Metabolism generates carbon and nitrogen fluxes in forest ecosystems. Carbon enters vegetation in photosynthesis, thereafter it is utilised in the formation of new cells or for energy needs. Litter formation moves it into the pool of organic compounds in the soil and finally microbes release carbon back into the atmosphere as CO2 (Picture 1). Nitrogen enters vegetation through root uptake as ions. It is converted into amino acids and thereafter utilised for the synthesis of proteins in the functional substances. Nitrogen in proteins in senescent tissues is either returned as amino acids for the synthesis of new functional substances or added into the pool in the soil to be broken down into ammonium ions by microbes. Thus nitrogen is circulating in forest ecosystems. (Picture 2) The nitrogen ion fluxes between a forest ecosystem and its surroundings are small. This fact makes the ammonium ion flux from microbes important for the stand growth.
Picture 1. Carbon compound fluxes in forest ecosystem from atmosphere via leaves and microbes back to atmosphere. Boxes indicate amounts, arrows indicate flow or conversion of carbon compounds and double circles indicate processes.
Picture 2. Circulation of nitrogen compounds in forest ecosystem from available nitrogen in the soil into metabolism of trees and microbes back to available nitrogen. Boxes indicate amounts, arrows indicate flow or conversion of carbon compounds and double circles indicate processes.
Trees, ground vegetation and soil form the forest ecosystem in MicroForest. Trees play a dominant role and they are divided into size classes. In the model trees consist of whorls that are the functional units of a tree. Needles, water transport and nutrient uptake systems make up the whorls. These systems provide water and nitrogen for the needles. There are structural regularities between the needles, their water transport and nitrogen uptake systems.
All carbon and nitrogen fluxes presented in the figures 1 and 2 are treated in MicroForest. Carbon and nitrogen balance equations for each whorl are the core of the model. They include unknown masses of needle and fine root growth. These two unknown masses can be easily solved from the balance equations.
The measuring system SMEAR II in Hyytiälä has been planned and constructed to measure all relevant carbon and nitrogen fluxes in a Scots pine stand. The data measured at SMEAR II is very useful in determination of the parameter values in MicroForest.
We have measured six test stands in Hyytiälä in Finland and five in Estonia. MicroForest is able to predict very well, without estimation, the development of each size classes during 20 – 40 years from the initial state, about 0.5 – 1 m tall seedlings in all of the test stands. The first test with these eleven stands (Hari et al 2008) discovered shortcomings in the ground vegetation sub model. The correction of these shortcomings improved the prediction considerably. This will be reported in the book “Physical and Physiological Forest Ecology” edited by Hari and Kulmala, Springer 2011.
Hari, P., Salkinoja-Salonen, M., Liski, J., Simojoki, A., Kolari, P., Pumpanen, J., Kähkönen, M., Aakala, T., Havimo, M., Kivekäs, R. and Nikinmaa, E. 2008. MicroForest. In: Hari, P. and Kulmala, L.(eds.). Boreal Forest and Climate Change. Advances in Global Change Research 34: 433-478. Springer.