Agricultural land abandonment and rural depopulation are frequent phenomena in many parts of the developed world that often result in considerable conservation benefits. Although settlements are hotspots of alien species that may threaten ecosystem recovery, no study to date has systematically assessed the persistence and spread of cultivated alien plants following the abandonment of rural settlements. By examining 190 farmsteads abandoned between 1956 and 2005 in central Hungary, we show that cultivated species can remain for decades at abandoned settlements, with many species occurring in similar frequency in long-ago and recently abandoned farmsteads. Many species spread vegetatively, and persistence through time was not related to estimated longevity for woody species. Furthermore, by analysing vegetation samples from the surrounding landscape, we found that some of these cultivated species also occurred outside farmsteads in areas where they had not been planted, most often in tree plantations. In addition, the number of escaped cultivated species occurring in tree plantations was positively related to farmstead density, suggesting a prominent role of farmsteads as a source. Our results suggest that rural settlements and rural depopulation provide a special opportunity for cultivated alien plants. These special habitats serve as incubators, where many cultivated species can survive long-term, and even spread to the surrounding landscape. We conclude that farmsteads have a long-lasting local and landscape-scale legacy, and imprint a unique signature on the flora of their broader region.
Over the past few decades, agricultural change and rural depopulation have occurred in many parts of the developed world (Bazzaz 1975; Pascarella et al. 2000; Hölzel et al. 2002; Gellrich and Zimmermann 2007), often with positive conservation outcomes (Harmer et al. 2001; Hölzel et al. 2002; Csecserits et al. 2011; Wiezik et al. 2013). This agricultural change usually includes a decline in arable land, as has been the case in 23 of 29 European countries between 1993 and 2008, including all Mediterranean and Eastern European states (Storkey et al. 2012). These former agricultural lands are sometimes used for urban development or intensive forestry, but very often are simply abandoned, allowing natural succession to occur (Harmer et al. 2001; Chinea 2002; Mottet et al. 2006). The secondary ecosystems that emerge during this succession are sometimes similar in composition, structure, and function to historical vegetation and thus have a considerable conservation value (e.g. Hölzel et al. 2002; Wiezik et al. 2013). However, the high number and abundance of alien species often differentiate these ecosystems from their undisturbed counterparts (Pascarella et al. 2000; Von Holle and Motzkin 2007).
Although agricultural change is often accompanied with rural depopulation, most studies have merely assessed the fate of abandoned arable lands (e.g. Bazzaz 1975; Harmer et al. 2001; Cramer et al. 2008; Klanderud et al. 2010), with few focusing on the fate of the abandoned settlements (Dambrine et al. 2007; Vojta 2007; Muchiru et al. 2009). Although settlements cover only a tiny portion of the landscape, they can be important because they are unique in both abiotic and biotic features. Settlements are characterised by a highly transformed and heterogeneous environment, with altered environmental conditions including climate, hydrology, and soils (Gilbert 1989; Pickett et al. 2011), which is favourable for the establishment and spread of many alien species (Loram et al. 2008; Wang et al. 2011). In addition, intentional introduction in settlements involves not only a few major crops, but also a wide variety of horticultural, ornamental, and amenity species. Thus, the proportion of alien species in settlements can be very high, ranging from 20 to 60 % of all species (Thompson et al. 2003; Pyšek et al. 2004; Loram et al. 2008).
Even if rural depopulation is a frequent phenomenon in many parts of Europe (Skowronek et al. 2005; Plieninger 2006; Bell et al. 2009), complete abandonment takes place mostly in small villages and, in particular, in scattered farmsteads, which we define as a farmhouse and outbuildings with associated yard and garden, located out of village and used as a residence. These smallest settlements are typical throughout the cultural landscape in some parts of Europe and are associated with the traditional small-scale farming system. In some densely populated regions, such as the Netherlands, farmsteads change function and serve as residences for people working in cities (Van der Vaart 2005). However, in many Mediterranean and Eastern European rural landscapes, farmsteads are abandoned in great number. For example, Plieninger (2006) reported that in a dehesa landscape of Cáceras province, Spain, building density declined from 1.2 to 0.6/km2 between 1956 and 1998. In the Great Hungarian Plain, the number of people living outside cities and villages declined from 1.1 million after the World War II to 200,000 in 1990 (Duró 2004). Although the total area and associated propagule pressure of these rural farmsteads may be relatively small compared to larger settlements, their dispersed distribution in the landscape increases their importance, especially for biological invasions (Sax and Brown 2000; Lockwood et al. 2005).
