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

Abstract

We examined the effects of forest harvesting on the net exchange of methane (CH 4 ) 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 CH 4 (-0.03 to -2.6 mg CH 4 ·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 CH 4 . 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 CH 4 . Average emission rates from harvested soils ranged from 3 to 11 mg CH 4 ·m -2 ·d -1 . Harvested soils were net sources of CH 4 for at least 1 year after the harvest. However, the duration of the post-harvest CH 4 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 CH 4 sources (0.2 mg CH 4 ·m -2 ·d -1 ) or net CH 4 sinks (-0.4 mg CH 4 ·m -2 ·d -1 ). Soil CH 4 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 CH 4 flux from sink to source.