Plant biodiversity-productivity relationship as defined in ecological research often represents the correlation between the number of plant species (i.e., taxonomic diversity) and the sum of their above-ground biomass production in a community (Whittaker and Niering 1975; Hector et al. 1999; Tilman et al. 2001). However, in context of the natural environment, the loss of species resulting in the potential loss of function of an ecosystem is highly debatable (Lavers and Field 2006; Grace et al. 2007; Paquette et al. 2018; van der Sande et al. 2018; van Moorsel et al. 2018; Wu et al. 2018; Ammer 2019; Yang and Liu 2019; Fayiah et al. 2019; Craven et al. 2020; Yue et al. 2020; Mahaut et al. 2020). Ecological communities appear to have diverse existing relationships between taxonomic diversity and productivity, which may be positive (Bai et al. 2007), neutral (Grace et al. 2007; Adler et al. 2011; Burley et al. 2016), or even negative (Rose and Leuschner 2012; Zhang et al. 2016; Wu et al. 2018). In natural environments the numerous factors known to contribute to the variance in the taxonomic diversity-productivity relationships include spatial scale (Bond and Chase 2002; Cardinale et al. 2004; Harrison et al. 2006), grazing (Dingaan et al. 2016), species pool (McKenna and Yurkonis 2016), plant-soil feedbacks (Kulmatiski et al. 2012), and evolutionary history (Pärtel et al. 2007).
The scale-dependency of taxonomic diversity-productivity relationships in ecological communities mainly result from distinct factors at the local and regional scales (Weiher 1999; Bond and Chase 2002; Harrison et al. 2006; Yamamura 2006; Ni et al. 2007; Chisholm et al. 2013; Bracken et al. 2017; Oehri et al. 2017; Li et al. 2019). Plant taxonomic diversity-productivity relationships at local scales (among field sampling plots in a relatively homogeneous environment) are expected to be positive when the number of species is low with a high probability of local species niche complementarity (whereby different species possesses different traits that utilize available resources differently) (Bond and Chase 2002). Whereas it is assumed to be negative when the number of species is high, with a high probability of species overlap in terms of resource utilization (Bond and Chase 2002). Regional scales when compared to local scales often contain distinct environmental gradients or ecological systems, which are often presumed to have stronger taxonomic diversity-productivity relationships due to their high environmental heterogeneity-based regional species diversity (Bond and Chase 2002). However, the strong correlation between taxonomic diversity and productivity at the regional scale might be a result of common responses to relevant environmental drivers or their internal linkages (Ma et al. 2010; Jing et al. 2015). A majority of studies conducted previously have focused on the correlation between taxonomic diversity and productivity without accounting for the effects of confounders such as abiotic factors (Ni et al. 2007; Li et al. 2019) or have not explicitly evaluated the direct correlation between taxonomic diversity and productivity after controlling other relevant covariates (Maestre et al. 2012). Hence, a comprehensive understanding of the intensity of taxonomic diversity-productivity relationships at regional scales or local scales is lacking.
Compared to taxonomic diversity, functional diversity, which measures the mean or variance of single or multiple trait values of species in a community, might be more strongly related to productivity since functional traits take a direct part in the processes by which plants absorb, utilize, and transform essential resources (such as water and nutrients) (Cadotte et al. 2009; Flynn et al. 2011; Liu et al. 2015; Brun et al. 2019). Indeed, traits such as community-weighted mean (CWM), reflect the magnitude of the aggregation of trait values among species in a community, and higher CWM of functional traits may result in higher or lower productivity (Cadotte and Hillebrand 2017). Trait variance, which reflects functional complementarity (e.g., facilitation) or divergence (e.g., competition) among species in a community, may also have a positive or negative influence on productivity (Cadotte and Hillebrand 2017). Meanwhile, phylogenetic diversity, which is calculated using the phylogenetic relationship among species in a community is considered to be a parameter reflecting their functional similarity, and may have a closer relationship with productivity in comparison to taxonomic diversity (Cadotte et al. 2013). However, empirical studies to assess the relationship of functional and phylogenetic diversity with productivity at both local and regional scales in comparison with taxonomic diversity have not been reported till date.
In order to address the above-mentioned research questions, we surveyed 60 quadrats in three different meadow habitats, each measuring 0.5 m × 0.5 m, situated along an elevational gradient in the Yulong Mountains of Southwest China. For each species in the quadrats, we weighed their above-ground stem dry weight, while simultaneously recording a number of functional traits like maximum plant height (cm), leaf carbon, nitrogen, and phosphorus content (mg/g). A molecular phylogenetic tree for all the species in the quadrats was constructed using the DNA sequences of the coding genes rbcL and matK, respectively. For each quadrat, we also evaluated soil chemical properties such as pH, organic carbon (C, mg/g), total nitrogen (N, mg/g), and total phosphorus concentration (P, mg/g). In this study, we considered each meadow to be equivalent to the local scale, while all meadows taken together were considered to represent the regional scale. For each observed assemblage, we calculated the species richness, Shannon’s evenness, and a series of functional and phylogenetic diversity metrics. Using multi-model inference and structural equation modeling, we aimed to test the following hypotheses: (1) plant taxonomic diversity-productivity relationship is stronger at the regional scale than at the local scale; however, the strong relationship at the regional scale might be driven by the responses of plant taxonomic diversity and productivity to the common abiotic covariates, and (2) functional and phylogenetic diversity have more robust relationships with productivity than taxonomic diversity over both local and regional scales.