Understanding how trees help each other in Mediterranean forests

Study reveals surprising teamwork among trees in Mediterranean forests. Water plays a key role, with mixed-species forests thriving in wet conditions but struggling in drought.

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Outside perspective of experimental site, Macomer, Italy. Photo: T. Chahine

Mediterranean forests, known for their biodiversity and resilience, face significant challenges due to climate change, particularly regarding water availability. A recent study conducted in Sardinia, Italy, by the University of Sassari (UNISS) and the Italian National Research Centre (CNR), provides critical insights into how different tree species interact under varying water conditions, offering important implications for forest management and restoration.

Research overview

In this experiment, researchers studied 12 Mediterranean tree species, arranging them in plots with varying species compositions to observe their growth and interactions. The primary objective was to test the Stress Gradient Hypothesis (SGH) in these forests. The SGH suggests that in high-stress environments (e.g., drought), species are more likely to engage in facilitative interactions (mutual help), whereas in low-stress environments (e.g., sufficient water), competitive interactions predominate.

Key findings

1. Species interactions and water availability

Contrary to the SGH, the study found that trees in well-watered conditions exhibited more positive interactions and superior growth compared to those in drier conditions. Specifically, in high-water conditions, mixed-species plots demonstrated enhanced growth and health, suggesting that water availability amplifies the benefits of species diversity. These findings imply that adequate water supply is crucial for maximizing the positive effects of biodiversity.

The experimental site in Sardinia demonstrates ecological coexistence among 4 distinct species. Photo: T. Chahine

2. Impact of drought

In contrast, under drier conditions, mixed-species plots did not perform as well as expected. The reduced positive interactions under drought stress suggest that increased aridity may undermine the benefits of biodiversity, highlighting a potential vulnerability of Mediterranean forests to climate change.

3. Tree species-specific responses

The study also revealed species-specific responses to water availability and species mixing. Some species, such as Quercus ilex and Pinus halepensis, showed significant benefits from mixed-species interactions in well-watered conditions, while others exhibited less pronounced responses. Understanding these differential responses is essential for developing targeted forest management practices.

Implications for forest management

The findings of this study have several important implications for forest management and restoration strategies:

1. Promoting biodiversity

Encouraging species diversity can enhance forest resilience and productivity, particularly in environments with adequate water supply. This underscores the importance of maintaining and promoting biodiversity within forest ecosystems.

2. Water management

Effective water management is vital for optimizing the benefits of biodiversity. Ensuring that forests have sufficient water resources can help sustain healthy and diverse tree communities, which in turn support overall ecosystem stability and function.

3. Adaptation to climate change

As climate change progresses, Mediterranean forests are likely to experience increased water stress. Understanding how tree species interact under different water conditions can inform adaptive management practices, making these forests more resilient to climatic shifts.

4. Species-specific strategies

Given the variable responses among tree species, management practices should be tailored to the specific needs and interactions of these species. For instance, incorporating drought-tolerant species or those that benefit significantly from mixed-species interactions in well-watered conditions could optimize forest health and productivity.

Inside view of the experimental plot showcasing the density of the diverse plant species. Photo: T. Chahine

Conclusion

This study provides valuable insights into the complex interactions between tree species in Mediterranean forests and the critical role of water availability. By interpreting how trees help each other under varying environmental stresses, the research offers essential guidance for forest management and restoration. As we continue to address the challenges posed by climate change and biodiversity loss, such studies are vital for developing effective, science-based strategies to protect and sustain our forest ecosystems.

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Tony Chahine
With a PhD in Forest Engineering, my background includes managing forest ecosystem restoration projects in Lebanon for an NGO. Researching biodiversity's role in Mediterranean ecosystem adaptation was my focus during my Ph.D. in Italy. Currently, as a climate advisor at Euro-Mediterranean Center on Climate Change (CMCC) in Italy, my work centers on assessing climate change impacts on soil health and biodiversity.