Developing countries such as China have experienced fundamental changes in recent decades, while gains in wealth have been achieved at a severe cost to the environment (Liu et al. 2018; Xu et al. 2018). Air pollution is a major threat to people’s health, particularly in urban areas (Przybysz et al. 2014; Yan et al. 2016), due to particulate matter (PM) emissions linked to traffic and industry (Simon et al. 2016). Coarse particles can cause lung diseases, while smaller particles, such as PM2.5, can be inhaled more deeply into the lungs (Beckett et al. 2000; Liang et al. 2016) and have been widely recognized as more harmful to human health. Air pollution is estimated to contribute to at least five million premature deaths each year (WHO 2014).
Vegetation can reduce particle concentrations and thereby improve air quality (Litschke and Kuttler 2008; Liu et al. 2013; Shi et al. 2017; Weerakkody et al. 2018a). A leaf is the main organ for particle deposition (Weerakkody et al. 2018b) and can even biodegrade or transform pollutants into less or nontoxic molecules with its habituated microbes and endophytes (Zhang et al. 2017). Recognizing what kind of plants are more powerful in capturing airborne PM is important but challenging because the retention capacity of leaves in terms of accumulating PM is influenced by a variety of factors, such as the concentration of atmospheric PM (Luo et al. 2020), days after rainfall (Xu et al. 2020), wind speed (Beckett et al. 2000), leaf stage (Nguyen et al. 2015) and sampling season (Zhang et al. 2017), which are linked with surface moisture (Wang et al. 2013; Sun et al. 2020). Therefore, it is necessary to investigate what kind of trait could improve species’ PM adsorption abilities (Janhäll 2015; Shao et al. 2019). For example, species with smaller leaves, more complex leaf structure, waxy leaves and hairier leaves usually capture PM more efficiently (Beckett et al. 2000; Dzierżanowski et al. 2011; Weerakkody et al. 2017; Sun et al. 2018; Liu et al. 2019). These findings have important significance for selecting greening tree species, which is the primary task in the construction of urban forests.
In comparison to ecosystem functions, environmental stress tolerance is sometimes even more important in greening tree species selection (Chaudhary and Rathore 2018, 2019; Przybysz et al. 2021). However, there are very few studies investigating species’ PM adsorption abilities in relation to species’ environmental stress tolerance, such as shade tolerance and drought tolerance. Therefore, it is of great importance to test whether species with high PM adsorption abilities are fast- or slow-growing species and whether they are resource-demanding species. Because light is the major limiting resource in moist forests (Zhao and He 2016), we specifically sought to determine how PM adsorption abilities are correlated with the light requirements of tree species (Question 1).
Theoretically, plant traits can track environmental changes and reflect the adaptive strategies of plants (Liu et al. 2021). Dust accumulation reduces the light availability of leaves (Zhu et al. 2019) and may inhibit chlorophyll biosynthesis (Chen et al. 2015) and stomatal conductance (Lewis et al. 2017), which is important for plant photosynthesis (Hetherington and Woodward 2003; Yu et al. 2018). Consequently, species with a greater ability to accumulate PM might need to be more shade tolerant from the perspective of evolution. We therefore hypothesized that species with high PM accumulation abilities should be able to adapt to poor light environments (Hypothesis 1).
In forest ecology, there is a well-established trade-off between the survival rate in deep shade and the growth rate in bright light (Adler et al. 2014). Fast-growing species that rapidly acquire resources thrive in rich light environments, while slow-growing species that conserve resources are dominant in poor light environments (Lohbeck et al. 2013). This acquisitive-conservative trade-off points to the extremes of a continuum in plant design, and the position of a species along this continuum can be quantified by its functional traits (Poorter et al. 2008; Wright et al. 2010). For instance, an acquisitive species tends to have a high photosynthetic capacity, dark respiration rate, specific leaf area (SLA) and leaf nitrogen content (LNC) and hence a high growth rate. In contrast, conservative species tend to have thicker leaves and a higher wood density (WD), reducing the volumetric stem growth rate but facilitating leaf and stem protection and high survival rates (Chave et al. 2009). In this study, we tested whether species with high PM adsorption abilities are characterized by conservative or acquisitive functional traits (Question 2).
Conservative species commonly have high WD and leaf thickness but a low SLA and photosynthetic capacity, suggesting a low growth rate strategy (Firn et al. 2019). Low growth rate species tend to produce durable leaves with long life spans (Adler et al. 2014). The tough leaves of these species may be associated with complex leaf surface structure and waxes, which could trap PM (Dzierżanowski et al. 2011). Therefore, we hypothesized that species with high PM accumulation capacities should be characterized by conservative traits (Hypothesis 2).
If these two hypotheses were confirmed, then we would test the third question relating to which traits are good predictors of the PM accumulation capacities of species (Question 3). The functional traits measured in this study, including photosynthetic capacity, leaf length, SLA, WD, LNC and leaf phosphorus content (LPC), represent relatively easily measured characteristics that can be obtained for large numbers of species (Li et al. 2021). Relationships among functional traits and PM adsorption capacities may reflect direct effects but could also result from trait coevolution (Reich et al. 2003). Therefore, we used structural equation modelling (SEM) to investigate the relationships among a suite of traits and PM adsorption capacities of different size fractions.
To test the first two hypotheses and establish linkages between plant functional traits and ecosystem functioning (PM adsorption in this study), we sampled leaves from fourteen evergreen tree species (Fig. 1) and experimentally measured their PM adsorption capacities. We then quantified shade tolerance and measured six functional traits of these species and compared them with their PM adsorption abilities.