Researchers in Croatia Track How Pruning Changes Olive Growth and Oil Accumulation

A three-year study in a commercial olive grove in Croatia’s Dalmatia region has found that heavily pruned olive trees accumulated oil in their fruit more rapidly than unpruned trees, adding new evidence to the benefits of opening dense canopies.

The research, conducted by a team from the University of Zadar as part of the OLIVE-EduTech project, examined how different pruning intensities affect olive tree growth, flowering, fruit production and oil accumulation.

Šime Marcelić, an agronomist in the university’s Department of Ecology, Agronomy and Aquaculture, presented the findings during a field event at the Lepur family’s olive grove and mill near Benkovac.

See Also: Dalmatian Agronomists Experiment with New Pruning Methods

The event drew students, olive growers and agricultural specialists from Zadar and neighboring Šibenik-Knin counties, as well as visitors from Bosnia and Herzegovina.

The research was conducted in an 18-hectare Croatian olive grove owned by Lecor, a company owned by Branimir Lepur. The property contains about 4,000 organically cultivated trees growing in shallow, rocky red soil at an elevation of about 150 meters.

The grove is divided into two sections, with trees planted 22 and 12 years ago.

“We are right on the boundary between Ravni Kotari and Bukovica,” Lepur said. “Nothing had been planted here before.”

The experiment began three years ago, when researchers carried out substantial reform pruning to reshape selected trees into a polyconic vase, an open-canopy training system widely used in modern olive cultivation.

The trees were divided into groups receiving light, moderate, or heavy pruning, along with an unpruned control group, allowing researchers to monitor their recovery and physiological responses.

“Our observations so far in Lepuri show that olive trees respond differently after pruning in terms of vegetative growth, flowering and oil accumulation,” Marcelić said.

“In pruned trees, we recorded stronger growth of new shoots and more favorable flowering characteristics, including a higher proportion of hermaphroditic, or functionally bisexual, flowers,” he added. “This is an important indicator of fruit-setting potential, although by itself it does not guarantee a higher yield.”

Researchers also examined how canopy density affects tree health. Opening the canopy allows more sunlight and air to circulate through the tree, reducing conditions favorable to the development of peacock spot, a common fungal disease of olive trees.

The project uses near-infrared spectroscopy and optical sensors to monitor fruit development following different pruning treatments. The NIR analyzer allows researchers to repeatedly measure oil content in the fruit throughout the season and track the rate at which oil accumulates.

Marcelić said one of the most important aspects of the research is measuring oil content on a dry-matter basis rather than relying on the percentage of oil in fresh fruit.

As olives ripen, drought or strong bora winds can cause them to lose water. That can make the percentage of oil in fresh fruit appear to rise even when the fruit is not actually accumulating additional oil. Measuring oil relative to dry matter removes the effect of changing water content.

The results have challenged some long-held assumptions among local growers.

“Measurements so far indicate that oil accumulates more rapidly in the fruit of heavily pruned olive trees than in fruit from unpruned and unirrigated trees,” Marcelić said. “In some comparisons, the difference in the change in oil content approaches twice the value.”

However, he emphasized that the research is continuing. “Through systematic measurements across multiple sampling dates and seasons, we want to confirm the actual magnitude of the pruning effect,” he said.

Despite the findings, some Dalmatian olive growers remain reluctant to adopt the polyconic vase system, largely because substantial reform pruning can temporarily reduce production.

“Part of the resistance comes from insufficient knowledge of this training system and how the tree recovers after pruning,” Marcelić said. “Growers are also cautious because of yield: heavy, radical pruning to renew the canopy can reduce production during the first three years.”

“For many, that short-term loss is difficult to accept, particularly when the harvest represents an important source of income,” he added. “Renewal is therefore postponed, even though the tree becomes increasingly difficult to maintain and harvest over time.”

Marcelić said attitudes are gradually changing as growers see the techniques demonstrated in working groves.

The underlying principle is hardly new. Marcelić cited a passage by Vrsalović, written in 1901, describing the olive as a tree that bears fruit on shoots exposed to sunlight rather than on those buried in dense shade.

“The goal of the polyconic, or multi-conical, vase is to bring light to a larger proportion of the fruiting branches, not to strip the tree bare,” Marcelić said. “Leaves that receive sufficient light can photosynthesize efficiently and supply the fruit with the substances needed for growth and oil synthesis.”

He cautioned that pruning practices must be adapted to local conditions to avoid exposing trunks to excessive sunlight and possible sunburn.

The OLIVE-EduTech project also brings together researchers from several disciplines. Agronomists monitor growth, flowering, fruit, and pruning; geographers use spatial technologies to track changes in tree canopies and the grove; chemists analyze fruit and oil; and education specialists help translate the findings into practical guidance for growers.

“The University of Zadar is a public institution whose task is to create and transfer knowledge,” Marcelić said. “For us, that means educating students, but also sharing research results with the people who can apply them in their olive groves.”

“Only when we connect these data can we show growers what a particular intervention means for the tree and how to carry it out properly,” he added.

Marcelić cited the Olive Pruning School, established by former students with the university’s cooperation and support, as an example of that knowledge transfer. Participants learn through practical courses to assess individual trees and understand the reasoning behind each cut.

The aim, he said, is for OLIVE-EduTech to follow a similar path, moving from measurements and scientific findings to workshops and ultimately to better decisions in the grove.

Following the field presentation, participants visited the Lepur estate’s modern olive mill, where a dedicated cooling system is used during processing to help preserve delicate aromas and naturally occurring antioxidants.

Lepur said oils produced at the estate from Dalmatian and selected olive varieties have recorded polyphenol concentrations as high as 1,500 milligrams per kilogram, a result he said was confirmed by an analysis from the Faculty of Chemistry and Technology at the University of Split.

The mill also includes a reception and tasting room used for workshops, visiting groups and local olive growers waiting for their fruit to be processed during the autumn and winter harvest.

The event was the first in a series of public presentations planned under OLIVE-EduTech, which organizers said is intended to turn research findings into practical knowledge that growers can apply in their orchards.

“Through cooperation between olive growers and the University of Zadar, we want to continue raising the bar for quality and striving for the excellence for which oils from Zadar growers are already recognized beyond Croatia,” said Toni Družijanić, president of the Zadar County Olive Growers Association, a co-organizer of the project.