Interaction intensity and importance along two stress gradients: adding shape to the stress-gradient hypothesis

Area of Science:

• Ecology
• Plant-plant interactions
• Community ecology

Background:

• The stress-gradient hypothesis (SGH) posits that positive interactions prevail over negative ones under severe environmental conditions.
• Testing the SGH involves analyzing how pairwise interactions change along environmental severity gradients.
• Recent research suggests the severity-interaction relationship (SIR) might be unimodal, not strictly monotonic.

Purpose of the Study:

• To investigate the shape of the severity-interaction relationship (SIR) between two plant species.
• To determine if the interaction between Azorella selago and Agrostis magellanica shifts from negative to positive with increasing environmental severity.
• To compare the SGH predictions with observed interaction patterns along different severity gradients.

Main Methods:

• Studied the interaction between the cushion plant Azorella selago and the grass Agrostis magellanica on sub-Antarctic Marion Island.
• Compared A. magellanica performance with and without A. selago along altitudinal (4-8 km) and wind exposure (0.4 km) gradients.
• Quantified interaction intensity, absolute intensity, and importance across varying environmental severities.

Main Results:

• Along altitudinal gradients, the relative intensity of the A. selago-A. magellanica interaction increased with altitude, forming a plateau-shaped SIR.
• Agrostis magellanica performance shifted from negative to positive in the presence of Azorella selago under higher environmental severity.
• Wind exposure gradient also showed increased positive interaction intensity with greater severity for most performance measures.

Conclusions:

• The study provides strong evidence for a plateau-shaped SIR, supporting the SGH under specific non-resource severity gradients.
• The findings indicate that the net outcome of plant-plant interactions can switch from negative to positive across both short and long environmental gradients.
• The results highlight the importance of considering the shape of the SIR, particularly unimodal or plateau forms, when testing the stress-gradient hypothesis.