Although definition of NPP is a difference between GPP and plant respiration, it is one of the most difficult quantity to measure, especially in forests and multi-storey vegetation canopies. GPP can not be measured directly and plant respiration is difficult to quantify in presence of soil respiration and its sensitivity to various environmental factors.
Field estimates of NPP are based on different assumptions and sum-total of various biomass gain and loss terms over two successive biomass measurements. Ideally NPP should be measured for both above- and belowground components. However, considering complexities involved in the belowground biomass measurements, it is either considered constant or is estimated as theoretical proportion of aboveground biomass.
Aboveground biomass increments in case of woody vegetation is estimated by measuring diameter at breast height and relating it to biomass through harvest-based allometric regression equations. Ideally these equations have to be calibrated for a site under investigation, but in absence of harvest data, literature-based equations can be used. However, choice of these equations can lead to large errors in the final estimates. The biomass loss terms can be estimated by leaf-fall, branch-fall collection in traps.
In case of herbaceous vegetation, biomass measurements are done simply by destructive sampling, wherein all the above (and possibly belowground) vegetation is cut and dried to estimate NPP over a considered time interval.
Measurements of NPP can further be complicated if other biomass loss terms such as herbivory (grazing, insect attack), root-rotting, fruit and flowering bodies loss and volatile organic compound, exudates loss are considered.
Reference:
Clark, D. A. et al. Measuring net primary production forests: concepts and field methods. Ecol. Appl. 11, 15 (2001).