Morphological and Physiological Consequences of a Positive Plant Interaction

It is known that plants can create acceptable environments for other plant species that may be unable to physiologically adjust to harsh physical conditions in isolation. In this study, we explore the morphological and physiological responses of a positive interaction on a common New England salt marsh shrub, Iva frutescens L. In previous experiments, we have shown that Iva benefits from the presence of the black rush, Juncus gerardi Loisel. Juncus removal in the lower marsh caused increases in soil salinity and anoxia, which resulted in lower Iva biomass, growth, and survival. We hypothesize that the positive effect of Juncus on Iva is a consequence of (1) the turf morphology of Juncus shading soil surfaces, thereby minimizing salt accumulation, and (2) the increased soil oxygen content surrounding Juncus caused by radial oxygen loss from its rhizopshere. However, the specific morphological and physiological benefits conferred on Iva are unknown. We conducted experiments in the salt marsh to isolate the effects of Juncus neighbors, waterlogging, and salt water stress on Iva. We found that Iva, when exposed to salty, oxygen—deprived soil conditions created in the absence of Juncus, responded more negatively to salinity than waterlogging, although the interaction between salinity and waterlogging was often significant. Iva without neighbors showed decreases in plant height, leaf density, leaf area, and flower density compared to controls or plants with fresh—water additions. Leaves of Iva without neighbors responded to salt stress and waterlogging by increasing leaf thickness and succulence. The increase in soil salinity of plants without neighbors caused plants to have correspondingly low water potentials, indicating that they were severely water stressed. Physiologically, Iva plants had a low mean photosynthetic rate, transpiration rate, water—use efficiency, and stomatal conductance compared to controls or Iva with fresh—water additions. Low water—use efficiency in Iva without neighbors was correlated with stomatal closure but also with high intercellular CO₂ concentrations. The mechanisms behind this apparent anomaly may include both nonstomatal inhibition of photosynthesis as well as patchy stomatal closure, although it is uncertain which predominates from our data. Our results show that the presence of Juncus, with its superior ability to withstand waterlogging and salt stress, can create a hospitable environment for Iva, extending its distribution to lower intertidal habitats in which it does not have the morphological or physiological plasticity to normally cope.