Data on the current distribution of A. projectellus were gathered from the literature (Motiejūnaitė et al. 2011; Wrzosek et al. 2017; Banasiak et al. 2019b), herbaria, the Global Biodiversity Information Facility (GBIF 2025a, 2025b) and our own extensive field survey. Direct use of all GBIF records could be limited because of the autocorrelation of several closely located records. Thus, exclusively records for which the precise geographic coordinates were available were used in analyses. From a total of 447 locations of A. projectellus gathered in this study, the multiple species records from each grid cell (in the same 21.62 km² equator) were removed. A final database included 325 occurrence data (Fig. 1). Both maps were generated in ArcGis 10.8 software powered by Esri. The distribution of the fungus in Poland was presented based ona map of counties (‘powiat’ of which there are 380 in Poland) and voivodships (16) as basic spatial units (Fig. 2).

In September 2019 we published an online questionnaire on the species, which was active till March 2024, and supplemented with a question about dishes made from the species in 2023. The survey was completely anonymous. We asked about general information (like gender, age, place of life, and place of birth) data specific to a given record (georeferences and date of observation), year of first observation, experience in the mushroom collection (number of collected species, number of known fungi), ethnomycological issues (name, that gatherers use, how they perceive with fruitbodies) and others. The full questionnaire is attached as a Suppl. material 1. The link to the questionnaire was published on social media, mainly on various Facebook fungi enthusiast forums. This yielded 165 answers (three questionnaires were excluded due to the nonserious nature of the responses). Additionally, interviews were done on the Baltic coast near Stilo, where the species was first observed in Poland, at a site popular among mushroom pickers (112 interviews). To confirm the correct identification of A. projectellus interviewers added the photo showing fruitbody in case of an online survey. For people interviewed in the forest, this confirmation was performed by naked eye, based on the content of their basket. Some go there specially to see or collect the species and they are advanced mushroom pickers. Altogether, 274 people provided meaningful information, either online or during field interviews, between 2019 and 2024.

To compare our data with existing data on the species distribution, we used our own first author’s (M.P.) unpublished data as well as available literature. Towards the end of our study, we encountered an international survey on the distribution of A. projectellus organized by Inaturalist (https://www.inaturalist.org/observations?project_id=aureoboletus-projectellus-spread-in-europe&subview=map&verifiable=any) but did not use it in statistical comparison, as some of our information suggested that some of our respondents later also filled in the Inaturalist questionnaire.

Spearman’s rank correlation was performed using PAST 4.03 (Hammer and Harper 2003) to evaluate the correlation between the collectors’ origin and the number of reports detected in a given regions. Because of the reverse relationship between the number of collectors and the number of records (collectors travelled specifically to the Pomorskie region knowing that the fungus occurs there, rather than encountering it incidentally and potentially spreading it further), we excluded Pomorskie from the analysis. We also present trends in the observed number of fungus records in Poland, expressed per time, distance, and cumulatively. We assessed the overall trend in the cumulative number of records over time by fitting both exponential and curvilinear functions. These estimators of introduction rate are among the most long-standing and frequently utilised indicators of biological invasions. (Liebhold and Tobin 2008; Tittensor et al. 2014; Seebens et al. 2017). We assumed that fungus was first reported in Europe in 2007 (Motiejūnaitė et al. 2011).

The anonymized data matrix was deposited in the Scientific Data Repository of Rzeszów University (see Data Availability section).

