Patterns of NPP, GPP, respiration, and NEP during boreal forest succession

We combined year-round eddy covariance with biometry and biomass harvests along a chronosequence of boreal forest stands that were 1, 6, 15, 23, 40, similar to 74, and similar to 154 years old to understand how ecosystem production and carbon stocks change during recovery from stand-replacing crown fire. Live biomass (C-live) was low in the 1- and 6-year-old stands, and increased following a logistic pattern to high levels in the 74- and 154-year-old stands. Carbon stocks in the forest floor (C-forest floor) and coarse woody debris (C-CWD) were comparatively high in the 1-year-old stand, reduced in the 6- through 40-year-old stands, and highest in the 74- and 154-year-old stands. Total net primary production (TNPP) was reduced in the 1- and 6-year-old stands, highest in the 23- through 74-year-old stands and somewhat reduced in the 154-year-old stand. The NPP decline at the 154-year-old stand was related to increased autotrophic respiration rather than decreased gross primary production (GPP). Net ecosystem production (NEP), calculated by integrated eddy covariance, indicated the 1- and 6-year-old stands were losing carbon, the 15-year-old stand was gaining a small amount of carbon, the 23- and 74-year-old stands were gaining considerable carbon, and the 40- and 154-year-old stands were gaining modest amounts of carbon. The recovery from fire was rapid; a linear fit through the NEP observations at the 6- and 15-year-old stands indicated the transition from carbon source to sink occurred within 11-12 years. The NEP decline at the 154-year-old stand appears related to increased losses from C-live by tree mortality and possibly from C-forest floor by decomposition. Our findings support the idea that NPP, carbon production efficiency (NPP/GPP), NEP, and carbon storage efficiency (NEP/TNPP) all decrease in old boreal stands.