Soil seed banks are vital for the long-term survival of the diversity and dynamics of plant communities (Bakker, 2001). The presence of wastelands in cities contribute significantly to the biodiversity in urban areas (Godefroid et al., 2007). However, the dynamic in space of soil seed banks in wasteland during the process of urbanization is poorly documented.
In our study, the seed density in urban wasteland of Shanghai is much lower than urban wasteland in Munich, German (Albrecht et al., 2011) and Tianjin, China (Mo, 2013). This discrepancy may be due to the longer time span since abandonment compare to our study. We speculate that as the duration of abandonment increases, more species may be able to produce stable seed banks, which could consequently promote an increase in the number of species within the soil seed bank. The urban wasteland in Munich and Tianjin has been abandoned for 20 and 40 years, respectively. But in this study, the abandonment time of the study site was no more than 15 years. Additionally, when comparing with formal urban greening, the urban wastelands, an informal urban greening, own higher seed density. For example, compare to our study, the soil seed density of urban forest parks in Chongqing (Xu, 2016), Beijing (Xue et al., 2023), Shenyang and Fushun (Zhang & Gao, 2009), showed lower seed density. The main reason could be that in formal urban green spaces, there's a significant amount of human management, making it difficult for weed populations to form stable communities.
The seed density in urban wasteland exhibits distinct seasonal variation, with seed density in spring being notably lower than that in autumn. Our autumn sampling was in November, when the seeds of most summer annual and perennial plants got mature and many of them forming a transient seed bank with a large proportion of germinable seeds (Thompson & Grime, 1979). The sampling in spring was in April, when the seeds in the transient seed bank have already germinated, resulting in a lower seed density.
Our study found that the seeds bank of wasteland in Shanghai are dominate by annuals plants and small growth type species. Many studies about soil seed bank have found similar results (Fischer et al., 2013; Fu, 2016; Li et al., 2018; Xu, 2016; Xue et al., 2023). Annuals plants and small growth type species are generally highly competitive and plasticity, and can survive in the soil for a long time. The dominance of annuals in the seed bank of Shanghai's wastelands, as observed in our study, suggests a rapid turnover of plant species that may be indicative of the dynamic and transitional nature of these urban ecosystems (Poppenwimer et al., 2023). Rapid urbanization process leading a higher intensity and frequency human interference and resulting in plant communities difficult to succeed to the perennial stage (Li, 2009). The prevalence of small growth type species in the seed bank could be attributed to their adaptive advantages in wastelands, such as requiring less space and resources, allowing them to thrive in the potentially nutrient-poor and disturbed soil conditions often found in these areas. In addition, plants of small growth types that crawl and grow have strong adaptability to trampling, maintaining their normal reproductive and transmission abilities, and surviving in urban areas (Tian et al., 2008). As observed in our study, there is no significant differences across the urban-rural gradient in the distribution of life forms and growth types. This indicates a certain level of ecological homogeneity in the studied wastelands. These wastelands may not have experienced the same environmental pressures or land-use changes that typically drive shifts in species distributions and they have similar microclimatic conditions or soil properties. These environmental factors are beneficial for a particular set of growth forms, regardless of the urban or rural status of the site.
It is generally believed that in the early stages of succession or frequently disturbed habitats, the dominant species of vegetation are mostly anemochory, whose seeds are mostly small and dormant, making them easy to bury in soil seed banks (Liu et al., 2022). Albrecht et al (2011) found that the number of anemochory and zoochory dominated urban wasteland soil seed banks. Our study showed that the major seed dispersal type of soil seed bank is autochory and anemochory. This indicates that the wasteland in this study is in the early stage of succession and is subject to increased interference. However, due to the short abandonment period, the species with zoochory dispersal mechanisms have not yet formed a stable seed bank but favor autochorous plants. Additionally, seeds from autochorous plants are typically larger and more viable in stressful environments, which could also be one of the reasons they dominate. The larger seed size may provide more resources for the developing embryo, increasing the seed's chances of successful germination and establishment, especially in the challenging conditions often found in urban wastelands (Westoby et al., 1996).
Our finding of dominant position of native species and no significant difference in the proportions of native versus exotic species along the urban-rural gradient challenges the common assumption that urbanization invariably leads to an increase in exotic species at the expense of natives (Figueroa et al., 2020; King & Buckney, 2001; Londe et al., 2017). Similar results have been found in the research of Albrecht et al. (2011) and Zhao et al. (2023), where the proportion of native species in soil seed banks is higher than that of exotic species. A high proportion of native species in aboveground vegetation communities with no significantly different along the urban-rural gradient were found in Shanghai in previous studies (Tian et al., 2008; Zhang, 2017). It suggests that the native species in our study area may be more resilient to urbanization pressures or that exotic species have not yet established a strong foothold in the local seed bank.
Different with our previous study’s finding that plant species diversity followed a unimodal curve along the urban-rural gradient.in Shanghai (Tian et al., 2014), we did not find clear trend of species diversity of seed bank of wasteland soil along urban-rural gradient. And this very low similarity between soil seed banks and the standing vegetation has been widely recognized (Moore, 1980; Vandvik et al., 2016; Yang et al., 2021). In addition, the life span of wastelands is highly variable when facing numerous developmental pressures (Harrison & Davies, 2002). In the short to mid-term, wastelands are partly protected from human-induced disturbance by their negative image, which may favor biodiversity (Godefroid et al., 2007). This suggests that the response of seed banks to urbanization lag with changes of aboveground plants as it can offset fluctuations in seed yield and quality when the population is disturbed (Doroski et al., 2020; Pakeman & Small, 2005). Within the current urbanization context of Shanghai, the effects of environmental shifts and anthropogenic disruptions on plant communities can be somewhat mitigated by the presence of soil seed banks. In addition, due to the random management of wasteland (Sukopp, 2002), the ability of urban wasteland soil seed banks to resist interference may change with the time of abandonment.
In short, we did not find any significant difference along urban-rural gradients for functional composition (i.e. the proportion of annual plant species, small growth type species, exotic species, and dispersal modes), species diversity and species composition, excepting that marginal significant for autumn species composition among urban-rural gradients. It is important to consider that the relative homogeneity in the seed bank characteristics we observed could be a transient state influenced by recent environmental changes or disturbances. Therefore, long-term monitoring and comparative studies with other urban ecosystems would be necessary to determine whether these patterns persist over time or are subject to change.