Estimating net primary production of boreal forests in Finland and Sweden from field data and remote sensing

We calculated annual mean stem volume increment (AMSVI) and total litter fall to produce forest net primary production (NPP) maps at 1‐km² and half‐degree resolutions in Finland and Sweden. We used a multi‐scale methodology to link field inventory data reported at plot and forestry district levels through a remotely sensed total plant biomass map derived from 1‐km² AVHRR image. Total litter fall was estimated as function of elevation and latitude. Leaf litter fall, a surrogate for fine root production, was estimated from total litter fall by forest type. The gridded NPP estimates agreed well with previously reported NPP values, based on point measurements. Regional NPP increases from northeast to southwest. It is positively related to annual mean temperature and annual mean total precipitation (strongly correlated with temperature) and is negatively related to elevation at broad scale. Total NPP (TNPP) values for representative cells selected based on three criteria were highly correlated with simulated values from a process‐based model (CEVSA) at 0.5°× 0.5° resolution. At 1‐km² resolution, mean above‐ground NPP in the region was 408 g/m²/yr ranging from 172 to 1091 (standard deviation (SD) = 134). Mean TNPP was 563 (252 to 1426, SD = 176). Ranges and SD were reduced while the mean values of the estimated NPP stayed almost constant as cell size increased from 1‐km² to 0.5°× 0.5°, as expected. Nordic boreal forests seem to have lower productivity among the world boreal forests.