THE BIOLOGY OF FOREST GROWTH EXPERIMENT - LINKING WATER AND NITROGEN AVAILABILITY TO THE GROWTH OF PINUS-RADIATA

The Biology of Forest Growth (BFG) study in a 10- to 14-year-old Pinus radiata stand comprised detailed investigations of stand growth, water balance, soil and tree N cycling, canopy dynamics and stand growth modelling. The BFG study: (a) developed a method for measuring soil mineral N dynamics in situ; (b) developed the water stress integral, an index of temporally integrated water stress; (c) identified annual average needle litter N concentration as a useful measure of N uptake by and N status of P. radiata stands; (d) demonstrated a marked positive interactive effect of water and N availability on the rate of canopy development, stand foliage carrying capacity, light-use efficiency and above-ground net primary productivity; (e) identified a water stress threshold at a soil water content of 40% of 'plant-available water' below which the level of tree water stress is controlled by soil water content and above which the degree of water stress is determined by N status and soil temperature; (f ) demonstrated that the leaf area index (LAI) of P. radiata stands varies markedly both within and between years, and that variations in LAI can be monitored indirectly using light transmission techniques provided that account is taken of the surface area of other stand components (dead foliage, branches, boles); (g) quantified the water use of irrigated plantations and its dependence on LAI; (h) quantified the effects of water and N availability on N retranslocation in foliage, and demonstrated the importance of retained foliage for the provision of N for new growth; (i) elucidated mechanisms resulting in prolonged (greater than 8 year) growth responses to N fertilisation, including long-term increases in soil N mineralisation rates; (j) constructed and evaluated a biological model (BIOMASS) of forest stand growth. The BIOMASS model estimates total CO2 uptake by a forest stand from intercepted radiation and photosynthetic properties of the foliage. Net Primary Production is obtained by subtracting respiration rates of the component parts of trees. BIOMASS simulated well the observed growth patterns of trees, especially stem and foliage components. Close similarities between the processes involved in transpiration and CO2 Uptake, and good correspondence between calculated and measured soil water balances over a 4 year period, increased confidence in the total CO2 uptake predictions of the model. Research needed to improve tree growth models is briefly discussed. It is argued that long-term multidisciplinary research, such as the BFG study is necessary for advancing understanding of the links between site conditions and forest productivity.