More trees do not always mean more birds, and a Japanese study found grassland species fell by over 70% near shelterbelts, showing that restoring habitat can sometimes reduce biodiversity

Most of us were probably taught that planting trees is good for the planet, good for the climate, good for wildlife, good for all of it. So it might come as a surprise to learn that in some landscapes, planting trees can actually cause bird populations to plummet dramatically. That is the precise argument of a new study, and the implications reach far beyond Japan, where the research was done.

According to a study, ‘Shelterbelts support edge birds but limit grassland and wetland specialists in agricultural landscape,’ published in the Journal of Environmental Management by a multidisciplinary team of researchers from institutions including Hiroshima University, the University of Tokyo, Kyoto University, and the University of British Columbia, rows of trees planted on farmland as windbreaks, known as shelterbelts, significantly reduce the abundance and diversity of open grassland and wetland bird species. The results are a blow to one of the most common assumptions in agricultural conservation, that planting trees on farmland is an easy win for biodiversity.

What are shelterbelts and why you see them everywhere
Shelterbelts are lines of trees or shrubs that are planted along field edges to protect crops from strong winds. They are a familiar sight on farmland across the world, and conservation programs in the US, Europe and Asia have long promoted the planting of woody vegetation, mostly on the assumption that it is good for wildlife.


According to the Journal of Environmental Management study, most of the research supporting the assumption comes from dry croplands and grasslands in Europe and North America. But very little of it examines what happens in wet agricultural landscapes, such as rice paddies, which are common across Asia and provide critical habitat for migratory birds that travel long distances each year.

Where the research was done
The team from Hiroshima University conducted their fieldwork on farmland surrounding Lake Kahokugata on the western coast of central Japan. Large tracts of rice paddies, lotus fields, cultivated cropland, and pastureland punctuate its landscape. It lies on the East Asian-Australasian Flyway, one of the world's major bird migration routes. Nearly 300 bird species have been recorded in the area. Wintering species come in the colder months and breeding species in the summer.

The team conducted bird surveys using a point-count method in February and March 2021 and in June 2023, comparing sites next to shelterbelts with open sites approximately 1 km away.

A dramatic drop and a clear trade-off
The effects were dramatic. According to the Journal of Environmental Management study, abundance of grassland birds was more than 70 percent lower at sites adjacent to shelterbelts compared to open sites about one kilometer away. Bird diversity in the wetlands also declined near the rows of trees.

Masumi Hisano, assistant professor at Hiroshima University's Graduate School of Advanced Science and Engineering and the study's corresponding author, said, “A useful way to think about this is that shelterbelts act like ecological walls.”

The study found benefits of shelterbelts for birds that prefer shrubby edge habitat. But for species that nest and feed in open spaces, such as skylarks and lapwings, the rows of trees actually reduce the usable landscape and increase exposure to predators. It also has strong winter winds and storms. That is why the shelterbelts are so common in that region.

Why Americans should pay attention
If you’ve noticed fewer meadowlarks or sparrows in rural America over the years, this research is relevant context. According to the study ‘Decline of the North American avifauna,’ published in Science, North America has lost a net total of almost three billion birds since 1970, a decrease of about 29% from the 1970 bird population. Some of the hardest hit were grassland birds.

A review published in Agriculture, Ecosystems and Environment by Stanton et al. , found that populations of 57 of 77 farmland-associated bird species, about 74%, declined in North America between 1966 and 2013, with some of the steepest declines among grassland species; the authors identified habitat loss as a major driver of agricultural intensification.

In that context, the Hiroshima findings provide a useful warning: even conservation-minded interventions such as tree planting can quietly worsen conditions for already-struggling open-habitat bird species, if placement isn’t carefully considered.

It's not about planting fewer trees; it's about planting them smarter
The researchers are clear this is not an argument against shelterbelts per se. The real issue is how and where trees are incorporated into agricultural landscapes.

“Biodiversity-friendly farmland management must balance structural complexity with the ecological needs of open-habitat species, especially in landscapes where wetlands have already been heavily modified by humans,” Hisano said.

Birds whose natural wetland areas have been lost or degraded are already finding alternative habitats in agricultural wetlands. To add shelterbelts to those landscapes without regard for the bird communities that already live there risks compounding an already serious problem.

Future work should examine the effects of specific shelterbelt characteristics, such as width, height, spacing, configuration and tree species composition, on bird communities in different regions and seasons, the researchers say. They also want to learn more about how these structures affect predator behavior and habitat connectivity, which can have ripple effects through entire bird populations.

“Rather than promoting a single solution, such as planting more trees everywhere, we aim to support landscape-level planning that combines open habitats and woody features in ways that sustain diverse bird communities,” Hisano said.

That’s a message worth sitting with, for farmers, policymakers and conservationists in the US, where the pressure to bring back bird populations is already high.