Understanding variation in the light environment
To understand the effects of light limitation on small plant species in a herbaceous plant community, we first characterized variation in light penetration within-plots, among-plots, and across time through the growing season. Despite existing assumptions that light is not heterogeneously distributed within herbaceous communities, light penetration across plots in our community ranged from 0.3-72.4% (Fig. S1A). Light penetration also varied within plots and across the growing season, and the degree of variance in light penetration among intraplot, interplot, and temporal measures was similar (Fig. 3). Light penetration significantly increased from June to July as well as from June to August but did not differ between July and August; thus, maximum canopy development was reached by July.
Analyses of light penetration through the canopy of natural herbaceous plant communities are rare (e.g., Kelly and Canham 1992); those that exist are often conducted on communities that have been subjected to an experimental treatment. These include various fertilization (e.g., Urbas and Zobel 2000; Semchenko et al. 2012; Borer et al. 2014), watering (e.g., Semchenko et al. 2012), mowing (e.g., Urbas and Zobel 2000; Bachmann et al. 2018), herbivory (e.g., Izaguirre et al. 2006; Borer et al. 2014), and shading or illumination treatments (e.g., Urbas and Zobel 2000; Izaguirre et al. 2006; Semchenko et al. 2012). Varied experimental treatments, along with differences among studies in how the light environment is quantified (e.g., 5cm from the ground by Kelly and Canham 1992; 3-150 cm from the ground by Bachmann et al. 2018), make it difficult to compare across studies, and to get a general sense of light environments in herbaceous communities. However, when species richness was manipulated via seeding, analyses of old-field meadow and grassland communities have described ranges of percent light penetration similar to our community, with averages as low as 13% in 8-species mixtures (Spehn et al. 2000), 20-32% in 16-species mixtures (Bachmann et al. 2018), and 3% in 32-species mixtures (Spehn et al. 2000). Within our community, total species richness ranged from 13-29 among plots; thus, the ranges of light penetration and species richness recorded in our non-experimental study site are comparable to findings from experimental herbaceous study sites.
Because the herbaceous canopy lacks shrubs and trees, it is a widely held view that light is both highly abundant and relatively homogenous within herbaceous communities. However, average light penetration within our community (5-29%) is comparable to 18-27% light penetration through a mixed wood temperate forest (Bartemucci et al. 2006) and, for some plots, 0.4-6.7% light penetration through a tropical forest (Valladares et al. 2002). While herbaceous canopies are likely slower to close, many small plant species in herbaceous communities persist underneath these canopies. While shade tolerance is generally accepted as supporting the ability of small plant species to survive in forest understories, it remains an open question whether shade tolerance plays a similar role in herbaceous plant communities.