Carbon sequestration patterns and forest product yield of two southern pine stands over 75 years of uneven-aged management

Background/Question/Methods

Some of the most important ecosystem services of managed forests are their ability to sequester carbon (C) while simultaneously producing commercial wood products, wildlife habitat, ecological functionality, and recreational opportunities.  However, because of the recency of this issue, few field studies have been able to document the influences of silvicultural practices on C dynamics.  Rather than examining long-term ecological impacts via computer simulations, we adapted inventories and harvest data (pines > 10 cm DBH only) from two uneven-aged southern pine stands on the Crossett Experimental Forest (Arkansas, USA) as the source of our data.  These loblolly (Pinus taeda) and shortleaf (Pinus echinata) pine-dominated stands have been consistently monitored for 75 years.  Over the decades, both have been commercially harvested dozens of times (annually from 1936-1968, then every 5-7 years between 1968-2011), primarily removing large-diameter (sawtimber-sized) pines.  To document the long-term trends of stand-level C sequestration, we applied some basic allometric relationships between tree size, biomass, and C content.  The sequestration contribution of durable wood products (mostly dimensional lumber and plywood panels) was then derived from the relationship between harvested lumber yield and C storage.

Results/Conclusions

The initially better stocked of the two stands contained approximately 34.5 MgC/ha in aboveground live pine C when the study was established in 1934, only modestly different than the average standing C content (37.8 ± 4.3 MgC/ha) of this stand during the next seven decades and the most current inventory of this stand (29.8 MgC/ha in 2012).  The other stand, chosen for its lower initial pine stocking, had approximately half the standing pine C stocks (18.2 MgC/ha) in 1934, but quickly rebounded to a long-term average of 34.3 MgC/ha (± 8.0 MgC/ha) and 29.1 MgC/ha in 2012.  During this 70+ year period, each stand yielded approximately 167 MgC/ha in durable wood products, or just over 2.2 MgC/ha/yr.  Not surprisingly, frequent harvest entries moderated the fluctuations in stand-level C storage, while longer cutting cycles produced more dramatic variation.  The sustainability of these uneven-aged southern pine stands, coupled with their contributions to C storage, high-value wood product yield, and other ecological functions suggest that multiple ecosystem services can be achieved under this management regime.