Complex forest dynamics indicate potential for slowing carbon accumulation in the southeastern United States

Abstract

Over the past century forest regrowth in Europe and North America expanded forest carbon (C) sinks and offset C emissions but future C accumulation is uncertain. Policy makers need insights into forest C dynamics as they anticipate emissions futures and goals. We used land use and forest inventory data to estimate how forest C dynamics have changed in the southeastern United States and attribute changes to land use, management, and disturbance causes. From 2007-2012, forests yielded a net sink of C because of net land use change (+6.48 Tg C yr(-1)) and net biomass accumulation (+75.4 Tg C yr(-1)). Forests disturbed by weather, insect/disease, and fire show dampened yet positive forest C changes (+1.56, +1.4, +5.48 Tg C yr(-1), respectively). Forest cutting caused net decreases in C (-76.7 Tg C yr(-1)) but was offset by forest growth (+143.77 Tg C yr(-1)). Forest growth rates depend on age or stage of development and projected C stock changes indicate a gradual slowing of carbon accumulation with anticipated forest aging (a reduction of 9.5% over the next five years). Additionally, small shifts in land use transitions consistent with economic futures resulted in a 40.6% decrease in C accumulation.

Figure 1. Eleven state study region in the southeastern United States.

Maps were generated using ArcGIS 10.0 (www.esri.com/software/arcgis).

Figure 2. Forest carbon stock changes (Tg C yr⁻¹) resulting from land use dynamics (right side) and forest dynamics within forest land uses (left side).

Line thickness is proportional to the flow. Standard errors are available in Tables S3 and S4.

Figure 3

(a) C stock density change (Mg ha⁻¹ yr⁻¹) by 5-year stand age class (left y, based on initial plot measurements) for forest stands that were not cut between measurements. Forest area in each time 1 age class (right y). (b) C stock density change (Mg ha⁻¹ yr⁻¹) by 5-year stand age class (left y) for all forest stands. Harvested forest area in each age class (right y).

Figure 4. Forest C stock change (Tg C yr⁻¹) from 2012 to 2017 based on integrated land use – forest age structure model.

Scenario 1: forest remaining as forest from 2007-2012 projected to 2017. Scenario 2: Forest remaining as forest from 2012–2017 plus additions to forest land from other land uses. Land use transitions to forest were assumed to occur at the 2007–2012 rates. Scenario 3: Forest remaining as forest from 2012–2017 plus additions to forest land from other land uses. The 2007–2012 land use transition rates for agriculture to forest were decreased by 10% and the transitions from forest to developed were increased by 10%. All other rates remained at observed levels. Scenario 4: Forest remaining as forest from 2012–2017 plus additions to forest land from other land uses. The 2007–2012 land use transition rates for agriculture to forest were reversed.