Prescribed prairie burns boost reproduction only in large populations, study finds

Fires are commonly used to maintain plant diversity in prairies worldwide. But do small fragmented prairie plant populations see the same boost in reproduction from burning as large interconnected populations? New research by Jared Beck and colleagues published in Proceedings of the National Academy of Sciences suggests they do not.

Scientists studied how prescribed burns affected narrow-leaved purple coneflower (Echinacea angustifolia) across 35 remaining prairie fragments in western Minnesota over six years. They tracked over 6,000 plants, burning some populations experimentally. These burned sites ranged from just 3 to nearly 4,000 flowering purple coneflowers.

The study found that prescribed burns doubled the percentage of purple coneflowers flowering and increased their flower head counts by 58% in all populations, regardless of size. However, the effects on seed production depended on population size. In sites with fewer than 20 flowering purple coneflowers, burns did not consistently increase pollination or annual seed production. Only in larger populations with over 20 plants did fires boost mating opportunities enough to raise individual seed production by 71%.

The researchers conclude that habitat loss may reduce the reproductive benefits of fire for small prairie plant populations. Accounting for these differing responses based on population size will be necessary for managing declining prairie species. The study shows the value of long-term field experiments to uncover critical population size thresholds, beyond the prairies of Minnesota. Beck and colleagues write:

Our findings have important implications for plant conservation in historically fire-dependent ecosystems worldwide. Many fire-dependent ecosystems have experienced extensive habitat loss and fragmentation. North American Longleaf pine systems and tallgrass prairie have both been reduced to less than 3 percent of their historic extent. Accelerating rates of land conversion and habitat loss threaten fire-dependent biodiversity hotspots such as South African fynbos and Brazilian cerrado. Increasingly, conservation practitioners advocate for the use of prescribed fire in remaining patches of fire-dependent biomes to maintain habitat structure and promote species diversity. Moreover, conservation and restoration efforts are often limited by seed availability and prescribed fire can be an effective tool for increasing seed yields. This study reveals that habitat loss and fragmentation may constrain the beneficial effects of fire in some plant populations

Beck et al. 2023.

Further research is needed to fully understand why fire-stimulated flowering does not translate to fitness gains in https://botany.one/2018/07/consequences-of-habitat-fragmentation-on-the-reproductive-success-of-two-tillandsia-species-with-contrasting-life-history-strategies/”>fragmented habitats. However, these initial findings challenge assumptions about prescribed burning equally aiding small and large populations of prairie plants.

READ THE ARTICLE
Beck, J., Waananen, A. and Wagenius, S. (2023) “Habitat fragmentation decouples fire-stimulated flowering from plant reproductive fitness,” Proceedings of the National Academy of Sciences, 120(39). Available at: https://doi.org/10.1073/pnas.2306967120.

Cover: Echinacea angustifolia. Image: Canva.