Contrasting responses to drought of forest floor CO2 efflux in a Loblolly pine plantation and a nearby Oak‐Hickory forest

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Abstract
Forest floor CO2 efflux (Fff) depends on vegetation type, climate, and soil physical properties. We assessed the effects of biological factors on Fff by comparing a maturing pine plantation (PP) and a nearby mature Oak‐Hickory‐type hardwood forest (HW). Fff was measured continuously with soil chambers connected to an IRGA during 2001–2002. At both sites, Fff depended on soil temperature at 5 cm (T5) when soil was moist (soil moisture, θ>0.20 m3 m−3), and on both T5 and θ when soil was drier. A model (Fff (T5, θ)) explained 92% of the variation in the daily mean Fff at both sites. Higher radiation reaching the ground during the leafless period, and a thinner litter layer because of faster decomposition, probably caused higher soil temperature at HW compared with PP. The annual Fff was estimated at 1330 and 1464 g C m−2 yr−1 for a year with mild drought (2001) at PP and HW, respectively, and 1231 and 1557 g C m−2 yr−1 for a year with severe drought (2002). In the wetter year, higher soil temperature and moisture at HW compared with PP compensated for the negative effect on Fff of the response to these variables resulting in similar annual Fff at both stands. In the drier year, however, the response to soil temperature and moisture was more similar at the two stands causing the difference in the state variables to impel a higher Fff at HW. A simple mass balance indicated that in the wetter year, C in the litter–soil system was at steady state at HW, and was accruing at PP. However, HW was probably losing C from the mineral soil during the severe drought year of 2002, while PP was accumulating C at a lower rate because of a loss of C from the litter layer. Such contrasting behavior of two forest types in close proximity might frustrate attempts to estimate regional carbon (C) fluxes and net C exchange.