Seedling recruitment correlates with seed input across seed sizes: implications for coexistence

Abstract

Understanding controls on recruitment is critical to predicting community assembly, diversity, and coexistence. Theory posits that at mean fecundity, recruitment of highly fecund small-seeded plants should be primarily microsite limited, which is indicated by a saturating recruitment function. In contrast, species that produce fewer large seeds are more likely to be seed-limited, which is characterized by a linear recruitment function. If these patterns hold in nature, seed predation that disproportionately affects larger-seeded species can limit their establishment. We tested these predictions by comparing recruitment functions among 16 co-occurring perennial forb species that vary by over two orders of magnitude in seed size. We also assessed how postdispersal seed predation by mice influenced recruitment. We added seeds at densities from zero to three times natural fecundity of each species to undisturbed plots and examined spatial variation in recruitment by conducting experiments across 10 grassland sites that varied in productivity and resource availability. Consistent across two replicated years, most species had linear recruitment functions across the range of added seed densities, indicative of seed-limited recruitment. Depending on year, the recruitment functions of only 19-37% of target species saturated near their average fecundity, and this was not associated with seed size. Recruitment was strongly inhibited by rodent seed predation for largeseeded species but not for smaller-seeded species. Proportional recruitment was more sensitive to spatial variation in recruitment conditions across sites for some small-seeded species than for large-seeded species. These results contradict the common belief that highly fecund smallseeded species suffer from microsite-limited recruitment. Rather, they imply that, at least episodically, recruitment can be strongly correlated to plant fecundity. However, proportional recruitment of small-seeded species was inhibited at productive sites to a greater extent than large-seeded species. Results also show that in a system where the dominant granivore prefers larger seeds, low-fecundity large-seeded species can suffer from even greater seed-limited recruitment than would occur in the absence of predators.

Trade‐offs between seed size and biotic interactions contribute to coexistence of co‐occurring species that vary in fecundity

“…We previously reported initial results from a seed addition experiment in grasslands in western Montana where we found that the recruitment of species possessing functional traits including large‐seededness, taller height, lower C/N ratio and lower water use efficiency were less negatively affected by bunchgrass competition than smaller‐seeded species that were less tall, had higher C/N ratios and higher water use efficiency (Maron, Hajek, Hahn, & Pearson, 2018). However, within the context of the same experiment, we also demonstrated that post‐dispersal seed predation by mice had greater suppressive effects on the recruitment of larger‐seeded species compared to smaller seeded ones, thereby working in the opposite direction of bunchgrass competition (Maron et al, 2018; Maron, Hajek, Hahn, & Pearson, 2019). In temperate grasslands, old fields and other habitats, a growing body of work indicates that rodents focus on larger‐seeded species, disproportionately suppressing their recruitment compared to smaller‐seeded species (Dylewski, Ortega, Bogdziewicz, & Pearson, 2020; Brown & Heske, 1990; Larios, Pearson, & Maron, 2017; Maron, Pearson, Potter, & Ortega, 2012; Reader, 1993).…”

“…Fourth, are spatial differences in the performance of individual species across a productivity gradient manifest at the community level by shifting CWMs for traits? Finally, one important initial result of our prior work was that regardless of whether we manipulated interactions or not, species with higher annual fecundity (and hence that had more seeds initially added to plots) experienced greater recruitment than species with lower annual fecundity (Maron et al, 2018, 2019). Thus, does bunchgrass competition or seed predation equalize or exaggerate initial differences in relative abundances of target species that are related to fecundity?…”

“…Here, we build on this previous work (Maron et al, 2018, 2019) by following the fate of species and communities that assembled within our experimental plots beyond recruitment. We examine patterns of cumulative recruitment (i.e.…”

“…recruitment of species over years 1 and 2 of our experiment [year 1 results were previously reported; Maron et al., 2018]), as well as adult plant establishment 3 years after seed addition. Specifically, we address the following questions not considered previously (Maron et al, 2018, 2019). First, does competitive tolerance vary predictably based on seed size, height or LMA, and does seed size alone predict vulnerability to seed predation?…”

Voles mediate functional trait diversity along a resource gradient

“…Historically, research on plant community assembly has focused on evaluating how the abiotic environment acts a ‘filter’ selecting for those species and traits that are best suited for particular environments. Although much work has evaluated trait–abiotic environment relationships (Cornwell & Ackerly, 2010; Kraft et al., 2011; Spasojevic & Suding, 2012), fewer studies have examined how herbivores may mediate these effects (Eskelinen et al., 2012; Maron et al., 2019). The little work that has been performed has focused on large grazers.…”