Here, we examine the role of abandoned farmsteads in supporting the long-term persistence and spread of cultivated alien plants in a region undergoing agricultural abandonment in central Hungary. Specifically, we (1) document the dynamics of farmstead abandonment, (2) compare the occurrence of cultivated species at farmsteads abandoned at different times, (3) quantify the occurrence of these cultivated species in major habitat types in the surrounding landscape, and (4) test if the occurrence of these species in habitats where they were not planted is related to farmstead density in the surroundings. We hypothesise that (1) rural settlements may serve as invasion sources because they initially harbour many alien species, and (2) once abandoned, they can support the long-term survival of many alien species because they are associated with heterogeneous and highly altered abiotic conditions, where cultivated species are the founding populations.
The Kiskunság is the 7,500 km2 central part of the Danube-Tisza Interfluve, a large alluvial fan of the River Danube in central Hungary, with an elevation ranging from 90 to 140 m. The region is characterised by the overall dominance of coarse-textured sandy soils. The climate is temperate with a clear continental and minor Mediterranean influence. The mean annual temperature is 10.5 °C, and the mean monthly temperature ranges from −2 °C in January to 21 °C in July. Mean annual precipitation is around 500–550 mm, peaking in June (Kovács-Láng et al. 2000).
The potential vegetation of the region is forest–steppe (Zólyomi 1973–1974), but by the 17th century, the region was almost completely deforested (Biró 2008). Following depopulation during the Turkish occupation (16–17th centuries), small-scale farming became the dominant land-use and settlement structure, and rural population density and grassland-to-cropland conversion increased until the early 20th century. A decline in the rural population and small-scale farming, as well as an associated land and farmstead abandonment, started after the World War II, accelerating in the 1960s and 1970s due to a decline in the water table and socioeconomic changes, and continues today. In the last 10–15 years alone, 10,000–12,000 farmsteads were abandoned in this region (Csatári and Kanalas 2006). The rate of land abandonment shows great spatial variation, but a recent analysis suggests that it can be above 50 % at both the regional and local scale (Biró et al. 2013).
Study sites and sampling
Four study sites, each 36 km2 and located 8–40 km from each other, were selected in the centre of the region. The large size of the sites was necessary to capture the heterogeneity of the region (Csecserits et al. 2011) and to include all major habitat types (natural grasslands, natural forests, old fields, forest plantations, and arable lands) in each site.
The earliest map that showed individual farmsteads from 1956 was used as a starting point to locate once existing farmsteads in the landscape. This time coincides well with the start of the region’s depopulation. Of these farmsteads, we identified those that were abandoned by 2005 using high-resolution aerial photos taken in 2005 (Anonymous 2005), and subsequently ground truthed their status in 2007. We found 86–260 farmsteads per site that were abandoned between 1956 and 2005, and from these we randomly selected 60 (a total of 240) in each site. In each farmstead we classified their state as (1) building remains (buildings, walls), (2) mounds of collapsed buildings, and (3) converted to some other land-use type (ploughed). In total, 190 were scored as categories 1 and 2, and 50 were ploughed and converted to vineyards, cropland, or forest plantations (category 3).
We recorded all cultivated alien species, including agricultural, ornamental, amenity, and medicinal species at all the 190 unploughed farmsteads in 2007. Non-cultivated aliens were not included in the survey. Species nomenclature followed the latest Hungarian checklist (Király 2009), and for species not covered in this volume, we used an earlier, but more thorough checklist (Soó 1964–1980).
