Interactions among plants frequently vary in strength and sign along latitudinal gradients, and this variation is often attributed to environmental differences. Yet in addition to abiotic factors, biotic factors may also vary along these gradients; consumers may be more prevalent at lower latitudes or plants may be adapted to the environmental conditions in which they live. I conducted several studies during my PhD to determine how and why plant-plant interactions vary along the California coast. Contrary to the Atlantic coast, environmental factors seemed to have little predictable effect on species interactions in California marshes, while local adaptation and variation in the abundance of herbivores may better explain interactions between the dominant and subordinate plant species.
Relevant papers: Noto and Shurin 2015 (Local adaptation), Noto and Shurin 2017 (Abiotic factors)