Use of a process-based model to describe spatial variation in Pinus radiata productivity in New Zealand

The process-based model CenW was used to model the growth of Pinus radiata in New Zealand. The model was parameterised by comparison with data from 101 permanent sample plots, consisting of 1309 individual observations of stands at different ages of up to 32 years. The data covered the range of conditions under which P. radiata is grown in New Zealand. Observations consisted of stand density, and height and/or basal area. Input parameters consisted of site water-holding capacity, soil texture and soil nitrogen concentration, and daily records of minimum and maximum air temperatures, precipitation, solar radiation and atmospheric humidity. Using the same plant parameters for the full data set from all locations, we obtained excellent correspondence between predictions and observed data. Model efficiencies ranged from 0.828 to 0.892 for four growth indices that were either directly observed (height, basal area) or derived from the data (stand volume, mean stem diameter).Within New Zealand, stand productivity was found to be particularly sensitive to mean annual air temperature and mean total annual precipitation, with optima reached from 12-15 °C and 1500-2000 mm, respectively. Productivity exhibited little sensitivity to site fertility as soil nutrition was generally adequate for most sites. Spatial projections showed highest productivity in the moderately wet, warm northern and western regions of the North Island and lowest for cold sites at higher elevation, dry eastern areas of the South Island and the extremely wet sites on the West Coast of the South Island. The model runs provided robust estimates of productivity across the diverse environmental conditions in New Zealand.     

Why does rainfall affect the trend in soil carbon after converting pastures to forests?: A possible explanation based on nitrogen dynamics

When trees are planted onto former pastures, soil carbon stocks typically either remain constant or decrease, with decreases more common in regions with higher rainfall. We conducted a modelling analysis to assess whether those changes in soil carbon, especially the interaction with rainfall, could be understood through consideration of nitrogen balances. The study was based on simulations with the whole-system ecophysiological model CenW which allowed explicit modelling of both carbon and nitrogen pools and their fluxes through plants and soil organic matter.