FOR2060 1nd Edition Lecture 7 Outline of Last Lecture I. Why Are Forests Less Grazed Than Other Ecosystems?II. LitterfallA. Aboveground LitterfallB. Belowground LitterfallIII. Detritivores A. Detritus B. The Detritus Food Web C. Saprotrophic Organisms D. Assimilation Efficiency E. ComminutionIV. What Factors Control the Decomposition of Litter?A. Litter Quality B. TemperatureC. Soil MoistureD. Soil pH Outline of Current Lecture V. Residence TimeVI. Nutrient Cycling A. Geochemical CycleB. Biogeochemical CycleC. Biochemical Cycle D. Essential Nutrient E. Nutrient Movement VII. MycotrophyA. Ectomycorrhizae B. Arbuscular Mycorrhizae VIII.Nutrient Limitations A. Liebig’s Law of the Minimum B. Nitrogen Current LectureV. Residence TimeThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.The residence time is the ratio of the mass of forest floor litter to annual litterfall. For example: Forest Floor LitterAnnual Litter FallA low residence time means that the litter is turning over quickly and a high residence time means that the litter is turning over slowly. In a tropical rainforest there is very little litter on theforest floor; the trees produce leaves but the litter breaks down very quickly. The residence timein a rainforest can be less than a year because a tropical rainforest meets all of the requirementsfor fast litter break down. In a boreal forest the residence time can be up to 300 years because you have short growing seasons with evergreens that have low quality tissues. This combined with the cold temperatures, dry soil, and alkali soil leads to a very slow litter decomposition rate. VI. Nutrient CycleThe flow of energy is intrinsically linked to elemental cycling. The energy enters the system, flows through the system, and is ultimately lost from the ecosystem. It is not reused or cycled once it has been converted to heat. The chemical elements involved in this energy flow are returned into the non-living part of the ecosystem once their association with the energy in an organic molecule is over. Once released nutrients become available for plant uptake. The nutrients can also move to another ecosystem or end up in long-term storage. There are three major types of cycling. Geochemical Cycling is the exchange of chemicals between ecosystems. This occurs on a large spatial scale. An example of this type of cycling is when rivers carry nutrients from forests to oceans. Biogeochemical cycling is the exchange of chemicals within anecosystem. This occurs on a smaller spatial scale. An example is the movement of nutrients or materials down a hill but not out of the ecosystem. Biochemical cycling is the redistribution of chemicals within individual organisms. Time and size scales are much smaller because they occur within individual organisms as part of active metabolic processes. An example would be the internal nutrient cycles in plants and animals. An essential nutrient is an element required by plants for normal growth, development, and completion of its life cycle. These nutrients cannot be substituted for/by other chemical compounds. There are macronutrients, which are required in large quantities and micronutrients, which are only required in trace quantities. (CHOPKNS CaFe ClZn MoB CuMn Mg – way to remember the nutrients; the bolded are macronutrients). Plants allocate a large proportion of their NPP in the growth and maintenance of their roots. One way, probably the most important way, to move nutrients is called mass flow, which is the movement of nutrients dissolved in the soil solution as it moves to the plant root. It occurs when a plant takes up water from the soil and takes up nutrients that are in the water. One wayto move nutrients is diffusion, which is the movement of nutrients in response to a concentration gradient. Another way of nutrient movement is called root interception. As roots proliferate and push through “new” soil, they encounter unexploited nutrients. This is most likely just enhanced diffusion. VII. MycotrophyMycotrophy is defined as nutrient uptake via mycorrhizae, which is a symbiotic association of mycorrhizal fungi and the roots of higher plants. The plants provide the fungi with carbohydrates and the fungi enhance the plant’s access to soil resources. The fungi vastly expand the area available for the absorption of water and mineral. Fungal enzymes also help liberate nutrients bound to organic matter and soil minerals. Ectomycorrhizae form with pines, oaks, and eucalyptus. There are thick hyphal mats that surround the root. The Hyphae penetrate the cortex and form a network called a Hartig net around the cortical cells. This increases the surface area of the roots and maximizes the contact of the cells to maximize nutrient transfer between cells. Arbuscular mycorrhizae form with herbaceous vegetation, some woody species, and tropical trees. They produce characteristic vesicles and arbuscules. They form long roots because the external hyphae can extend great distances from the