Cultivated species were recorded within and at the margin of former yards and gardens around the farmstead buildings. Thus we did not have a uniform sampling unit (e.g. plot), but we used a functional unit (farmstead). Even when yard or garden boundaries were not recognisable, cultivated plants could be clearly associated with a farmstead, as neighbouring farmsteads were separated with arable, ex-arable, pasture land or forest plantation.
To assess the time of abandonment for each farmstead, we checked maps and aerial photos between 1956 and 2005. We found data sources from 4 years: 1956 (map; Anonymous 1956); 1978 (map; Anonymous 1978); 1989 (map; Anonymous 1989); and 2005 (aerial photo; (Anonymous 2005), where individual farmsteads were reliably recognizable. Based on these maps and aerial photos, we determined the number of active farms at each of these 4 years, and estimated the time of abandonment for each of the surveyed farmstead. This resulted in three age classes for abandoned farmsteads: (1) abandoned between 1956 and 1978 (old farmsteads); (2) abandoned between 1978 and 1989 (medium-aged farmsteads); and (3) abandoned between 1989 and 2005 (young farmsteads).
To assess the occurrence of cultivated species outside farmsteads, we used 85 vegetation plots (400 m2) from a previous study (Csecserits et al. 2011) sampled at the same four sites, that covered all major habitat types (natural grasslands: 24 plots, natural forests: 23 plots, old fields: 33 plots, forest plantation: 35 plots). Vegetation plots closer than 50 m to an active or abandoned farmstead location were excluded from analyses. Vegetation plots were evenly distributed among sites and randomly selected within sites and habitat types, and no replicate plots were sampled in the same vegetation patch. All species occurring in these plots were recorded in 2006–2007 (Csecserits et al. 2011), but here we only use data of species that escaped from cultivation. Finally, we tested if the number of cultivated species in vegetation plots was related to nearby farmstead density, distance to the nearest village, or distance to the nearest road, as these landscape elements may be sources of spreading cultivated species. Only forest plantation plots were used in this analysis, as this was the habitat type with most cultivated species. Farmstead density was calculated within 500 m from each forest plantation plot, as this distance has been reported to include most dispersal events for a similar situation for one of the spreading species (Juglans regia, Lenda et al. 2012).
We used generalized linear models (GLMs) with Poisson error structure and log link function (Crawley 2007) to look at (a) the effect of time since abandonment (age classes) and site on the number of cultivated species at abandoned farmsteads, (b) the effect of habitat type and site on the number of cultivated species in different habitats in the landscape, and (c) the effect of the number of farmsteads within 500 m, the distance from the nearest village, the distance from the nearest road, and site on the number of cultivated species in forest plantations. In the latter case we applied one-side tests because we had a priori hypotheses of relationship directions, being positive with number of farmsteads and negative with increasing distance from the nearest village and nearest road. Because interaction terms were not significant (p > 0.15 for all tests) they were removed from the model following Crawley (2007). Thus the final fitted equation was E(y) = exp(a + b1X1 + b2X2 + b3X3), where y = number of cultivated species, a = site specific intercept, and X1, X2 and X3 = explanatory variables (two or three depending on the test). Tukey’s post hoc test (Hothorn et al. 2008) was used to test for differences among categories of categorical predictors, such as age classes and habitat types.
We examined the relationship between individual species occurrences and age classes using GLMs (with age class and site being explanatory variables) using binomial error structure and logit link function, because of the binary nature of the response variable (i.e. presence/absence of species). The intermediate age class was omitted from the analyses due to low farmstead numbers. Species with fewer than eight occurrences in the two age classes combined were not tested, because our preliminary test (testing of hypothetical species that occurred only in one age class) showed that these rare species could not have a statistically significant association. Benjamini–Hochberg correction (Verhoeven et al. 2005) was used to account for the increase of false discovery rate due to the high number of individual tests (testing each species separately).
We used Chi square test to examine if there was a relationship between life form (herbaceous vs. woody species) and long-term persistence at farmsteads (significant association with recently abandoned farmsteads vs. no association with age of abandonment). In a second test, we asked if the life span of woody species ((short (< 100 years) vs. long (> 100 years)) was related to long-term persistence at farmsteads (significant association with recently abandoned farmsteads vs. no association with age of abandonment). Life span estimates were obtained using a combination of data sources including an online database (USDA NRCS 2013), a local compendium of horticultural plants (Tóth 2012), and expert knowledge (G. Kósa personal communication).
