Task 2 - Plant Shading Stress

Effects of prolonged shading on Spartina and Juncus

In the fall 2007 through spring 2008 we tested the plant responses of Spartina and Juncus to shade levels typically caused by piers, as measured under both the 30 actual piers and the 36 experimental pier simulators. Plants were received from a nursery and housed at the Coastal Ecosystems Group (CEG) experimental facilities and greenhouse until the beginning of the shading experiment (Fig 2). Productivity of Spartina and Juncus were evaluated in the shading experiment using a multi-parameter plant health assessment protocol. Measurements of of plant size, shoot counts and leaf counts, as well as PAM fluorescence were done daily for week 1 and then weekly for the 1st month to measure stress response levels in these 2 species as a result of the shading effect. The 8 treatments were: control (no shading), 30%, 55%, 80%, 90%, 95%, 99% and 100% shading. PAR data was collected in each treatment by HOBO microstation loggers at 15 minute intervals.

Responses to shading were similar in both Spartina alterniflora and Juncus roemerianus, with tolerance up to 80% shading compared to controls (no shading) not appearing substantially different. Shading levels greater than 80% caused declines in survival and health of both species (Fig 7). 90% shading caused some mortality and plants looked visually stressed. Shade levels of 95, 99, and 100% caused mortality in most or all plants of both species by 4-6 weeks. No flowers were produced by Juncus plants that had experienced 80% or greater shading, suggesting that most piers will stop this plant’s reproduction in the affect area (Fig 8). Photos of plant responses to 8 shading levels are available on the internet: http://ocean.st.usm.edu /~w546990/PierShading/index_Shading.htm. Results from this approach indicate that above-ground production declined with increasing shading levels, based on fewer shoots and leaves per pot. Below-ground biomass also declined with increasing shading, most root production occurred in the control plants. This is cause for concern as shading levels measured under existing piers in coastal MS ranged from 80-100%, with most structures causing greater than 90% shading. This level of shading was demonstrated to kill plants in this experiment.

Plant shade stress responses were measured for the following metrics:

1. 1. Survival - Shading great than 80% caused substantial mortality

   2. Morphology - Shading greater than 50% caused stunted growth (lower stem height, fewer leaves per shoot)

   3. Chl fluorescence - F'/Fm' and Fv/Fm were both reduced in the more shaded treatments

   4. Chl content - Chl a increased up to about 70-80% shading and then declined again in a hyperbolic response.

   5. CNP contents - Nutrient stress was not evident during the shading stress duration.

Recovery of plants that survived the 3 month shading stress was monitored for an additional 6 months and all metrics recovered to control levels, suggesting that removal of piers (e.g. after a storm) allows marsh plant recovery to proceed relatively rapidly.