Dunes under attack: temporal trends of iceplant invasion (Carpobrotus sp. pl., Aizoaceae) in different landscape contexts. An insight on Mediterranean coasts

Carpobrotus acinaciformis, C. edulis (Aizoaceae) and their hybrids, native to South Africa, are considered as one of the worst Invasive Alien plants (IAPs) along the Mediterranean coastal dunes causing irreversible damage in ecosystem biodiversity and loss of ecosystem services [1]. The Convention on Biological Diversity (https://www.cbd.int/) and the Regulation (EU) no. 1143/2014 on Invasive Alien Species propose a common strategy to monitor the invasion and to contrast the negative impacts of IAPs [2]. The invasion of these two species and their hybrids with similar impacts, referred here as Carpobrotus spp., is a highly dynamic process shaped by the interplay of biotic factors (e.g. competition with native communities, spread by mammals or birds), abiotic ones (e.g. coastal erosion and accumulation, seashore distance) and anthropic pressure (e.g. dune trampling, land take, artificial infrastructures) [3,4]. Due to the widespread presence of Carpobrotus spp. on Mediterranean coasts and the huge threat it poses to biodiversity, important efforts have been devoted to mapping it [5], studying its relation with biodiversity [6] and analyzing its spatial pattern in the landscape [7]. However multi-temporal studies exploring the mutual relationship between landscape dynamics and alien species expansion or contraction are still lacking. In this context, the present work sets out to investigate landscape pattern changes occurred in areas invaded by Carpobrotus spp. (i.e. composition, configuration) during the last decade (2012-2022), and to examine the relationships among such spatial pattern changes and invaded patches dynamics. We produced fine-scale (1:2000) land-cover/vegetation maps for two time steps (years 2012 and 2022) on invaded coastal dune in central Mediterranean (Lazio region, Italy). We mapped 95 invaded sites with a round buffer of 100 m radius by visual inspection of aerial orthophotos in Google Earth at the fourth level of CORINE land cover legend. Overall landscape change was analyzed by a transition matrix and summarized them by a chord-diagram [8]. We classified the mapped areas into two categories: EXPCAR gathering sites in which alien expansion occurs (e.g. increasing cover of Carpobrotus spp. patches in 2022) and REDCAR grouping sites with a reduction of alien cover over time (e.g. decreasing cover of Carpobrotus spp. patches in 2022). For each area and time step, we calculated a set of non-redundant landscape metrics (LM) depicting the spatial pattern of the whole coastal mosaic (LMland) and of each land cover class (LMclass). In particular: the percentage of the landscape covered by each class (PLAND), patch density (PD), edge density (ED), mean patch area (AREA_MN), Shannon index (SHDI) and Simpson’s index (SIDI) [9] and we tested their temporal difference through non-parametric pair-wise Wilcoxon rank test. Then, we quantified changes in pattern metrics occurring in each cover category by computing ΔLM = 2022LM - 2012LM and we explored the partial dependence of the changes on Carpobrotus spp. spatial pattern (ΔLMCAR) in the coastal dune landscape variables without multicollinearity problem and Variance Inflation Factor (VIF) lesser of three (ΔLMclass, ΔLMland) using a Random Forest model (RF) and Partial Dependence Plot (PDP) [10]. In general, the transition matrix confirms natural dynamism of coastal dune during the last 10-years with comparable shifts towards adjacent land cover classes. Landscape analysis over time evidenced important changes on Carpobrotus spp. spatial pattern during the last decade (2012-2022) and such changes varied across coastal dune tracts in which invasion is expanding (EXPCAR) or contracting (REDCAR). In EXPCAR areas a quite stable coastal context was evidenced, while in REDCAR intense erosion process was detected. Focusing on Carpobrotus spp., in EXPCAR areas, the chord diagram evidenced an increment of CAR class extension substituting HDV class, and on REDCAR areas CAR class decreased and was replaced mainly by SEA and partially recovery by HDV. Consequently, the spatial pattern of Carpobrotus spp. patches significantly changed over time with opposite trends, all landscape metrics intensify in EXPCAR areas causing increase in pressure and impacts on HDV, while in REDCAR areas all landscape metrics show a contraction and simplification of invasion due to very high fragmentation caused by erosion process. Moreover, all RF models and PDP, adequately described the relation of temporal changes in Carpobrotus spp. composition (ΔPLANDCAR) and configuration (ΔPDCAR, ΔEDCAR, ΔAREA_MNCAR) with changes on the spatial pattern of the surrounding landscape (ΔLMclass and ΔLMland) with R2 values greater than 0.405. The different trends of Carpobrotus spp. expansion or reduction denote the heterogeneous contribution of class and landscape dynamics in determining its spread. Here for the first time, we quantify the multi-temporal effects of landscape pattern and processes to shape Carpobrotus spp. invasion discerning different trends in increase and contraction of invasion [11]. These information concerning IAP occurrence needs to be supported by the knowledge on landscape processes to identify effective conservation and management strategies as claimed by international and regional regulations on biodiversity.