All statistical analyses were conducted using R, version 2.15.0 (R Development Core Team 2012).
Farmstead density varied considerably among sites, but decreased steadily through time at all sites, with the average density decreasing from 8.1/km2 in 1956 to 3.8/km2 in 2005 (Fig. 1a). Over half of the recently abandoned (after 1989) farmsteads had building remains in 2007 (Fig. 1b), whereas few old farmsteads (abandoned before 1978) had building remains. Only about 30 % of the old farmstead sites were ploughed following abandonment and thus subject to major land-use change, with all the others being simply set aside and not used intensively.
We found a total of 77 cultivated species at the 190 abandoned farmsteads, including 56 woody species (trees, shrubs, or vines; Online Resource 1). A total of 39 species occurred at farmsteads abandoned at least 30 years ago. The number of cultivated species on abandoned farmsteads was negatively related to the time since abandonment (χ2 = 66.333, d.f. = 2, p < 0.001, n = 190), with more cultivated species occurring at young and medium-aged farmsteads compared to old ones. Old farmsteads harboured an average of seven species (Fig. 2), compared to about ten species at young and medium-aged abandoned farmsteads which did not differ in the number of cultivated species.
Of the 34 species frequent enough for species-level analysis (Table 1), 15 were associated with young farmsteads, 18 showed no association, and one species had a higher frequency at old farmsteads (Table 1). A total of 24 species were observed to spread vegetatively at abandoned farmsteads (Table 1; Online Resource 1), and 11 species occurred outside farmsteads, in plots sampled in the surrounding landscape (Table 1; Online Resource 1), being Morus alba, Ribes aureum, Juglans regia, Persica vulgaris, Ailanthus altissima, Armeniaca vulgaris, Gleditsia triacanthos, Acer negundo, Eleagnus angustifolia, Parthenocissus inserta, and Hedera helix. All these species occurring outside farmsteads were woody.
We found a significant relationship between life form of cultivated species and association with recently abandoned farmsteads (χ2 = 8.494, d.f. = 1, p = 0.004, n = 34), with herbs being associated with recently abandoned farmsteads more frequently than expected from a random pattern (Table 1). We found no significant relationship between estimated life span of woody species and association with young farmsteads (χ2 = 0.163, d.f. = 1, p = 0.686, n = 27).
The number of cultivated species found in the surrounding landscape differed among major habitat types (χ2 = 125.789, d.f. = 3, p < 0.001, n = 115), with the most being in forest plantations, and fewest in old fields and grasslands (Fig. 3). The number of cultivated species in forest plantations was positively related to the number of farmsteads within 500 m (b1 = 0.1358, z = 3.93, p < 0.001), but not related to the distance from the nearest village (b2 = 0.0003, z = −2.183, p = 0.985) and distance from the nearest road (b3 = −0.0003, z = 0.378, p = 0.706).
The rate of farmstead abandonment and the fate of abandoned farmsteads
The decline in farmstead density we quantified parallels observed trends of rural depopulation throughout Europe (Csatári and Kanalas 2006; Plieninger 2006). Based on the average abandonment of 4.3 farmsteads/km2 in the last 50 years, we roughly estimate that there are 30,000 abandoned farmsteads across the 7,500 km2 of the Kiskunság region. Together with active farmsteads, these sites likely played, and continue to play, a more important role in spreading alien species than villages or roads, as our analysis found that the number of cultivated species in forest plantations was related to farmstead density, but not to distance from nearest villages or roads. The tens of thousands of abandoned farmsteads scattered throughout the region, each harbouring 7–10 alien plant species that have survived abandonment, provide an ideal setting for both long-term persistence and spread of potentially invasive species.
