Spontaneous vegetation on walls

In general, in urban two-dimensional land environment, spontaneous plants total 129 species in 101 genera from 48 families in Ningbo City32, 107 species in 89 genera from 32 families in Shanghai downtown33, 95 species in 75 genera from 37 families in Xi’an City14, and 128 species in 98 genera from 32 families in Beijing Olympic Forest Park34. In our study, walls with comparatively harsher conditions, supported 239 vascular plant species belonging to 172 genera and 75 families. Similarly, a study of the extant ancient city wall of Nanjing City found 159 species in 125 genera of 70 families13. A recent study of modern urban walls (stone, concrete or brick) in Christchurch and Dunedin cities, New Zealand, found 117 vascular and non-vascular species in 41 families, and 70 species in 26 families respectively23. It seems that wall habitats perform well compared with general urban environment. Urban walls, although not conventionally considered as habitats, still show heterogeneity in terms of their physical environmental features, including substrate (stone or brick), the micro-scale profile of the wall surfaces (crack size, brick size, etc.), and the surrounding environment. Additionally, the studied Chinese cities are in the subtropical humid climate zone, which has rich species diversity and moist air is suitable for plants on a vertical space. Furthermore, under the current urban landscaping and greening culture, spontaneous species in parks and green space are often regarded as harmful weeds. Therefore, they are removed by high frequency maintenance. In comparison, walls as vertical space are less disturbed and thus such plants can grow.

The dominant families were Compositae and Urticaceae. This is consistent with earlier studies on spontaneous vegetation. Unlike two-dimensional land environment, more fern families, such as Dryopteridaceae, Pteridaceae, Dennstaedtiaceae, Sinopteridaceae and Aspleniaceae, inhabit walls. This finding confirms the results on the ancient stone wall of the Nanjing City. Even compared with Southern Hemisphere cities with a typical oceanic climate23, the warm temperate continental Chongqing has Dryopteridaceae and Aspleniaceae fern families in common. These results reflect the uniqueness of walls as potential “habitats”.

The dominant species are Pteris vittata (78.3%), Pteris multifida, (49.3%), seedlings of Broussonetia papyrifera (45.4%), Cyrtomium fortune (42.1%), and seedlings of Ficus virens (41.5%). Obviously, ferns predominate. Compared with two-dimensional land environment in cities at a similar latitude and similar climate, the dominant species are distinct. For example, the dominant species in Ningbo City include Digitaria sanguinalis, Oxalis corniculata, Acalypha australis, Cynodon dactylon, Alternanthera philoxeroides and Conyza canadensis32. The dominant species in Hangzhou City include Broussonetia papyrifera, Koelreuteria paniculata, Pterocarya stenoptera, Ophiopogon japonicus, Humulus scandens and Hedera nepalensis35. Conspicuously, spontaneous species in two-dimensional land environment are concentrated in xerophytes; some of them are invasive (e.g., Humulus scandens). Such species easily achieve dominance and accelerate population declines of urban biodiversity.

The dominant species on walls in Chongqing City are very similar to the walls of other cities with similar climatic conditions, such as cities in the Pearl River Delta region, where Pteris vittata, Pteris multifidi and Broussonetia papyrifera are dominant36. Likewise, adjacent Hongkong has Broussonetia papyrifera as the dominant species10. Additionally, Broussonetia papyrifera, Pteris multifidi and Cyrtomium fortune were dominant in the stone wall of Nanjing city13. The varied microclimate and socio-environmental factors around these sites and regions influence and differentiate species composition37,38. However, analogous to two-dimensional land environment, we speculate that there are also some species present because of certain traits such as seed dispersal and reproduction, can be distributed widely on wall habitats. This speculation requires further exploration in cities with different environment. Analysis of the functional traits of these spontaneous species (e.g., life form, dispersal type, morphological type) are needed.

