Historical reconstruction of the invasions of four non-native tree species at local scale: a detective work on Ailanthus altissima, Celtis occidentalis, Prunus serotina and Acer negundo

Abstract

Reconstructing the history (spatio-temporal patterns) of biological invasions at a small spatial scale is challenging, notably because the required data are often not available in sufficient quantity and quality. In this study, we present a mixed approach using six different data sources to explore the spreading history of four non-native invasive tree species, Ailanthus altissima, Celtis occidentalis, Prunus serotina and Acer negundo in a high conservation value foreststeppe habitat with an area of 1000 ha (Peszér Forest, Central Hungary). We carried out a literature search, compiled all the archived and currently valid data of the National Forestry Database (NFD) in a GIS database, conducted a full-coverage field survey, mapped all the large/old tree specimens and carried out annual ring counts, performed a hotspot analysis on the abundance data provided by the field survey and gathered local knowledge. Each of these approaches proved indispensable and their complementary use made it possible to reconstruct the invasion history of all four tree species. According to the available source literature, P. serotina was first planted in the area in 1937 and the first known occurrence of A. altissima could also be traced back to the 1930s. The examination of large specimens of C. occidentalis and querying the NFD for data related to A. negundo provided evidence that these species have been present in the area since at least the 1940s. However, based on the NFD and local knowledge, it is certain that the rapid expansion of the four tree species occurred simultaneously and only around the turn of the millennium, with a lag of at least 60-70 years. The exploration of local knowledge revealed three possible explanations, which interestingly also coincided in time. With the change in the political regime, the intensity of forest use started to decrease in the 1990s, the population of game was drastically reduced at the end of the decade and droughts became more frequent from 2000 onwards. The field survey clearly showed that these tree species were 2-3 times more prevalent and abundant than the relevant NFD data indicated. Finally, the primary hotspots of A. altissima and A. negundo overlapped with the locations of their first known occurrences, while in the case of C. occidentalis and P. serotina, they did not. However, local knowledge revealed that the former two had been ignored since at least the 1950s, while the latter two were occasionally planted until the 1990s. It is likely that the primary hotspots of C. occidentalis and P. serotina indicate the locations of these undocumented plantations.