Abstract. Planted pine forests (plantations) in the southeastern United States are an
important component of the continent’s carbon balance. Forest carbon dynamics are affected
by a range of factors including climatic variability. Multiyear droughts have affected the
region in the past, and an increase in the frequency of drought events has been predicted. How
this increased climatic variability will affect the capacity of the region’s pine plantations to
sequester carbon is not known. We used eddy covariance and biometric approaches to
measure carbon dynamics over nine years in two slash pine plantations (Pinus elliottii var
elliottii Englm) in north Florida, consisting of a newly planted and a mid-rotation stand.
During this time, the region experienced two multiyear droughts (1999–2002 and 2006–2008),
separated by a three-year wet period. Net ecosystem carbon accumulation measured using
both approaches showed the same trends and magnitudes during plantation development. The
newly planted site released 15.6 Mg C/ha during the first three years after planting, before
becoming a carbon sink in year 4. Increases in carbon uptake during the early stages of stand
development were driven by the aggrading leaf area index (LAI). After canopy closure, both
sites were continuous carbon sinks with net carbon uptake (NEE) fluctuating between 4 and
;8Mg Cha1yr1, depending on environmental conditions. Drought reduced NEE by ;25%
through its negative effects on both LAI and radiation-use efficiency, which resulted in a larger
impact on gross ecosystem carbon exchange than on ecosystem respiration. While results
indicate that responses to drought involved complex interactions among water availability,
LAI, and radiation-use efficiency, these ecosystems remain carbon sinks under current
management strategies and climatic variability. Variation within locations is primarily due to
major disturbances, such as logging in the current study and, to a much lesser extent, local
environmental fluctuations. When data from this study are compared to flux data from a
broad range of forests worldwide, these ecosystems fill a data gap in the warm-temperate zone
and support a broad maximum NEE (for closed-canopy forests) between 88C and 208C mean
annual temperature.
