. ×subsphaerica GAND. (C. ×subsphaerica s.str., C. kyrtostyla auct. non FINGERH., C. ×raavadensis RAUNK., C. ×fallacina KLOKOV, C. ×heterodonta POJARK.) 2 × 3.2 C. ×domicensis HRABĚTOVÁ (C. ×plagiosepala POJARK.).