Title: Physiological Response of Red Mangrove Seedlings Under Differing Light Regimes The Mangrove Blues

We studied the physiological differences between sun and shade seedlings of Rhizhophora mangle, the red mangrove. Using the seedlings and their leaves, five experiments were made to determine whether true adaptations were available to shade seedlings to cope with higher stress. The reason why the shade seedlings were in higher stress is because they have to deal with the salt and intense competition for sunlight to photosynthesize. We measured seedling density, stomata density, transpiration rates, succulence, and chlorophyll content. A seedling density count was conducted to assess the amount of seedlings per square meter of the sun and shade areas. The count showed there was a significant difference in which there where more seedlings per meter square of sun area then shade area. Stomata density was done to determine how much stomata was on a square cm on the leaf back side. A higher stomata density rate may indicate that there is more gas exchange occurring which in turn would bring in more carbon dioxide to be used in photosynthetic reaction. There was no significant difference in the stomata density of shade versus sun leaves. A potometer was used to measure transpiration rates of sun and shade leaves. A higher level of transpiration would point to more water uptake by the leaf which may indicate a more efficient xylem system. There was no significant difference in the transpiration rates of shade versus sun leaves. Leaf succulence was tested to find out the amount of water per cm square of each leaf. There was significantly higher succulence is shade as compared to sun leaves which may help prevent dehydration in saline water. Our chlorophyll experiment was done to find out the total chlorophyll there was per cm square of leaf. Having more chlorophyll aids in capturing more light for photosynthesis. The chlorophyll in the sun and shade leaves where significantly different; there was more chlorophyll in the shade leaves than in the sun leaves. In conclusion the five experiments pointed to shade seedling adaptations to cope with the extra stress. Out of the five experiments, only one was genetically affected which is the stomata density; the environment affected the other four. Introduction General background on mangroves and their significance Mangroves are one of the most important species of trees in south Florida. Mangroves have a large affect on the fishing economy and also help prevent erosion (Heald & Odum 1969). Mangroves are also an important species of trees is because it is an important source of firewood and timber. Mangroves also provide habitat for many of South Florida’s animals. There are only three true mangroves in South Florida they are the red mangrove (Rhizophora mangle), white mangrove (Languncularia racemosa), and also the black mangrove (Avicennia germians). There is also an associate mangrove called the buttonwood (Canocarpus erectus). Mangroves are found through out the tropics and usually live near the water and coastal areas. Every mangrove species in south Florida has its own unique characteristics. For example the red mangroves is the closest to the shoreline. Then the black mangrove is found finally, the white mangrove is usually further into the shoreline, but they are not in perfect lines and are scattered around the area. The mangroves are one of the few trees that can survive under salt water. After the seeds of the mangrove are germinated when they fall of the tree and are known as viviparous propagules. These viviparous propagules are “born live” in that they are germinated seeds. Propagules can survive on water approximately 6 to 8 months. When the propagule lands on reachable soil it will begin to grow into an adult and grow seedlings, which can form another mangrove forest. Lack of mangrove understory; the prevailing theories Mangroves have no understory (Janzen 1985). Janzen (1985) hypothesized that other plants could not handle the salt stress and low light stress at the same time as the mangroves could. He also said that mangroves were evergreens and that is why you don’t find them farther north. Lugo (1986) also said there was no understory because of other stress factors like nutrients, oxygen, and freshwater. Further, Snedaker and Lahmann (1988) believed that there was no understory because these other plants could not evolve in this habitat. Since mangroves were the first tropical plants to evolve saltwater they grew to out compete other species for sunlight. So when other plant tried to evolve there was too much stress and lack of sunlight that they died off. Considering this lack of understory, we are interested in how mangrove seedlings survive in this environment and what physiological adaptations they may develop to handle the stress. We believe that shade seedlings will be more effective in terms of photosynthesis and water use to compensate for physical stress than the sun seedlings. In our study we will test the following hypotheses: H1) Seedling density will be more abundant in sunny areas than inb shady areas because there is more sunlight. H2) Sun leaves will have a higher stomatal density than the shade leaves because there is a higher rate of photosynthesis. H3) Shade leaves will have greater succulence than sun leaves because they are under more stress. H4) Sun leaves will have a higher rate of transpiration because there is a higher rate of photosynthesis. H5) There will be a higher amount of chlorophyll in shade leaves then in sun leaves because they are under more stress. Methods Site description of Matheson Hammock We chose Matheson Hammock Park and marina as our studies site because it was a park with a lot of mangroves in different conditions. We sampled mangroves on 7/10 & 7/21. The temperature was really hot and humid. The forest floor was generally sunlight because not much complete canopy after Hurricane Andrew. We chose two study areas sun/shade. We accessed the sun site by a pavement road that we walked down was dominated by red mangroves. The sun field site was very dense and hard to walk through. Nearby our gathering area there was a creek about 20 ft away. In the field site were a lot of crabs moving around. It was high tide during sampling and our study site was partially flooded. The shade study site did not have a fully closed canopy. The site was quite dense with adult trees and prop roots and we had a hard time getting around. There was a creek located about 15 ft away from our study site. Also the ground was much higher and dryer then the sunny site. Another interesting thing is that there were not as many crabs in the shady site than in the sunny site. Field and lab protocols Abundance assessment and leaf harvesting On July 10, 2000 we assessed mangrove seedling abundance in sun and shade plots at Matheson Hammock. A 1m2 quadrat was haphazardly tossed to calculate seedling density in each plot. We counted mangrove seedlings inside each square quadrat. We first went to a sunny area where we estimated about 80-85% of full sunlight reaching the seedlings. We observed and studied two adjacent patches with most red mangroves, with Bostrychia sp. (red alga) on their prop roots. We then sampled a shaded area where we calculated about 20% of full sunlight reaching seedlings. The soil in this particular area was covered with unknown algae. Results were analyzed to determine if there was a difference between the number of red mangrove seedlings growing in the shaded area than the sunny area. Red Mangroves leaves were collected on 7/10 and 7/21 from each study area for use in lab experiments. We collected numerous leaves from different seedlings and placed them in ziploc bags. These leaves were collected in both, shade and sun areas and were stored in a lab refrigerator.