Dieback and Replacement of Riparian Trees May Impact Stream Ecosystem Functioning

Area of Science:

• Ecology
• Forest Science
• Environmental Science

Background:

• Alders (Alnus spp.) are vital nitrogen-fixing riparian trees enhancing stream decomposition via nutrient-rich litter.
• Phytophthora ×alni infection causes alder dieback, creating opportunities for invasive black locust (Robinia pseudoacacia) establishment.
• Vegetation shifts from healthy alder to infected alder or black locust dominance can disrupt stream ecosystem processes.

Purpose of the Study:

• To investigate how alder dieback and replacement by black locust affect leaf litter decomposition and associated microbial decomposer communities in streams.
• To compare the impacts of healthy alder, P. ×alni-infected alder, and black locust on fungal decomposition dynamics.

Main Methods:

• A six-week microcosm experiment was conducted using three leaf litter mixtures: healthy alder-poplar-ash, infected alder-poplar-ash, and black locust-poplar-ash.
• Leaf litter decomposition rates, fungal biomass, fungal reproduction (sporulation), and fungal assemblage structure were measured bi-weekly.
• Nutrient concentrations of the leaf litter were analyzed to correlate with decomposition rates.

Main Results:

• Leaf litter decomposition was highest in mixtures with infected alder and lowest in those with black locust, linked to litter nutrient content.
• Fungal assemblages and sporulation rates were significantly different between alder-based and black locust-based litter mixtures.
• Changes in decomposition and fungal communities were evident even during the alder infection stage.

Conclusions:

• Alder loss and replacement by black locust significantly alter stream ecosystem processes, including decomposition and microbial community structure.
• The shift impacts fungal decomposer assemblages and their functional roles in nutrient cycling.
• Conservation of healthy riparian alder forests is essential for maintaining stream ecosystem health and function.