Effects of time since fire on flower production of a key nectar source, butterfly milkweed (Asclepias tuberosa)

Background/Question/Methods

Most perennial forbs of tallgrass prairie are thought to be fire-adapted, and many have been shown to resprout after fire.  However, the effects of fire on flower production are poorly known for most prairie perennials, including butterfly milkweed (Asclepias tuberosa). This plant, as its name suggests, is commonly visited by butterflies, but also by other pollinators.  We sought to determine the effects of fire on flower production of this plant, in grazed prairies as well as ungrazed prairies, in order to better understand how fire can affect floral resource availability for pollinators, and to determine if that relationship is altered by cattle grazing.  Toward these ends, we applied prescribed fire on a 3 year rotation to multiple tallgrass prairies having natural populations of butterfly milkweed.  All fires were conducted between mid-winter and early spring of 2012-2014.  In summer 2012, during the peak blooming of butterfly milkweed, we used stratified random sampling to select and mark over 200 plants.  Each plant had experienced fire either 0, 1 or 2 years prior.  We counted total number of flowering stems on each plant, as well as the number of flowers and buds on the three longest stems in midsummer of 2012-2014. 

Results/Conclusions

Our preliminary results indicate that as time since fire increased, there was a decrease in flowering stem production and flower production per stem in ungrazed pastures. In other words, floral output was highest in the year of a fire (time since fire = 0 years), but in years 1 and 2 after fire, floral output declined.  In most grazed pastures, we found a similar trend: fire stimulated an increase in floral output, which declined in subsequent years.  However, in one grazed pasture where cattle density was increased for a few months of 2014, this trend did not hold up. Instead, recently burned prairie had the lowest floral output. Examination of the individual plants suggests that the cattle consumed many of the stems before they could flower.  Our findings suggest that fire can stimulate floral output of butterfly milkweed, which is likely to increase foraging efficiency for floral visitors such as butterflies and bees, and thus might also enhance abundance of these insects.  However, it appears that the positive effect of fire on butterfly milkweed floral output can be nullified by cattle stocked at high rates.