Background/Question/Methods Animal-mediated seed dispersal is critical for maintaining plant populations. However, these mutualisms can be strongly altered by climate change, and our ability to predict future responses is limited because the mechanisms underlying variation in seed dispersal are not well understood. In this study, we assessed (1) how changes in climate affect the outcome of seed dispersal of bloodroot (Sanguinaria canadensis), as well as how variation in (2) disperser identity and activity and (3) plant traits mediate such effects. Bloodroot is a perennial herbaceous plant that is widely distributed throughout eastern deciduous forests and produces seeds with lipid-rich appendages (“elaiosomes”) specialized for attracting ants as dispersers. Bloodroot is named for the red-orange alkaloid-rich latex it exudes from multiple tissues (including fruits) as a defense against herbivores, which could also potentially influence seed dispersal interactions. Using seed depots, we tested for variation in rates of dispersal of locally collected bloodroot seeds across seven sites spanning 314 m in elevation, corresponding to a ~2°C change in mean annual temperature. We documented the identity and activity of seed-dispersing ants as well as fruit chemical and morphological traits, including latex exudation from fruits and the size of seeds and elaiosomes. Results/Conclusions Although the activity of seed-dispersing ants significantly increased with elevation (7.16-fold), there was no corresponding increase in removal of seeds by ants, suggesting that higher-elevation seeds are relatively less attractive. Moreover, while the likelihood of latex exudation from fruits did not depend on elevation, seeds sourced from latex-exuding fruits were significantly less likely to be dispersed (44%). Seed mass marginally increased with elevation (30%) and latex exudation (8%), which may partially explain the decline in removal of these seeds, since they likely require more energy to transport. However, there were no significant changes in elaiosome fresh mass or elaiosome:seed ratio. Future work will test whether changes in seed dispersal rates are correlated with fruit chemical traits, including the concentration and diversity of both attractive fatty acids and defensive alkaloids in seeds and elaiosomes. Overall, our results indicate that changes in climate may alter seed traits important for attracting dispersers, but seed disperser activity can also change in the opposite direction, perhaps limiting the degree of variation in the net outcome of this mutualism. This work contributes to a broader understanding of the mechanisms underlying variation in seed dispersal and may facilitate predictions of the effects of future climate change.
