| Abstract: | The surface water table is an important factor determining soil chemical, physical and biological processes, and thus affects the functions of forested wetlands. The objective of this study was to assess surface water table dynamics from timber harvesting through early forest plantation establishment in a coastal plain wetland area located in the southeastern United States. Simulated harvesting patterns included two replicates of clear-cutting when soils were dry (dry-weather harvest), three replicates of clear-cutting when soils were wet (wet-weather harvest), and one replicate of uncut control in three 20 ha wetland loblolly pine (Pinus taeda L.) forests of ages 20, 23 and 25 years. After harvesting, two site preparation levels (non-bed and bed; bedding is a tillage process of preparing a series of parallel ridges) were randomly assigned to both dry-weather and wet-weather harvested plots, while an additional level (mole-plow+bed) was assigned only to the wet-weather harvested plots. The harvest treatments were designed to create a broad gradient of surface soil disturbance, while the site preparation treatments were done to encompass a range of site drainage and aeration conditions. Areal changes in soil bulk density, macro- and total porosities, and saturated hydraulic conductivity following harvesting were quantified. The depths of water table were recorded at monthly intervals on a 20 m×20 m grid across the 15 clear-cut and three uncut control plots (a total of 1409 PVC slotted wells) over 6 years (1992–1998), subdivided into five periods: pre-harvest, post-harvest, site preparation, and first year and second year after forest plantation establishment. The results showed that compared to the uncut control, the surface water table depth during a 1-year post-harvest period rose 14 cm for the dry-weather harvested site and 21 cm for the wet-weather harvested site. The difference in the water table rise between the two harvest treatments was small during the dormant season (<2 cm) but large during the growing season (>10 cm). These results indicate the large influence of tree removal on the surface hydrology in forested wetlands and the strong impact of wet-weather harvesting on transpiring ground vegetation due to a larger surface area of soil disturbance. Bedding initially lowered water tables on both dry-weather and wet-weather harvested sites. However, this effect decreased rapidly during the first 2 years after forest plantation establishment. Among all treatments, the dry-weather harvested sites without bedding presented the fastest recovery of water table depth to that of the non-harvested references, suggesting that bedding may have been a further disturbance with respect to wetland surface hydrology. |