The survival and persistence of cultivated species at abandoned farmsteads
We found many (77) cultivated species that survived the cessation of cultivation, but they form a heterogeneous group that can be subdivided based on survival time following abandonment and spreading ability. First, species that are confined to young abandoned farmsteads (e.g. Malus domestica, Pyrus communis, Tulipa gesneriana, Narcissus poeticus, Iris germanica; Table 1; Online Resource 1), including almost all non-woody species, constitute only the short-term legacy of human habitation. Yet, as these species have a demonstrated ability to survive without cultivation they should be put on a watch list of noteworthy species (Mack 2005), and deserve casual alien species status (Richardson et al. 2000). Second, species that are confined to farmsteads, but are still there decades after abandonment (old farmsteads) deserve special attention. Notably, species whose frequency is similar in old and young farmsteads or are capable of vegetative spread (Syringa vulgaris, Prunus domestica, Yucca filamentosa, Spirea x vanhouttei; Table 1.) have a high potential to persist long-term at old farmstead locations, and should thus be viewed as naturalised species (Richardson et al. 2000). After the deterioration of buildings, these species may, in fact, serve as the only remaining marker or “memento”of once existing farmsteads. Third, species that also occurred outside farmsteads, within habitats that they had not been planted, have reached the final invasion stage: landscape spread (Theoharides and Dukes 2007; Blackburn et al. 2011), and pose the greatest threat (e.g.: Morus alba, Juglans regia, Ailanthus altissima, Gleditsia triacanthos; Table 1).
In the context of plant invasions, persistence at abandoned farmsteads is a special situation. Alien species here were purposefully introduced and cultivated, and they needed only resist or tolerate the arrival of other species from the surrounding landscape. As in an incubator, in the establishment phase (before abandonment), cultivation serves as a buffer against demographic and environmental stochasticity (Mack 2005). Only after the species establish and mature are they exposed to a harsher, more competitive environment (abandonment). Thus being a founder population (Yodzis 1978), this is a much more favourable situation for alien species than when they need to establish in natural communities.
The life form composition of cultivated species found at abandoned farmsteads differs considerably from that of both urban floras and national lists of aliens, with a very strong bias towards woody species. We did not survey active farms, but woody species typically make up 20–30 % of alien floras in settlements (Pyšek et al. 2004; Knapp et al. 2010), in accordance with woody species contributing less than 20 % of all alien species within national species lists (Pyšek et al. 2002; Balogh et al. 2003; Jiang et al. 2011). In contrast, 56 of 77 (73 %) of cultivated species we found at abandoned farmsteads were woody. The finding that short-lived woody species did not decline faster than long-lived ones shows that we did not simply see the longer-term survival of long-lived woody species. Also, the high number of woody species that spread vegetatively indicates their capability for long-term persistence. Finally, the most clear proof of the success of woody species is that all species that spread in the landscape were woody. The fast decline of cultivated herbaceous species may be due to the lack of disturbance and active management, and the resulting closure of the herb layer. In addition, although the potential vegetation of the region is forest steppe (Zólyomi 1973–1974), forests were cleared in the middle ages (Biró 2008). Thus natural forest fragments and forest species are scarce and natural forest recovery is slow, which is a favourable setting for alien woody species; a situation similar in many agricultural landscapes undergoing land abandonment.
The spread of cultivated species
The presence of species in habitats where they were not planted, combined with the positive relationship documented between farmstead density and the number of cultivated species, suggests that some of these cultivated species do spread in the landscape and farmsteads may be the source. These spreading species may constitute the landscape-scale legacy of declining rural settlements. In addition to the species that are currently spreading, the high number of species that persist or slowly decline at abandoned farmsteads, combined with the large number of introduction points (farmsteads), provides a potential for future spread due to environmental change (Walther et al. 2009), microevolutionary change (Lee 2002), or changing species interactions (Lenda et al. 2012). As an example, one of our spreading species, Juglans regia, has existed for centuries in many parts of Europe and its relatively recent spread in Poland has been attributed to land abandonment and changed behaviour of its main disperser, the rook (Corvus frugilegus) (Lenda et al. 2012). The landscape-scale spread of Yucca filamentosa, a species that spreads vegetatively and shows no sign of decline through time, is currently prevented by the lack of its co-evolved pollinator (Powell 1992). This limitation, however, may change in the future as happened to Ficus microcarpa, a native species to southern Asia. Ficus microcarpa had been cultivated for many decades in Florida before it started to spread after the arrival of its specific pollinator (Nadel et al. 1992), and became an invasive species (Caughlin et al. 2012).
