Israel, in the mid-1900s, planted millions of pine trees, and ecologists later found that the Mediterranean shrubs that grew back underneath decided which birds would move in

Walk into two pine forests that look nearly the same from the outside, and you might hear totally different things. One is alive with chirps and trills and rustling wings. The other is strangely empty, like wind through bare trunks. According to a study titled ‘Understory vegetation in planted pine forests governs bird community composition and diversity in the eastern Mediterranean region,’ published in Forest Ecosystems by researchers Uzi Dagan and Ido Izhaki, the difference almost always comes down to one thing: what's growing under the trees.

The two-year study tracked birds across 20 plots of pine forest in northern Israel, all of which were more than 50 years old and grew in similar climates. The only major difference was the understory, the layer of shrubs, small trees, and climbing plants that fills the space between the forest floor and the pine canopy. In some plots, the undergrowth was thick and tangled. Others had little but dirt and a few blades of grass. The bird numbers between the two were not even close.

The forest floor makes or breaks the neighborhood
In two years, researchers counted 1,476 birds in 33 species. Forests with a well-developed shrub layer had an average of about two to three times as many birds per visit, in addition to significantly greater species richness and diversity. Where shrubs were allowed to grow in, the complexity of how different bird species interacted with each other was also much greater.

Some species were pickier than others. The European robin was not found in the bare, shrub-free forest plots but was common in the two forest types with a rich understory. Blackbirds and great tits were also almost absent in shrubless plots. The most recorded bird in the study was the Sardinian warbler, a species generally known for avoiding pine plantations, appearing in particularly high numbers wherever thorny shrubs and fruit-bearing climbers had become established. The difference in forest types persisted through fall, winter, and spring, but was most pronounced in fall and winter when shelter and fruit are most important to birds.

It is not really about the trees at all
For decades, forest managers have worked under the assumption that if a stand of pines looks healthy, then the forest is healthy, period. This research contests this idea. In a long-term US Forest Service study of a developing pine plantation in the southeastern US, ‘Neotropical Migratory Bird Communities in a Developing Pine Plantation,’ bird abundance and diversity in a young pine stand steadily increased as shrub and hardwood vegetation developed below the canopy, peaked when the shrub layer matured, and then declined again as the pines grew tall enough to shade it out.

In the study, birds were censused annually from four 250-by-80-inch transects in a young pine plantation from age 2 to 17, allowing the authors to track a full succession sequence. Bird abundance was lowest and diversity poorest at age 2, rose as shrubs developed, peaked around ages 10–11, then fell again as canopy closure shaded out the lower deciduous layer.

That finding is important in the United States, where pine plantations cover tens of millions of acres in the South alone. A study in Forest Ecology and Management of southeastern US pine plantations showed that stands that are kept open through thinning and prescribed fire, rather than left to grow into dense, closed-canopy plantations, support species like the prairie warbler and indigo bunting that depend on structurally varied, sunlit habitat. The specifics are different from the Israeli study, since American open-pine systems are generally biased towards grassy or thinned understories rather than shrubby ones, but the main lesson is the same: a forest that appears uniform and neat from a distance can be a much quieter place underneath than a messier one.

A systematic review of the literature was performed, and structural data from 20 loblolly pine studies in seven southeastern states were synthesized, with emphasis on stands during the first five years after planting and at mid-rotation thinning stages. They ran a 60-year simulation on a more than 22,000-ha sawtimber landscape and found that suitable habitat for the four focal species can be present but is often highly ephemeral. The review itself was limited by sparse data on variables such as overstory DBH and distance to nearest suitable patch.

Why this matters beyond birdwatching
Birds aren't just pleasant background noise. They are often treated by scientists as bioindicators. Their presence or absence tells scientists about the health of an ecosystem. A forest that supports a diversity of birds usually supports insects, small mammals, and healthy soil processes that people rarely think about but quietly benefit from, including natural pest control and seed dispersal.

For the millions of Americans living near managed forests, state parks, or reforested land, this research provides a straightforward way to think about the woods around them. A dense, layered, slightly wild-looking forest floor is not neglected. Often a sign of a forest doing its job. A manicured forest, with bare trunks and clear sight lines, may be quieter than it seems.

The fix is simpler than it sounds
The researchers behind the Israeli study aren’t suggesting ripping out pine plantations or stopping the use of timber. They argue for a more modest approach: allow natural shrub growth to establish itself where it already can, and consider thinning in stands where it has been suppressed. Over time, this alone can transform a monoculture of pine into what ecologists call a semi-natural forest, which still supplies timber and recreational use but is also home to a lot more life.

The next time a walk in the woods seems unusually quiet, it might be worth looking down before looking up. Pines are rarely the whole story. Who shows up to sing is usually determined by what’s growing in the shadows beneath them.