Future Tree Prize winner Anneleen Geussens is researching a widely used forest management strategy for her master’s thesis: thinning.
‘European beeches are most comfortable with minimal management. Pedunculate oaks can tolerate more intensive management’, is how Anneleen Geussens summarises the conclusion of her thesis. The former master’s student in land management at KU Leuven won the 2024 Future Tree Prize for best student thesis in the field of forest ecology and forest management, with her research into the influence of forest management on drought stress. ‘[Geussens] keeps all forest managers on their toes by scrutinising their most powerful management tool – thinning’, the jury report states.
Is thinning – selectively felling trees to give others more space – beneficial or detrimental to the drought stress experienced by pedunculate oaks (Quercus robur) and European beeches (Fagus sylvatica)? To investigate this, Geussens studied trees in the Sonian Forest and Meerdaal Forest. ‘These are two of the largest forests in Flanders, with large numbers of oaks and beeches. I examined a total of 35 oaks and 37 beeches there, growing in locations with varying degrees of management intensity’, says Geussens. She determines drought stress using so-called dendrometers. ‘That is a device you drill into the tree. It measures fluctuations in stem diameter. The stem naturally grows thicker as the tree grows, but there are also daily fluctuations. These arise because water evaporates through the leaves during the day, causing the stem to shrink. At night, the roots take up water again, so the stem expands. During drought, more water evaporates than the tree can absorb overnight, and the tree experiences drought stress.’
Some of the dendrometers are already in place, but Geussens also gets to go out into the field herself. ‘That was a welcome break from studying at a desk. Nice to be outside, and you also get to see for yourself what the data collection looks like.’ Back at the desk, Geussens reads out the data and combines it with various weather factors in a statistical model.
The outcome is that European beeches cope best with drought in a dense forest, whilst oaks thrive in forest sections with a somewhat more open character. ‘Oaks and beeches therefore respond in very different ways, which was also broadly in line with our expectations beforehand’, says Geussens. ‘Beeches are more sensitive to changes in air humidity. The lower it is, the more the beech suffers. In a dense forest, a microclimate develops in which humidity and temperature remain relatively stable. The beech benefits from that. When heavy thinning has taken place, the microclimate weakens and external factors have greater influence. Oaks, by contrast, are most comfortable with moderate to intensive management’, Geussens explains.
Stomata
This comes down to the difference in water strategy between the two tree species, Geussens explains. ‘In drought conditions, oaks are more inclined to close their stomata. They then lose no water. The drawback, however, is that they fix less CO2 and effectively begin to starve over time. Beeches employ the opposite strategy: they keep their stomata open so they can continue to absorb CO2, but lose a great deal of water in the process.’ Because of this strategy, beeches benefit particularly from a stable microclimate; without one, they quickly feel the effects of drought. For oaks, more intensive thinning is most effective: fewer surrounding trees means less competition for water.
Title: The effects of forest management intensity on drought stress in pedunculate oak and European beech
Author: Anneleen Geussens
Category: master’s thesis
Programme: land management at KU Leuven
Supervisor: Bart Muys
Grade: 18/20
Special mention: winner of the Future Tree Prize
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