More Trees Don’t Mean More Birds: New Research Challenges Assumptions

Planting trees along farmland is widely promoted as a straightforward way to boost biodiversity. But a new study from Hiroshima University shows the reality is far more complicated, especially in agricultural wetlands where many bird species depend on open, unobstructed landscapes.

Researchers studying farmland around Lake Kahokugata on Japan's central western coast found that shelterbelts do not benefit all birds equally. While they create suitable habitat for some species, they can simultaneously reduce the abundance and diversity of birds that rely on open grassland and wetland environments.

Shelterbelts as Ecological Walls

Many agricultural conservation programs encourage farmers to plant trees and hedgerows to support biodiversity. However, most research backing their value has been conducted in arable and grassland systems across Europe and North America. Far less is known about their effects in wet agricultural landscapes such as rice paddies, which are widespread across Asia.

"The central question of our study was whether shelterbelts provide equal benefits to all farmland birds in wetland landscapes, or whether they create trade-offs that disadvantage open-habitat species," said lead author Masumi Hisano, associate professor at Hiroshima University's Graduate School of Advanced Science and Engineering.

The study area around Lake Kahokugata includes extensive rice fields, lotus fields, arable land, and pastures. The region regularly experiences strong winter winds, making shelterbelts a common landscape feature. The lake itself sits along the East Asian–Australasian Flyway and nearly 300 bird species have been recorded in the area.

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Winners and Losers

Field surveys were conducted in February–March 2021 and June 2023 using point-count methods. The results revealed a clear ecological trade-off.

Shelterbelts supported birds associated with shrubby edges and woodland margins. At the same time, they reduced grassland bird abundance by more than 70% near shelterbelts compared to open sites roughly one kilometer away.

Rethinking Tree-Planting Conservation

The study argues for smarter placement. Researchers call for landscape planning that deliberately balances open habitats with wooded elements, rather than promoting blanket tree-planting as a universal conservation tool.

"Biodiversity-friendly farmland management must balance structural complexity with the ecological needs of open-habitat species," Hisano said.

This is particularly relevant for wetland agricultural systems in Asia, where conservation policy has often borrowed frameworks developed for very different European or North American landscapes. Rice-field ecosystems support a distinct community of birds.

Implications for Policy

The findings challenge a common assumption embedded in agri-environment schemes: that more vegetation always means more biodiversity. In open wetland systems, the opposite can be true for a significant portion of the bird community.

Future conservation programs, the researchers suggest, should assess the existing landscape structure before recommending tree planting, and prioritize maintaining large blocks of open habitat where wetland and grassland specialists can persist.