This year, many of the forests in my main study site (the Tallgrass Prairie Preserve in Osage County, Oklahoma) are demonstrating something I have not witnessed in my 15-year familiarity with the area. The forest understory is yellowing. Some particular features:
=20 1. In some locations, the entire understory seems to be affected - consisting of dozens of plant species. 2. It is most pronounced in mesic forests, including those on relatively steep slopes. 3. It is practically absent in the most upland forests 4. It is not particularly pronounced in floodplain forests, but is still evident. 5. Grasslands and wetlands are completely unaffected. 6. Different plant species are affected differently in severity and symptoms. =20 7. Some species have strong mosaic symptoms, some are yellowing at the tip, some are turning uniformly yellow, and some are wilting.=20 8. Woody plants (including tree seedlings) and herbs are affected. Grasses do not seem to be affected as severely. 9. Only a few overstory trees displayed signs. 10. Herbs (of the same species as those in the closed forest) in treefall gaps did not exhibit the symptoms strongly. 11. So far, there is no clear mass mortality of the understory - but there is obvious declining vigor. =20 My hypothesis is: 1. the root zone in the forest understory is experiencing marked hypoxia due to phenomenal spring rainfall. =20 2. The treefall gaps, grasslands, and canopy trees have enough transpiration to prevent this. 3. The wetland and floodplain plants have the aerenchyma to handle the conditions. 4. The symptoms may be caused by plant pathogens responding to a weakened plant, causing differences in symptoms =20 Does this hypothesis seem reasonable? Has anyone noticed such an occurrence elsewhere? The main concern I have for this explanation is that forests on slopes (where one might expect good soil drainage) seem to be the most strongly affected. But the sogginess of the spring has been remarkable. =20 =20 =20 Michael W. Palmer, Botany Department, Oklahoma State University 104 LSE Stillwater OK 74078 USA 405-744-7717 fax:405-744-7074 LABORATORY FOR INNOVATIVE BIODIVERSITY RESEARCH AND ANALYSIS: http://ecology.okstate.edu/Libra/ <http://ecology.okstate.edu/Libra/> =20 OSU Botany: http://botany.okstate.edu/ <http://botany.okstate.edu/> OSU Ecology: http://ecology.okstate.edu <http://ecology.okstate.edu/> =20 =20
