Forest ecosystems play an essential role in biodiversity conservation and carbon cycling (Pan et al. 2011; King et al. 2012). Species-rich subtropical forests cover an extensive area and are crucial in regulating global carbon cycling and maintaining high biodiversity (Li et al. 2019). Evidence is mounting that diverse forest communities generally accumulate biomass more rapidly than species-poor ones (Jucker et al. 2014; Liang et al. 2016). However, changing biotic and abiotic conditions are forcing ecosystems across thresholds into alternative stable, global environmental change and biodiversity loss strongly threaten the sustainability of ecosystem functions and services they underpin (Loreau et al. 2001; Worm et al. 2006; Ives and Carpenter, 2007; Cardinale et al. 2012). Our understanding of biodiversity and ecosystem functioning and their drivers in natural communities remains limited (Zhang and Chen 2015; Fotis et al. 2017). Therefore, more detailed studies are needed to better understand the mechanisms linking biodiversity to ecosystem functioning, especially for species-rich subtropical natural forests.
Recently, studies have revealed that species diversity may enhance productivity/biomass through a variety of mechanisms. For example, the niche complementarity hypothesis assumes that increasing diversity enhances forest productivity through niche differentiation and facilitation (Tilman et al. 1997). In contrast, the selection probability effect proposes that higher species richness increases community productivity through an increased chance of possessing highly productive species (Hooper et al. 2005). An alternative but not mutually exclusive mechanism to the niche complementarity is the mass-ratio hypothesis (Grime 1998), which states that the most dominant species in the community drive the ecosystem processes by means of their traits. Moreover, species with acquisitive traits lead to faster carbon capture ability, while species with conservative traits possess a higher long-term carbon sequestration strategy (Díaz et al. 2009). Plant functional traits are the key aspects shaping forest biomass dynamics (Díaz et al. 2007; Lohbeck et al. 2015). Species-level differences are important in structuring highly diverse communities (Kraft et al. 2008). Functional trait trade-offs are useful metrics for understanding community response to global change (Kimball et al. 2016). In addition, previous studies reported that plant biomass accumulation predicted by phylogenetic diversity is stronger than by species richness and functional diversity (Cadotte et al. 2008; Liang et al. 2019).
In addition to species diversity, stand structural attributes in natural forests also have a strong influence on biomass and may interfere with the relationship between species diversity and biomass. Structural variability may influence ecosystem processes and functioning. Stand structural complexity increases light capture ability, light-use efficiency, plant water and nutrients use efficiency, promoting the accumulation of biomass in forest ecosystems (Hardiman et al. 2011). Forest biomass is intrinsically related to tree size. A study in natural boreal forests found that tree size inequality links to diversity and aboveground biomass and regulates above-ground biomass and species diversity by interactions among individuals (Zhang and Chen 2015). Studies at global and regional scales have shown that large-diameter trees comprise a large fraction of the stand basal area and biomass in many forests (Paoli et al. 2008; Lutz et al. 2012, 2018; Slik et al. 2013). Mensah et al. (2020) pointed out that structural complexity and large-sized trees explain shifting species richness and carbon relationship across vegetation. Stand density and age are more important modulators of forest productivity than diversity (Ouyang et al. 2018). Recent studies defined a stronger role of stand structure attributes over diversity in shaping forest biomass or productivity patterns (Fotis et al. 2008; Yuan et al. 2018).
It is noteworthy that environmental variation is also a key regulator of productivity/biomass in forests. Topography, for example, represents many aspects of microenvironmental changes. Topographic characteristics such as elevation, slope, and aspect influence microclimate, which are known to drive tree species distributions and abundances (McEwan and Muller 2006; Murphy et al. 2015), influence aboveground biomass (Valencia et al. 2009; McEwan et al. 2011), and forest dynamic (Bellingham and Tanner 2000; Robert 2003). For instance, topography plays a significant role in determining live tree biomass in tropical forests of Amazonia (de Castilho et al. 2006). Poor environmental conditions were found to have strongly limited forest biomass/productivity (van der Sande et al. 2017). In addition, soil type, soil water potential, and nutrient cycling are affected by topography, affecting tree biomass accumulation. Microclimate created by topography also changes stand attributes and leaf characteristics.
In this study, we set up plots at different elevation gradients in three subtropical forests. This ultimate goal is to determine how multiple diversity, plant functional traits, stand structure attributes, and topographic factors affect biomass in subtropical forests. Specifically, we address four questions. First, we ask (i) is there a positive relationship between multiple diversity and biomass in a subtropical natural forest? Second, we incorporate data on key plant functional traits related to tree growth to ask (ii) is biomass influenced through the mass-ratio effects? Third, we ask (iii) how do stand structural attributes affect biomass of subtropical forests and maintain the diversity-biomass relationship? Finally, we ask (iv) how do topographic factors drive biomass other biotic factor? To answer these research questions, we used bivariate relationships, multiple linear regression, variation partitioning analysis and structural equation model based on existing research theories (Fig. 1a) to quantify the relative importance of multiple diversity, functional traits, stand structure, and topographic factors as the best predictors of variation in biomass.