Biomass accumulation over the first 150 years in coastal Oregon Picea-Tsuga forest

Production and mortality are the component processes that together determine the biomass dynamics of forests. Due to the significant role of forests in the global carbon cycle, it is important to assess how these two processes affect the maximum biomass attained by forests, as well as the dynamics leading up to and following peak biomass. We address these questions for two sets of plots in Picea stichensis-Tsuga heterophylla forest on the northern Oregon coast: that originated from a catastrophic wildfire in the 1840s, using new data on dynamics of live trees and stocks of coarse woody debris (CWD). The set of plots closest to the ocean and occupying steeper, more dissected terrain with areas of thin soils has lower biomass, lower net primary production (NPP) of bole wood and higher, tree mortality as a fraction of standing biomass. The two sets of plots have similar CWD levels, most of which has accumulated in the last 25 yr. The present dispar ity in biomass between the two sets of plots appears to be the result of lower NPP on the low-biomass plots for the entire 140+ yr history of the forest. Over the 58 yr that the high-biomass plots have been measured (from stand age 85 to 143 yr), NPP of bole wood has declined by 41%. Only ca. 6% of this decline can be accounted for by an increase in maintenance respiration of woody tissues. For both sets of plots relative constancy of biomass in the long term appears likely, due to a short time lag in tree regeneration, asynchronous tree mortality and little overall decline in NPP of bole wood in recent decades. However, since tree mortality as a fraction of stranding biomass is higher on the low-biomass plots, and NPP of hole wood is slightly lower, the difference in biomass between the two sets of plots should increase if current rates of production and mortality persist.