Facilitation within plant species: the generality, mechanisms and effects and potential effects of intraspecific facilitation

Competition has long been thought to be dominant in ecology since Darwin and is widely studied. However, there is more and more evidence for positive interactions in the past two decades, plants can also help each other through many mechanisms like shading, hydraulic lift, fertility islands, mycorrhizal fungal networks, protection from herbivore and decreasing the performance of a third species. The relationship between competition and facilitation is summarized by stress gradient hypothesis (SGH) to alternate from competition to facilitation across a benign to harsh environmental gradient. The prominent difference between competition and the increasing body of facilitation studies is that facilitation has been mostly observed or studied in an interspecific context, but studies from an intraspecific perspective are relatively quite rare.

To study the intraspecific facilitation, literature review, green house experiments and models will be combined. We have:

• reviewed the literature to find out evidence for intraspecific facilitation and compiled all the mechanisms of facilitation in various stressful environments, e.g., radiation, salt, nutrient, grazing, water, wind, cold, thermal, drought, precipitation, acidity and etc.
• conducted greenhouse experiments which aim to test whether facilitation process can occur under strong niche overlap and find out how will intraspecific facilitation differ for individuals with different stress tolerance.

We predicted that:

• 1) positive interactions can overwhelm competition for conspecifics in the stressful environments.
• 2) Stress-Gradient- Hypothesis, which predicts relative magnitude of facilitation and competition will vary inversely across gradients of stress, may still hold for intraspecific facilitation.
• 3) Salt-sensitive mutants should demonstrate stronger facilitative response to conspecific neighbors.

We will also build IBM (Individual Based Model) and use the model and experiments to study the potential effects of facilitation on the plant allometry, population size hierarchies and population dynamics.

Main Investigators: Ruichang Zhang & Katja Tielbörger