Understanding edge effects on seed production

Katherine Hulting, from Michigan State University’s W.K. Kellogg Biological Station and an EEB member, discusses her

article: Habitat edges decrease plant reproductive output in fragmented landscapes

Habitat fragmentation is widespread globally. However, understanding how to conserve in fragmented landscapes is challenging. Breaking habitats apart leads to multiple changes, such as decreased connectivity and increased edge habitat, making it difficult to disentangle the effects in natural landscapes. Because of these co-occurring changes to habitats, there are several mechanisms for how populations could be affected by fragmentation – connectivity loss could impact dispersal and rescue effects, while habitat edges could impact populations through changes in abiotic conditions.

For plant populations, understanding how fragmentation alters demographic processes can be important for predicting conservation outcomes. While we know that fragmentation can change colonization/extinction dynamics, we know less about other processes like reproductive success. Given that reproductive output is a component of population growth, changes in seed production may have implications for population persistence, particularly for seed-limited species.

We tested how connectivity and habitat edges affect plant reproductive output in the Savannah River Site Corridor Project, one of the largest and longest running habitat fragmentation experiments. Our experiment consists of patches of longleaf pine savanna habitat within a pine plantation matrix, either connected by a corridor or unconnected and varying in edge amount. Because all habitat patches are the same size, this experiment provides the unique opportunity to study connectivity and edge amount independently of habitat area.

To understand how these processes affect plant populations, in 2007 we established populations of five plant species in each habitat patch at varying distances from the forest edge. We chose species that did not previously occur in our experiment – three wind-pollinated grasses (Anthaenantia villosa, Aristida beyrichiana, and Sorghastrum secundum) and two insect-pollinated forbs (Carphephorus bellidifolius and Liatris earlei). Three years after the plants were established, we measured flowering, pollination rate, and seed production of each individual to understand how connectivity, patch edge amount, and distance from the forest edge affect plant reproductive output.

We found that habitat edges consistently decreased plant reproductive output through impacts on flowering. For all plant species, individuals were more likely to flower and produce more flowering structures farther from the forest edge. For four out of five species, this increase in flowering resulted in a higher seed production farther from the edge. Our results suggest that reproductive output is often decreased by edge proximity, which could have implications for population demography in fragmented landscapes.

Because habitat edges have unique microclimate conditions, plant responses to habitat edges were likely driven by these changed abiotic conditions. In our experiment, with forested matrix and open-habitat patches, edges tend to be cooler and shadier because of increased canopy cover. All our focal species are longleaf pine savanna understory species, adapted to high light environments with less accumulated leaf litter, which suggests that plant flowering and seed production may have been decreased because of unfavorable conditions at edges.

The advantage of using experimentally established plant populations within a larger fragmentation experiment is that we have insights from other studies that use the same system. For our two forb species (Carphephorus and Liatris), previous work in our experiment found that seed production was the most important demographic predictor for population growth. We also know that seed dispersal distances and directions are altered by habitat edges for our focal species. Paired with our findings, these results highlight that habitat edges are important for these plant populations, potentially shaping population growth and spread. Our work underscores the significance of considering habitat edges in conservation and restoration efforts, which will strengthen conservation planning in fragmented areas.

See the full story in the Journal of Ecology Blog.