The finding that cultivated alien species spread most often into forest plantations is in agreement with other studies, and has important implications regarding ongoing land use changes in Europe. Chytrý et al. (2005) also found that broad-leaved plantations are among the richest habitats in terms of alien species. The deep ploughing applied when establishing these forest plantations (Magyar 1961) makes it very unlikely that anything that was planted before survives. On the other hand, this ploughing before tree planting and in the early years of tree growth may facilitate the establishment of spreading species, as disturbance often facilitates the spread of invasive species (Burke and Grime 1996). Furthermore, by providing nesting and resting places, trees attract birds, which are the likely dispersers of the fleshy-fruited species (Gosper et al. 2005). Natural forests may also attract birds and thus seeds, but are relatively undisturbed, which provides a possible explanation why the number of cultivated species in natural forests is intermediate between that of forest plantations and grasslands. The increase in forest cover in the region (Farkas 2006), and also in many parts of Europe (Skowronek et al. 2005; Mottet et al. 2006) may thus be favourable for the spread of the bird-dispersed cultivated species, such as Morus alba, Ribes aureum, Juglans regia, Persica vulgaris, Armeniaca vulgaris, Gleditsia triacanthos, Elaeagnus angustifolia, Parthenocissus inserta. This suggests that the observed spread of alien species is facilitated by changing land use (Lenda et al. 2012).
Some of the cultivated species that is spreading in the landscape may become permanent members of the regional flora, as it happened to other cultivated species in history. It is generally accepted that ancient Romans introduced many cultivated species in the occupied territories (Martinez 2005), some of which survived also in the wild. An example is sweet chestnut (Castanea sativa), which was cultivated and spread by the Romans, and then established in the wild, leading to a huge and persistent range expansion (Conedera et al. 2004). We argue that some of our cultivated species may be on the same path, and even if many of them are likely to fail, some may succeed and persist in the wild even in the long run.
Although human population declines have repeatedly occurred throughout history, reports on the ecological legacy of abandoned settlements are rare. Altered vegetation has been reported 40 years after the abandonment of mountainous villages in the Czech Republic (Vojta 2007), 60 after the abandonment of African pastoral settlements (Muchiru et al. 2009), and even almost two millennia after the abandonment of Roman settlements in France (Dambrine et al. 2007) compared to control sites not occupied by settlements in the surrounding landscape. Based on our results, we argue that rural settlements and rural depopulation provide a special opportunity for cultivated alien species. Abandoned settlements serve as an incubator, where many cultivated species can survive for decades after abandonment while the habitat becomes seminatural, and may even persist for the long-term and become permanent components of the flora. Some species are already spreading to new habitats, and others have the potential to spread in the future. Our results suggest that these small but numerous and scattered rural settlements may have a prominent role in this spread, which can be further facilitated by the ongoing expansion of forest plantations in the study region and elsewhere in Europe. Clearly, our results indicate that rural farmsteads have a long-lasting local and landscape-scale legacy, and imprint a unique signature on the regional flora.
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We thank Jánosné Pándi for assistance with the field work. Jeff Dukes and Emily Rauschert commented on earlier versions of the manuscript and improved English. We are grateful to Ben Hoffmann for detailed suggestions on the final version of the manuscript. Financial support to Károly Penksza came from Research Centre of Excellence- 17586-4/2013/TUDPOL.
Communicated by B.D. Hoffmann.
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Pándi, I., Penksza, K., Botta-Dukát, Z. et al. People move but cultivated plants stay: abandoned farmsteads support the persistence and spread of alien plants. Biodivers Conserv 23, 1289–1302 (2014). https://doi.org/10.1007/s10531-014-0665-y
- Land abandonment
- Land use change
- Plant invasion
- Rural depopulation