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Premise Herbivores directly influence plant reproduction by damaging reproductive tissue which reduces seed production in consumed plants. Consumption of reproductive tissue may also indirectly depress reproduction in unconsumed plants by isolating unconsumed individuals from prospective mates and reducing pollination. However, empirical support for such hypothesized indirect effects remains limited. Methods To investigate potential indirect effects of herbivory on mating opportunities and pollination, we quantified spatial patterns of white-tailed deer (Odocoileus virginianus) herbivory and reproductive outcomes in Lilium philadelphicum (Liliaceae). We mapped 708 flowering plants, monitored deer herbivory, and examined how distance to prospective mates influenced rates of ovule fertilization, a proxy for pollen receipt. Results Most floweringL. philadelphicum plants (83%) failed to produce seed. Deer consumption of flowers prevented seed production in 35% of individuals. The probability of deer herbivory declined with distance to flowering conspecifics. Sixty-five percent of the remaining, unconsumed plants failed to produce seed due to apparent pollen limitation. While ovule fertilization rates declined with multiple predictors quantifying distances to flowering conspecifics, isolation metrics that excluded plants consumed by deer significantly improved predictions of ovule fertilization. Conclusions Our results offer empirical support for the hypothesis that herbivory reduces pollination and seed production in unconsumed plants by altering spatial mating opportunities. This feedback between deer herbivory and distance-dependent pollination contributes to widespread reproductive failure in L. philadelphicum, though the extent to which deer reduce seed production varies with plant isolation. Herbivory may exacerbate pollen limitation in other species by isolating unconsumed plants from prospective mates. 

Data and R code necessary to replicate figures and analyses presented in the following manuscript: Beck and Wagenius. In press. Herbivory exacerbates pollen limitation by isolating unconsumed plants from prospective mates. American Journal of Botany. 

Periodic fire enhances seedling recruitment for many plant species in historically fire-dependent ecosystems. Fire is expected to promote recruitment by generating environmental conditions that promote seedling emergence and survival. However, fire may also increase flowering and seed production. This makes it difficult to distinguish the effects of microsite conditions from seed availability in observational studies of seedling recruitment. Experiments that manipulate seed inputs across a representative range of conditions are needed to elucidate how seed availability versus microsite conditions influence post-fire seedling recruitment and plant demography. We experimentally manipulated time since fire across 36 patches of remnant tallgrass prairie distributed across 6400 ha in western Minnesota (USA). Over two years, we sowed 11,057Echinacea angustifolia(Asteraceae) seeds across 84 randomly placed transects and tracked 974 experimentally sown seedlings to evaluate how time since fire influenced seedling emergence and survival after experimentally controlling for variation in seed inputs. We also quantified six environmental variables and evaluated whether these covariates were associated with seedling emergence and survival. Fire influenced both seedling emergence and seedling survival. Seedlings emerged from approximately 1 percent of all seeds sown prior to experimental burns. Seeds sown one year after experimental burns emerged at 15 times the rate of seeds sown in the fall before burns, but emergence then declined as time since fire increased. Sowing seeds at high densities reduced rates of seedling emergence but increased overall recruitment. Increases in litter depth were associated with reduced emergence. Meanwhile, the probability that seedlings survived to late summer was greatest when they emerged 0-1 years after fire. The probability of seedling survival decreased with litter depth and increased with the local density of conspecific seedlings. Our findings experimentally support widespread predictions that fire enhances seedling recruitment by generating microsite conditions favorable for seedling emergence and survival – especially by increasing the light available to newly emerged seedlings. Nevertheless, recruitment also increased with seed inputs indicating that both seed availability and microsite conditions influence post-fire recruitment. Explicitly discriminating between seed-limitation and microsite-limitation is critical for understanding the demographic processes that influence plant population dynamics in historically fire-dependent ecosystems. 

Many plant species in historically fire-dependent ecosystems exhibit fire-stimulated flowering. While greater reproductive effort after fire is expected to result in increased reproductive outcomes, seed production often depends on pollination, the spatial distribution of prospective mates, and the timing of their reproductive activity. Fire-stimulated flowering may thus have limited fitness benefits in small, isolated populations where mating opportunities are restricted and pollination rates are low. We conducted a 6-y study of 6,357 Echinacea angustifolia (Asteraceae) individuals across 35 remnant prairies in Minnesota (USA) to experimentally evaluate how fire effects on multiple components of reproduction vary with population size in a common species. Fire increased annual reproductive effort across populations, doubling the proportion of plants in flower and increasing the number of flower heads 65% per plant. In contrast, fire’s influence on reproductive outcomes differed between large and small populations, reflecting the density-dependent effects of fire on spatiotemporal mating potential and pollination. In populations with fewer than 20 individuals, fire did not consistently increase pollination or annual seed production. Above this threshold, fire increased mating potential, leading to a 24% increase in seed set and a 71% increase in annual seed production. Our findings suggest that density-dependent effects of fire on pollination largely determine plant reproductive outcomes and could influence population dynamics across fire-dependent systems. Failure to account for the density-dependent effects of fire on seed production may lead us to overestimate the beneficial effects of fire on plant demography and the capacity of fire to maintain plant diversity, especially in fragmented habitats. 

Beck and Wagenius. Herbivory exacerbates pollen limitation by isolating unconsumed plants from prospective mates. Contains data and R code necessary to replicate figures and analyses presented in the manuscript. This data set includes spatially explicit data on multiple stages of reproduction for N = 708 Lilium philadelphicum individuals. We documented spatial patterns of deer herbivory, capsule formation, and seed set enabling us to test how proximity to prospective mates influences both rates of herbivory and pollination as well as interactions between herbivory and pollination. We found rates of both deer herbivory and seed set increase when conspecific flowering plants are nearby. However, proximity to unconsumed neighbors better predicts variation in seed set than measures of proximity that include both consumed and unconsumed individuals revealing that deer exacerbate pollen limitation by increasing the distance to prospective mates. This finding reveals how herbivores can indirectly depress reproduction by altering the spatial distribution of prospective mates. This data set will be made publicly accessible as soon as the associated manuscript is accepted. 

Data and code for: Beck, Waananen, and Wagenius. 2023. Habitat fragmentation decouples fire-stimulated flowering from plant reproductive fitness. Proceedings of the National Academy of Sciences. Contains metadata, data, and R code necessary to replicate figures and analyses presented in the manuscript.