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.