Eco-physiological analysis of spatial and temporal variations in photosynthetic properties within a Fagus crenata Blume crown on Naeba Mountain [Japan]

Interest in measuring and predicting forest photosynthesis is increasing under the context of climate change. Modeling canopy photosynthesis requires the understanding of spatial and temporal variations of foliage photosynthetic properties. However, due to the complexity of canopy structure, a detail description of canopy physiology requires intensive investigation. Thus, such a canopy complexity has been simplified. For example, "big-leaf models" assume that the canopy structure and physiology behave similar to a single leaf. However, such simplification may lead to large error. Before the simplification, we must understand the extent of the variations and evaluate the sensitivity to canopy photosynthesis. The primary objects of this study are as follows: (1) to understand the spatial and temporal variations in foliage photosynthetic properties, (2) to clarify the factors determining those variations, (3) to analyzo the sensitivity of canopy photosynthesis to the variations and to evaluate eco-physiological implication. The main study site is located at a second-growth beech (Fagus crenata Blume) forest of 70 years old at an altitude of 900m above sea level on the eastern slope of Mt. Neaba. A 20-m X 30-m plot was set up in the forest, and a steel tower (22 m height) was constructed in the center of the plot enabling us to access the entire crown of the selected tree, which has a height of 21.5 m, a diameter of 26.5 cm at breast height and a crown radius of 2.0 m. In order to examine the spatial variation of photosynthetic properties, the crown of the selected tree was separated into several sections by vertical, horizontal directions and crown azimuth. For each section, photosynthetic properties (i.e. maximum carboxylation rate Vcmax, light saturated net photosynthetie rate; Pnmax, leaf nitrogen on an area basis ; Narea, leaf mass per area; LMA) and light environment (relative photosynthetic photon flux density; rPPFD) were measured. To characterize the temporal variations, the diurnal, seasonal and inter-annual changes in photosynthetic properties were measured.