Copyright © 2015 by the Italian Society of Silviculture and Forest Ecology
doi: 10.3832/ifor1201-008

Collection: 3rd International Elm Conference, Florence (Italy - 2013)
“The elms after 100 years of Dutch Elm disease”
Guest Editors: A. Santini, L. Ghelardini, E. Collin, A. Solla, J. Brunet, M. Faccoli, A. Scala, S. De Vries, J. Buiteveld

Review Papers

Ulmus laevis in the Iberian Peninsula: a review of its ecology and conservation

Martin Venturas ⁽¹⁾, Pablo Fuentes-Utrilla ⁽²⁾, Rosana López ⁽¹⁾, Ramón Perea ⁽¹⁾, Victoria Fernández ⁽¹⁾, Antonio Gascó ⁽³⁾, Paula Guzmán ⁽¹⁾, Meng Li ⁽¹⁾, Jesús Rodríguez-Calcerrada ⁽¹⁾, Eva Miranda ⁽¹⁾, Jorge Domínguez ⁽¹⁾, Guillermo González-Gordaliza ⁽¹⁾, Elena Zafra ⁽¹⁾, Miriam Fajardo-Alcántara ⁽¹⁾, Juan A Martín ⁽¹⁾, Richard Ennos ⁽⁴⁾, Nikos Nanos ⁽¹⁾, Juan J Lucena ⁽⁵⁾, Salustiano Iglesias ⁽⁶⁾, Carmen Collada ⁽¹⁾, Luis Gil ⁽¹⁾

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Introduction 

The European white elm (Ulmus laevis Pallas) is a hardwood deciduous tree which grows in river margins and damp bottomland forests, tolerating flooding for some periods of the year ([17]). Ulmus laevis belongs to section Blepharocarpus ([107]), in contrast to the other two native European elms, U. minor Mill. and U. glabra Huds., which belong to the Ulmus section. Ulmus laevis has been considered to be naturally distributed across Europe, from Ural Mountains to eastern France, and from southern Finland to the Caucasus, Balkans and southern France ([16]). The loss of suitable habitats due to human-induced changes in riparian forests, combined with the effect of Dutch elm disease (DED), has compromised the survival of many U. laevis populations ([16]). It is estimated that only 1 % of the elms still remain alive in Germany ([57]), whereas the white elm is considered an endangered species in northern Belgium ([95]), southern France ([91]), and Finland ([18], [93]). Moreover, many small isolated populations throughout its distribution range are at risk of genetic drift ([16]). As such, this species has been identified as needing specific conservation measures in Europe ([18]).
According to Flora Ibérica ([65]), U. laevis is an established alien species in Spain which was introduced as an ornamental; a conclusion based on its small population sizes and scarcity. This is the opinion which has prevailed among botanists, and thus, limited efforts were initially undertaken towards white elm conservation in Spain. However, the presence of U. laevis in Spain was cited for the first time by Lapeyrouse ([47]) in the Pyrenees. Following this, it was also observed in Asturias ([73]), and as a result, was included in the first Iberian Floras ([108], [4]). Nevertheless, other botanical studies failed to include this species ([19], [49], [46], [12], [79]) or considered it as an introduced species ([11]). More recently, attending to several white elm stand characteristics, certain authors considered that U. laevis could be native to the Iberian Peninsula ([81], [2]).
Following the second DED pandemic ([10]), several small, scattered, previously unknown populations of U. laevis were detected during the field surveys carried out by the Spanish Elm Conservation and Breeding Programme (SECBP, Technical University of Madrid, Spanish Environmental Ministry - Fig. 1). The location and characteristics of some of these newly located populations indicated that human introduction was highly unlikely. The fact that 41 of these 52 stands had not been previously detected was quite surprising, but can be explained by: (i) some stands growing in difficult access areas; (ii) difficulty in distinguishing U. laevis from field elm (U. minor) or wych elm (U. glabra) morphologically, unless the tree has flowers or fruits; and (iii) lack of interest in reporting its presence, given that it was considered an alien species. In addition, U. laevis trees were more easily detected after DED pandemics because they remained alive, whereas other elms died. Despite white elms being highly susceptible to DED ([77], [82]) they can survive the disease by an escape mechanism: bark beetles (Scolytus spp.), the propagation vectors of DED, prefer feeding on U. minor and U. pumila L. ([102], [103]) due to the chemical composition of their bark ([71], [60]).

Fig. 1 - Distribution of Ulmus laevis populations in Spain. Populations previously cited in the literature (blue dots) are differentiated from those detected by the Spanish Elm Conservation and Breeding Programme (red dots). Modified from Venturas et al. ([98]).

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Determining whether or not small marginal populations of a species are native is complicated, especially in Mediterranean riparian forests, as they have been deeply transformed by humans over the last 4000 years ([33], [94]). Indeed, all elm species were initially considered to be introduced in the Iberian Peninsula due to the lack of evidence for their presence in the first fossil records ([37]). Following this, and based on taxonomic traits, Richens & Jeffers ([78]) established that U. glabra was native to northern Spain, and considered that if U. minor was to be indigenous, it would only be so in the eastern half of the Iberian Peninsula. When the number of palaeobotanic records increased and Ulmus spp. pollen was found all over Spain ([25], [50]), fossil remains were assumed to belong to U. glabra in the mountain areas, and to U. minor in the lowlands. Contrary to the hypothesis put forth by Richens & Jeffers ([78]), palaeobotanic remains which appeared in non-mountainous areas of western Spain were assumed to be from U. minor, despite the fact that pollen and wood from European elms cannot be morphologically distinguished at species level ([80], [86]). Therefore, after the discovery of new U. laevis populations, the following question arose: could part of the pollen from western Spain belong to U. laevis?
Herein, we review the process followed by SECBP to demonstrate the nativeness of U. laevis in Spain, and the species’ natural distribution, ecology, conservation status and recovery possibilities.