Pollination and Pollen Limitation in Mayapple (Podophyllum Peltatum L.), A Nectarless Spring Ephemeral.

Abstract

Mayapple (Podophyllum peltatum L.) is a common clonal understory herb in temperate eastern North America. Its fecundity is pollen-limited because its flowers are nectarless, and native pollinators do not collect its pollen. I conducted field studies in southeastern Michigan to determine mayapple’s compatibility system and whether neighboring plants facilitated its pollination. I tested for facilitation by correlating the degree of pollen limitation with the abundances of neighbors and measuring whether the removal of neighboring flowers increased pollen limitation of fecundity.

Mayapple populations in four sites were self-incompatible (SI), but all clones in one site were self-compatible (SC). This difference could reflect genetic differences or possibly differences in inbreeding depression due to resources. The site with SC had the highest light availability and outcross fruit set, suggesting that abortion of inbred ovules may be lower under high resources, resulting in expression of SC.

Visitation to mayapple flowers was consistently low (0.03-0.06 visits/flower/hour), and fruit set was pollen-limited (pollen supplementation increased fruit set 3 – 18-fold) in all three years of this study. Based on regressions of pollen limitation violets facilitated fruit set in 2005 and 2007 but reduced seed set in 2005. Garlic mustard and spring beauty reduced fruit set in 2005. Except for violets in 2007, co-flowering species did not affect pollen limitation in 2006 or 2007. Floral removal did not change pollination success in 2006, confirming that neighbors neither facilitated nor competed with mayapple for pollination in that year.

Neighboring plants could also reduce mayapple fecundity through interspecific pollen transfer (IPT). The addition of Phlox divaricata pollen did depress fruit set, but Geranium maculatum pollen did not. However, foreign pollen was rare on mayapple stigmas suggesting that IPT is unlikely to be important in the field.

Mayapple could also facilitate or compete with neighboring plants for pollination. However, correlations showed no effect of mayapple on the pollination success of wild geranium, and hand-pollination with mayapple pollen did not significantly depress fruit or seed set.

In a review of studies on pollination facilitation, I propose that future studies employ similar methods and measure effect sizes for comparisons and meta-analyses.