Pollination and herbivory influence plant allocation pattern: Within- and among-year effects in Chamerion angustifolium

Background/Question/Methods

Plants that require pollination interact with two types of insect foragers: pollinators and herbivores. Plasticity in plant traits such as flowering and growth that mediate interactions with insect foragers, along with allocation trade-offs among plant traits, may allow herbivores and pollinators to influence each other ecologically. The potential for such plant plasticity-mediated interactions between foragers is particularly high for perennial and asexually reproducing species where effects might be expected both within and across years. Pollinator effects especially, which occur later in the growing season and influence allocation to resource-heavy reproductive structures, may be lost when considering only within year effects. However, little previous work has addressed plasticity in response to both types of foragers or followed these effects across years.

In hillside meadows near the Rocky Mountain Biological Lab (CO), we used two separate two-year field experiments to characterize plastic plant responses in growth and flowering in Chamerion angustifolium (fireweed) to herbivores and pollinators, and herbivore and pollinator responses to plant size and flower number, within and across years. In experiment one, we manipulated herbivore damage and pollen receipt and measured plant responses; in experiment two we manipulated plant size and flower number, and measured herbivore damage and pollination. 

Results/Conclusions

Within years, herbivory decreased flowering and increased stem number, whereas pollen receipt did not affect flower or inflorescence number, but did increase plant height. Across years, herbivory increased stem number, while pollen receipt decreased reproductive effort. Pollen receipt and herbivory had non-opposing effects on size and reproduction, indicating that the presence of insect foragers reduces reproductive allocation and increases size allocation both within- and among-years.

We found no evidence that pollinating and herbivorous insect foragers respond similarly to stem or flower number: removal of stems and flowers affected only herbivores. Stem removal treatments, which reduced aboveground biomass by half, increased herbivore damage when coupled with either small plant size or high reproductive effort. Both stem and flower removal treatments had almost entirely positive effects on allocation to size and reproduction across years. This suggests that plants are able to compensate for biomass removal in the subsequent year. 

These experiments emphasize the need for across-year studies, particularly for perennial species with multiple seasons to store resources and to flower; single-year studies may result in misinterpretations of the ultimate causes of variation in reproductive effort or size, or underestimate the effect of insect foragers on plant traits.