Nitrogen Productivity or Photosynthesis Minus Respiration to Calculate Plant Growth?

One approach to calculate plant growth rate is from models of photosynthesis, respiration and allocation. This requires that processes with characteristic time constants of seconds to minutes be scaled to hours or days. Another approach is to use aggregate models defined at the time scale of growth, hours and days. I use such an aggregate model, the nutrient productivity, to compare the performance of the two approaches on growth experiments with small, nitrogen-limited birch plants. The problems of error aggregation when using the large number of parameters required to scale from the detailed level of photosynthesis and respiration to the aggregate level of growth are in this case such that whole plant growth rate is more accurately predicted with the nutrient productivity model.