For all species found in each city, the spontaneous species on walls in Chongqing have little resemblance to those on the stone wall of Nanjing city13, with less than 25% in common. This may be related to the fact that the local “seed bank” varies in these cities. For example, Ficus virens is a well-known civic tree in Chongqing and is widely cultivated in almost all types of green space. This tree belongs to the Moraceae, genus Ficus, which are generally hardy, vigorous and prolific in growth and regeneration, relying on seeds and propagules such as cuttings. Their fruits, figs, are favored by a wide range of frugivores that are capable of dispersing the seeds. Unsurprisingly, it can survive and settle in barren walls and grow into a dominant species. However, Ficus virens is not reported in cities at a similar latitude such as Nangjing City13 and Jinzhou City39. Likewise, Pteris vittata is also a common species in greening and landscaping in Chongqing. Therefore, there is a rich local “seed bank”. Plants can easily “escape” from urban green space and “spill over” into the vertical space by spores or ramets. This finding of walls having similar species to the surrounding environment is consistent with other studies7. This finding is incompatible with the “Urban Cliff Hypothesis”. Instead, our results in Chongqing are inclined to represent the mass effect, whereby species establish in some unsuitable habitats simply because of the abundance of propagules in the surrounding environment that ensures that some individuals establish successfully40. With walls, because species abundance is greatly different between urban green space (squeezed with species) and bare walls, species migrate from source to sink habitats.

Influences on spontaneous species on walls

This study involved two types of influence and 13 indicators. However, the explanatory power of all four wall characteristics was low, less than 30%. The unexplained residual may come from aspects of the complexity of Nature. Plant species’ introduction, settling and community assembly are complex processes. On one side, introduction is affected by the species’ characteristics (such as species’ adaptation to their environment) and stochastic mechanisms across the natural environment. On the other side, the process is affected by the wall’s characteristics and the surrounding land use. Studies support the importance of land use on a large scale to species richness on a local scale41,42. Specifically, the study of the stone wall of Nanjing city confirmed that sampled walls located among hills, botanical gardens and urban green space tended to produce high species richness and abundance because of nearby seed sources. Although we have considered adjacent influences in this study, only the human disturbance degree and land use type were selected to measure this. More factors measuring adjacent land use and landscape patterns are lacking, such as metrics describing the composition and configuration of the surrounding landscape. Further empirical studies are therefore required to understand the coupling mechanism of wall attributes and surrounding land use on wall vegetation patterns.

As expected, wall attributes contribute the largest explanation. Among all variables, vine coverage, wall shade and wall height were the most important factors influencing vegetation composition and distribution. Vine coverage and wall shade are directly related to water availability. Rainfall is a direct source of water for plant growth. According to our observations, making full use of moisture from the air and wall is more important for spontaneous species on walls. Therefore, some plants were found having aerial roots, e.g., Ficus virens and other Ficus species. Subtropical cities generally have high relative humidity, like Chongqing whose annual average relative humidity is 70–80%. This allows plants to absorb adequate water from the air. However, summers are scorching and solar radiation is high (Chongqing is well-known as one of China’s “Four Big Furnaces”). Especially in today’s context of extreme weather events associated with global climate change, creating a damp, humid environment is a key for plant species’ settlement.

As a mountain city short of construction land, Chongqing has a higher density of buildings than plains cities. No matter the wall aspect, the lower part of walls is not easily exposed to sunlight and generally grows more species, while the higher wall part has less shading and may be exposed to cold winds, so it is harsh area for plant species to settle. This finding is consistent with results from a study of the ancient walls of Jingzhou City, where shadowed sides of walls had more species than the sunny side39. In highly dense cities, the wall aspect cannot determine sun exposure, instead wall height, to some extent, represents sun availability. Therefore, variable wall aspects showed no influence on wall species, which agrees with a study in Hongkong7.

Wall is a novel yet undefined habitat, where nutrients that plants require almost all come from the external environment. Therefore, the plants are greatly disturbed by externalities and often have a typical randomness and stochasticity. Compared with general urban two-dimensional land environment, the composition and distribution of spontaneous species on walls represents high uncertainty and high species turnover rate and, presumably, the influences are varied. Therefore, a long-term, tracking study over different seasons is needed.

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