Testing hypotheses related to species size and light penetration
According to the size-advantage hypothesis, larger species are
competitively superior to smaller species, mainly due to their ability
to intercept light, shading plants below (e.g., Schwinning and Weiner
1998; Anten 2005; Tracey and Aarssen 2014; Tracey et al. 2017).
Consistent with this, we found that light penetration significantly
decreased with mean plot height (Fig. 5). Importantly, the height of
species in sample plots, as measured here, left approximately 90% of
variation in plot-level light penetration unexplained. Some of this
unexplained variation is inevitably due to the way in which we estimated
plot height – future assessments based on measuring the heights of all
plant species in plots would be of benefit but were not possible for
this study. Regardless, this result highlights that while tall species
have the capacity to shade smaller plants, the biological significance
of this advantage may be greatly reduced by the size distributions of
tall species in plots, and by other factors that contribute to variation
in composition within herbaceous plant communities. Inevitably, other
factors, like the available soil nutrients, can limit the height of the
canopy by controlling which species can persist (Borer et al. 2014).
Large species with large minimum reproductive size thresholds (Tracey
and Aarssen 2019) may be less likely to persist in low soil nutrient
settings and self-thinning among tall plant species may create regular
gaps, allowing light to reach smaller species (Schamp and Aarssen 2014).
Thus, nutrient limitation and competition among tall species may
decrease the abundance of large plant individuals per area,
simultaneously increasing light penetration and altering the importance
of light as a previously limited resource. Herbivory can also impact
canopy development directly, and indirectly by influencing plant
investments in herbivore defense.
We found evidence that small species abundance and richness are limited
by light penetration; these patterns were consistent when we considered
reproductive plants and small species were defined as those below the
median height of species in the community, but inconsistent when small
species were defined as those in the first quartile of species heights
in the community. These findings were less consistent across both
definitions of small species when we did not focus on reproductive
plants (Table S1). Importantly, the coefficients of determination
(R2 ) for regressions between light penetration
and our response variables, when significant, ranged between 8-11%
(Fig. 6). Furthermore, only 4.2% of the variation in the composition of
small species, defined using the median, was explained by light
penetration (Fig. 7). Low light penetration had a significant, but
meagre impact on small species abundance and richness in this community.
These results, in combination with the above results showing that large
species contribute only slightly to patterns of light penetration,
provide useful context for understanding the conundrum of small species
diversity in natural systems in spite of experimental observations that
small species are at a competitive disadvantage. First, light
penetration in herbaceous systems is relatively low, but less so earlier
in the growing season (i.e., June in this study system). It is possible
that some small species take advantage of higher light penetration early
in the growing season. This is consistent with observations that smaller
plant species often flower earlier in the season (Du and Qi 2010; Sun
and Frelich 2011; Segrestin et al. 2020). Second, within-plot variation
in light penetration, reflecting heterogeneity in plot canopy structure,
had no impact on small species abundance and richness, suggesting that
local sunflecks are relatively unimportant in maintaining small species
in herbaceous communities. This may be because plots with more variation
in canopy structure tended to be less shaded on average, reducing the
role of sunflecks as a critical source of light. Third, the fact that
small species richness is only modestly impacted by light penetration
suggests that many small species possess some mixture of shade avoidance
and shade tolerant strategies that allow them to persist in shaded
conditions. Shade tolerance may contribute to increased reproductive
economy in smaller species (Aarssen 2008). Shade tolerant species
implement morphological and physiological traits which increase
efficiency of carbon gain and stress tolerance, allowing them to persist
under low light conditions (Valladares and Niinemets 2008).
Additionally, small species richness, defined using the
1st quartile, was no lower when light penetration was
lowest, indicating that the richness of the 25% smallest species in the
community was maintained under increasingly low light availability.
Adaptation to low light levels via phenotypic plasticity in
morphological traits may explain the persistence of small species
richness under low light (Ballaré 1994; Urbas and Zobel 2000; Hallik et
al. 2009; Niinemets et al. 2015), as some individuals may express traits
associated with greater light capture (e.g., high specific leaf area;
Valladares and Niinemets 2008), thus maintaining species presence under
reduced abundance. The characteristics of the light environment in our
community, and likely the characteristics of many resident small species
ensure that any disadvantage they suffer in competition for light is
minimal.
The effects of shading on small species abundance and richness were more
consistently observed and stronger in general when we focused on
reproductive individuals in sample plots. This demonstrates that
although ‘median’ small species were able to persist under low light
with no detected loss in abundance or richness, some were not successful
enough to reach reproductive maturity. The presence of flowering
individuals in a sample plot is evidence of a species’ capacity to
successfully establish in that environment. Our results make it clear
that light penetration does indeed affect which small species can find
success, but its impact on small species abundance and richness is
relatively minor (8-15% of variation explained). Our results also
support the contention that focusing censuses on flowering species when
studying coexistence, can clarify plant community dynamics (Schamp et
al. 2016, Schamp and Jensen 2019).