The first documented record of A. projectellus in Europe was from the Curonian Spit in Lithuania in 2007 (Motiejūnaitė et al. 2011). The authors suggested that fruitbodies of the fungus may have been observed in this region as early as the 1980s; however, this claim cannot be substantiated with existing literature or available data. In Poland, the earliest record dates to before 2011 (Wrzosek et al. 2017), as confirmed in interviews gathered in this study. According to ethnomycological survey data, the fungus has been present in northern Poland since the late 1980s. Nonetheless, its earlier presence in Poland is not supported by long-term monitoring of mycobiota in the Pomeranian region (Stasińska and Sotek 2003; Stasińska 2005). Therefore, we consider 2007 as the potential starting point of the species’ expansion in Poland. Since then, the distribution range of the fungus has steadily grown, now extending over 600 km inland and encompassing almost all voivodships. Previously, the species had been recorded in 32 out of 380 counties of Poland. Our questionnaire-based survey confirms its presence in 60 counties. The fungus had previously been reported in 20 of these. In addition, this study revealed 56 new locations of the fungus in Poland, primarily in the southeastern part of the country.

The expansion pattern of A. projectellus generated based on ethnomycological interviews (Fig. 3A) corresponds to that of a typical invasion curve, illustrating the phases (Blackburn et al. 2011) and stages (Heger and Treplof 2003) of an alien organism’s invasion. During the initial stage (since its introduction to 2012), the fungus was recorded in only a few locations, all situated closely to the Baltic seashore (Wrzosek et al. 2018). In 2012, A. projectellus was reported for the first time in central Poland. The trend in the cumulative number of records over time, reflected by both exponential and curvilinear functions, indicates a rapid increase in fungus reports (Fig. 3A). Since 2017, the species has entered a rapid expansion phase, characterized by a sharp increase in population size and wide dispersal. Numerous new occurrences were reported in southeastern Poland – over 500 km from the original introduction site (Fig. 3B). The last year of interviews, 2023, showed the highest number of records, which increased in north, central and south Poland (up to 100 km, 100–400 km, and beyond 500 km, respectively, Fig. 3B). The cumulative curve of A. projectellus records (Fig. 3A) has not yet plateaued, suggesting that the species’ range is still expanding. This ongoing spread likely reflects continued colonization of new areas, with the fungus successfully dispersing, establishing, and reproducing across a broad geographic range, including sites at both short and long distances from the original introduction point. Based on the framework proposed by Blackburn et al. (2011), the observed expansion pattern of A. projectellus supports its classification as an invasive fungal species.

Out of several alien plant species that have found culinary use in Poland, A. projectellus is the first exotic fungus to be widely eaten. As far as plants are concerned, the prime example of an invasive species used for food is Robinia pseudoacacia, introduced to Poland at the turn of the 18^th and 19^th century, whose flowers have been popularly made into fritters (Łuczaj 2010). Some other invasive plants, such as Galinsoga spp. (Diaz-Belancourt et al. 1999) eaten in its native South America and Impatiens glandulfiera eaten in its native Kashmir (Srivastava 1988), have only recently become an object of culinary interest in foraging and herbalist circles due to the popularity of wild food in general (Łuczaj, personal observations).

Intercontinental invasions of other Boletaceae and truffles have not yet been documented, either in biogeographical or ethnobiological terms (Hall et al. 1998; Murat et al. 2008; Dickie et al. 2016). Only the occurrence of these species has been recorded, however without deeper insight into how the appearance of new edible species influenced the culinary culture. Mushroom users tend to be conservative in their choice of species (Łuczaj and Nieroda 2014), however the A. projectellus was quickly ‘accepted’ as edible. What made it so easy? First of all, their ‘classic’ shape with spongy hymenophores (Boletaceae and Suillaceae) is regarded as relatively safe in Poland, i.e. either edible or, if bitter, slightly toxic. Education on edible mushrooms often starts from this piece of information (Łuczaj and Nieroda 2014). Once the most daring mushroom experts established that the species is not bitter and is actually tasty, it was quickly classified as edible. Also its colour resembles the edible Boletus edulis, Suillus spp. Imleria badia and Xerocomellus spp., the most commonly collected fungi in Poland. Thus, the new species was considered a variation in a folk genus of several edible species with common characteristics (for explanation of ‘folk genus’ see Berlin (1992)). Unfortunately, it cannot be legally sold as, according to European Union regulations, it is a ‘novel food’, not used in the E.U. before 1997, due to which its introduction to EU markets would require more complicated procedures and tests (see Novel Food website).

