Dense afforestation reduces plant-pollinator network diversity and persistence

Tree plantations are considered as a solution to reduce the impacts of climate change and can enhance biodiversity. Consequently, many tree planting schemes around the world have been started to achieve these dual objectives. However, many of these tree plantations are being implemented without proper design or post-plantation management, often overlooking potential long-term effects on biodiversity. Therefore, it is essential to identify which aspects of tree plantations can negatively impact biodiversity. Such knowledge is vital to design new plantations and manage existing ones, such that they do not pose threats or additional costs to the conservation of natural ecosystems. To this end, we conducted an observational study in the Mediterranean heathland habitat of the southwestern Iberian Peninsula. This treeless habitat, locally known as herriza, has been planted with pine trees until the onset of the 21st century. This historical tree plantation presents a unique natural experiment to assess the long-term effect of tree cover, measured as canopy openness, on several community properties of plant, pollinators and their network of interactions. Our results reveal a strong positive relationship between canopy openness and floristic diversity and abundance. This means that, as we increase tree cover, plant diversity is reduced. We found this has consequences for pollinator diversity and plant-pollinator networks, the latter exhibiting declines in stability. Furthermore, we reveal the importance of woody blooming plants in comparison to non-woody ones which, despite their greater importance for pollinators, they are the most impacted. These findings underscore the importance of tree cover for severely affecting multiple properties of plant-pollinator networks at different levels of organization. Overall, this knowledge indicates that high tree cover in plantations conducted 50 years ago is incompatible with maintaining and conserving plant-pollinator networks in natural treeless habitats, at least in the herriza. Actions that want to avoid negative long-term effects of tree plantations on plant-pollinator communities should consider existing biodiversity before planting and refrain from achieving high tree cover values.Read the free Plain Language Summary for this article on the Journal blog.