Mixing oak and pine trees in Mediterranean forests increases aboveground hydraulic dysfunctions

Area of Science:

• Forest ecology
• Plant physiology
• Climate change adaptation

Background:

• Mediterranean forests face increasing drought stress.
• Tree species diversity is hypothesized to improve resilience.
• The impact of diversity on tree hydraulics across seasons is poorly understood.

Purpose of the Study:

• To investigate how tree species diversity affects seasonal hydraulic traits in Mediterranean forests.
• To compare hydraulic performance in monospecific versus multispecific stands.
• To assess the role of species interactions in drought response.

Main Methods:

• Studied unmanaged monospecific and multispecific stands of four co-existing tree species (Pinus and Quercus) over two years.
• Measured in-situ hydraulic traits including leaf water potential (Ψpd, Ψmd), hydraulic conductivity (KS, KL), and percentage loss of conductivity (PLC).
• Analyzed seasonal dynamics of traits, focusing on differences between stand types and species.

Main Results:

• Multispecific stands generally showed adverse effects: lower predawn and midday leaf water potential (Ψpd, Ψmd), reduced hydraulic conductivity (KS, KL), and higher percentage loss of conductivity (PLC), particularly for pines.
• These negative impacts were present throughout the growing season but intensified during dry summer periods.
• Oaks (Q. faginea) showed rare beneficial responses to diversity only after severe water stress.

Conclusions:

• Mixing oak and pine species in Mediterranean forests can exacerbate hydraulic impairments, especially during dry seasons.
• This suggests potential vulnerability of mixed forests to future climate change and increased drought.
• Strategies for enhancing forest resilience need to consider species-specific drought tolerance and interactions.