An interesting phenomenon mentioned by one of the respondents is the fact that internet mushroom forums require members to use official Polish names and ban them for using local spontaneous names. Thus, the spontaneously created common name ‘heather bolete’ may be replaced by ‘golden bolete’, as the species has now been moved to the genus Aureoboletus, meaning ‘golden bolete’.

Aureoboletus projectellus is already sold in markets even as far as Rzeszów in south-eastern Poland (Ł.Ł.– personal observations; Fig. 4). This happens even though the species is not on the official list of fungi species that are allowed to be sold to the public (Kotowski 2016; Minister of Health of the Republic of Poland 2022). The list is updated every few years, and the question if this species should be sold in markets will divide mycofans. On the one hand, it belongs to the Boletaceae group, prized in Polish cuisine and sought-after as food. On the other, its sales in different places may facilitate the spread of spores, when we still do not know how its presence might affect fungal communities.

The collection of A. projectellus has become an element of ‘foraging tourism’. This is a newly emerging type of tourism where visitors not only seek new culinary ingredients but gather them themselves (de Jong and Varley 2018). Overall, the large number of questionnaires submitted from different parts of the country demonstrates the great power of citizen science in ecological and ethnobiological research. Citizen science programs using internet questionnaires and apps have previously been used in biogeographical research (e.g. Devictor et al. 2010; Kumar et al. 2019) and most studies dealing with the biogeography of invasive fungi in Europe have been made thanks to information gathered in public databases, like GBIF, or iNaturalist (Pietras et al. 2018, 2021). However, they were only recently introduced in ethnobiology (Vandebroek and Albuquerque 2024). Questionnaires sent to citizens were issued in a number of eastern European countries in the 19^th century and brought and immense amount of data (Łuczaj and Köhler 2014; Köhler 2015; Kalle et al. 2022) about plant uses. The Polish Ethnographic Atlas also used questionnaires sent by local data collectors when collecting data on the use of wild edible plants and fungi, as well as medicinal plants, since 1948 (Łuczaj 2010). This study follows similar approach by gathering part of the data in the form of an online questionnaire.

Several management strategies have been proposed to mitigate the spread of ectomycorrhizal fungi into new areas. One such strategy to slow the invasion of edible fungi is reducing human-related dispersal (Dickie et al. 2016). Previous studies have shown that climatic conditions in Europe are favourable to A. projectellus, making its spread nearly inevitable (Banasiak et al. 2019a), furthermore, restricting the collection of fungi appears unrealistic in the context of Polish cultural and culinary traditions. Any attempt to limit fungal foraging in Poland could also intensify conflicts related to future invasive species management. The collected data show that a great number of people from different regions of Poland have visited the large A. projectellus sites on the Baltic coast, which may have been an important vector of the spread of the fungus in inland Poland (Table 2). However, we found no correlation (Spearman’s rank) between the collectors’ origin and the number of reports detected in a given regions. For example, the fungus is now often reported in the Podkarpackie region, where no people who had collected the fungus in the north of Poland were encountered. Conversely, the questionnaire yielded several people from Śląskie and Dolnośląskie who collected the fungus in the north, but hardly any localities of this fungus are known from those regions. This shows that even if spreading the spores by mushroom-pickers may be an additional facilitating factor, the main vector in the species’ invasion may be an animal vector or wind.

The decision on whether to further advertise the species’ edibility should depend upon the results of studies investigating the impact of A. projectellus on native fungal communities. If it is very detrimental to them, people should be educated and discouraged from eating and collecting in order to slow down its spread, as commerce, especially across long distances, dramatically increases the spread of invasive organisms (Lenda et al. 2014). Otherwise, it could be added to the list of commercially utilized mushrooms as it is sold in some markets anyway. Unfortunately, currently not enough data is available to assess the impact of Aureoboletus projectellus on the local mycobiota, due to which the species is better referred to as exotic instead of invasive.