The foraging decisions of individuals are contingent upon diverse factors, including body condition, resource distributions, and predation risk. Behavioral studies of non-photosynthetic organisms frequently demonstrate that the presence of stressors (e.g. predators) can influence an individual’s foraging behavior. For vascular plants, a common foraging decision is where to grow roots in relation to soil nutrient distributions. Most plants express some degree of foraging precision, where roots are concentrated in high nutrient patches in a manner generally consistent with an ideal free distribution. The degree to which root foraging behavior is influenced by herbivory risk is unknown, though of great relevance for understanding ecological interactions and the provisioning of ecosystem services. Here we used a novel digital image correlation (DIC) based methodology to non-destructively observe in-situ root foraging decisions in Helianthus annuus, the common sunflower. Stressed (leaf removal) and non-stressed plants were grown in experimental arenas in which nutrient distributions were manipulated. The use of successive images and the DIC processing algorithm allowed us to measure root displacement at a high degree of spatial and temporal resolution. Using these data, we were able to test whether leaf removal reduced foraging precision, and whether any observed changes were transient or fixed.
Results/Conclusions
Unstressed plants demonstrated a high degree of foraging precision with root growth concentrated in high resource patches. The imposition of a stressor, leaf removal, caused a nearly immediate loss of precision in root placement, with new root growth equally allocated among high and low-quality patches. This shift in foraging decision quality was not due to a cessation of root growth following leaf removal, and instead was the result of shifts in the placement of new roots relative to the unstressed plants. This decline in foraging precision was immediate, but also transient. After approximately 120 hours, the stressed plants returned to high-precision root placement, again favoring growth in high quality patches. These results represent the first demonstration of stress-sensitivity in root foraging decisions and add to a growing body of literature demonstrating the complexity of behavioral response in plants. If root foraging efficiency is generally reduced following stressful events, this could alter numerous ecological processes, including resource-based competitive interactions among neighboring plants and primary productivity. These results reinforce the importance of timing in understanding the fundamental ecology of plants, where a focus on end-point metrics may miss the underlying complexity of plant life.