Jerusalem, 22 September, 2026 (TPS-IL) — Planting trees to restore degraded land or improve ecosystems may not always increase vegetation, a new Israeli Study has found. Scientists at the Hebrew University of Jerusalem have developed a mathematical model showing when trees can help grasses grow and when shade and competition for resources can instead suppress them.
Ecologists have long observed that trees and shrubs can facilitate the growth of grasses and other plants in dry environments but suppress them under wetter conditions. The New Study provides a quantitative explanation for how that relationship can change, with potential implications for afforestation, dryland restoration, grazing management and the use of land for solar panels.
The study by Oded Hollander, Dr. Yair Mau and Dr. Niv DeMalach, published in the peer-reviewed journal Proceedings of the National Academy of Sciences, brings several competing effects of trees into a single mathematical framework. The model identifies conditions under which the balance can shift from trees benefiting grasses to competing with them.
The researchers separated the effects of trees into two main mechanisms: their canopies and their roots.
Canopies reduce the amount of sunlight reaching plants beneath them, potentially limiting photosynthesis. But shade can also lower temperatures and reduce evapotranspiration, helping retain water in the soil. Tree roots can compete with grasses for water, while under dry conditions they can also redistribute water from deeper soil layers toward shallower ones through a process known as hydraulic redistribution.
When Trees Help
The model showed that these opposing effects can produce different outcomes depending on rainfall and other conditions. Under dry conditions, water-related benefits can outweigh the costs of reduced sunlight and competition for water. As rainfall increases, however, the benefit of conserving or redistributing water becomes less important, while the effects of reduced sunlight and competition can become more significant.
Rainfall alone, however, does not determine whether trees will ultimately help or suppress grasses.
In the model, the transition from facilitation to competition occurs when water is no longer the only factor limiting herb growth at higher rainfall levels, or when woody plant density increases with precipitation. In the latter case, greater tree density produces stronger effects on the vegetation growing beneath the canopy.
The researchers said their framework helps explain why previous field studies have reached different conclusions about the relationship between woody plants and grasses.
“Woody cover is rapidly changing due to mortality, shrub encroachment, and afforestation, reshaping herbaceous communities and ecosystem functioning worldwide,” the researchers wrote.
The model suggests that the ecological effect of adding trees depends on local conditions rather than following a single pattern. Rainfall, woody plant density and the factors limiting plant growth can all influence whether trees facilitate or compete with grasses.
The findings could have practical implications for afforestation, dryland restoration and grazing management. Drylands cover about 40% of the Earth’s land surface, and their herbaceous vegetation provides important forage for livestock and supports biodiversity.
The framework may also apply to situations in which vegetation is shaded by structures other than trees. Because the model examines how reduced sunlight affects plant growth alongside changes in water availability, it could potentially be used to study vegetation beneath shaded hillsides, buildings or solar panels.
That could be relevant to agrivoltaic systems, which combine agriculture or grazing with solar-energy production. The study does not directly test solar panels, however; the connection is an extension of the model’s treatment of shading rather than a finding from a solar-panel experiment.