1 INTRODUCTION

Stand structure and species composition of forests are shaped by demographic rates of individual trees (Clark, 2010; Hart et al., 2016; Iida et al., 2014). Among these, survival provides the most important link between ecosystem functions and community assembly (Leibold et al., 2017). The often-observed positive biodiversity–ecosystem functioning (BEF) relationship with regard to productivity can be due to increased growth or increased survival of individuals in more diverse plant communities (Barrufol et al., 2013; Marquard et al., 2009; Tilman et al., 2014). However, while a large number of BEF studies have explored biodiversity effects on individual growth, very few have studied such effects on individual survival (but see Kirui et al., 2008; Van de Peer et al., 2016; Yang et al., 2017), potentially due to the short-term nature of most of these studies.

Biodiversity is expected to promote plant survival and thus species coexistence (Grossman et al., 2018; Isbell et al., 2011; Neuner et al., 2015) due to three mechanisms. In diverse stands under environmental fluctuations, a ‘buffering effect’ could result from asynchronous responses of different species to these fluctuations, so that rarely all individuals are simultaneously exposed to the same mortality risk. A ‘performance-enhancing effect’ could result when species with high performance under particular environmental conditions at each time have greater survival, so that the mean community performance increases (Yachi & Loreau, 1999). In the absence of environmental fluctuations, trees in mixtures may still survive better due to lower inter- than intra-specific competition, which is expected when trees have complementary niches, also referred to as ‘complementarity effect’ (Tilman et al., 2014). However, so far these three mechanisms have mostly been studied for productivity (Ammer, 2019), especially in grassland experiments. Survival and its relation to buffering, performance-enhancing and complementarity effects have far less been studied (but see Luo & Chen, 2011).

Differential survival responses among species to biodiversity over time might be interactively affected by functional traits and interannual variation in climatic conditions. Survival rates under particular environmental conditions may be predictable to a certain extent based on functional traits regulating the acquisition of light, water or nutrient resources (Van de Peer et al., 2016). Previous studies have reported that species adapted to less favourable environmental conditions (e.g. low nutrient or water availability) generally have enhanced resource-holding capacity (e.g. higher specific leaf area, higher stomatal density, higher hydraulic conductivity) and lower biomass loss than species adapted to better environmental conditions (Laughlin et al., 2018; Maire et al., 2015; O'Brien et al., 2014; O'Brien et al., 2017; Pu et al., 2020; Sterck et al., 2003; Sterck et al., 2006). In addition, functional traits are often used to place species along an acquisitive to conservative strategy gradient, ranging from deciduous, early-successional species with thin leaves, high specific leaf area (SLA) and high hydraulic conductivity to evergreen, late-successional species with thick leaves, low SLA, low hydraulic conductivity and high leaf nutrient concentration (Anderegg et al., 2016; Fichtner et al., 2017; Schnabel et al., 2021; Wright et al., 2004). As a consequence, if the functional traits of a species do not match the corresponding environmental conditions, its survival rate may be reduced. However, because environmental conditions, in particular climatic conditions, can vary between years, it is necessary to study effects of functional traits and environmental conditions on individual survival in a time-dependent manner (Adler et al., 2006; Ammer, 2019). This requires the study of survival rates obtained from interval-censored data (Sparling et al., 2006), because it is more difficult to accommodate time-dependent covariates such as yearly precipitation in studies of survival times (Egli & Schmid, 2001).

Here, we used a long-term, interval-censored dataset from a large forest BEF experiment in subtropical China (BEF-China) to analyse annual survival rates of individuals trees from age 3 to 12 (years) among 39 subtropical tree species with contrasting functional traits as a function of stand diversity, age and yearly precipitation (Figure 1a). Stand diversity was represented by tree species richness in the experiment and varied from monocultures to 2-, 4-, 8- and 16-species mixtures. Each diversity level was represented by different species compositions, with each species occurring at each richness level. There were 469 plots (25.8 × 25.8 m), each planted with 400 trees of which the central 16 were yearly censused (Figure S1). We tested if stand diversity, that is species richness, could promote tree survival and how survival rates were modified by species functional traits and yearly climatic conditions. Our specific hypotheses were (Figure 1b): H1, stand diversity increases annual survival rate and this effect strengthens with age. The second part of H1 was based on our previous observation that positive diversity effects on stand biomass and productivity increased with stand age due to species complementarity (Huang et al., 2018). H2, different species show differential survival responses to stand diversity and these responses change with age. H3, differential survival responses of species with age may depend on functional traits and annual climatic conditions such as precipitation.