Trees beside vines: who has access to soil water?

Trees in vineyards promise several benefits at once: shade, greater biodiversity and potentially a more balanced water regime. Yet beneath the surface, trees and vines do not stand independently of one another. Both explore the soil and take up water and nutrients – and can get in each other’s way. A study from Geisenheim published in 2026 makes this competition measurable, at least for three specific vineyards and a single year of investigation.

The study by Michael Bruch, Lukas Fichtl and Matthias Friedel asks what happens in vines when a row of trees stands nearby. Its findings are not a general verdict on agroforestry in viticulture, but a close examination of specific distances, sites and measurement times.

Three sites, three distances

The researchers studied three vineyards in Geisenheim in 2025. Field maples, birches or plane trees stood beside the vine rows. The vines compared grew four, six or 18 metres from the tree row. Measurements included the vines’ water status, the opening behaviour of their stomata and berry characteristics.

Stomata are small pores on the leaf surface. Through them, the plant takes in carbon dioxide and releases water vapour. When water is scarce, they can close more tightly; this limits water loss but also restricts gas exchange. So-called pre-noon water potential, a measure of the leaf’s water status, was also more negative on average near the trees in the study: more so at a distance of four metres than at six and 18 metres.

This pattern suggests that, under the conditions studied, vines in the immediate vicinity of the tree rows faced greater water scarcity. It does not, however, prove that every tree deprives every vine of water. The measurements reflect a particular period and specific sites; differences between the vineyards influenced several results.

Roots grow away

The view below ground is particularly revealing. At one site, the team excavated six Riesling vines and mapped their structural roots in three dimensions. The roots tended to grow away from the tree row; fewer substantial root axes were found growing towards the trees. This is consistent with an interpretation of competition for water. However, mapping six vines at just one site cannot show how roots develop in every vineyard or under different soil conditions.

“Who has access to soil water?” is therefore not a question with a simple answer. Water is not divided between plants as if from a fixed store. Rainfall, soil depth and structure, root distribution, tree species and weather patterns determine what is accessible in each case. The vine itself also adapts its growth. In the trial, the tree rows stood four metres from the first vine row; the findings cannot readily be applied to other layouts.

Benefits and competition at the same time

Agroforestry – combining trees and shrubs with agricultural crops – can alter the microclimate. Tree canopies can provide shade and shelter from wind; under certain conditions, deep-reaching roots could access water from deeper soil layers. Hydraulic lift is the term for the potential movement of deeper water into drier upper soil layers. Whether vines benefit from this, or whether the trees mainly compete with them, depends on the specific conditions.

Another study, from Bordeaux and also published in 2026, offers a different finding that is not directly comparable: during an unusually wet growing season in 2024, it found no evidence of increased competition for water between trees and vines. It examined a different system and different questions. Taken together, the studies above all highlight how important weather and experimental conditions are when assessing the findings.

For growers, this does not amount to a blanket recommendation either to plant trees or to avoid them. The Geisenheim study provides evidence of possible competition for water and root space at close distances. Whether this brings an advantage or a disadvantage for a particular vineyard can only be assessed by considering the site, tree species, spacing and soil over several years. Precisely this long-term perspective remains an important next task for research.