Citation: Vagge, I.; Chiaffarelli, G. The Alien Plant Species Impact in Rice Crops in Northwestern Italy. Plants 2023, 12, 2012. https:// doi.org/10.3390/plants12102012 Academic Editors: Kamil Najberek, Wojciech Solarz, Barbara Tokarska- Guzik and Damian Chmura Received: 21 March 2023 Revised: 12 May 2023 Accepted: 15 May 2023 Published: 17 May 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). plants Article The Alien Plant Species Impact in Rice Crops in Northwestern Italy Ilda Vagge * and Gemma Chiaffarelli Department of Agricultural and Environmental Sciences, University of Milan, Via Celoria 2, I-20133 Milan, Italy * Correspondence: ilda.vagge@unimi.it Abstract: Alien species represent one of the causes of biodiversity loss, both in natural and anthropic environments. This study contributes to the assessment of alien species impact on Western Po Plain rice field cultivations, referring to different agricultural management practices and ecological traits. Flora and vegetation were studied (the latter through the phytosociological method), and α-biodiversity was estimated through Shannon and Simpson Indices. Results highlighted a significant floristic contingent depletion and increase in therophyte and alien components, compared to pre- existing studies (1950s); higher α-biodiversity levels in organic farms, compared to conventional farms, but also a higher invasive alien species percentage. The high deterioration of the territorial– landscape context appears to play a major role in shaping these patterns. Some of these alien species are particularly aggressive (e.g., Murdannia keisak), as confirmed by two experimental rice field plots which were left unharvested, continuously flooded, making it possible to assess the competitiveness between weed species. The detected weed vegetation is attributed to the Oryzo sativae-Echinochloetum cruris-galli association, already described for Southern Europe, with two different ecological and floristic variants. Future studies, by including other sites and framing their territorial–landscape context, may further complement this overview on the alien species distribution and behavior in rice fields, hence facilitating their strategic management. Keywords: biodiversity loss; alien invasion; alien flora; vegetation; weed species; phytosociology; organic farming; rice field; rice paddy 1. Introduction Alien species invasion is described as one of the causes of biodiversity loss by many sci- entific studies [1–10] and reports from world organizations such as the IUCN and UN [11–15]. This not only concerns natural and seminatural habitats, but also anthropic environments [3,9]. Biodiversity loss alters the structure of ecosystems, their functionalities, and leads to a decline in ecosystem services, economic losses and human health issues [16–20]. In Italy, 1628 alien plant taxa are documented (about 16% of total Italian flora) [21–26], and can be mainly found in anthropic settings, such as urban and agricultural areas, refor- ested areas and artificial water courses, where they often become the dominant component of vegetational associations. The Po Plain, a highly urbanized, industrialized and inten- sively cultivated lowland in Northern Italy, is the most affected Italian region, especially with respect to invasive alien species [9,21,26,27]. Here, alien species tend to constitute monospecific populations or to become predominant in some habitats. They significantly shape plant communities and threaten the autochthonous habitats, especially those of conservation interest [27,28]. Moreover, it has been shown that intensive, highly simplified, agricultural systems and anthropized territories represent a threat for the native flora while favouring alien species invasion [29]. Such territorial traits affect the vast majority of the Po Plain area, with oversized agricultural patches, often matched with anthropic linear infrastructures, not leaving room to natural and seminatural ecosystems. Plants 2023, 12, 2012. https://doi.org/10.3390/plants12102012 https://www.mdpi.com/journal/plants