Eugénie Mas, from the Smithsonian Tropical Research Institute (@MasPlantphys) discusses her article: Drought effects in Mediterranean forests are not alleviated by diversity-driven water source partitioning
Forest are unique ecosystems where each species of animal, plant, and microorganism interact together to provide many services, called ecosystem services. These services can benefit humans directly through wood production, but also indirectly because plants store nearly one third of the world’s anthropogenic carbon emissions each year. However, the worsening of drought events, especially in summer, puts this fragile ecosystem under threat, causing increased worldwide forest dieback. Therefore, it is essential to prepare forests for the future climatic conditions.
Picture of the Alto Tajo Natural Park in Spain. Photo by Alex Tunas.
Having the right neighbor
Favoring mixed forests over single-species forest is often advocated to reduce forest’s drought vulnerability. Different tree species could have positive interactions by sharing the resources aboveground or belowground. For instance, different rooting depths reduce the competition for water when the soil moisture decreases (e.g., water source partitioning). In contrast, trees with similar characteristics will compete for these resources. Therefore, tree-tree interactions can be beneficial for both, only one interacting species, or none of them. Moreover, beneficial interactions can shift from positive to negative when the environmental conditions deteriorate.
Theoretical example of positive (a. Niche partitioning and b. Facilitation) and negative (c. Competition) tree species interaction mechanisms occurring in mixed or monospecific forests.
How to assess species diversity effect in natural forest
Tree interactions are complex and depend on many factors, making it difficult to understand the role of tree species diversity on forest drought vulnerability. Many studies confirmed the power of diversity: the more different the neighbor, the more productive and resilient to droughts the forest is. However, these studies based their conclusions exclusively on aboveground hydraulic traits, such as transpiration or tree growth, ignoring the species interaction happening belowground (e.g., water source partitioning). Therefore, assessing the water uptake depth is important to get the complete picture of the tree interactions in natural forests.
Picture of a monoculture of oaks (left) and a mixture of four species (right). Photos by Alex Tunas.
Controversial results
Using a network of 30 permanent plots in Mediterranean forests with increasing tree species diversity (from monospecific to four-species mixtures), we examined the seasonal patterns of in-situ aboveground carbon and water relations, and belowground water sources for 265 trees from four pine and oak species over two years. We found that increasing species diversity in broadleaf and conifer mixtures induced strong soil water source partitioning between oak and pine species. As conditions became drier during the summer in mixed stands, oak species took up water from deeper soil sources, while pines were systematically limited to shallow ones. Despite significant belowground water source partitioning, stronger drought-induced reductions in aboveground hydraulic traits (e.g., transpiration) were still observed in diverse compared to monospecific plots. Hence, a different neighbor during droughts is not necessarily positive, especially if the tree species involved in the mixture is more drought vulnerable.
Soil aridity, leaf water potential at predawn (Ψpredawn), and stomatal conductance (gs) in function of water source partitioning for the oaks and pines species in monospecific and mixed forests.
Conclusion
Increasing biodiversity in forests could bring a lot of positive ecosystem services by improving resistance to pest outbreaks or increasing wood production. However, under drier soil moisture, the effect of species diversity is more nuanced because of its sensitivity to species identity, their characteristics, and environmental conditions. More research on different forest ecosystems (i.e., temperate, boreal, arid, etc.) and species mixtures are crucial to improve our understanding of the complex relationships between neighbors in natural forests in order to better prepare the forests for the challenges to come.