Abstract
Crataegus species are common and widespread shrubs or small trees across Europe, and are of considerable ecological and agricultural importance. However, controversy still surrounds the taxonomy and ecology of this genus, mainly due to frequent hybridization between sympatric species and introgression of genes. Considering the poor representation of Crataegus-related research in the Romanian botanical literature, we investigated the occurrence and morphology of native Crataegus species and their putative hybrids in north-western Romania. We collected herbarium specimens from three regions (Zărandului Mountains, Transylvanian Plain, Călata region), from a variety of habitats, including forests, grasslands and forest-grassland ecotones. Morphometric measurements were taken on a total of 34 fruit-, leaf- and stipule characters on generative shoots. We used PCA and Random Forest analysis to select for the ‘best’ characters differentiating between species and hybrids. We identified ten different Crataegus taxa, including species, subspecies, varieties and their hybrid nothotaxa: Crataegus monogyna (mainly in grasslands), C. rhipidophylla var. rhipidophylla, C. rhipidophylla var. lindmannii and C. laevigata subsp. laevigata, C. laevigata subsp. palmstruchii (mainly in forests), C. × kyrtostyla nothovar. domicensis, C. × media, C. × macrocarpa nothovar. macrocarpa and C. × macrocarpa nothovar. hadensis (mainly in forest openings and near forest edges). The most common taxa identified was C. × kyrtostyla nothovar. kyrtostyla (common in grasslands, rarer in forests). The best differentiating characters between species and hybrids were number of styles, fruit length, number of teeth on stipules, position of sepals on mature fruits, number of serrations on basal leaf lobe, ratio between serrate part of lower leaf lobe and the entire part, depth of basal sinus, and ratio between sepal length and width. Based on the results of the morphometric analysis and literature review, we constructed national identification keys of all Crataegus taxa identified in this study and other confirmed taxa present in Romania. Additionally, we have illustrated the most common six identified taxa (including hybrids).
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