The multiple stress gradient hypothesis: expansion of the revised stress gradient hypothesis using a mangrove and salt marsh study system

Plant interactions (e.g., competition, facilitation) are critical drivers in
community development and structure. The Stress Gradient Hypothesis (SGH)
provides a predictive framework for how plant species interactions vary inversely
across an environmental stress gradient, predicting that facilitation is stronger with
increasing levels of stress. The SGH has been supported in numerous ecosystems
and across a variety of stress gradients, but recent research has demonstrated
contradictory results. These discrepancies have led to SGH revisions that expand its
conceptual framework by incorporating additional factors, such as other stressor
types and variations in species life history strategies. In this dissertation, I examine
a further modification of the SGH by proposing and testing a Multiple Stress
Gradient Hypothesis (MSGH) that considers how plant interactions vary along a continuous gradient of two co-occurring stressors using mangrove and salt marsh
communities as a case study. In Chapter 1, I outline the predictive framework of a
MSGH, by creating a series of predictions of species interactions. The components
of the MSGH predict that stressors of similar types (e.g., resource and nonresource)
will have similar effects and be additive. On the other hand, varying
species life history strategies and life stages will lead to extremes of plant
interactions. In Chapter 2, I performed a series of experiments to test the various
components of the MSGH. In Chapter 3, I performed a large-scale observational
study to test whether multiple co-occurring stressors altered the cumulative effects
on plant interactions, and if these stressors should be grouped (e.g., resource and
non-resource, abiotic and biotic, etc.) to enhance predictability. From a series of
studies conducted herein, I concluded that co-occurring stressors are important
factors that control complex species interactions as shown in my MSGH modeling
approach. Further, future theories need to incorporate species-specific and stressor specific
grouping when modeling how species interactions shape communities.