Effects of forest harvesting on soil methane fluxes in Florida slash pine plantations

We examined the effects of forest harvesting on the net exchange of methane (CH4) between the atmosphere and sandy soils in two mature slash pine (Pinus elliottii Engelm. var. elliottii) plantations in northern Florida. Before each of the stem-only harvests, soils in these plantations were net sinks for CH4 (-0.03 to -2.6 mg CH4.m(-2).d(-1)) on all but one sampling date. On this one pre-harvest sampling date, soils in one plantation were net sources of CH4. This emission coincided with an average soil moisture content of 83% water filled pore space (WFPS), which was significantly greater than the average soil moisture contents (25-66% WFPS) for all pre-harvest sampling dates. After harvesting, soils in both plantations became net sources of CH4. Average emission rates from harvested soils ranged from 3 to 11 mg CH4.m(-2).d(-1). Harvested soils were net sources of CH4 for at least 1 year after the harvest. However, the duration of the post-harvest CH4 emission period was reduced by bedding the plantation soils, a typical post-harvest site preparation treatment. Bedded soils in harvested stands were either relatively small net CH4 sources (0.2 mg CH4.m(-2).d(-1)) or net CH4 sinks (-0.4 mg CH4.m(-2).d(-1)). Soil CH4 fluxes were highly correlated with soil moisture contents (r(2) = 0.66 and 0.71; significant at p < 0.05), which were strongly influenced by climate and forest management practices. For example, soil moisture contents for one of our sites, averaged over the pre-harvest, post-harvest, and post-harvest-plus-bedding periods were 46, 68, and 38% WFPS, respectively. Our results suggest that increased soil moisture in southern pine plantations induced by either precipitation events or forest management changes the direction of the soil CH4 flux from sink to source.