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.