Living on the edge of fragmented forests: distance, insects and fungi impact plant traits

Seedling establishment is a crucial bottleneck in plant population dynamics, but seedlings don't behave the same way fully-grown trees do.

How are species and ecosystems impacted by human-driven forest fragmentation? There are some fundamental theories that ecologists have found about seedling establishment around forests. For example, the abundance of fast-growing, resource-acquisitive species usually increases after fragmentation compared to slower growing, resource-conservative species. However, fast-growing, shade-tolerant species often have thinner leaves and might be more susceptible to herbivores and fungi.

Meghna Krishnadas and colleagues from Yale University, Centre for Cellular and Molecular Biology, India and Smithsonian Tropical Research Institute investigated how plant communities change with distance to the forest edges and tested if the absence of insects and fungi impact seedling establishment. They found that there were more small seeded plants closer to the edges and the abundance of those increased when they applied insecticides along the edge-to-interior gradient. These findings suggest that plant-plant and plant-enemy interactions mediate functional diversity around fragmented forest edges. Meghna Krishnadas and Liza Comita also recently reported that “edge effects” influenced seedling diversity throughout an annual cycle of seedling recruitment.

Tea plantation in Western Ghats, India. Source: Canva

Krishnadas and colleagues set up experiments in Western Ghats Biodiversity Hotspot, Karnataka, south India where 60% of the 3,600 ha landscape is fragmented due to tea plantations, roads and grassland. The researchers selected 15 locations and applied either fungicide, insecticide, water or no treatment at different distances from the forest edge (0–5 m, 20–30 m, 50–60 m, and 90–100 m) and set up 292 seed traps and monitored 730 seedling plots. 

They measured six functional traits (e.g. specific leaf area, leaf C:N ratio, wood density, seed size) of 72 tree species. They calculated the functional diversity along the edge-to-interior gradient and investigated the distance to the edge and trait–recruitment relationships.

Map showing the different locations, forest edges and experimental treatments used by Krishnadas and colleagues (2020).

The scientists found that seedling recruitment was mostly determined by two traits, specific leaf area (SLA) and seed size. Along the edge-to-interior gradient, the functional diversity of plants was higher at 90-100 m compared to 0-5 m from the edge. The insecticide treatment led to more small seeded  – probably acquisitive – species whilst fungicide application led to more, lower SLA – probably conservative – species at 90-100 m from edges. There were more species with smaller seeds close to the edges than larger seeded species.

This study found that there is an “edge effect” on plant traits around fragmented forests. The canopy cover did not appear to have an effect on functional diversity so it is likely that biotic (plant-enemies and plant-plant) interactions are the mediators of different traits (SLA and seed size) during seedling establishment.

“Increased recruitment of smaller seeded species near forest edges could persist if edge effects weaken post-dispersal biotic interactions that regulate seedling recruitment – a factor that may contribute to the ‘arrested succession’ of fragmented forests”, Krishnadas and colleagues wrote. 

This large-scale experiment in a biodiversity hotspot highlights that complex interactions between plants and their natural enemies need to be considered when trying to manage and restore these fragmented ecosystems. 

“Maintaining ecosystem health in human-modified forests might require that restoration efforts include the recovery of key ecological interactions that regulate community structure.”

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