Introduction
Evidence from neighbour removal studies clearly indicates that plant communities are highly competitive (e.g., Clements et al. 1929; Grime 1973; Tilman 1989; Gurevitch et al. 1992). Research also indicates that variation in plant functional traits is important in determining competitive ability and predicting the suppression of neighbour growth (Aarssen and Keogh 2002). For example, larger plant species are generally considered superior to smaller species when competing for pollinators (e.g., Donnelly et al. 1998), seed dispersal agents (e.g., Thomson et al. 2011), and for light (e.g., Weiner 1990; Schwinning and Weiner 1998). We refer to the competitive advantage commonly attributed to relatively tall plant species as the “size-advantage hypothesis” (e.g., Tracey and Aarssen 2014; Tracey et al. 2017).
Light is an essential resource, and light limitation can negatively impact plant growth (Bazzaz 1979; Tilman 1985; Grubb 1998). Light availability varies across the growing season (Bachmann et al. 2018), with canopy maturation (Ballaré 1994), and with community density (Givnish 1982). When competing species vary in size, competition for light is said to be size-asymmetric (Lamb et al. 2009; DeMalach et al. 2017), with larger plants expected to dominate light competition by intercepting more light per unit size than smaller plants (Anten and Hirose 1998; DeMalach and Kadmon 2017). This growth of larger species imparts additional light limitation on smaller species while also reducing their own likelihood of being shaded (Anten 2005). When soil resources are adequate, natural communities can support the growth of tall plant species, which are able to limit the light reaching smaller plant species. For example, soil fertilization can result in increased light limitation and reduced grassland diversity (Borer et al. 2014). However, despite a perceived size-advantage in terms of expected negative impact on small species survival, growth and reproduction, small plants remain both ubiquitous and abundant within herbaceous vegetation (Aarssen et al. 2006). There is also little evidence of deterministic organization of species according to size in herbaceous communities (Schamp et al. 2008). Recent research has shown that species with larger body size in fact do not dominate neighbourhood biomass production in old-field vegetation (Tracey et al. 2017), nor do they recruit more offspring from the soil seed bank (Tracey and Aarssen 2019).
Light limitation has been studied extensively in forests (Canham et al. 1990; Canham et al. 1994; Gilliam and Roberts 2003; Gommers et al. 2013), but whether the impact of light limitation in forests is comparable to that in herbaceous vegetation remains unclear. While grasslands and old-fields are largely dominated by herbaceous species, distinct canopy layers have attributed vertical complexity to forests (Parker and Brown 2000; Miedema et al. 2019). In contrast, researchers have implied higher abundance and even distribution of light within herbaceous communities. However, several studies counter this widespread assumption, and collectively argue that light heterogeneity exists in herbaceous communities (Bazzaz 1990; Kelly and Canham 1992; Ballaré 1994; Körner 1995; Liira et al. 2002; Heger 2016; Huber et al. 2021). Herbaceous vegetation exhibits a layered canopy structure (Körner 1995; Liira et al. 2002) which, according to experimental studies, can reduce light penetration to the ground level to as low as 3% (Spehn et al. 2000), comparable to the 18-27% observed in some forests (Bartemucci et al. 2006). However, we struggled to find many published assessments of light penetration in herbaceous communities. One study found a reduction in diversity of small plant species in experimental plots under fertilization as ground-level light penetration decreased (Borer et al. 2014). Other experimental studies have also assessed the impact of light availability on herbaceous vegetation (e.g., Semchenko et al. 2012; Bachmann et al. 2018); however, analyses have yet to examine light conditions in unmanipulated communities (e.g., absent of seeding, fertilization, watering treatments).
It remains unclear how small species persist in herbaceous vegetation despite their apparent disadvantage in light competition, but the ubiquity of small species, on every scale from regional floras to local neighbourhoods, suggests that these species either avoid or tolerate low-light conditions. In woodlands, species that remain shaded under the herbaceous canopy may minimize light requirements through high photosynthetic efficiency and low light compensation points (Boardman 1977; Givnish 1988; Ballaré et al. 1997; Valladares and Niinemets 2008). In this case, there may be little to no change in small species abundance or richness associated with the amount of shade cast by taller vegetation. Smaller species may also rely on light from canopy gaps and sunflecks, which are commonly observed in forests (Chazdon and Pearcy 1991), and are likely important within herbaceous communities as well. Greater heterogeneity in canopy height in herbaceous communities and higher frequency of disturbance can increase the incidence of canopy gaps and sunflecks, allowing more light to penetrate towards the soil surface, which may thus support a greater number of small species (Chesson and Huntley 1997; Roxburgh et al. 2004). Additionally, small species may rely on early season light availability prior to canopy closure; although seasonal variation in light has been understudied in herbaceous communities, small species have been found to flower earlier in the season (Du and Qi 2010; Sun and Frelich 2011; Segrestin et al. 2020). Early growth and flowering by small species may reduce competition for light, as well as for pollinators, allowing these species to coexist with larger, later flowering species (Jensen et al. 2019).
In this study, we sought to answer two important questions concerning the presumed advantage that larger species have in light competition within a temperate mesic old-field community. First, we examined patterns of light penetration through the standing vegetation to determine whether sample plots containing taller resident species have significantly reduced light penetration. Second, we tested whether the species abundance and richness of smaller plants were lower in sample plots where light penetration was lowest. The goal of this research was to better reconcile experimental evidence that large plant species enjoy a competitive advantage for light with the widespread prevalence of small plant species in natural communities.