Ailanthus altissima Forests Determine a Shift in Herbaceous Layer Richness: A Paired Comparison with Hardwood Native Forests in Sub-Mediterranean Europe

Abstract

Ailanthus altissima is an invasive alien species (IAS) present throughout Europe and included in the list of alien species of Union concern. In sub-Mediterranean areas of central Italy, there is a lack of knowledge about this invasive species and its interactions with the native forest ecosystems. We aim to find what are the main differences in vegetation structure and floristic diversity between A. altissima forests and native forests through the assessment of the principal ecological parameters that differ between the forest types. We performed 38 phytosociological relevés and sampling of ecological parameters in A. altissima forest communities and neighboring native forests. We analyzed how species richness, diversity, life forms, life strategies, structural characteristics, and ecological parameters changed in A. altissima forests compared with native ones. We found that in A. altissima forests, there is a shift in herbaceous layer richness, with a higher presence of annual ruderal herbs and the absence of herbaceous species linked to the forest environment. The ecological parameters that diverge from the native forests were total nitrogen, total carbon, and C/N ratio. A. altissima forest communities could threaten the biodiversity of the native forest ecosystems in the sub-Mediterranean landscape, favoring ruderal species and inhibiting the presence of typical forest species.

First syntaxonomical contribution to the invasive Ailanthus altissima (Mill.) Swingle forest communities at its southern limit in Europe

“…However, this is similar to what happens for the xerophilous and thermophilous communities of the order Euphorbio cyparissiae-Robinietalia of the class Robinietea shown in Vitkova and Kolbek (2010). Moreover, from an ecological point of view, the differences between A. altissima forests and the neighboring native forests were highlighted and proved for both R. pseudoacacia (Montecchiari et al 2020a) and A. altissima forests (Montecchiari et al 2020b). A great effort has been made in Europe to classify alien-dominated forest communities that previously were referred to different orders and classes.…”

“…Its first record for the Ital-ian territory dates back to 1760 in the Botanical Garden of Padua (Badalamenti et al 2012). Now is present in all the Italian administrative regions (Galasso et al 2018) classified as invasive alien species because it can constitute proper forest communities (Montecchiari et al 2020b;Viciani et al 2020) capable to impact native ecosystems and Natura 2000 sites (Lazzaro et al 2020). Moreover, according to the National Forest Inventory (Tabacchi et al 2007), which classifies in a single category the A. altissima and R. pseudoacacia and forests, these formations together occupy almost 250000 ha, equal to 2.23% of the total national wooded area.…”

“…The efficacy in gamic reproduction and dissemination (Knapp and Canham 2000;Motard et al 2011), agamic reproduction (Kowarik 1995;Saumel 2007, Von der Lippe et al 2013) and rapid growth, enable A. altissima to form nearly pure stands (Dihoru and Doniţă 1970;Montecchiari et al 2020b) and to have better competitive ability compared to the forest native species (Arnaboldi et al 2002;Fotiadis et al 2011;Höfle et al 2014;Costàn-Nava et al 2015). Moreover, the production of an allelopathic compound (Ailanthone) from the bark and leaves, can inhibit the germination of native species (Lawrence et al 1991;Bostan et al 2014).…”

“…Badalamenti et al (2012) reports that A. altissima avoids excessively clay soils or soils subject to prolonged water stagnation. Thanks to its highly competitive features A. altissima can establish in a wide variety of environmental conditions and is able to form dense forest population that can also impact soil properties and nutrient cycling (Vilà et al 2006;Gómez-Aparicio et al 2008;Castro-Díez et al 2009;Medina-Villar et al 2015;Motard et al 2015;Montecchiari et al 2020b). Despite the many ecological data available and its wide distribution in the Mediterranean and Temperate Europe, there is an important lack of floristic-vegetational studies on A. altissima forests in its meridional range of distribution, in sub-Mediterranean and Mediterranean areas.…”

Risks, benefits, and knowledge gaps of non-native tree species in Europe

“…were selected due to their potential economic contribution in rural areas (Catry et al, 2015;Pra et al, 2019) and even more so for soil stabilization, fire mitigation, and non-wood products (Palahi et al, 2008). These rapid and intensive introductions, combined with the favorable climatic conditions, have led to some negative outcomes with NNTs, as in the case of Robinia pseudoacacia (Vítková et al, 2020) and Ailanthus altissima (Montecchiari et al, 2020).…”