From natural forests to urban weeds

Post provided by Kun Song, Ying-Ji Pan & Yi-Chong Cui

Weed communities in urban area, Shanghai. An invasive species, Solidago canadensis (the individuals with yellow flowers in the photo), was the dominant species with few other species living together. Photo credit: Yi-Chong Cui.

This post refers to the article Phylogenetic and functional structures of plant communities along a spatiotemporal urbanization gradient: Effects of colonization and extinction by Cui et al. published in the Journal of Vegetation Science.

In China, most forests distributed in the east of Heihe-Tengchong Line occurs together with more than 96% population of the country. The dense population in this area brings intensive damage to the natural forest, especially accelerated by the fast economic development since the 1980s. Natural forest loss and degradation were considered as one of the important causes for the severe flooding which struck vast area of eastern China from north to south in the summer of 1998 and resulted in million homeless people and significant economic loss. Since then, forest research and protection became a very hot topic in China.

In the latter time of this year, Dr Da (the corresponding author of this paper) came back to China and established his lab in East China Normal University (ECNU) in Shanghai city. He was soon awarded with the national funds to investigate the forest vegetation of the middle and lower reaches of the Yangtze River. Several years later, he noticed that urbanization became another major issue affecting the environment and the natural ecosystem in China. Such an impact was especially severe in large cities like Shanghai, due to the fast population increase and urban expansion. Dr Da suggested that urban vegetation can be separated into three types: weed vegetation, remnant forests and artificial plantations (Oshawa et al., 1988). From 2005, his team began to take weeds as the model vegetation to reveal the response of plants to urbanization.

When we compared the weeds species composition along urban-rural gradients in Shanghai, one interesting pattern which we observed was that the proportion of monotypic genera (i.e. only one species recorded in a genus) was quite high (c. 70%) in urban areas. With many years’ research experience in subtropical/warm-temperate forests in China and Japan, Dr Da innovatively described this pattern as urban relict weeds analogous to the relic plant (i.e. Ginkgo biloba) in natural forests which survived from the ice age and reduced to a few or even to a single species in its relict lineages (Tang et al., 2018). A similar pattern was also found in the in Ha’erbin city when we compared the current weed community survey to the records in history before the intensive urbanization. We concluded that urbanization caused the loss of close-related species predicted by the widely accepted competition exclusion theory. Consequently, we expected that the phylogenetic structure of weed communities in a central urban area should be overdispersed. However, the statistical analysis of phylogenetic structure along urbanization gradients showed that the phylogenetic structure of weed became more clustered in the central urban area, and this contradiction between our theoretical prediction and observed results puzzled us.

In 2016 East China Normal University international forum for young scientists, Kun Song and Yichong Cui met with Dr Li who was introducing his work on phylogenetic and functional changes over forest succession by comparing the phylogenetic and functional similarities among colonists, extinct and resident species (Li et al., 2015). His idea to decompose the species changes during succession into new colonist species and local species extinction inspired us. In Shanghai and Ha’erbin, weed communities are at the pioneer stage of secondary succession after the destruction of natural vegetation. Both of extinction and colonization process also shape community structure of weeds. With increasing urbanization, extinction of native species and colonization of non-native species would become more intensive and governed urban weed community composition. If contributions of colonization and extinction to the changes in phylogenetic structures in urbanization can be distinguished, it will be critical for revealing the underlying key mechanism that rules weed assembly in urban areas. Kun Song and Yi-Chong Cui quickly exchanged our opinion with Dr Da and other lab members and finally wrote this paper.

It has been 15 years since our lab started to focus on urban weeds. More than 10 lab members have finished their Master or PhD theses on this topic, and many of them remained in the academic field after graduation. Through our efforts, more and more cities have been investigated for urban weeds along with the other urban vegetation types from different eco-regions of China, e.g. Hangzhou, Qingdao, Kunming and Chongqing. Our studies provide important records of urban vegetation and valuable suggestions for urban ecosystem management in China. We will keep working on revealing the key mechanisms that rule urban weed assembly.

References:

  • Li, S., Cadotte, M. W., Meiners, S. J., Hua, Z., Jiang, L., & Shu, W. (2015). Species colonisation, not competitive exclusion, drives community overdispersion over long-term succession. Ecology Letters, 18, 964–973. https://doi.org/10.1111/ele.12476
  • Ohsawa, M., Da, L.J., & Otuka, T. (1988). Urban vegetation-Its structure and dynamics. Integrated Studies in Urban Ecosystems as the Basis of Urban Planning (Ⅲ). Chiba, Chiba University.
  • Tang, C.Q., Matsui, T., Ohashi, H., Dong, Y.F., Momohara, A., Herrando-Moraira, S., … López-Pujol, J. (2018). Identifying long-term stable refugia for relict plant species in East Asia. Nature Communication, 9, 4488. https://doi.org/10.1038/s41467-018-06837-3.

About the authors:

Kun Song is the associate professor in East China Normal University and focuses on the natural forests and urban vegetation in subtropical area of eastern China.

Ying-Ji Pan is the PhD candidate in Leiden University, the Netherlands. His research interest mainly focuses on the plant adaptation strategies through the perspective of eco-physiological adaptive traits and leaf economics traits.

Yi-Chong Cui is the PhD candidate in East China Normal University. His research interest mainly focuses on assembly rules of weed communities in urban area.