Microscale Plant Diversity in Traditional Wood-Pastures

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Prepared by Thuy Hang Le, Stefan Zerbe, Adrian Indreica, Alessandro Bricca, Gabriel Tofan, Johann Nikolaus Janssen, Martin Sauerwein & Gianmaria Bonari

In October 2022, we made our first visit to the traditional wood-pastures of Transylvania, Romania, to conduct a preliminary survey of the study sites. This drone image shows the site in Mercheașa. Photo credits: A. Indreica, October 14th, 2022

Traditional wood-pastures are landscapes where large, scattered trees grow in open, grazed grasslands. These systems are biodiversity hotspots in many European regions, including Southern Transylvania (Romania). In 2023, we investigated how plant communities vary at a very fine spatial scale around solitary trees by surveying 90 vegetation plots arranged along a short gradient extending from directly beneath the canopy of 30 individual oak trees to the surrounding open grassland.

Along such a microenvironmental gradient, plant communities differed clearly. Plant communities beneath the canopy had lower species richness and diversity compared to fully open grassland. These differences were associated with the filtering effects of the specific microenvironmental conditions under the trees, which select for shade-tolerant species. By contrast, in open grassland, the lack of filtering by the solitary tree canopy triggers a competitive exclusion mechanism, leading to richer, more diverse plant communities dominated by light-demanding species. These patterns demonstrated how small changes in microenvironmental conditions can influence community assembly over even a short distance.

Example of sampling plots established during our fieldwork conducted from May to July 2023. For each selected tree, we established three 1 m × 1 m plots: the first adjacent to the trunk (“under”), the second at the canopy edge (“edge”), and the third outside the canopy (“outside”), placed at an equal distance from the previous two. Photo credits: T. H. Le, June 18th, 2023

A notable finding was the higher occurrence of woody seedlings beneath the canopy, due to specific microenvironmental conditions and reduced competition with grass species. This suggests that solitary trees act as “nurse plants”, facilitating regeneration of woody species in grazed landscapes. While this process contributes to natural dynamics, it may also promote shrub and tree encroachment when grazing intensity declines.

Our study highlights the important ecological roles of solitary trees in wood-pastures. They create microhabitats for different plant communities and facilitate the establishment of woody plants. These patterns underscore the importance of scattered trees in maintaining both gamma diversity and the natural dynamics of these landscapes. For conservation, it is essential to recognise the ecological importance of these trees while also address the potential for shrub and tree encroachment when grazing declines. A patchy and adaptive management approach, which preserves scattered trees while controlling woody growth where necessary, can help maintain the ecological and cultural integrity of traditional wood-pastures. 

Plant community composition in traditional wood-pastures varies along a microgradient: shaded, nutrient-rich zones beneath trees promote shade-tolerant, and woody species, whereas open grasslands host diverse, light-demanding pasture species

This is a plain language summary of the paper by Thuy Hang Le and colleagues, published in Applied Vegetation Science (https://doi.org/10.1111/avsc.70043)